WO2005086174A1

WO2005086174A1 - Nuclear reactor plant

Info

Publication number: WO2005086174A1
Application number: PCT/IB2004/004289
Authority: WO
Inventors: Tinus Vermaak; Ivan Drodskie
Original assignee: Pebble Bed Modular Reactor (Proprietary) Limited
Priority date: 2004-02-10
Filing date: 2004-12-28
Publication date: 2005-09-15

Abstract

A nuclear reactor plant (10) includes a reactor contained within a pressure vessel (12). A plurality of internal heat exchangers are positioned around the vessel (12). The plant includes a pump (23) which is connected in flow communication with the vessels (12) and a tank (40), positioned at an elevation which is above that of the heat exchangers (20), with which the outlets of the internal heat exchangers are in flow communication and an outlet (42) which is in flow communication with the pump (23). A continuously open flow path (52) extends between the storage tank (40) and a position upstream of the inlets (22) of the internal heat exchangers so that, in the event of pump failure, coolant can be fed, under the influence of gravity, from the storage tank (40) to the heat exchangers (20) to replace coolant which may be lost from the heat exchangers and thereby provide passive cooling capacity.

Description

NUCLEAR REACTOR PLANT

THIS INVENTION relates to a nuclear reactor plant.

Nuclear reactors of which the Inventors are aware include a reactor pressure vessel within which the core is contained and a containment structure within which the reactor pressure vessel is contained. A cooling system, referred to as a reactor cavity cooling system is provided to remove heat from the cavity defined between the reactor pressure vessel and the containment structure. Typically, use is made of a closed loop liquid cooling system which includes one or more coolant chambers, arranged around at least part of the reactor, and pump means for pumping the coolant into and through the coolant chambers.

According to the invention there is provided a nuclear reactor plant which includes a heat source; at least one internal heat exchanger positioned in proximity to the heat source, the internal heat exchanger having an inlet and an outlet through which liquid coolant can enter and leave the internal heat exchanger; a pump having a suction side and a discharge side, the discharge side being connected in flow communication with the inlet of the internal heat exchanger; at least one storage tank, at least part of which is positioned at an elevation which is above that of the internal heat exchanger, the storage tank having an inlet with which the outlet of the internal heat exchanger is in flow communication and an outlet which is in flow communication with the suction side of the pump; and a continuously open flow path extending between the storage tank and a position upstream of the inlet of the internal heat exchanger so that, in the event of pump failure, coolant can be fed, under the influence of gravity, from the storage tank to the heat exchanger to replace coolant which may be lost from the heat exchanger and thereby provide passive cooling capacity.

The coolant will typically be water and a vent may lead from the storage tank through which steam can be vented from the storage tank to atmosphere. The vent may incorporate a filter.

In one embodiment of the invention, the heat source may be a nuclear reactor core contained in a reactor pressure vessel and a plurality of internal heat exchangers may be provided, the heat exchangers being circumferentially spaced around the reactor pressure vessel in a cavity defined between the reactor pressure vessel and a containment shell or structure.

Each internal heat exchanger may be in the form of an elongate vertically extending vessel, the inlet being in the form of a pipe which enters the vessel at or adjacent to an upper end thereof and extends downwardly within the vessel to terminate in a discharge end which opens into the vessel and the outlet leading from the vessel at or adjacent the upper end thereof.

The storage tank inlet may be provided at a position at or towards the top of the storage tank, the storage tank outlet leading from the storage tank at an elevation which is lower than that of the storage tank inlet.

The discharge side of the pump may be connected to a main header which in turn may be connected, via feeder pipes, to a plurality of intermediate headers, each of which is in turn connected to the inlets of a group of the internal heat exchangers. The number of storage tanks may correspond to the number of intermediate inlet headers, each flow path extending from the associated storage tank to a position upstream of an associated intermediate inlet header.

