CA2887741A1

CA2887741A1 - Reactor containment cooling system and nuclear power plant

Info

Publication number: CA2887741A1
Application number: CA2887741A
Authority: CA
Inventors: Tomohiko Ikegawa; Takahisa Matsuzaki
Original assignee: Hitachi GE Nuclear Energy Ltd
Current assignee: Hitachi GE Nuclear Energy Ltd
Priority date: 2014-04-10
Filing date: 2015-04-09
Publication date: 2015-10-10
Anticipated expiration: 2035-04-09
Also published as: CA2887741C

Abstract

A reactor containment cooling system and a nuclear power plant that can be started using a passive method. A
reactor containment vessel with a biological shield wall disposed outside thereof, and an airtight containment cooling region therebetween. A containment coolant pool is formed by filling the containment cooling region with coolant, and a heat exchanger is disposed outside thereof.
An upstream-side pipeline connects a suppression chamber space and an upper end of the heat exchanger. A downstream-side pipeline connects the containment cooling region located below a water level in the containment coolant pool and a lower end of the heat exchanger. An externally open line connects at one end to a section of the downstream-side pipeline above the water level and open at the other end.

Description

A
TITLE OF THE INVENTION
REACTOR CONTAINMENT COOLING SYSTEM AND NUCLEAR POWER
PLANT
BACKGROUND OF THE INVENTION
1. Field of the Invention The present invention relates generally to reactor containment cooling systems and nuclear power plants, and more particularly, to those reactor containment cooling system and nuclear power plants that are suitable for boiling-water nuclear power generation.

2. Description of the Art Background For example, JP-1989-91089-A discloses a reactor containment cooling system suitable for removing heat energy released into a reactor containment vessel in case of any trouble with plants. The heat energy in this case is removed over a long period of time by use of the power of nature.
A function of the reactor containment cooling system disclosed in JP-1989-91089-A will be described below with reference to Fig. 7, which is a system diagram showing a configuration of a cooling system similar to the reactor containment cooling system. As shown in Fig. 7, the cooling system for a reactor containment vessel includes, outside the reactor containment vessel 10, a containment coolant pool 5 placed between the reactor containment vessel 10 and a biological shield wall 22. The pool water filling the containment coolant pool 5 is thermally connected to the I
pool water within a suppression pool 14 via a steel wall of the reactor containment vessel 10, the suppression pool 14 being housed in the reactor containment vessel 10.
The reactor containment vessel 10, enclosing a reactor pressure vessel 7, includes an upper drywell 11, a lower drywell 12, and a suppression chamber constituted by a suppression chamber spatial section 13 and the suppression pool 14 filled with a coolant, each divided from the others.
The upper drywell 11 and the suppression pool 14 are coupled to each other via a vertical vent pipe 17 and horizontal vent pipes 18. The upper drywell 11 and the lower drywell 12 are spatially connected through a communicating tube 19 disposed on the vertical vent pipe 17.
The vertical vent pipe 17 is opened in the upper drywell 11, and the horizontal vent pipes 18 are each opened in the suppression pool 14.
The cooling system for the reactor containment vessel has a function to assume an incident damaging a pipeline connected to the reactor pressure vessel 7, and this function works in the reactor containment vessel 10. The following discusses the function. Steam that has passed through the damaged main steam pipe 23 and then flown out from the reactor pressure vessel 7 into the upper drywell 11 or the lower drywell 12 is guided to the suppression pool 14 via the vent pipes 17 and 18 and the communicating tube 19.
The steam is then condensed in the suppression pool 14.
This condense operation prevents an increase in an internal pressure of the reactor containment vessel 10.

A coolant is injected into the reactor pressure vessel 7 by use of either a high-pressure core flooder (HPCF) system (not shown) serving as an active core coolant injection system or a low-pressure core flooder (LPFL) system (not shown). In the latter case, the injection is done after an internal pressure of the reactor pressure vessel has been reduced to a sufficiently low level by opening a main steam relief valve (not shown). Although the steam that has been conducted into the suppression pool 14 increases a temperature of the pool water, the incident can be calmed down or terminated by removing heat by use of a residual-heat removal (RHR) system (not shown).
However, if the RHR system should ever become unusable for a reason such as a loss of ultimate heat sink, heat removal from the suppression pool 14 may be hindered, which could in turn increase the internal pressure of the reactor containment vessel 10 and thus reduce integrity of the reactor containment vessel 10. Even if these conditions actually occur, removing the heat from the suppression pool 14 by the pool water in containment coolant pool 5 via the steel wall of the reactor containment vessel 10 will prevent an increase in the internal pressure of the reactor containment vessel 10, thereby ensuring the integrity of the reactor containment vessel 10 over a long period of time.
SUMMARY OF THE INVENTION
In the event of a rupture accident of the main steam pipe 23 connected to the reactor pressure vessel 7, the

3 employment of the reactor containment cooling system discussed above would allow the pool water within the containment coolant pool 5 to be used to remove heat from the suppression pool 14 that has been heated by the steam generated in the reactor pressure vessel 7. Accordingly, in case of a loss of an active heat-removal function of a pump/heat exchanger system due to a station blackout, the increase in the internal pressure of the reactor containment vessel 10 can be inhibited and hence the reactor containment vessel 10 can be stably cooled for a prolonged period.
If the pool water in the containment coolant pool 5 boils and/or evaporates to decrease in a water level, resulting deterioration of the reactor containment vessel 10 in heat transfer characteristics may cause it to lack a heat removal capability. A gap between the biological shield wall 22 and the reactor containment vessel 10 is dimensionally limited from a perspective of economy and workability, for which reason the containment coolant pool 5 is limited in width. In addition, in terms of preventing a collapse of the reactor containment vessel 10 due to an external water pressure, the water level in the containment coolant pool 5 needs to be maintained at substantially the same as or below the water level in the suppression pool 14.
It is, therefore, impossible to secure the amount of water in the containment coolant pool 5 by raising an initial water level.
The heat removal system for the reactor containment vessel using the containment coolant pool 5 presupposes

