JP2003130982A - Boiling water reactor of natural circulation type - Google Patents

Abstract

PROBLEM TO BE SOLVED: To separate two phases of gas and liquid, adequately remove the moisture of steam guided to a turbine and sufficiently lessen the entrainment of steam into a downcomer in a boiling water reactor of a natural circulation type. SOLUTION: This reactor has a shroud 25 surrounding a core 26 in a reactor pressure vessel 30, a chimney 24 communicating with the upper part of the shroud 25 and a downcomer part 34 formed between the chimney 24, the shroud 25 and the reactor pressure vessel 30. The internal diameter 20 of the chimney 24 is sufficiently wide, specifically 5.4 m or wider. The height 21 from the level of reactor water 6 to a main steam pipe 8 during the operation of the reactor is sufficiently large, specifically 2.5 m or larger. The width 22 of the downcomer part 34 is sufficiently wide, specifically 0.37 m or wider.

Description

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a natural circulation type boiling water reactor which obtains a driving force for circulating cooling water by a difference in density of cooling water. The present invention relates to a natural circulation type boiling water reactor capable of separating steam from cooling water without using the same. [0002] Regarding a natural circulation boiling water reactor,
Until now, JP-A-6-265665, JP-A-8-9
Many examples have been made, such as the publication of No. 4793.
FIG. 6 shows a conventional natural circulation type boiling water reactor. In FIG. 6, reference numeral 1 denotes a reactor pressure vessel, in which a reactor core 2 is stored. At the same time, a cylindrical shroud 5 is provided so as to surround the core 2, and a cylindrical chimney 3 is provided above the shroud 5 so as to be connected thereto. [0003] A downcomer 4 is formed inside the shroud 5 and the chimney 3 as an ascending flow path of cooling water, and outside the shroud 5 and the chimney 3 as a descending flow path of cooling water. Cooling water (hereinafter referred to as reactor water 6) naturally circulates through the downcomer 4, the lower plenum 7, the core 2, and the chimney 3. During the circulation, steam generated by the reactor water 6 receiving heat from the reactor core 2 is sent to a turbine (not shown) through a main steam pipe 8, and steam after working in the turbine is After the water is condensed, the water is returned to the reactor pressure vessel 1 via the water supply pipe 9. [0005] A control rod 10 is inserted into and removed from the core 2 to control the output of the core 2. Thus, in the conventional natural circulation boiling water reactor having the above configuration, the cylindrical shroud 5 and the cylindrical chimney 3 surrounding the core 2 in the reactor pressure vessel 1 are provided.
And a downcomer 4 serving as a cooling water flow path surrounded by the outer peripheral portions of the shroud 5 and the chimney 3 and the reactor pressure vessel 1, and a rising force due to the buoyancy of water and steam in the shroud 5 and the chimney 3 and the downcomer 4 The coolant is circulated using the head pressure as the driving force. The natural circulation type boiling water reactor is intended to simplify the equipment and reduce the construction cost from the existing boiling water reactor, and the above-mentioned conventional example is a separator provided in the current boiling water reactor. And a configuration in which the dryer is removed. [0007] However, the above-mentioned conventional example has the following problems. First, a current boiling water reactor will be described for comparison. Current boiling water reactors are generally configured as shown in FIG. That is, in the reactor pressure vessel indicated by reference numeral 11 in the drawing, a reactor core 12 is disposed at the center, and a cylindrical shroud 13 is disposed so as to cover the reactor core 12. A plurality of recirculation pumps 14 are disposed below the downcomers 19 in the gap between the shroud 13 and the reactor pressure vessel 11. [0008] The thermal energy generated by the nuclear reaction in the core 12 is obtained, and the cooling water flows upward in the shroud 13 as high-temperature and high-pressure steam. The mixed flow of water and steam, after the water is separated by the separator 15,
Further, it is introduced into a dryer 16, where moisture is removed therefrom, and then guided through a main steam pipe 17 to a turbine. The steam after driving and working the turbine is condensed, and this condensate passes through the main water supply pipe 18 and flows again into the downcomer 19 outside the shroud 13. As described above, in the current boiling water reactor,
