GB2564898A

GB2564898A - Cooling system for a nuclear reactor

Info

Publication number: GB2564898A
Application number: GB1712076.7A
Authority: GB
Inventors: Palmer James
Original assignee: Rolls Royce Power Engineering PLC
Current assignee: Rolls Royce Power Engineering PLC
Priority date: 2017-07-27
Filing date: 2017-07-27
Publication date: 2019-01-30
Also published as: GB201712076D0

Abstract

An emergency cooling system for a nuclear power plant 100 comprises: a first fluid circuit 18 connecting the reactor pressure vessel 12 to a first heat exchanger 20 located within the reactor containment vessel 15; and a second fluid circuit 26 connecting said first heat exchanger to one or more second, preferably air-cooled, heat exchangers 28 located outside of the containment. When it is measured that the temperature within the reactor vessel 12 is above a threshold, or when conditions therein are otherwise indicative of an emergency situation, a valve 22 is configured to open so as to allow the fluid, i.e. water, to flow through the first fluid circuit. The second heat exchangers are preferably elevated with respect to the first, and the first heat exchanger elevated with respect to the reactor, so as to allow a natural circulation of the coolant, driven by temperature variations therein, to be established within the fluid circuits.

Description

[1]

COOLING SYSTEM FOR A NUCLEAR REACTOR

The present disclosure concerns cooling systems for nuclear reactors and, in particular, emergency or supplementary cooling systems for nuclear reactors.

It is known to provide cooling systems for nuclear reactors. In the event of an incident in which the temperature of the reactor increases as a result of a loss of the duty cooling function, it is desirable to provide systems which can reduce the temperature of the reactor urgently to ensure safe operation of the reactor and the power plant as a whole.

According to a first aspect, there is provided a cooling system for a nuclear reactor, the cooling system comprising first and second fluid circuits and first and second heat exchangers, wherein the first fluid circuit is arranged to flow a first fluid from a reactor vessel to the first heat exchanger; the second fluid circuit is arranged to flow a second fluid from the first heat exchanger to the second heat exchanger; the first heat exchanger is arranged within a containment surrounding the nuclear reactor and configured to transfer heat from the first fluid in the first fluid circuit to the second fluid in the second fluid circuit; and the second heat exchanger is arranged outside of the containment of the nuclear reactor and configured to transfer heat out of the fluid in the second fluid circuit.

A cooling system according to the above aspect provides two-stage cooling to transfer heat from the reactor to outside of the containment, where temperatures are significantly lower than inside the containment. The cooling system avoids filling the containment with a potentially hazardous liquid or gas, such as steam, which may react with or damage components in the containment, or could escape from the containment more easily.

The first fluid circuit may be arranged completely within the containment.

The first fluid circuit and first heat exchanger may be arranged such that, in use, temperature variations in the first fluid around the first fluid circuit drive fluid flow through the first fluid circuit. In particular, the first heat exchanger, or at least a part thereof, may elevated from the reactor vessel. Thus, the first fluid circuit and first heat exchanger may be operable to remove heat from the reactor vessel automatically [2] without pumps or other fluid moving devices, which improves its operation as an emergency or contingency device when no power is available. The first fluid may, when heated by the reactor, rise up in the first fluid circuit to the first heat exchanger. Once heat has been exchanged from the first fluid to the second fluid in the first heat exchanger, the newly cooled first fluid may descend back to the reactor vessel to be reheated.

The first fluid circuit may comprise a valve for selectively preventing or permitting fluid flow though the first fluid circuit. The valve may be a fail-safe valve. In other words, the valve may be configured to automatically open in the event of a failure, emergency, or overheating incident at the nuclear reactor.

The valve may be configured to open when a temperature of the reactor vessel is above a predetermined threshold or when a measured parameter indicates a complete or partial loss of normal or duty cooling, for example when the reactor plant is not operating within a recognised control band. A control system may be provided to monitor the reactor or reactor vessel and control the valve.

The second fluid circuit and second heat exchanger may be arranged such that, in use, temperature variations in the second fluid around the second fluid circuit drive fluid flow though the second fluid circuit. In particular, the second heat exchanger, or at least a part thereof, may be elevated from the first heat exchanger. Thus, the second fluid circuit and second heat exchanger may be operable to remove heat from the reactor vessel automatically without pumps or other fluid moving devices, which improves the system’s operation as an emergency or contingency device when no power is available. The second fluid may, when heated by the first fluid in the first heat exchanger, rise up in the second fluid circuit to the second heat exchanger. Once heat has been exchanged away from the second fluid in the second heat exchanger, the newly cooled second fluid may descend back to the first heat exchanger to be reheated and continue flowing through the second fluid circuit.

The second heat exchanger may be arranged to transfer heat from the second fluid in the second fluid circuit to air. The air may be at ambient temperature. Accordingly, a large temperature difference or deltaT may exist between the second fluid and the air at the second heat exchanger. Thus, an increased heat transfer rate may be achieved without the use of further contained or high-pressure fluids such as water.

[3]

Furthermore, a higher deltaT reduces the surface area necessary at the second heat exchanger to achieve a given heat transfer rate.