4 system usage under a station blackout or other such beyond design-basis accidents or incidents that render active systems unusable. In case of a station blackout, the effectiveness of electric power restoration or of coolant injection into the containment coolant pool 5 from an alternative pump by operating staffs still can be expected.
Nevertheless, given the uncertainty of the restoration timing and the load upon the operating staffs, it is desirable that the exhaustion of the containment coolant pool 5 be prevented with the use of passive tools not depending upon external power or the operating staffs.
The present invention has been made with the foregoing in mind, and an object of the invention is to provide a reactor containment cooling system and nuclear power plant that can be started using a passive method without relying upon an active method that requires valve operation or pump activation.
Any one of the configurations described in the attached claims, for example, is adopted to solve the problems discussed above. The present invention includes a plurality of methods to solve the problems. An example of such methods includes the following: a reactor containment vessel made of steel, enclosing therein a reactor pressure vessel, a drywell space, a suppression chamber space, a suppression pool, a vent pipe, and a vacuum breaker; the reactor pressure vessel containing a core loaded with a plurality of fuel assemblies, the drywell space surrounding the reactor pressure vessel, the suppression chamber space divided from the drywell space by an airtight wall, the suppression pool formed by filling the suppression chamber space with coolant, the vent pipe connecting the drywell space and the suppression pool, the vacuum breaker configured to cause non-condensable gases to flow through to the drywell space from the suppression chamber, a biological shield wall disposed outside the reactor containment vessel;
a containment cooling region defined between the reactor containment vessel and the biological shield wall so as to have airtightness; a containment coolant pool formed by filling the containment cooling region with coolant; a heat exchanger disposed outside the containment cooling region, the heat exchanger being above a water level in the containment coolant pool; an upstream-side pipeline connecting the suppression chamber space and an upper end of the heat exchanger; a downstream-side pipeline connecting the containment cooling region located below the water level in the containment coolant pool and a lower end of the heat exchanger; and an externally open line connecting at one end thereof to a section of the downstream-side pipeline which is located above the water level in the containment coolant pool, the externally open line being opened at the other end thereof to an outside of the containment cooling region.
In accordance with the present invention, a cooling system of a containment coolant pool can be started using a completely passive method without relying upon an active method that requires valve operation or pump activation.
This allows the amount of coolant in the containment coolant pool to be maintained, thus a reactor containment vessel to be cooled stably and continuously over a long period of time, and consequently a highly reliable reactor containment cooling system and nuclear power plant to be provided.
BRIEF DESCRIPTION OF THE DRAWINGS
Fig. 1 is a system diagram showing a configuration of a reactor containment cooling system and nuclear power plant according to a first embodiment of the present invention;
Fig. 2 is a system diagram showing a configuration of a reactor containment cooling system and nuclear power plant according to a second embodiment of the present invention;
Fig. 3 is a system diagram showing a configuration of a reactor containment cooling system and nuclear power plant according to a third embodiment of the present invention;
Fig. 4 is a system diagram showing a configuration of a reactor containment cooling system and nuclear power plant according to a fourth embodiment of the present invention;
Fig. 5 is a system diagram showing a configuration of a reactor containment cooling system and nuclear power plant according to a fifth embodiment of the present invention;
Fig. 6A is a conceptual diagram that shows operation of a water-level control float valve constituting a portion of the reactor containment cooling system and nuclear power plant according to the fifth embodiment of the present invention, the operation being at a time of a high water level;
Fig. 6B is a conceptual diagram that shows operation of the water-level control float valve constituting a portion of the reactor containment cooling system and nuclear power plant according to the fifth embodiment of the present invention, the operation being at a time of a low water level;
Fig. 7 is a system diagram showing a configuration of a conventional cooling system for a reactor containment vessel;
Fig. 8 is a schematic configuration diagram that shows a nuclear power plant applying a reactor containment cooling system according to a sixth embodiment of the present invention;
Fig. 9 is a schematic configuration diagram that shows a feedwater regulator constituting a portion of the reactor containment cooling system shown in Fig. 8 and according to the sixth embodiment of the present invention;
Fig. 10 is an explanatory diagram that shows opening/closing operation of the feedwater regulator constituting a portion of the reactor containment cooling system shown in Fig. 9 and according to the sixth embodiment of the present invention;
Fig. 11 is a schematic configuration diagram showing a reactor containment cooling system according to a seventh embodiment of the present invention;
Fig. 12 is a schematic configuration diagram showing a reactor containment cooling system according to an eighth embodiment of the present invention; and Fig. 13 is a schematic configuration diagram showing a reactor containment cooling system according to a ninth embodiment of the present invention.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
Hereunder, embodiments of a reactor containment cooling system and nuclear power plant according to the present invention will be described with reference to part of the accompanying drawings.
First Embodiment Fig. 1 is a system diagram showing a configuration of a reactor containment cooling system and nuclear power plant according to a first embodiment of the present invention.
The reactor containment cooling system in the present embodiment is applied to a boiling-water nuclear power plant. Referring to Fig. 1, the boiling-water nuclear power plant includes a reactor pressure vessel 7, a core 8, a reactor containment vessel 10, and a reactor containment cooling system. The reactor containment vessel 10 is made of steel, having its outer surface airtightly shrouded by a biological shield wall 22.
The reactor pressure vessel 7 contains the core 8 loaded with a plurality of fuel assemblies (not shown). A
main steam pipe 23, a feedwater pipeline (not shown), and other elements are connected to the reactor pressure vessel 7.
The reactor containment vessel 10, enclosing the reactor pressure vessel 7, includes an upper drywell 11, a lower drywell 12, and a suppression chamber 13 constituted by a suppression chamber spatial section 13a and a suppression pool 14 filled with coolant, each divided from the others. The upper drywell 11 and the lower drywell 12 form a drywell space. The drywell space and the suppression chamber spatial section 13a are divided from each other by a diaphragm floor 15 and a pedestal 16, both of which are airtight walls. Inside the reactor containment vessel 10, nitrogen is substituted for air, whereby oxygen is eliminated for the sake of provision against a contingent hydrogen explosion.
The lower drywell 12 is a region directly under the reactor pressure vessel 7, and is surrounded by the annular suppression chamber 13. The pedestal 16 (airtight wall) is placed in the lower drywell 12, and supports the reactor pressure vessel 7 via a reactor pressure vessel support skirt 20. The lower drywell 12 is divided from the upper drywell 11 by the reactor pressure vessel support skirt 20.
The suppression chamber spatial section 13a is divided from the upper drywell 11 by the diaphragm floor 15 (airtight wall) disposed over the suppression chamber 13.
A vertical vent pipe 17 is placed inside the wall of the pedestal 16. The vertical vent pipe 17 is opened at one end to the upper drywell 11, and is connected at the other end thereof to horizontal vent pipes 18 in the coolant stored within the suppression pool 14. The horizontal vent pipes 18 are each opened in the suppression pool 14. The vertical vent pipe 17 is provided with a communicating tube 19, through which the upper drywell 11 and the lower drywell 12 are spatially connected. In the upper section of the pedestal 16 is placed a vacuum breaker 24 for causing non-condensable gases, generated in the suppression chamber spatial section 13a, to flow through to the lower drywell 12.
The reactor containment cooling system includes: a containment cooling region 6 that is an airtight region present between the reactor containment vessel 10 and the biological shield wall 22; a containment coolant pool 5 present inside the containment cooling region 6 and disposed outside the suppression pool 14; a heat exchanger 1 for cooling the pool water stored within the containment coolant pool 5; an upstream-side pipeline 2 connecting an upper section (upstream end) of the heat exchanger 1 and an upper section of the containment cooling region 6; a downstream-side pipeline 3 connecting a lower section (downstream end) of the heat exchanger 1 and a lower section of the containment cooling region 6; and an externally open line 4 formed into an L-shape, connecting at one end of the line 4 to the downstream-side pipeline 3, and opened upward to an external environment, the externally open line 4 thereby activating the heat exchanger 1 in a passive way.
The heat exchanger 1 includes an upstream-side header la disposed at its upper section and connected to the upper section of the containment cooling region 6 via the upstream-side pipeline 2, a downstream-side header lb disposed at its lower section and connected to the lower section of the containment cooling region 6 via the downstream-side pipeline 3, and a plurality of heat transfer pipes lc each connected at an upper end thereof to the upstream-side header la and at a lower end thereof to the downstream-side header lb.
The containment cooling region 6 is disposed outside the reactor containment vessel 10, and the heat exchanger 1 is disposed outside the containment cooling region 6. The lower end of the heat exchanger 1 and a connecting portion 21 at the one end of the externally open line 4 are disposed above the water level in the containment coolant pool 5.
In the reactor containment cooling system having the above configuration, a spatial section of the containment cooling region 6 is filled with air under normal operating conditions of the nuclear power plant. Likewise, the upstream-side pipeline 2, the heat exchanger 1, and the externally open line 4 are internally filled with air. By contrast, an internal region of the downstream-side pipeline 3 that is below the water level in the containment coolant pool 5 is filled with water, and an internal region above the water level in the containment coolant pool 5 is filled with air.
Such a state of the nuclear power plant under normal operating conditions, that is, an initial state of the reactor containment cooling system, is automatically accomplished from a natural balance of pressures without any need for particular operation or motive power. For the sake of enhancing heat-removal capability of the reactor containment vessel, the section for connecting the upstream-side pipeline 2 and the containment cooling region 6 should preferably be disposed near an upper end of the containment cooling region 6. In addition, for a purpose of submerging the internal region of the downstream-side pipeline 3 as deeply as possible into the water, the section for connecting the downstream-side pipeline 3 and the containment cooling region 6 should preferably be disposed near a lower end of the containment cooling region 6.
One structural feature of the first embodiment of the present invention resides in that the reactor containment cooling system's configuration includes no valves or pumps.
Accordingly, failures due to valve operation or pump activation can be eliminated fundamentally. Another structural feature of the first embodiment of the present invention resides in that the internal region of the downstream-side pipeline 3 that is below the water level in the containment coolant pool 5 is submerged in the water.
This prevents back flow of steam and air from the downstream-side pipeline 3 to the heat exchanger 1, thus allowing for stabilized flow rate of steam and water in the direction having the order of the upstream-side pipeline 2, heat exchanger 1, and downstream-side pipeline 3.
Operation of the reactor containment cooling system and nuclear power plant, based on an example of assuming a loss-of-coolant accident (LOCA) situation due to a severe breakage of one main steam pipe in the first embodiment of the present invention, will be described below.

Referring again to Fig. 1, if the main steam pipe 23 breaks, steam that has been generated in the reactor pressure vessel 7 flows out from the break. As a consequence, the water level 9 and pressure in the reactor pressure vessel 7 both decrease. A main steam isolating valve (not shown) is fully closed by virtue of a signal denoting an excessive flow rate of the steam inside the main steam pipe. A scram signal is then generated by the main steam isolating valve closing signal, followed by all control rods being inserted into the core 8. The reactor is thus shut down.
After the reactor has been shut down by the scram, decay heat associated with nuclear decay of fission products (FPs) present in the core 8 causes steam to continue to be generated and hence the amount of coolant in the reactor pressure vessel 7 to keep decreasing although an amount of the decay heat, being as small as several percent or less of rated heat output, decreases exponentially with time. For these reasons, there is a need to continuously cool the core 8 by maintaining the water level 9 within the reactor pressure vessel 7 and mitigate any increase in the internal pressure of the reactor containment vessel 10 by condensing the steam occurring in the core 8, and thus to vent the decay heat from the reactor containment vessel 10 to the outside the system.
The condensation of the steam occurring is first described here. Upon steam flowing out from the break in the main steam pipe 23 to the upper drywell 11, the upper drywell 11 increases an internal pressure thereof, which lowers the water level in the vertical vent pipe 17. After the water level in the vertical vent pipe 17 has thus been lowered to a level lower than a steam outflow port in the horizontal vent pipes 18, nitrogen and steam flow into the suppression pool 14 from the upper drywell 11. The nitrogen that has flown into the suppression pool 14 accumulates in the suppression chamber spatial section 13a to increase the internal pressure of the suppression chamber spatial section 13a. However, a rate of volume between the upper drywell 11 and the suppression chamber spatial section 13a is designed to fall within an appropriate range to control increase rate in pressure below a certain level.
If for a reason such as an event of flow of a non-condensable gas into the suppression chamber spatial section 13a, the pressure in the drywell space becomes a negative pressure with respect to that of the suppression chamber spatial section 13a, the vacuum breaker 24 will open and thus the non-condensable gas that has flown into the suppression chamber spatial section 13a will flow through to the lower drywell 12. This helps prevent an increase in differential pressure between the internal pressure of the suppression chamber spatial section 13a and that of the drywell space.
The steam that has flown into the suppression pool 14 is condensed back to water by subcooled water present in the suppression pool 14. The increase in the internal pressure of the reactor containment vessel 10 due to the generation of the steam can be prevented.
The maintenance of the water level 9 in the reactor pressure vessel 7 and the heat removal from the reactor containment vessel 10 will now be described. In case of severe LOCA, when an external power source or emergency power source is available, use of a residual-heat removal (RHR) system, which is one of major constituent elements of an emergency core-cooling system (ECCS), allows coolant injection into the reactor pressure vessel 7 (LPFL), spray injection into the reactor containment vessel 10, and cooling of the suppression pool 14. As long as the water level 9 is maintained above the core 8 by injecting coolant into the reactor pressure vessel 7, stable cooling of the core 8 will be continued. Furthermore, removing heat from the reactor containment vessel 10 will safely terminate the severe LOCA.
In addition, assuming a loss of ultimate heat sink (LUHS), where seawater used for cooling becomes unusable, is postulated as an example of a beyond design-basis accident in terms of an idea of defense in depth, a heat removal function of the RHR system will become unusable. This may cause progressive increase in temperature of the water inside the suppression pool 14 and progressive increase in internal pressure of the reactor containment vessel 10. The following describes how the reactor containment cooling system and nuclear power plant according to the first embodiment of the present invention operates under such a beyond design-basis accident.