Since the cooling water heated in the reactor core 12 and converted into a gas-liquid two phase is separated into a gas phase and a liquid phase by the separator 15 and the dryer 16, the steam guided to the turbine through the main steam pipe 17 is sufficiently removed of moisture. On the other hand, the entrainment of steam into the cooling water descending the downcomer is sufficiently small. However, in the conventional natural circulation type boiling water reactor, a separator and a dryer are omitted, and there is no conventional example in which separation of heated cooling water into two gas-liquid phases is sufficiently considered. Moisture is not sufficiently removed from the steam guided to the turbine, and the entrainment of the steam into the cooling water descending the downcomer is not sufficiently suppressed. If the steam guided to the turbine does not sufficiently remove moisture, the turbine may be deteriorated due to erosion or the like. Also, if the entrainment of steam into the cooling water descending the downcomer is not sufficiently suppressed, the high-temperature cooling water circulates through the core, causing a problem in the heat balance of the core. An object of the present invention is to sufficiently separate cooling water heated in a reactor core into two phases of gas and liquid even if a separator and a dryer are omitted, and to introduce the cooling water to a turbine so that deterioration on the turbine side does not occur. It is an object of the present invention to provide a natural circulation type boiling water reactor capable of sufficiently removing the moisture content of steam generated and keeping the entrainment of steam into cooling water descending downcomers sufficiently to maintain the heat balance of the core. . [0013] The principle of the present invention is that a cooling water ascending flow path is provided at a central portion in a reactor pressure vessel, and the cooling water ascending flow path is provided around an outer periphery of the ascending flow path. In the natural circulation boiling water reactor having an inlet of a main steam pipe connected to the reactor pressure vessel above the cooling water level during the operation of the reactor, The allowable steam flow rate at which the free interface is stable, which is obtained from the relationship between the pressure of the allowable steam flow rate that can maintain the stability of the free interface of the gas-liquid two phase and the operating pressure of the reactor, is converted into the inner diameter of the rising channel Having an ascending flow path inner diameter equal to or larger than the value obtained as a height from the cooling water level to the inlet of the main steam pipe, necessary for obtaining the desired steam moisture at the inlet of the main steam pipe. Up to the inlet of the main steam pipe at the operating pressure change and the cooling water level Has a height equal to or higher than the height corresponding to the allowable steam flow rate determined from the relationship with the height, and as the width of the downcomer, determined from the relationship between the allowable upper limit of steam entrainment into the downcomer and the downcomer cooling water flow rate. The width is greater than the downcomer width. Such a principle is developed and described in a specific embodiment as a means for solving the invention. In order to achieve the above object, in the embodiment of the present invention, a shroud is provided in a pressure vessel, and The coolant is heated by the core arranged in the shroud, the heated coolant rises up the chimney following the inside of the shroud and the upper part of the shroud, and the cooling water separating the steam further flows between the chimney and the shroud and the pressure vessel. It is a natural circulation type boiling water reactor which descends the formed downcomer part, further ascends into the core from the lower part of the core, circulates, and uses the differential pressure due to the difference in cooling water density inside and outside the shroud and chimney as the driving force for cooling water circulation. In a natural circulation boiling water reactor without a separator for separating steam from water and a dryer for drying steam, the inner diameter of the chimney is made sufficiently large. Specifically, it is 5.4 m or more. In addition, the height from the cooling water level during operation to the main steam pipe is sufficiently increased. Specifically, the length is 2.5 m or more. And make the width of the downcomer part sufficiently large. Specifically, it is 0.37 m or more. Gas-liquid separation when the separator and the dryer are omitted is based on gravity. Therefore, in order to sufficiently remove the moisture of the steam guided to the turbine, the flow velocity