The outlets of the internal heat exchangers in each group may be connected to an intermediate outlet header which in turn is connected to the inlet of the storage tank associated with the group of internal heat exchangers. Hence, each group of internal heat exchangers will have associated therewith an intermediate inlet header, an intermediate outlet header and a storage tank.

The flow path may extend between the storage tank and the associated feeder pipe at a position upstream of the associated intermediate inlet header. The position of the storage tank outlet is typically selected such that, in use, when the pump is operating, i.e. when the cooling system is operating in an active mode, the pressure owing to the static head of coolant in the storage tank is approximately equal to the pressure of coolant in the feeder pipe at the point at which the flow path opens into the feeder pipe. Hence, no or little coolant will flow from the feeder pipe through the flow path into the storage tank.

The outlets of the storage tanks may be connected in flow communication with a common outlet header which in turn is connected in flow communication with the suction side of the pump.

An external heat exchanger, i.e. a heat exchanger positioned outside the containment structure, may be connected in flow communication between the outlet header and the suction side of the pump thereby to form a closed loop reactor cavity cooling system.

In a preferred embodiment of the invention, there are eighteen intermediate inlet headers, eighteen intermediate outlet headers and eighteen storage tanks, with four internal heat exchangers associated with each of the intermediate inlet headers thereby providing a total of seventy two internal heat exchangers.

Each flow path may include flow restriction means. The flow restriction means may include an orifice.

The invention will now be described, by way of example, with reference to the accompanying diagrammatic drawings.

In the drawings: Figure 1 shows a three-dimensional view of part of a nuclear reactor plant, in accordance with the invention, the containment structure within which a reactor pressure vessel is contained being omitted; Figure 2 shows schematically a flow diagram illustrating the operation of a reactor cavity cooling system forming part of the plant of Figure 1 ; Figure 3 shows a three-dimensional view of part of the reactor cavity cooling system of Figure 2; and Figure 4 shows a three-dimensional view, on an enlarged scale, of part of the reactor cavity cooling system.

In the drawings, reference numeral 10 refers generally to part of a nuclear reactor plant in accordance with the invention. In the embodiment shown, the plant 10 is in the form of a process plant.

The plant 10 includes a heat source which, in the embodiment shown, is the core of a nuclear reactor which core is contained within a reactor pressure vessel 12. The reactor pressure vessel 12 is itself contained within a containment structure, e.g. a concrete shell or citadel 14, part of which is shown in Figure 2 of the drawings. An annular cavity 15 is defined between an outer surface of the reactor pressure vessel 12 and an inner surface of the citadel 14.

The plant 10 further includes a power generation circuit, part of which is generally indicated by reference numeral 16 (Figure 1 ). The power generation circuit 16 is connected to the reactor pressure vessel 12 by suitable piping, the details of the power generation circuit 16 are not required for an understanding of the present invention and are not described in any detail.

The plant 10 includes a reactor cavity cooling system, generally indicated by reference numeral 18 for removing heat from the cavity 15. The system 18 includes a plurality of internal heat exchangers in the form of stand pipes 20 which extend vertically and are arranged at circumferentially spaced positions in the cavity 15 around the reactor pressure vessel 12, as shown in Figures 1 and 3. Each stand pipe 20 has an inlet 22 and an outlet 24. The system 18 further includes a pump 23 having a suction side and a discharge side, the discharge side of the pump being connected in flow communication with the inlets 22 as described in more detail below. More particularly, the outlet or discharge side of the pump is connected via a pipe 25, which extends through a wall of the citadel 14, in_. flow communication with a main header 28 positioned inside the citadel 14. The main header 28 in turn is connected via feeder pipes 29 to a plurality of intermediate inlet headers 30. Each intermediate inlet header 30 is connected to the inlet 22 of a plurality of the vessels or stand pipes 20. As can best be seen in Figure 2 of the drawings, each inlet 22 is in the form of a length of pipe 32 which enters the stand pipe 20 at or adjacent an upper end thereof and extends downwardly within the stand pipe 20 to terminate in a discharge end 34 which opens into the stand pipe 20. Each outlet 24 leads from the stand pipe 20 at a position at or towards the top thereof from where it feeds to an intermediate outlet header 36. The plant 10 includes a plurality of the intermediate outlet headers 36 each of which feeds via a line 38 into an associated storage tank 40. Each storage tank 40 has an inlet 41 defined by the end of the line 38 which opens into the storage tank 40 at or towards the top of the tank 40. The storage tank 40 further includes an outlet 42 which leads from the storage tank 40 at an elevation below that of the inlet but above the bottom of the storage tank 40 and typically from the top half thereof. If desired the outlet 42 can be formed by a pipe which extends internally to a position adjacent the bottom of the storage tank 40 and anti-siphoning devices may be fitted if warranted by operating conditions.