=
If severe LOCA occurs to release steam from the main steam pipe 23 into the reactor containment vessel 10, the nitrogen existing in the upper drywell 11 and the lower drywell 12 will migrate into the suppression chamber spatial section 13a via the vertical vent pipe 17 and the horizontal vent pipes 18. In a case where the severe LOCA further carries on, substantially all the amount of nitrogen in the upper drywell 11 and the lower drywell 12 would migrate into the suppression chamber spatial section 13a, accompanied by the result that the upper drywell 11 and the lower drywell 12 become filled with the steam released from a breakage portion of the main steam pipe 23 connected to the reactor pressure vessel 7.
A flow of the steam into the suppression pool 14 via the vertical vent pipe 17 and the horizontal vent pipes 18 raises the temperature of the water in the suppression pool 14. A resulting difference in the pool water temperature between the suppression pool 14 and the containment coolant pool 5 serves as a driving force for heat release from the suppression pool 14 to the containment coolant pool 5 via the steel wall of the reactor containment vessel 10.
Consequently the pool water in the suppression pool 14 is cooled and that of the containment coolant pool 5 is heated.
Since the pressure in the reactor containment vessel rises to a maximum of nearly three atmospheres under the LOCA, the pool water in the suppression pool 14 exceeds 100 C in saturation temperature. By contrast, since the containment cooling region 6 is connected to the atmosphere via the externally open line 4, the containment cooling region 6 has a pressure equal to the atmospheric pressure and thus the pool water in the containment coolant pool 5 boils at 100 C. Steam generated in the containment coolant pool 5 is conducted, along with the air that has filled the containment cooling region 6 before the steam was generated, into the heat exchanger 1 via the upstream-side pipeline 2.
Of the air and steam that have been conducted into the heat exchanger 1, only the steam is condensed by the heat exchanger 1. The resulting condensate falls into the downstream-side pipeline 3, whereas only the air is released from the other end of the externally open line 4.
The connecting portion 21 at the one end of the externally open line 4 is placed downstream of the heat exchanger 1 and the inside of the downstream-side pipeline 3 is filled with water. For this reason, steam does not outflow into the externally open line 4 via the downstream-side pipeline 3, while the steam generated in the containment cooling region 6 is always introduced into the externally open line 4 via the upstream-side pipeline 2 and the heat exchanger 1. In accordance with the present embodiment, the possibility that the steam generated in the containment cooling region 6 would be released to the outside without involving the heat exchanger 1 can be eliminated fundamentally. This in turn prevents a loss of the pool water in the containment coolant pool 5 due to release of water vapor via the externally open line 4.
As described above, only air is pushed out to the outside from the other end of the externally open line 4, so that the containment cooling region 6 becomes nearly filled with steam. Once this state has been reached, heat transfer from the high-temperature steam in the upper drywell 11 to the 100 C low-temperature steam in the containment cooling region 6 via the steel wall of the reactor containment vessel 10 becomes more efficient than under the nearly air-filled state of the containment cooling region 6. The result is that heat becomes more efficiently removable from the reactor containment vessel 10. Additionally, the heat exchanger 1 can maximize its heat removal performance since air, which is a non-condensable gas, is eliminated.
Furthermore, in the event that the steam in the containment cooling region 6 condenses and the internal pressure therein lowers for some reason, the containment cooling region 6 will not collapse due to an outside air pressure since the reactor containment cooling system and nuclear power plant according to the first embodiment of the present invention can take in air via the externally open line 4.
In the first embodiment of the present invention, the reactor containment cooling system and nuclear power plant can start and operate the cooling system for the containment coolant pool 5 using a completely passive method without relying upon an active method such as valve operation or pump activation. This allows the amount of water in the containment coolant pool 5 to be maintained, thus the reactor containment vessel 10 to be cooled stably and continuously over a long period of time. Consequently, the reactor containment cooling system and nuclear power plant of high reliability can be provided.
Second Embodiment Hereunder, a second embodiment of a reactor containment cooling system and nuclear power plant according to the present invention will be described with reference to part of the accompanying drawings. Fig. 2 is a system diagram showing a configuration of the reactor containment cooling system and nuclear power plant according to the second embodiment of the present invention. Referring to Fig. 2, the same reference numbers as those shown in Fig. 1 denote the same elements and detailed description of these elements is thus omitted.
In the second embodiment of the reactor containment cooling system and nuclear power plant according to the present invention, the reactor containment cooling system's configuration is almost the same as that of the first embodiment, as shown in Fig. 2. However, the system configuration of the second embodiment differs from that of the first embodiment in that: a check valve 25 for permitting fluid flow in a direction from the downstream-side pipeline 3 to the outside and stopping fluid flow in a direction from the outside to the downstream-side pipeline 3 is disposed on the externally open line 4; a vacuum breaker 26 for the reactor containment cooling system, adapted to establish communication between the containment cooling region 6 and the outside, is disposed above the water level in the containment coolant pool 5 in the biological shield wall 22; and a fluid discharge destination from the other end of the externally open line 4 is changed to an exhaust tower 27 enabling a fluid to be discharged to the outside from a sufficiently high position.
Operation in the present embodiment when assuming beyond design-basis accidents will now be described. The beyond design-basis accidents here mean occurrences of both a severe breakage of one main steam pipe, which is a loss-of-coolant accident (LOCA), and a loss of ultimate heat sink (LUHS), where the ultimate heat sink cannot be used. The functions of the reactor containment vessel 10, and an event transition under a severe LOCA not associated with LUHS are the same as in the first embodiment, and thus, description of these items is omitted. The following description, therefore, focuses primarily upon operation of the reactor containment cooling system under a combination of the severe LOCA and LUHS.
If a severe LOCA occurs and steam is released from the main steam pipe 23 into the reactor containment vessel 10, nitrogen present in the upper drywell 11 and the lower drywell 12 will migrate into the suppression chamber spatial section 13a via the vertical vent pipe 17 and the horizontal vent pipes 18. In a situation that the severe LOCA further continues, almost all the amount of nitrogen in the upper drywell 11 and the lower drywell 12 would migrate into the suppression chamber spatial section 13a, accompanied by the upper drywell 11 and the lower drywell 12 becoming filled with the steam released from breakage of the main steam pipe 23 connected to the reactor pressure vessel 7.
A flow of the steam into the suppression pool 14 via the vertical vent pipe 17 and the horizontal vent pipes 18 increases the temperature of the pool water in the suppression pool 14. A resulting difference in pool water temperature between the suppression pool 14 and the containment coolant pool 5 serves as a driving force for heat release from the suppression pool 14 to the containment coolant pool 5 via the steel wall of the reactor containment vessel 10. Consequently the pool water in the suppression pool 14 is cooled and that of the containment coolant pool 5 is heated.
Since the pressure in the reactor containment vessel rises to a maximum of nearly three atmospheres under the LOCA, the pool water in the suppression pool 14 exceeds 100 C in saturation temperature. By contrast, since the containment cooling region 6 is connected to the atmosphere via the externally open line 4, the containment cooling region 6 has a pressure equal to the atmospheric pressure and thus the pool water in the containment coolant pool 5 boils at 100 C. The generated steam is conducted, along with the air that has filled the containment cooling region 6 before the steam was generated, into the heat exchanger 1 via the upstream-side pipeline 2. Of the air and steam that have been conducted into the heat exchanger 1, only the steam is condensed by the heat exchanger 1. The resulting condensate falls into the downstream-side pipeline 3, whereas only the air is released from the externally open line 4 via the check valve 25.
The connecting portion 21 at one end of the externally open line 4 is placed downstream of the heat exchanger 1 and the inside of the downstream-side pipeline 3 is filled with water. For this reason, steam does not flow into the externally open line 4 via the downstream-side pipeline 3 and the steam generated in the containment cooling region 6 is always introduced into the externally open line 4 via the upstream-side pipeline 2 and the heat exchanger 1. In accordance with the present embodiment, a possibility that the steam generated in the containment cooling region 6 would be released to the outside without involving the heat exchanger 1 can be eliminated fundamentally. This in turn prevents a loss of the coolant in the containment coolant pool 5 due to release of water vapor via the externally open line 4.
In the present embodiment, since the check valve 25 is placed on the externally open line 4, the air that has been pushed out to the outside does not flow back into the heat exchanger 1. The heat exchanger 1 is capable of always maximizing its heat removal performance.
As described above, only the air is pushed out to the outside from the other end of the externally open line 4, so that the containment cooling region 6 becomes nearly filled with steam. Once this state has been reached, heat transfer from the high-temperature steam in the upper drywell 11 to the 100 C low-temperature steam in the containment cooling region 6 via the steel wall of the reactor containment vessel 10 becomes more efficient than under the nearly air-filled state of the containment cooling region 6. The result is that the heat becomes further efficiently removable from the reactor containment vessel 10.
Additionally, the heat exchanger 1 is capable of maximizing its heat removal performance since air, which is a non-condensable gas, is eliminated.
Furthermore, in the event that the steam in the containment cooling region 6 condenses and the internal pressure of containment cooling region 6 goes down for some reason, the containment cooling region 6 would not collapse due to an outside air pressure since the reactor containment cooling system and nuclear power plant according to the present embodiment is adapted to take in outside air and non-condensable gases (cause these fluids to flow through) via the vacuum breaker 26 for the containment cooling system.
Moreover, in a case where small-scale leakage from the reactor containment vessel 10 made of steel occurs and any gases containing fission products (FPs) leak out into the containment cooling region 6, the present embodiment would guide the leaking gases to the exhaust tower 27 via the externally open line 4, thus releasing the gases to the outside from a sufficiently high position. The risk of external radiation exposure will be further reduced as a result.