of the steam rising from the chimney through the water surface must be suppressed to stabilize the water surface, and the water level from the cooling water level during operation to the main steam pipe It is effective to make the height sufficiently large. Further, the flow velocity of steam rising from the chimney through the water surface is simply represented by (steam generation amount / chimney cross-sectional area), so that it is effective to increase the inner diameter of the chimney. In order to sufficiently suppress the entrainment of steam into the cooling water descending the downcomer, it is effective to suppress the flow velocity of the cooling water descending the downcomer portion. Since the flow rate of the cooling water is simply represented by (total circulation flow rate / downcomer area), it is effective to make the width of the downcomer sufficiently large. As described above, it is possible to reduce the construction cost by omitting the separator and the dryer from the existing boiling water reactor, and to sufficiently separate the cooling water heated in the reactor core into two gas-liquid phases, Natural circulation type boiling water that sufficiently removes the moisture of the steam introduced to the turbine so that deterioration on the turbine side does not occur, and keeps the entrainment of the steam into the cooling water descending downcomers sufficiently low to maintain the heat balance of the core. -Type nuclear reactor is obtained. Embodiments of the present invention will be described below in detail with reference to the drawings. In FIG. 1, cooling water is supplied as reactor water 6 inside a reactor pressure vessel 30 of a natural circulation boiling water reactor. In the reactor pressure vessel 30, a cylindrical shroud 25 surrounding the reactor core 26 below the surface of the reactor water 6 and a cylindrical chimney 24 continuous above the shroud 25 are incorporated. The inside of the shroud 25 and the chimney 24 is an upflow channel for reactor water, and the outside is a downcomer 34 as a downflow channel for reactor water between the shroud 25 and the inner wall surface of the reactor pressure vessel. The horizontal cross-sectional shape of the flow path cross section of the downcomer 34 has an annular shape. An inlet of the main steam pipe 8 is connected to a position of the reactor water 6 at a height 21 from a water level (cooling water level) during the operation of the reactor. A water supply pipe 9 is connected to the reactor water 6 at a level lower than the water level. A core 26 is provided in an area surrounded by the shroud 25. The output of the core 26 is controlled by the vertical movement of a control rod 27 that can enter and exit the core 26. Therefore, when the core 26 generates heat corresponding to the heat output controlled by the control rod 27, the reactor water 6
Is heated by the reactor core 26 and rises in the chimney 24 as a gas-liquid two-phase flow, and the steam rises from the water surface of the reactor water 6 and enters the inlet of the main steam pipe 8 to be supplied to the turbine. The steam which has been condensed is condensed and supplied again to the downcomer 34 in the reactor pressure vessel 30 at a lower temperature than the reactor water 6 in the chimney 24 by a water supply pipe. The steam rises from the water surface of the reactor water 6, and by the time the steam reaches the inlet of the main steam pipe 8, the moisture in the steam drops to the water surface of the reactor water 6 and the inlet of the main steam pipe 8 The moisture of the steam flowing into the inside decreases. As described above, since there is a temperature difference between the reactor water in the chimney and the reactor water in the downcomer, the reactor water becomes an upward flow in the chimney due to a temperature-dependent density difference, and steam is generated at the reactor water level. After being discharged upward, a circulation enters the downcomer and becomes a downward flow, and flows into the core again from below, and the circulation is continued. The difference between the first embodiment of the present invention shown in FIG. 1 and the conventional natural circulation type boiling water reactor shown in FIG. 6 is that the inner diameter 20 of the chimney 24 is large and that the reactor water 6 (cooling water) is not used. The height 21 from the water level to the main steam pipe 8 is large, and the width 22 of the downcomer portion is large. The evaluation results for the above three design dimensions when the thermal output is about 900 MW in the present embodiment are shown. FIG. 2 shows the change in the allowable vapor flow rate with respect to the pressure that can maintain the stability of the free interface between the two phases.