A vent line 44 leads from the top of the storage tank 40 and opens to atmosphere. A filter 46 is provided in the vent line 44. The outlets 42 of the storage tanks 40 are connected via lines 48 to an outlet header 50 which in turn is connected via a pipe 51 , which extends through the wall of the citadel 14, to an external heat exchanger (not shown) and then to the suction side of the pump 23.

A line 52 defines a flow path extending between a lower end of each storage tank 40 and the associated feeder pipe 29 at a position upstream of the associated intermediate inlet header 30 which feeds into the stand pipes 20 the outlets 24 of which are in turn connected in flow communication with the storage tank 40. Flow through the line 52 may be restricted by means of an orifice 54 provided at the end of the line 52 connected to the storage tank 40. If desired normally open orifice isolating valves (not shown) may be fitted to line 52.

In the embodiment shown, seventy two stand pipes are arranged around the reactor pressure vessel 12. The plant 10 includes eighteen intermediate inlet headers 30 and eighteen intermediate outlet headers 36, each pair of inlet and outlet headers 30, 36 is connected in flow communication with four of the stand pipes 20. The plant further includes eighteen storage tanks 40. Each storage tank 40, intermediate inlet header 30, intermediate outlet header 36 and four of the stand pipes 20 are connected together as a unit. In use, during normal or active operation of the reactor cavity cooling system, i.e. when the pump is operational, water is fed from the pump to the main header 28. From there it is distributed via the feeder pipes 29 to the intermediate inlet headers 30. The water is then distributed from each intermediate inlet header 30 to the four associated stand pipes 20 through the inlets 22 where it picks up heat. The heated water is discharged from the stand pipes 20 through the outlets 24 from where it is fed to the intermediate outlet headers 36. The water leaving the intermediate outlet header 36 is fed to the associated storage tank 40 into which it is discharged. The water exiting the storage tanks 40 through the outlets 42 is fed to the header 50 and from there to the external heat exchanger and back to the pump 23.

The size of the orifice 54 in feed line 52 is selected to suit the required water flow during passive operation , as well as, the required resistance flow into the storage tank 40 during active operation, i.e. when the pump 23 is operating. Further, the storage tank 40 is positioned at an elevation above the point at which the feed line 52 enters the feeder pipe 29 such that the static head of coolant within the storage tank 40 is approximately equal to the pressure of coolant in the feeder line 29 such that no or little coolant flows from the feeder line 29 through the feed line 52 and into the storage tank 40.

Hence, when in normal operation, the water is re-circulated around the reactor cavity cooling system to remove heat from the citadel.

However, in the event that active cooling is lost, e.g. if the pump ceases to operate the reactor would be tripped but would continue to radiate decay heat. It is therefore important that heat continue to be removed from the citadel 14. In this situation, the pressure in the feeder pipes 29 would decrease and water would be fed under the influence of gravity from the storage tank 40 through the orifice 54 and line 52 into the associated header 30. From there the water would be fed into the stand pipes 20 in which it would be heated. As the temperature of the water in the stand pipes 20 increases, steam may be generated which is fed via the header 36 and line 38 into the storage tank 40 and vented via the filter 46 to atmosphere. The water level in the stand pipes 20 is maintained by water being fed from the storage tanks 40 through the lines 52 into the stand pipes 20 to replace water lost by the formation of steam. If desired, water in the storage tank can be supplemented through a water make-up line 56 thereby providing sufficient cooling capacity for the plant 10 for an extended period of time to enable action to be taken to restore active cooling.