The second embodiment of the reactor containment cooling system and nuclear power plant according to the present invention yields substantially the same advantageous effects as those achievable in the first embodiment of the present invention.
Third Embodiment Hereunder, a third embodiment of a reactor containment cooling system and nuclear power plant according to the present invention will be described with reference to part of the accompanying drawings. Fig. 3 is a system diagram showing a configuration of the reactor containment cooling system and nuclear power plant according to the third embodiment of the present invention. Referring to Fig. 3, the same reference numbers as those shown in Figs. 1 and 2 denote the same elements and detailed description of these elements is thus omitted.
In the third embodiment of the reactor containment cooling system and nuclear power plant according to the present invention, the reactor containment cooling system's configuration is almost the same as that of the first embodiment as shown in Fig. 3. However, the system configuration of the third embodiment differs from that of the first embodiment in that: the heat exchanger 1 is disposed in an air-cooling vertical duct 28; the connecting portion 21 at one end of the externally open line 4 is disposed on the upper section of the downstream-side header lb of the heat exchanger 1; and the other end of the externally open line 4 is opened downward at a position lower than the heat exchanger 1. The air-cooling vertical duct 28, a cylindrical flow passageway with its upper and lower sections being open to the atmosphere, is adapted to rectify a flow of air to enhance natural circulation force.
The cylindrical flow passageway can take various sectional shapes, such as circular and rectangular.
Operation in the present embodiment when assuming beyond design-basis accidents will now be described. The beyond design-basis accidents here mean occurrences of both a severe breakage of one main steam pipe, that is, a loss-of-coolant accident (LOCA), and a loss of ultimate heat sink (LUHS) where the ultimate heat sink cannot be used. The present embodiment is substantially the same as the first embodiment except for the placement of the air-cooling vertical duct 28, a change of a fluid discharge destination from the externally open line 4, and a change of connection of the externally open line 4 to the downstream-side header lb of the heat exchanger 1. Detailed description of the present embodiment is therefore omitted and the following description focuses only upon advantageous effects obtained through the above changes.
Condensation of steam inside tubes of the heat exchanger 1 causes a temperature of air outside the tubes of the heat exchanger 1 to rise. Since hot air has a density smaller than that of cold air, the heated air outside the tubes of the heat exchanger 1 rises by reason of buoyancy, and in addition, cold air is supplied from below automatically. That is to say, so-called natural circulation force can be obtained. If the heat exchanger 1 is placed alone without the air-cooling vertical duct 28, however, sufficient natural circulation force may not be obtained since the air that has risen because of its buoyancy may lose the buoyancy as a result of rapidly being mixed with a large amount of air present above the heat exchanger 1. The placement of the heat exchanger 1 inside the air-cooling vertical duct 28 allows limitation on the amount of the air which mixes with the heated air, and thereby enables the cooling system to obtain stronger natural circulation force than that without the air-cooling vertical duct 28.
Additionally in the present embodiment, an opening in the other end of the externally open line 4 is disposed at a lower side of the air-cooling vertical duct 28. This enables air in the containment cooling region 6, heated via the reactor containment vessel 10, and hot steam that has not been condensed in the heat exchanger 1 to be released to a vicinity of an entrance of the air-cooling vertical duct 28. When the hot steam and hot air that have been released to the vicinity of the entrance of the air-cooling vertical duct 28 ascend inside the duct 28 by reason of their own buoyancy, the hot steam and the hot air can both generate a natural circulation force different from that generated outside the tubes of the heat exchanger 1.
In this way, the present embodiment makes it possible to generate strong circulation force inside the air-cooling vertical duct 28 to enhance steam-condensing performance of the heat exchanger 1.
If the externally open line 4 is disposed with the opening of the other end directed downward as in the present embodiment, the pool water in the containment coolant pool 5 could accidentally flow out for some reason. In the present embodiment, therefore, the connecting portion 21 at the one end of the externally open line 4 is disposed on the upper section of the downstream-side header lb of the heat exchanger 1 that has a larger volume than the downstream-side pipeline 3 and other pipes or tubes. In this way, a risk of accidentally releasing condensate is reduced.
The third embodiment of the reactor containment cooling system and nuclear power plant according to the present invention yields substantially the same advantageous effects as those achievable in the first embodiment of the present invention.
Additionally in the third embodiment of the reactor containment cooling system and nuclear power plant according to the present invention, since the heat exchanger 1 is disposed inside the air-cooling vertical duct 28, the heat removal performance of the heat exchanger 1 is further enhanced. The reactor containment cooling system and nuclear power plant of high reliability can be thus provided.
Fourth Embodiment Hereunder, a fourth embodiment of a reactor containment cooling system and nuclear power plant according to the present invention will be described with reference to part of the accompanying drawings. Fig. 4 is a system diagram showing a configuration of the reactor containment cooling system and nuclear power plant according to the fourth embodiment of the present invention. Referring to Fig. 4, the same reference numbers as those shown in Figs. 1 to 3 denote the same elements and detailed description of these elements is omitted.
In the fourth embodiment of the reactor containment cooling system and nuclear power plant according to the present invention, the reactor containment cooling system's configuration is almost the same as that of the first embodiment, as shown in Fig. 4. However, the system configuration of the fourth embodiment differs from that of the first embodiment in that: a heat exchanger coolant pool 29 is provided to cool a heat exchanger 1; and the heat exchanger 1 is submerged in heat exchanger coolant pool water 30 of the heat exchanger coolant pool 29. The heat exchanger coolant pool 29 is disposed with its bottom surface above the water level in a containment coolant pool

5. A connecting portion 21 at one end of an externally open line 4 is a part of a downstream-side pipeline 3 extending at almost all its sections to the outside of the heat exchanger coolant pool 29, the connecting portion 21 being disposed above the water level in the containment coolant pool 5. The other end of the externally open line 4 opens upward.