In the region above the graph 23 in FIG. 2, the free interface of the gas-liquid two-phase becomes unstable and the moisture of the steam guided to the turbine via the main steam pipe 8 increases, and the region below the graph 23 is the gas-liquid 2
The free interface of the phases is stabilized, and the moisture of the steam guided to the turbine via the main steam pipe 8 is reduced. As shown in FIG. 2, the allowable steam flow rate at the operating pressure of the boiling water reactor is 0.68 m / s. As described above, the steam flow rate is simply represented by (steam generation amount / chimney sectional area). Under the conditions of the first embodiment in which the heat output is about 900 MW, the inner diameter of the chimney 24 and the inner diameter 20 of the chimney are 5 .4m. Therefore, if the chimney inner diameter 20, which is the inner diameter of the ascending flow path of the reactor water 6, is 5.4 m or more, a stable water surface of the reactor water 6 can be obtained. FIG. 3 shows the steam flow rate and the height 21 from the water level (cooling water level) of the reactor water 6 to the main steam pipe 8 when the steam moisture at the inlet of the main steam pipe 8 is set to 0.1%. 6 is a graph showing the relationship of. Here, as in the current case, the steam moisture is 0.1
% Was considered the upper limit. FIG. 3 shows that as the steam flow rate increases, the height 21 from the water level of the reactor water 6 (cooling water level) necessary for suppressing the moisture content of the steam to the main steam pipe 8 increases. Here, the allowable steam flow rate of 0.68 m shown above.
The height 21 from the necessary water level (cooling water level) of the reactor water 6 to the main steam pipe 8 at the time of / s is 2.5 m 2. Therefore, if the height from the water level of the reactor water 6 (cooling water level) to the main steam pipe 21 is 2.5 m or more, the above moisture can be suppressed to the allowable upper limit or less. FIG. 4 shows the reactor water 6 descending the downcomer 34 part.
It is a graph which shows the relationship between the flow velocity of (cooling water), and the entrainment of steam in the downcomer. FIG. 4 shows that steam entrainment increases as the downcomer cooling water flow rate increases. Here, the allowable upper limit value of steam entrainment is 0.25.
%, The corresponding downcomer cooling water flow velocity is 0.24m
/ S. This corresponds to the width 22 of the downcomer 34 of 0.37 m under the conditions of the first embodiment. Therefore, when the width 22 of the downcomer 34 is 0.37 m or more, the entrainment of steam can be suppressed to the allowable upper limit or less. Regarding the three design dimensions described above,
A comparison will be made between a boiling water reactor having a current heat output of about 3900 MW, a conventional natural circulation boiling water reactor, and the first embodiment of the natural circulation boiling water reactor of the present invention. First, as for the chimney inner diameter 20, the first embodiment of the present invention has a size approximately equal to the shroud inner diameter of the existing boiling water reactor. This is a large size considering that the heat output is less than 1/4. On the other hand, in the conventional natural circulation type boiling water reactor, there is no specific detailed design example. Just do it. In the first embodiment of the present invention, in order to secure a large chimney diameter 20, the outer diameter of the core 26 and the inner diameters of the shroud 25 and the chimney 24 do not necessarily match, and a relatively large diameter is provided between the shroud 25 and the chimney 24 and the core 26. There may be intervals. Next, with respect to the height 21 from the water level of the reactor water 6 (cooling water level) to the main steam pipe 8, the first embodiment of the present invention has the same height as that of the existing boiling water reactor. Has become. Considering that the heat output is set to 1/4 or less, the size is large. On the other hand, in the conventional natural circulation type boiling water reactor, there is no particularly detailed design example, but only the size is changed in proportion to the output from the present. Regarding the width 22 of the downcomer 34, the first embodiment of the present invention is smaller than that of the existing boiling water reactor, but it is also large considering that the heat output is reduced to 1/4 or less. Say size. In the conventional natural circulation type boiling water reactor, there is no particularly detailed design example, but only the size is changed in proportion to the power from the present.