The inventors believe that a major advantage associated with the invention is that the switch from active to passive cooling happens automatically with no electrical, mechanical or human intervention required thereby ensuring the reliability of the system and the safety of the plant.

Claims

1. A nuclear reactor plant which includes a heat source; at least one internal heat exchanger positioned in proximity to the heat source, the internal heat exchanger having an inlet and an outlet through which liquid coolant can enter and leave the internal heat exchanger; a pump having a suction side and a discharge side, the discharge side being connected in flow communication with the inlet of the internal heat exchanger; at least one storage tank, at least part of which is positioned at an elevation which is above that of the internal heat exchanger, the storage tank having an inlet with which the outlet of the internal heat exchanger is in flow communication and an outlet which is in flow communication with the suction side of the pump; and a continuously open flow path extending between the storage tank and a position upstream of the inlet of the internal heat exchanger so that, in the event of pump failure, coolant can be fed, under the influence of gravity, from the storage tank to the heat exchanger to replace coolant which may be lost from the heat exchanger and thereby provide passive cooling capacity.

2. A plant as claimed in claim 1 , in which the heat source is a nuclear reactor core contained in a reactor pressure vessel and a plurality of internal heat exchangers is provided, the heat exchangers being circumferentially spaced around the reactor pressure vessel in a cavity defined between the reactor pressure vessel and a containment shell or structure.

3. A plant as claimed in claim 2, in which each internal heat exchanger is in the form of an elongate vertically extending vessel, the inlet being in the form of a pipe which enters the vessel at or adjacent to an upper end thereof and extends downwardly within the vessel to terminate in a discharge end which opens into the vessel and the outlet leading from the vessel at or adjacent the upper end thereof.

4. A plant as claimed in claim 2 or claim 3, in which the storage tank inlet is provided at a position at or towards the top of the storage tank, the storage tank outlet leading from the storage tank at an elevation which is lower than that of the storage tank inlet.

5. A plant as claimed in any one of claims 2 to 4, inclusive, in which the discharge side of the pump is connected to a main header which in turn is connected, via feeder pipes, to a plurality of intermediate headers, each of which is in turn connected to the inlets of a group of the internal heat exchangers.

6. A plant as claimed in claim 5, in which the number of storage tanks corresponds to the number of intermediate inlet headers, each flow path extending from the associated storage tank to a position upstream of an associated intermediate inlet header.

7. A plant as claimed in claim 5 or claim 6, inclusive, in which the outlets of the internal heat exchangers in each group are connected to an intermediate outlet header which in turn is connected to the inlet of the storage tank associated with the group of internal heat exchangers.

8. A plant as claimed in claim 7, in which the outlets of the storage tanks are connected in flow communication with a common outlet header which in turn is connected in flow communication with the suction side of the pump.

9. A plant as claimed in claim 8, in which there are eighteen intermediate inlet headers, eighteen intermediate outlet headers and eighteen storage tanks, with four internal heat exchangers associated with each of the intermediate inlet headers thereby providing a total of seventy two internal heat exchangers.

10. A plant as claimed in claim 7 or claim 8, in which an external heat exchanger, positioned outside the containment structure is connected in flow communication between the outlet header and the suction side of the pump thereby to form a closed loop reactor cavity cooling system.

11. A plant as claimed in any one of claims 2 to 10, inclusive, in which the or each flow path includes flow restriction means.

12. A plant as claimed in claim 11 , in which the flow restriction means includes an orifice.

13. A plant as claimed in any one of the preceding claims, in which the coolant is water and a vent leads from the storage tank through which steam can be vented from the storage tank to atmosphere.

14. A plant as claimed in claim 13, in which the vent incorporates a filter.