Operation in the present embodiment when assuming beyond design-basis accidents will be described as follows.
The beyond design-basis accidents here mean occurrences of both a severe breakage of one main steam pipe, which is a loss-of-coolant accident (LOCA), and a loss of ultimate heat sink (LUHS), where the ultimate heat sink cannot be used.
Except for the heat exchanger coolant pool 29 and the heat exchanger coolant pool water 30, the present embodiment is substantially the same as the first embodiment. Detailed description of the present embodiment is therefore omitted and the following description focuses only upon advantageous effects obtained through related changes.
An amount of heat needed to be removed by use of the heat exchanger 1 varies with time. As described in the previous embodiments of the present invention, after a scram the core 8 continues to release decay heat, it is necessary to continuously reduce post-accident decay heat and cool the reactor containment vessel 10. The amount of decay heat released will exponentially decrease with time.
Accordingly, a large amount of heat removal is required at an initial stage of the accident, while a small amount of heat removal is required after an elapse of a certain deal of time from the occurrence of the accident. When an air-cooled type of heat exchanger 1 is used as in the first to third embodiments, since there is a need to determine a size (heat transfer area) of the heat exchanger 1 so that a large amount of decay heat can be removed at the initial stage of the accident, the size of the heat exchanger 1 is estimated to be relatively large.
In the present embodiment, the heat exchanger 1 is submerged in the heat exchanger coolant pool water 30. In general, heat transfer efficiencies of subcooled water and cooled water are at least twenty times as high as that of air, which means that the post-accident initial large amount of heat removal can be realized with a relatively small-size heat exchanger.
Continued post-accident removal of decay heat with the heat exchanger 1 raises the temperature of the heat exchanger coolant pool water 30, resulting in the water 30 boiling. Boiling reduces the water level to cause the heat exchanger 1 to become exposed above the water level in the heat exchanger coolant pool water 30. The exposed section of the heat exchanger 1 is consequently subjected to steam cooling, changing from water cooling. Although this change reduces the amount of heat removal with the heat exchanger 1, since the amount of decay heat to be removed decreases after a certain amount of time from the occurrence of the accident, the heat exchanger 1 with its heat removal capability decreased still can operate. After becoming completely exposed above the water level, the heat exchanger 1 starts functioning as an air-cooled heat exchanger, as in the other embodiments of the present invention.
As described above, the present embodiment maintains substantially the same advantageous effects as those of the first embodiment. On top of that, the heat exchanger 1 can be customized to have a size allowing air-cooling removal of the amount of decay heat that has decreased after the elapse of a certain time from the occurrence of an accident, instead of a size allowing air-cooling removal of the large amount of decay heat at the initial stage of the accident.
Accordingly, the heat exchanger 1 can be reduced in its dimensions, while at the same time a reactor containment cooling system and a nuclear power plant which are highly reliable and economically efficient are provided.
The fourth embodiment of the reactor containment cooling system and nuclear power plant according to the present invention yields substantially the same advantageous effects as those achievable in the first embodiment of the present invention.
Although not shown in the present embodiment, natural circulation force can likewise be enhanced by disposing the air-cooling vertical duct 28 in a divided inner region of the heat exchanger coolant pool 29 as in the third embodiment. Adoption of such a configuration will enhance heat removal performance of the heat exchanger 1 serving as an air-cooled heat exchanger after the heat exchanger 1 has changed from its original water-cooled type to an air-cooled type, and further reduction in the size of the heat exchanger 1 will be achievable as a result.
In addition, if as in the second embodiment, the check valve 25 is disposed on the externally open line 4 and also the vacuum breaker 26 for the reactor containment cooling system is disposed on an upper section of the containment cooling region 6, then back flow of air from the externally open line 4 to the heat exchanger 1 can be prevented. A risk of heat exchanger performance deterioration due to inflow of air into the heat exchanger 1 can be thus reduced.
Fifth Embodiment Hereunder, a fifth embodiment of a reactor containment cooling system and nuclear power plant according to the present invention will be described with reference to part of the accompanying drawings. Fig. 5 is a system diagram showing a configuration of the reactor containment cooling system and nuclear power plant according to the fifth embodiment of the present invention. Fig. 6A is a conceptual diagram that shows operation of a water-level control float valve constituting a portion of the reactor containment cooling system and nuclear power plant according to the fifth embodiment of the present invention, the operation being at a time of a high water level. Fig. 6B is a conceptual diagram that shows operation of the water-level control float valve constituting a portion of the reactor containment cooling system and nuclear power plant according to the fifth embodiment of the present invention, the operation being at a time of a low water level. Referring to Figs. 5 to 6B, the same reference numbers as those shown in Figs. 1 to 4 denote the same elements and detailed description of these elements is omitted.
In the fifth embodiment of the reactor containment cooling system and nuclear power plant according to the present invention, the reactor containment cooling system's configuration is almost the same as that of the fourth embodiment as shown in Fig. 5. However, the system configuration of the fifth embodiment differs from that of the fourth embodiment in that: a heat exchanger coolant pool 29 is expanded in a downward direction, with its bottom surface being substantially flush with a bottom surface of a reactor containment vessel 10; a coolant injection line 31 is disposed for connecting a neighborhood of a bottom section of the heat exchanger coolant pool 29 and a neighborhood of a bottom section of a containment coolant pool 5; a coolant injection valve 32 with an automatic water-level control function is disposed at an end of the coolant injection line 31 that connects to the containment coolant pool 5; a connecting portion 21 at one end of an externally open line 4 is disposed on an upper section of a downstream-side header lb of a heat exchanger 1; and the other end of the externally open line 4 opens downward to the outside, at a position below the downstream-side header lb of the heat exchanger 1.
The automatic water-level control function of the coolant injection valve 32 controls opening/closing of the coolant injection valve 32 in accordance with a particular water level in the containment coolant pool 5, and thereby controls the amount of water supplied from the heat exchanger coolant pool 29 to the containment coolant pool 5.
The present embodiment uses a water-level control float valve as the coolant injection valve 32 having such a control function. The coolant injection valve 32 will be described below with reference to Figs. 6A and 6B.
Referring to Figs. 6A and 6B, the coolant injection valve 32 includes a lid 32a for blocking/unblocking an open end of the coolant injection line 31 that is positioned inside the containment coolant pool 5, a float 33 that floats on the surface of the pool water in the containment coolant pool 5, and a rod 32b of an inverse L-shape that couples the lid 32a and the float 33 together. The lid 32a has a lower end pivotally supported by a lower end of the open end of the coolant injection line 31 that is positioned inside the containment coolant pool 5, and the coolant injection valve 32 closes when the lid 32a takes an upright position to cover the entire open end of the coolant injection line 31 that is positioned inside the containment coolant pool 5. The coolant injection valve 32 opens when the lid 32a inclines, turning on an axis of its lower end, from the upright position and uncovers the open end of the coolant injection line 31, the open end being positioned inside the containment coolant pool 5.
When the water level in the containment coolant pool is high as in Fig. 6A, the coolant injection valve 32 closes since the lid 32a coupled to the float 33 by way of the rod 32b assumes the upright position using buoyancy of the float 33. Conversely, when the water level in the containment coolant pool 5 is low as in Fig. 6B, the float 33 moves downward and the lid 32a coupled thereto by way of the rod 32b inclines turning on the axis of its lower end, so that the coolant injection valve 32 opens. In this way, the coolant injection valve 32 is adapted to establish communication or cutoff of the coolant injection line 31 automatically, depending upon the water level in the containment coolant pool 5.
Operation in the present embodiment when assuming beyond design-basis accidents will now be described. The beyond design-basis accidents here mean occurrences of both a severe breakage of one main steam pipe, which is a loss-of-coolant accident (LOCA), and a loss of ultimate heat sink (LUHS), where the ultimate heat sink cannot be used. Except for the coolant injection line 31, the coolant injection valve 32, and arrangement of the externally open line 4, the present embodiment is substantially the same as the fourth embodiment. Detailed description of the present embodiment is therefore omitted and the following description focuses only upon advantageous effects obtained by conducting changes upon the above elements.
For example, if the heat removal performance of the heat exchanger 1 does not suffice for the removal of decay heat, steam that has not been condensed by the heat exchanger 1 is released to the outside via the externally open line 4. In this case, a decrease in the water level in the containment coolant pool 5 may lower cooling performance for the reactor containment vessel 10, as well as integrity of the reactor containment vessel 10.
In the present embodiment, when the water level in the containment coolant pool 5 decreases, the coolant injection valve 32 at the end portion of the coolant injection line 31 that is positioned in the containment coolant pool 5 opens automatically to supply the heat exchanger coolant pool water 30 within the heat exchanger coolant pool 29 to the containment coolant pool 5, thereby preventing the cooling performance of the reactor containment vessel 10 from deteriorating.
When the water level in the containment coolant pool increases to a definite position, the coolant injection valve 32 closes automatically to prevent an oversupply of coolant from migrating to the containment coolant pool 5.
This function causes the water level in the containment coolant pool 5 to remain substantially at the same level as, or below, the water level in the suppression pool 14, and hence inhibits a collapse of the reactor containment vessel due to the water pressure in the containment coolant pool 5.
If the blocking function of the float valve ever fails to operate, however, resulting increase in the water level in the containment coolant pool 5 might lead to a collapse of the reactor containment vessel 10. In the present embodiment, the opening in the other end of the externally open line 4 is disposed below the one end thereof to completely eliminate the likelihood of such a collapse.
Accordingly, if the water level in the containment coolant pool 5 increases, superfluous water is released to the outside via the connecting portion 21 at the one end of the externally open line 4. This release enables a maximum water level in the containment coolant pool 5 to be limited to the height of the connecting portion 21 located at the one end of the externally open line 4.
If the externally open line 4 is disposed with the opening of the other end directed downward as in the present embodiment, the pool water in the containment coolant pool 5 could accidentally flow out for some reason. In the present embodiment, therefore, the one end of the externally open line 4 is connected to the connecting portion 21 on an upper section of the downstream-side header lb of the heat exchanger 1 that has a larger volume than the downstream-side pipeline 3 and other pipes or tubes. In this way, the risk of accidentally releasing the condensate is lowered.
The fifth embodiment of the reactor containment cooling system and nuclear power plant according to the present invention yields substantially the same advantageous effects as those achievable in the first embodiment of the present invention.
Additionally, the fifth embodiment of the reactor containment cooling system and nuclear power plant according to the present invention includes both the coolant injection line 31 for connecting the neighborhood of the bottom section of the heat exchanger coolant pool 29 and the neighborhood of the bottom section of the containment coolant pool 5, and the coolant injection valve 32 with the automatic water-level control function. The embodiment can therefore prevent, while maintaining substantially the same advantageous effects as those of the fourth embodiment, the cooling performance of the reactor containment vessel 10 from deteriorating in the event that the water level in the containment coolant pool 5 increases. As a result, the reactor containment cooling system and the nuclear power plant can be further enhanced in reliability.
Furthermore, since the fifth embodiment of the present invention includes both the coolant injection line 31 for connecting the neighborhood of the bottom section of the heat exchanger coolant pool 29 and the neighborhood of the bottom section of the containment coolant pool 5, and the coolant injection valve 32 with the automatic water-level control function, the fifth embodiment, while at the same time maintaining substantially the same advantageous effects as those of the fourth embodiment, enables the heat exchanger 1 to be customized to have a lower heat-removal performance. This renders the reactor containment cooling system and the nuclear power plant highly economical.
The sixth to ninth embodiments of a reactor containment cooling system according to the present invention will now be described with reference to part of the accompanying drawings. The embodiments of the reactor containment cooling system according to the present invention have been made for solving following problems.
In conventional advanced boiling water reactors (ABWRs), the steam generated in the reactor pressure vessel from the decay heat occurring in the core after a reactor scram is introduced into the suppression pool in the reactor containment vessel and condensed therein to prevent pressurization of the reactor pressure vessel and the reactor containment vessel. Simultaneously, steam introduced into the suppression pool drives a pump of a reactor isolation cooling system and activates the pump to pump up coolant from the suppression pool. This coolant is then injected into the reactor pressure vessel for continued flooding and cooling of the core.
In this way, the decay heat that has occurred in the core moves from the reactor pressure vessel to the reactor containment vessel. When a pump of a residual-heat removal system is in operation, the coolant that has been heated to a high temperature in the suppression pool by the movement of the decay heat is delivered to a heat exchanger located outside the reactor containment vessel by the pump. The decay heat is removed from the system through the heat exchanger to prevent an internal temperature and pressure of the reactor containment vessel from rising. If the residual-heat removal system cannot be operated for a reason such as a station blackout, however, long-term gentle increase in the internal temperature and pressure of the reactor containment vessel may affect a radioactive materials containment function thereof.
Accordingly, for example the disclosure in JP-1992-125495-A is proposed as a system that removes decay heat by passively cooling a reactor containment vessel under a station blackout or other accident situations without using active devices such as pumps. The system described in JP-1992-125495-A includes an outer circumferential pool around an outer circumference of a steel-made reactor containment vessel. The system uses a temperature difference between the outer circumferential pool and a suppression pool to transfer heat to the outer circumferential pool with an outer surface of the reactor containment vessel as a heat transfer surface and eventually release the steam evaporated from pool water of the outer circumferential pool to the outside. This system can also supply, to the outer circumferential pool, a portion of coolant within a coolant pool of a gravity-driven emergency core-cooling system, provided on an upper section of a wall of concrete construction, and elevate the water level in the outer circumferential pool under an accident situation to enhance heat sink performance of the outer circumferential pool.
When the outer circumferential pool is used to passively cool the reactor containment vessel as in the system of JP-1992-125495-A, if all the amount of pool water in the outer circumferential pool evaporates, the outer circumferential pool will lose its cooling function.
Accordingly, it is imagined that the outer circumferential pool have its coolant-storage capacity increased to continue to cool the reactor containment vessel over a long period of time. Methods for achieving this could be setting a high outer-circumferential pool water level in advance and extending the outer circumferential pool outward.
If water level of the outer-circumferential pool is set high, a pressure corresponding to a hydraulic head due to the high water level is applied to an outer wall surface of the reactor containment vessel. In terms of preventing excessive pressurization of the reactor containment vessel, there is a limit to setting the outer-circumferential pool water level high, which makes it difficult to obtain an enough coolant storage capacity to allow long-term cooling of the reactor containment vessel.
The outward extension of the outer circumferential pool, by contrast, causes problems of construction period extension and increases in construction costs since the reactor building containing the outer circumferential pool needs enlarging. For a small-size or middle-size reactor in particular, for which active adoption of an outer circumferential pool is expected, the enlarging of the reactor building is likely to seriously impair an advantage of a small-size or middle-size reactor's short construction period.
In addition, if the reactor containment vessel is passively cooled using the outer circumferential pool, when the pool water in the outer circumferential pool reaches a boiling point, boiling heat transfer will cause heat transfer from the reactor containment vessel. Since the boiling heat transfer removes heat more efficiently than convective heat transfer of sub-boiling pool water, the amount of heat removal after the boiling point is reached increases relative to that achievable before the boiling point is reached. For this reason, small-size outer circumferential pools are advantageous over large-size ones because the former allows the pool water to reach the boiling point earlier and correspondingly enhances cooling capabilities.
In this way, various disadvantages arise if the coolant storage capacity of the outer circumferential pool itself is increased in an attempt to achieve the long-term continuation of reactor containment vessel cooling.
Supplying the coolant to the outer circumferential pool could be a choice to cool the reactor containment vessel over a long period of time without enlarging the outer circumferential pool. To allow for a station blackout and other contingencies, it is desirable that supplying the coolant to the outer circumferential pool be realizable without a pump or other active devices. In addition, if the supply of the coolant to the outer circumferential pool increases the water level in the outer circumferential pool, this increase will result in the same as if the high water level of the outer-circumferential pool were set.
Specifically, a pressure corresponding to a hydraulic head due to increase in water level will be applied to the outer wall surface of the reactor containment vessel. There will be a need, therefore, to limit the supply of the coolant to the outer circumferential pool for suppressed pressurization of the reactor containment vessel.
In the system of JP-1992-125495-A, a portion of the coolant in the coolant pool of the gravity-driven emergency core-cooling system, provided on the upper section of the wall of concrete construction, is gravitationally supplied to the outer circumferential pool. To enable the long-term cooling of decay heat, therefore, a coolant pool with a capacity large enough to hold the amount of coolant that facilitates long-term refilling of the outer circumferential pool, in addition to the amount of coolant for cooling the core, needs to be disposed on the upper section of the wall of concrete construction. Disposing such a coolant pool of a large capacity on the upper section of the concrete wall, however, is difficult to accomplish in terms of aseismic performance. In this system, therefore, a coolant pool of a large capacity cannot be installed and the long-term cooling of the reactor containment vessel is difficult.
If a coolant pool of a large capacity is disposed on the upper section of the concrete wall, the reactor containment vessel and the reactor building that contains it will need enlarging, which will cause substantially the same problems as those associated with the extension of the outer circumferential pool.
The sixth to ninth embodiments of the present invention have been made for solving the above problems, and an object of the embodiments is to provide a reactor containment cooling system adapted to passively cool a reactor containment vessel over a long period of time without a need to enlarge a reactor building containing an outer circumferential pool and the reactor containment vessel, the system being further adapted to suppress pressurization of the reactor containment vessel.
Sixth Embodiment Hereunder, a sixth embodiment of a reactor containment cooling system according to the present invention will be described with reference to Figs. 8 to 10.
Figs. 8 to 10 show the sixth embodiment of the reactor containment cooling system according to the present invention, Fig. 8 being a schematic configuration diagram that shows a nuclear power plant applying the reactor containment cooling system according to the sixth embodiment of the present invention, Fig. 9 being a schematic configuration diagram that shows a feedwater regulator constituting a portion of the reactor containment cooling system according to the sixth embodiment of the present invention shown in Fig. 8, Fig. 10 being an explanatory diagram that shows opening/closing operation of the feedwater regulator constituting a portion of the reactor containment cooling system according to the sixth embodiment of the present invention shown in Fig. 9.
Referring to Fig. 8, the nuclear power plant includes a reactor pressure vessel 102 containing a core 101, and a reactor containment vessel 103 with the reactor pressure vessel 102 housed therein. The reactor containment vessel 103, having airtightness, is constructed so that in case of radioactive materials leakage from the reactor pressure vessel 102, the reactor containment vessel 103 keeps radioactive materials contained inside it to minimize any impacts upon an ambient environment. The reactor containment vessel 103 is, for example, made of steel and formed into a substantially cylindrical shape.