Although these three design dimensions naturally vary depending on the design conditions, there is a simple relationship that if the heat output of the reactor increases, these design dimensions also increase, and if the heat output decreases, these design dimensions also decrease. Lord. As mentioned earlier, the first
Although the embodiment of the present invention assumes a considerably small heat output of 900 MW, which is 1/4 or less of the current heat output of 3900 MW, the above three design dimensions of the natural circulation boiling water reactor are respectively larger than this. It is necessary that the size is equal to or larger than the value shown above, whereby the moisture of the steam guided to the turbine can be sufficiently removed, and the entrainment of the steam into the cooling water descending the downcomer 34 can be suppressed to a sufficiently small value. Next, a second embodiment of the present invention will be described with reference to FIG. FIG. 5 shows that the control rod 28 is
9 is an embodiment of inserting from above. Other configurations are the same as those of the first embodiment. Thus, compared to the control rod 27 inserted from below in the first embodiment, the safety is improved because gravity acts in the insertion direction. In addition, gravity can be used as a power source of the scram for inserting the control rod 28 at a high speed for emergency furnace shutdown, and a seal structure by a control rod drive mechanism penetrating the lower part of the core becomes unnecessary. Construction costs can be reduced by simplifying the equipment. At the same time, in the present embodiment, the separator and the dryer are omitted from the existing boiling water reactor, and there is no problem of the interlocking with the control rod drive mechanism and the like, and a simple system configuration is possible. As described in detail above, according to the present invention,
Separation of the cooling water heated in the core into two gas-liquid phases despite the elimination of the separator and dryer, enabling the reduction of construction costs by eliminating the separator and dryer from the current boiling water reactor To sufficiently remove the moisture of the steam led to the turbine so that deterioration on the turbine side does not occur, and keep the steam entrained in the cooling water descending the downcomer to keep the heat balance of the core sufficiently small. A natural circulation boiling water reactor can be provided.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a schematic view of a natural circulation boiling water reactor according to a first embodiment of the present invention. FIG. 2 is a graph showing a change in allowable vapor flow rate with respect to pressure which can maintain the stability of a free interface between two phases of gas and liquid. FIG. 3 is a graph showing a relationship between a steam flow rate and a height from a cooling water level to a main steam pipe. FIG. 4 is a graph showing a relationship between a flow rate of cooling water descending a downcomer portion and entrainment of steam in the downcomer. FIG. 5 is a schematic diagram of a natural circulation boiling water reactor of a second embodiment according to the present invention. FIG. 6 is a schematic view of a conventional natural circulation boiling water reactor. FIG. 7 is a schematic diagram of a current boiling water reactor. [Description of Signs] 6 ... reactor water, 8 ... main steam pipe, 9 ... water supply pipe, 20 ... chimney inner diameter, 21 ... height from cooling water level to main steam pipe, 22
... width of downcomer part, 24 ... chimney, 25 ... shroud, 26, 29 ... core, 27, 28 ... control rod, 30 ... reactor pressure vessel, 34 ... downcomer.

Continuation of front page    (72) Inventor Komaki Moriya             3-1-1, Sachimachi, Hitachi City, Ibaraki Pref.             Within the Nuclear Power Division of Hitachi, Ltd.

Claims (1)

Claims: 1. A circulation path having an inner cooling water ascending flow path and an outer cooling water descending flow path is formed by a cylindrical chimney incorporated in a reactor pressure vessel. In a natural circulation boiling water reactor in which an inlet of a main steam pipe is connected to a part of the reactor pressure vessel above cooling water in the reactor pressure vessel, an inner diameter of the chimney is 5.4 m or more. The height from the water level during the operation of the cooling water to the inlet of the main steam pipe is 2.5.
m, and the width of the descending flow path is 0.37 m or more.