The reactor containment vessel 103 internally includes a drywell 104 encircling the reactor pressure vessel 102, and a wetwell 105 including a suppression pool 106 disposed in a lower section of the reactor containment vessel 103. The drywell 104 and the wetwell 105 are divided as independent partitions from each other by a diaphragm floor 107, and intercommunicate via a vent pipe 108. The vent pipe 108 is configured to conduct steam released into the drywell 104 in the event of a contingency such as a loss-of-coolant accident into the suppression pool 106.
A main steam safety relief valve (not shown) is disposed on a pipeline (not shown) that is connected to the reactor pressure vessel 102. The main steam safety relief valve allows steam to escape from the inside of the reactor pressure vessel 102 to the suppression pool 106 if an internal pressure of the reactor pressure vessel 102 rises to a certain level or higher.
The nuclear power plant includes a residual-heat removal system. The residual-heat removal system, mainly constituted by a heat exchanger (not shown) placed outside the reactor containment vessel 103, and a pump (not shown) driven by supply of electric power, discharges decay heat from the system. The heat decay is discharged by delivering pool water of the suppression pool 106, which has been heated by transfer of the decay heat generated in the core 101, to the heat exchanger located outside the reactor containment vessel 103 by the pump.
The nuclear power plant, also including a concrete-made biological shield wall (wall surface) 111 that is placed outside the reactor containment vessel 103 with a clearance (for example approximately 1 m), is constructed so that the coolant can be stored within a space present between the biological shield wall 111 and the reactor containment vessel 103. The biological shield wall 111 can be constructed as a wall surface that forms part of the reactor building.
The nuclear power plant further includes a reactor containment cooling system equipped with an outer circumferential pool 112 in which the coolant is stored in the space between the biological shield wall 111 and the reactor containment vessel 103. The reactor containment cooling system cools the reactor containment vessel 103 by naturally releasing the decay heat accumulated within the suppression pool 106 into the coolant of the outer circumferential pool 112 via the reactor containment vessel 103.
When heat is transferred to the outer circumferential pool 112 via the reactor containment vessel 103 to cool the pool water in the suppression pool 106 as in the above cooling system, as the water level in the outer circumferential pool 112 is raised, enlargement of a heat-transfer area between the reactor containment vessel 103 and the coolant in the outer circumferential pool 112 increases the amount of heat removal. It is thus desirable that the water level be as high as possible. A particular difference in hydraulic head between the coolant in the outer circumferential pool 112 and the pool water in the suppression pool 106 may cause an outer wall surface of the reactor containment vessel 103 to be excessively pressurized. In the reactor containment cooling system, therefore, the water level in the outer circumferential pool 112 is limited to a height of a previously-set level H, for example a height of approximately 1 meter above a normal operating water level in the suppression pool 106. Both of controlling the pressurization of the reactor containment vessel 103 and increasing the heat transfer area are thereby simultaneously achieved.
A discharge hole 113 for limiting the water level in the outer circumferential pool 112 and discharging the coolant within this pool 112 outside the biological shield wall 111 is disposed in the biological shield wall 111 at a predetermined height above the set level H. The discharge hole 113 also functions to release decay heat from the system by discharging from the system the steam that has been generated in the outer circumferential pool 112 by way of the heat transfer from the suppression pool 106.
A make-up coolant tank 115 for supplying a make-up coolant to the outer circumferential pool 112 is installed outside the biological shield wall 111 in the nuclear power plant. The make-up coolant tank 115 and the outer circumferential pool 112 are connected through a connecting tube 116. The make-up coolant tank 115 is installed substantially flush with the reactor containment vessel 103 (the outer circumferential pool 112). The make-up coolant tank 115 has the position of its initial water level T set to higher than the position of an initial water level P in the outer circumferential pool 112.
The connecting tube 116 is connected at one end thereof to a lower lateral end of the outer circumferential pool 112 and at the other end thereof to a lower lateral end of the make-up coolant tank 115. Thus a large portion of the make-up coolant stored within the make-up coolant tank 115 can be supplied to the outer circumferential pool 112.
On the connecting tube 116 is placed a feedwater regulator 120, which is a valve regulating a supply rate of the make-up coolant from the make-up coolant tank 115 to the outer circumferential pool 112 in accordance with the particular water level in the outer circumferential pool 112. The feedwater regulator 120 is constructed to close when the water level in the outer circumferential pool 112 is the same as or higher than the previously set level H and to open when the water level is lower than the previously set level H.
A configuration and operation of the feedwater regulator constituting a portion of the reactor containment cooling system according to the sixth embodiment of the present invention will be described below with reference to Figs. 9 and 10.
Figs. 9 and 10 show the configuration and open/close operation of the feedwater regulator, Fig. 9 being a diagram showing an open state of the feedwater regulator, Fig. 10 being a diagram showing a close state of the feedwater regulator. Arrows in Figs. 9 and 10 denote directions in which a valve driving mechanism 123 turns. Referring to Figs. 9 and 10, the same reference numbers as those shown in Fig. 8 denote the same elements and detailed description of these elements is omitted.
Referring to Figs. 9 and 10, the feedwater regulator 120 is a float valve that opens or closes, depending upon the water level in the outer circumferential pool 112, and mainly includes, for example, a float 121 that moves vertically in accordance with the particular water level in the outer circumferential pool 112, the valve driving mechanism 123 that is connected to the float 121 via a rod 122 and turns as the float 121 moves vertically, and a valve body (not shown) that is driven by the valve driving mechanism 123 and opens/closes the connecting tube 116. The feedwater regulator 120 is provided in an end portion of the connecting tube 116 that is positioned inside the outer circumferential pool 112.
The float 121 of the feedwater regulator 120 moves downward as the water level in the outer circumferential pool 112 decreases below the set level H, as shown in Fig.
9. This downward change in a position of the float 121 turns the valve driving mechanism 123 in one direction (indicated by an arrow) via the rod 122, thus opening the valve body.
On the other hand, when the water level that has been below the set level H rises, the float 121 correspondingly moves upward as shown in Fig. 10. This upward change in the position of the float 121 causes the valve driving mechanism 123 to turn in an arrow-indicated direction opposite to the above-mentioned direction via the rod 122, and to operate in a direction to close the valve body. In the feedwater regulator 120, buoyancy of the float 121, length of the rod 122, a turning angle of the valve driving mechanism 123, and other parameters are adjusted in advance to completely close the connecting tube 116 when the water level in the outer circumferential pool 112 reaches or exceeds the set level H.
In this fashion, the feedwater regulator 120 operates only by the passive buoyancy of the float 121 and limits the water level in the outer circumferential pool 112 to the set level H. In other words, the feedwater regulator 120 requires no supply of electric power and allows the supply rate of the make-up coolant to the outer circumferential pool 112 to be regulated without active devices.
Operation of the reactor containment cooling system according to the sixth embodiment of the present invention will be described below with reference to Figs. 8 to 10.
Referring to Fig. 8, the pressure in the drywell 104 rises in case of the loss-of-coolant accident or any other incidence, where a part of the reactor pressure vessel 102 or pipelines suffers damage and steam is released into the drywell 104. The released steam is guided into the pool water in the suppression pool 106 through the vent pipe 108 and condenses in the pool water, and heat of the steam moves to the suppression pool 106. Accordingly, an increase in the internal pressure of the reactor containment vessel 103 is prevented.
In addition, when the internal pressure of the reactor pressure vessel 102 rises to or above a fixed value, the main steam safety relief valve (not shown) opens, whereby the steam in the reactor pressure vessel 102 is conducted into the pool water of the suppression pool 106 and condenses therein and the heat of the steam moves to the suppression pool 106. Accordingly, increases in the internal pressures of the reactor pressure vessel 102 and the reactor containment vessel 103 are prevented.
In this way, in case of a loss-of-coolant accident or an excessive rise in the internal pressure of the reactor pressure vessel 102, the heat of the steam in the drywell 104 or the reactor pressure vessel 102 moves to the suppression pool 106. If the heat that has moved from the steam increases the water temperature of the suppression pool 106 and the pool water boils and evaporates, the internal temperature and pressure of the reactor containment vessel 103 eventually increase so that the heat in the pool water needs removal.
In the event that the residual-heat removal system operates, the pool water within the suppression pool 106 will be delivered to the heat exchanger (not shown) located outside the reactor containment vessel 103 by the pump (not shown). The heat in the suppression pool 106 will then be removed from the reactor containment vessel 103. This prevents the internal temperature and pressure of the reactor containment vessel 103 from increasing.

If an event not activating the residual-heat removal system arises, the water temperature in the suppression pool 106 begins to rise, but in the present embodiment, the reactor containment cooling system starts operating. That is to say, the heat of the heated pool water in the suppression pool 106 is transmitted to the low-temperature coolant in the outer circumferential pool 112 via the wall of the reactor containment vessel 103 and then removed outside the reactor containment vessel 103. In this way, the pool water in the suppression pool 106 is passively cooled without a pump or other active devices, whereby the internal temperature and pressure of the reactor containment vessel 103 are prevented from increasing.
Assuming that the residual-heat removal system cannot be operated for a long term for a reason such as a continued station blackout, the suppression pool 106 (the reactor containment vessel 103) needs to be cooled over a long period of time. In this case, the heat transfer from the suppression pool 106 causes the coolant in the outer circumferential pool 112 to be gradually heated to boil.
The steam generated during resulting evaporation of the coolant is consequently released from the discharge hole 113 to the outside of the biological shield wall 111. When no more of the coolant evaporates, the cooling function of the outer circumferential pool 106 turns out to be a substantial stop. In the present embodiment, however, as shown in Fig.
9, when the coolant in the outer circumferential pool 112 evaporates and the water level decreases below the set level H, the feedwater regulator 120 opens automatically, thus activating the make-up coolant tank 115 to supply the make-up coolant to the outer circumferential pool 112. Since the initial position of the water level T in the make-up coolant tank 115 is higher than that of the water level P (the set level H) in the outer circumferential pool 112, the make-up coolant in the make-up coolant tank 115, because of its own weight, is supplied without an active device and without supply of electric power for driving the active device.
Conversely when the supply of the make-up coolant causes the water level to rise above the set level H as shown in Fig. 10, the feedwater regulator 120 closes automatically to stop the supply of the coolant. This prevents the water level in the outer circumferential pool 112 from rising above the set level H, hence controlling the pressurization of the reactor containment vessel 103 due to the differential head between the suppression pool 106 and the outer circumferential pool 112.
As described above, since the feedwater regulator 120 opens or closes automatically without the supply of electric power, depending upon the water level in the outer circumferential pool 112, the supply of make-up coolant to the outer circumferential pool 112 can be continued passively even if a situation requiring the long-term cooling of the reactor containment vessel 103 arises by reason of a station blackout or the like. The pressurization of the reactor containment vessel 103 can be controlled at the same time.

In a case where the feedwater regulator 120 ever seizes up under its open state, normally the make-up coolant in the make-up coolant tank 115 would be supplied to the outer circumferential pool 112 gravitationally until the water level in the outer circumferential pool 112 has equaled that of the make-up coolant tank 115. However, the discharge hole 113 at the predetermined height in the biological shield wall 111 reliably prevents the water level in the outer circumferential pool 112 from rising above the predetermined height. This, in turn, reliably prevents an excessive pressurization of the reactor containment vessel 103 due to the differential hydraulic head between the suppression pool 106 and the outer circumferential pool 112.
In addition, if the connecting tube 116 breaks midway between the suppression pool 106 and the outer circumferential pool 112 for a reason such as an earthquake, in the present embodiment, since the feedwater regulator 120 is provided at the end portion of the connecting tube 116 that is disposed inside the outer circumferential pool 112, as long as the feedwater regulator 120 is blocking the connecting tube 116, the coolant in the outer circumferential pool 112 can be prevented from flowing out from the break. This makes it unnecessary to connect the connecting tube 116 to an upper section of the outer circumferential pool 112 allowing for such an outflow of the coolant within the outer circumferential pool 112 due to the breakage of the connecting tube 116. In other words, the connecting tube 116 may instead be connected to a lower end portion of the outer circumferential pool 112, which will enable a larger amount of the make-up coolant in the make-up coolant tank 115 to be supplied to the outer circumferential pool 112 than connecting the connecting tube 116 to the upper section of the outer circumferential pool 112. The connection of the connecting tube 116 to the lower end portion of the outer circumferential pool 112 will additionally make it unnecessary to provide a check valve for preventing the outflow of the coolant within the outer circumferential pool 112 due to the breakage of the connecting tube 116.
Furthermore in the present embodiment, because the make-up coolant tank 115 for supplying the make-up coolant to the outer circumferential pool 112 is installed outside the biological shield wall 111, this helps avoid enlarging the outer circumferential pool 112 and causes the coolant in the outer circumferential pool 112 to boil earlier than use of a large-sized outer circumferential pool. Compared to a large-sized outer circumferential pool, therefore, the outer circumferential pool 112 is capable of utilizing the boiling heat transfer that allows more efficient heat removal than the convective heat transfer of a sub-boiling water and thereby enhancing cooling capabilities.
Different from the present embodiment, if the make-up coolant tank 115 and the outer circumferential pool 112 are made to communicate without the feedwater regulator 120, although the make-up coolant can be supplied gravitationally, the water levels of the make-up coolant tank 115 and the outer circumferential pool 112 come to be equal each other. In this case, in terms of preventing the excessive pressurization of the reactor containment vessel 103 due to the differential hydraulic head between the suppression pool 106 and the outer circumferential pool 112, the water level in the outer circumferential pool 112 needs to be limited and hence that in the make-up coolant tank 115 is also to be correspondingly limited. For extended capacity of the make-up coolant tank 115, therefore, it is necessary to extend a floor area of the make-up coolant tank 115 since the height thereof cannot be increased from the above perspective. In this context, the make-up coolant tank 115 decreases in flexibility of its installation location in the nuclear power plant.
By contrast, in the present embodiment, the initial position of the water level T in the make-up coolant tank 115 can be set to be higher than that of the water level P
in the outer circumferential pool 112 since the feedwater regulator 120 allowing the supply rate of the make-up coolant to the outer circumferential pool 112 to be regulated in accordance with the water level in the outer circumferential pool 112 is provided on the connecting tube 116. The capacity of the make-up coolant tank 115 can be extended by increasing its floor area, its height, or both thereof. Briefly, a duration of the containment vessel cooling in the present embodiment using the outer circumferential pool 112 can be extended, and in addition, a shape and disposition of the make-up coolant tank 115 can be optimized.
As described above, in the sixth embodiment of the reactor containment cooling system according to the present invention, the make-up coolant is gravitationally supplied from the make-up coolant tank 115 disposed outside the wall surface (biological shield wall 111) serving as part of the reactor building to the outer circumferential pool 112. On top of that, the feedwater regulator 120 regulates the supply rate of the make-up coolant to the outer circumferential pool 112 in accordance with the water level in the outer circumferential pool 112. The reactor containment vessel 103 can therefore be passively cooled over a long period of time without the enlargement of the reactor building containing the outer circumferential pool 112 and the reactor containment vessel 103. The pressurization of the reactor containment vessel 103 can be controlled. Safety of the nuclear power plant consequently improves.
Moreover, since a float valve that opens or closes, depending on the water level of the outer circumferential pool 112, is employed as the feedwater regulator 120 in the present embodiment, in case of a contingency such as station blackout, the feedwater regulator 120 using only buoyancy to operate passively without the supply of electric power enables continued supply of the make-up coolant to the outer circumferential pool 112 and further enables the pressurization of the reactor containment vessel 103 to be controlled.

Seventh Embodiment A seventh embodiment of a reactor containment cooling system according to the present invention will now be described with reference to Fig. 11.
Fig. 11 is a schematic configuration diagram showing the seventh embodiment of the reactor containment cooling system according to the present invention. Referring to Fig. 11, the same reference numbers as those shown in Figs.
8 to 10 denote the same elements and detailed description of these elements is omitted.
The seventh embodiment of the reactor containment cooling system according to the present invention, shown in Fig. 11, differs from the sixth embodiment: whereas the float valve whose opening or closing depends on the water level of the outer circumferential pool 112 is employed as the feedwater regulator 120 forming a portion of the sixth embodiment, an electrically powered valve whose opening/closing control is based on a water level of an outer circumferential pool 112 is employed as a feedwater regulator 140 in the seventh embodiment.
More specifically, a level gauge 141 configured to measure the water level in the outer circumferential pool 112 is placed in the outer circumferential pool 112. The feedwater regulator 140 whose opening/closing is controlled in accordance with the water level that the level gauge 141 has measured is connected to the level gauge 141. The feedwater regulator 140 is provided at an end of a connecting tube 116 that is disposed inside the outer circumferential pool 112. The level gauge 141 and the feedwater regulator 140 are driven by a battery (not shown) and are adapted to operate even under a station blackout situation. In the present embodiment, without an active device such as a pump, opening the feedwater regulator 140 likewise allows a make-up coolant to be supplied from a make-up coolant tank 115 to the outer circumferential pool 112 because of the coolant's own weight.
The seventh embodiment of the reactor containment cooling system according to the present invention yields substantially the same advantageous effects as those obtained in the sixth embodiment.
In addition, in the present embodiment, since the feedwater regulator 140 is opened or closed depending on the water level measured by the level gauge 141, a supply rate of the make-up coolant to the outer circumferential pool 112 can be finely regulated in accordance with the particular water level in the outer circumferential pool 112.
Consequently, controlling the pressurization of a reactor containment vessel 103 resulting from a differential hydraulic head between the outer circumferential pool 112 and a suppression pool 106, and increasing a heat transfer area (heat removal amount) can both be achieved at the same time and accurately.
Eighth Embodiment An eighth embodiment of a reactor containment cooling system according to the present invention will now be described with reference to Fig. 12.
Fig. 12 is a schematic configuration diagram showing the eighth embodiment of the reactor containment cooling system according to the present invention. Referring to Fig. 12, the same reference numbers as those shown in Figs.
8 to 11 denote the same elements and detailed description of these elements is omitted.
The eighth embodiment of the reactor containment cooling system according to the present invention, shown in Fig. 12, differs from the sixth embodiment: whereas the make-up coolant tank 115 constituting a portion of the sixth embodiment is disposed substantially flush with the outer circumferential pool 112, a make-up coolant tank 155 in the eighth embodiment is disposed at a place higher than an installation surface of the outer circumferential pool 112 on the premises of the nuclear power plant. One end of a connecting tube 156 connecting the outer circumferential pool 112 and the make-up coolant tank 155 is coupled to a bottom surface of the make-up coolant tank 155 disposed at the high place.
The eighth embodiment of the reactor containment cooling system according to the present invention yields substantially the same advantageous effects as those obtained in the sixth embodiment.
In addition, in the present embodiment, since the make-up coolant tank 155 is disposed at a place higher than the installation surface of the outer circumferential pool 112 on the premises of the nuclear power plant, the cooling system will be less susceptible to entry of any tsunami waves or the like into the site.
Furthermore, since the connecting tube 156 is connected to the bottom surface of the make-up coolant tank 155 disposed at the high place, all the amount of make-up coolant in the make-up coolant tank 155 can be supplied to the outer circumferential pool 112. Thus, the cooling of a reactor containment vessel 103 by use of the outer circumferential pool 112 can be continued over a longer period of time.
Ninth Embodiment A ninth embodiment of a reactor containment cooling system according to the present invention will now be described with reference to Fig. 13.
Fig. 13 is a schematic configuration diagram showing the ninth embodiment of the reactor containment cooling system according to the present invention. An arrow in Fig.
13 indicates a direction in which a make-up coolant flows.
Referring to Fig. 13, the same reference numbers as those shown in Figs. 8 to 12 denote the same elements and detailed description of these elements is omitted.
Instead of the make-up coolant tank 155 constituting a portion of the eighth embodiment, the ninth embodiment of the reactor containment cooling system according to the present invention, shown in Fig. 13, employs a make-up coolant tank 165 constructed so that a make-up coolant can be supplied from the outside by use of an additional make-up coolant supply device 167.
For example, use of a fire engine as the make-up coolant supply device 167 is assumed here. In this case, the make-up coolant tank 165 includes a connecting plug (not shown) that can connect to a fire hose. This allows make-up coolant to be supplied from an external source of water to the make-up coolant tank 165. In addition, the make-up coolant tank 165 may be constructed so that water can be supplied by water-discharging operation from a fire engine.
Furthermore, the make-up coolant tank 165 is installed at a location easily accessible, which allows the make-up coolant supply device 167 such as a fire engine to be easily moved to a neighborhood of the make-up coolant tank 165. This, in turn, makes it easier to supply the coolant via the make-up coolant supply device 167 than to supply the coolant directly to an outer circumferential pool 112.
The ninth embodiment of the reactor containment cooling system according to the present invention yields substantially the same advantageous effects as those obtained in the eighth embodiment.
In addition, in the present embodiment, since the make-up coolant tank 165 is constructed so that the coolant can be supplied from the outside by use of the make-up coolant supply device 167, the make-up coolant can be supplied to the outer circumferential pool 112 over a longer period of time. Thus, the cooling of a reactor containment vessel 103 can be continued over a longer period of time.
Other Aspects The application of the present invention to a nuclear power plant of a boiling-water reactor equipped with a pressure suppression type of steel containment vessel including a suppression pool has been described by way of example in each of the sixth to ninth embodiments of the invention. The reactor containment cooling system according to each embodiment of the present invention, however, can be applied to reactors of both the boiling-water type and the pressurized-water type. The reactor containment cooling system can also be applied to reactors equipped with any other types of reactor containment vessels.
For example, if the reactor containment vessel does not contain a suppression pool, heat is transmitted to the outer circumferential pool via the wall surface of the reactor containment vessel that has been heated to a high temperature by steam convection inside the reactor containment vessel, so that extended cooling of the reactor containment vessel still can be achieved as long as the make -up coolant is continuously supplied to the outer circumferential pool.
Further, if the reactor containment vessel is made of concrete, although the concrete has a lower heat conductivity and lower heat removal efficiency than steel, the reactor containment vessel still can be cooled over a long period of time as long as the make-up coolant is continuously supplied to the outer circumferential pool.
While an example of using an electrically powered valve (see Fig. 12) serving as the feedwater regulator 140 has been shown and described in the seventh embodiment of the reactor containment cooling system according to the present invention, the feedwater regulator can instead be an air-driven valve or any other valve using a different driving source. In one example, the feedwater regulator may be driven by air that is supplied from an air container. In this case, substantially the same advantageous effects as achievable in the seventh embodiment will be obtained because the feedwater regulator is able to be opened or closed depending on the water level measured by the level gauge 141.
The present invention is not limited to the above first to ninth embodiments and may embrace varieties of modifications. The embodiments, for example, have only been described in detail for a better understanding of the invention and are therefore not necessarily limited to the configurations containing all the described constituent elements. In addition, part of the configuration of a certain embodiment may be replaced by the configuration of another embodiment and the configuration of a certain embodiment may be added to the configuration of another embodiment. Furthermore, part of the configuration of one of the embodiments may be added to, deleted from, and/or replaced by the other embodiments.

Claims

What is claimed is:

1. A reactor containment cooling system comprising:
a reactor containment vessel made of steel, enclosing therein a reactor pressure vessel, a drywell space, a suppression chamber space, a suppression pool, a vent pipe, and a vacuum breaker;
the reactor pressure vessel containing a core loaded with a plurality of fuel assemblies, the drywell space surrounding the reactor pressure vessel, the suppression chamber space divided from the drywell space by an airtight wall, the suppression pool formed by filling the suppression chamber space with coolant, the vent pipe connecting the drywell space and the suppression pool, the vacuum breaker configured to cause non-condensable gases to flow through to the drywell space from the suppression chamber, a biological shield wall disposed outside the reactor containment vessel;
a containment cooling region defined between the reactor containment vessel and the biological shield wall so as to have airtightness;
a containment coolant pool formed by filling the containment cooling region with coolant;
a heat exchanger disposed outside the containment cooling region, the heat exchanger being above a water level in the containment coolant pool;
an upstream-side pipeline connecting the suppression chamber space and an upper end of the heat exchanger;
a downstream-side pipeline connecting the containment cooling region located below the water level in the containment coolant pool and a lower end of the heat exchanger; and an externally open line connecting at one end thereof to a section of the downstream-side pipeline which is located above the water level in the containment coolant pool, the externally open line being opened at the other end thereof to an outside of the containment cooling region.

2. The reactor containment cooling system according to claim 1, further comprising:
a check valve disposed on the externally open line and only permitting a passage of a fluid in an external direction from the downstream-side pipeline; and a vacuum breaker for the containment cooling system, disposed on a section of the biological shield wall above the water level in the containment coolant pool, the vacuum breaker being configured to cause non-condensable gases to flow through from the outside of the containment cooling region to the containment cooling region.

3. The reactor containment cooling system according to claim 1 or 2, wherein the heat exchanger includes:
an upstream-side header disposed on an upper side of the heat exchanger and connected to the containment cooling region via the upstream-side pipeline; and a downstream-side header disposed on a lower side of the heat exchanger and connected to the containment cooling region via the downstream-side pipeline, and wherein the externally open line is connected at one end thereof to the downstream-side header of the heat exchanger.

4. The reactor containment cooling system according to any one of claims 1 to 3, further comprising an exhaust tower adapted to discharge a fluid from a high position to an outside, wherein a discharge destination of the other end of the externally open line is connected to the exhaust tower.

5. The reactor containment cooling system according to claim 3 or 4, further comprising a cylindrical air-cooling vertical duct with upper and lower sections opened to outside air, wherein the heat exchanger is placed inside the air-cooling vertical duct.

6. The reactor containment cooling system according to any one of claims 1 to 5, further comprising a heat exchanger coolant pool disposed outside the containment cooling region, wherein the heat exchanger is placed under coolant in the heat exchanger coolant pool.

7. The reactor containment cooling system according to claim 6, further comprising:
a coolant injection line connecting the heat exchanger coolant pool and the containment coolant pool, the coolant injection line being provided with a float valve adapted to regulate the water level in the containment coolant pool.

8. The reactor containment cooling system according to claim 7, wherein the opening in the other end of the externally open line is disposed below the one end of the externally open line.

9. A nuclear power plant comprising the reactor containment cooling system according to any one of claims 1 to 8.

10. A reactor containment cooling system comprising:
a reactor containment vessel housing a reactor pressure vessel, the reactor pressure vessel containing a core;

a wall surface disposed on an outer circumferential side of the reactor containment vessel via a clearance an outer circumferential pool with coolant stored in a space between the reactor containment vessel and the wall surface;
a make-up coolant tank installed outside the wall surface and configured to supply make-up coolant to the outer circumferential pool;
a connecting tube for connecting the outer circumferential pool and the make-up coolant tank; and a feedwater regulator disposed on the connecting tube and configured to regulate, in accordance with the water level in the outer circumferential pool, a flow rate of the make-up coolant supplied from the make-up coolant tank to the outer circumferential pool.

11. The reactor containment cooling system according to claim 10, wherein the feedwater regulator is disposed on an end portion of the connecting tube in the outer circumferential pool.

12. The reactor containment cooling system according to claim 11, wherein the feedwater regulator is a float valve that opens or closes depending on the water level in the outer circumferential pool.

13. The reactor containment cooling system according to claim 11, further comprising a level gauge placed in the outer circumferential pool and serving to measure the water level in the outer circumferential pool, wherein the feedwater regulator is a valve whose opening/closing control is based on the water level measured by the level gauge.

14. The reactor containment cooling system according to claim 13, wherein the feedwater regulator is an electrically powered valve adapted to be driven by a battery.

15. The reactor containment cooling system according to claim 13, wherein the feedwater regulator is a valve adapted to be driven by air.

16. The reactor containment cooling system according to any one of claims 10 to 15, wherein a discharge hole adapted to discharge the coolant within the outer circumferential pool is disposed at a predetermined height in the wall surface .

17. The reactor containment cooling system according to any one of claims 10 to 15, wherein the make-up coolant tank is disposed at a place higher than the outer circumferential pool, and wherein the connecting tube is connected to a bottom surface of the make-up coolant tank.

18. The reactor containment cooling system according to any one of claims 10 to 15, wherein the make-up coolant tank is adapted to be refilled with make-up coolant from outside.