The second heat exchanger may be arranged such that, in use, convective airflow occurs through the second heat exchanger. For example, the second heat exchanger may be arranged with one or more fins between or over which air can flow naturally as it is heated. In some examples, the one or more fins may extend substantially vertically, substantially horizontally, or at a predetermined sloped angle. The resulting updraft of air may draw further cool air into the second heat exchanger.

The second heat exchanger may be arranged to transfer heat from the second fluid in the second fluid circuit to a third fluid in a third fluid line or circuit. In some examples, one or further heat exchangers and fluid lines or circuits may be provided.

In some examples, the second fluid circuit may comprise more than one second heat exchanger.

Both the first fluid and the second fluid may be liquids in use. In other examples, only one of the first and second fluids may be a liquid in use. In particular, the first and second fluids may be water. In other examples, only one of the first and second fluids may be water. The first and second fluids may both be at pressures in excess of ambient pressure.

In use, the first fluid may have a maximum temperature of around 300-340 degrees centigrade and a maximum pressure of around 100-200 bar. It should be understood that the pressure and temperature of the fluid may reduce around the first fluid circuit or as the decay heat from the reactor reduces due to cooling.

In use, the second fluid may be have a maximum temperature of around 150-200 degrees centigrade and a maximum pressure of around 1-10 bar, or in particular, 3-5 bar. It should be understood that the pressure and temperature of the fluid may reduce around the second fluid circuit or as the decay heat from the reactor reduces.

The cooling system may further comprise one or more of a pump for pumping the first fluid along the first fluid circuit; and a pump for pumping the second fluid along the second fluid circuit.

[4]

In use, a temperature difference (deltaT) between the first fluid in the first heat exchanger and the second fluid in the first heat exchanger may be around 80-200 degrees centigrade.

In use, a temperature difference (deltaT) between the second fluid in the second heat exchanger and a third fluid in the second heat exchanger may be around 100-200 degrees centigrade.

The cooling system may be an emergency cooling system, a secondary cooling system, a supplementary cooling system, or a contingency cooling system.

According to a second aspect, there is provided a nuclear reactor apparatus comprising a nuclear reactor arranged within a reactor vessel, a containment, and a cooling system according to the first aspect. The nuclear reactor apparatus may be comprised within a nuclear power plant.

According to a third aspect, there is provided a method of cooling a nuclear reactor comprising: flowing a first fluid in a first fluid circuit from a reactor vessel to a first heat exchanger arranged within a containment surrounding the nuclear reactor; exchanging heat from the first fluid to a second fluid in a second fluid circuit via the first heat exchanger; flowing the second fluid from the first heat exchanger to a second heat exchanger outside of the containment via the second fluid circuit; and exchanging heat out of the second fluid via the second heat exchanger.

According to a fourth aspect, there is provided an emergency method of cooling a nuclear reactor comprising: maintaining a valve preventing fluid flow in the first fluid circuit in a closed position while the nuclear reactor is in a normal condition; opening the valve if an emergency condition occurs at the nuclear reactor to allow fluid flow around the first fluid circuit; and then performing the method of cooling a nuclear reactor according to the third aspect.

For the methods according to the third and fourth aspects, the methods may involve one, some, or all features of the cooling systems according to the first aspect.

The skilled person will appreciate that except where mutually exclusive, a feature described in relation to any one of the above aspects may be applied mutatis mutandis [5] to any other aspect. Furthermore except where mutually exclusive any feature described herein may be applied to any aspect and/or combined with any other feature described herein.

Embodiments will now be described by way of example only, with reference to the Figure, in which:

Figure 1 is a schematic illustration of a nuclear reactor apparatus comprising a cooling system.

The nuclear reactor apparatus 100 may be a nuclear power plant, or similar. The nuclear reactor apparatus comprises a nuclear reactor 10 arranged within a reactor vessel 12 (which may also be referred to as a reactor pressure vessel). Within the reactor 10 a controlled exothermic fission reaction takes place, which heats water within the pressure vessel 12. The heated water is flowed to a steam generator 14, via a hot leg 17 where it is used to heat water through a heat exchanger into steam, which is then used to drive a turbine to generate electricity (not shown). Once used within the steam generator 14, the water is returned to the pressure vessel 12 via a cool leg 13. Accordingly, the pressure vessel 12 and the steam generator 14 are connected in a circuit via the hot and cold legs 17, 13. The reactor apparatus 100 may form part of a nuclear power plant.

The reactor 10, pressure vessel 12 and the steam generator 14 are arranged within a larger vessel 15, known as a containment, which is a gas-tight reinforced shell. The containment 15 provides three main functions: confinement of the radioactive material in the reactor 10 from the outside atmosphere; protection of the reactor 10 from external disturbances or influences; and shielding of radiation emissions from the reactor 10.

In normal operation, the water passing through the reactor 10 and to the steam generator 14 provides sufficient cooling of the reactor 10 to maintain safe operation. However, in some instances, the temperature within the reactor vessel 12 can increase above normal or safe levels, for example if normal cooling to the reactor is degraded or lost.

[6]

Accordingly, a cooling system 16 is provided for cooling the reactor 10. The cooling system 16 comprises a first fluid circuit 18 which connects the interior of the reactor vessel 12 to a first heat exchanger 20. Both the first fluid circuit 18 and the first heat exchanger 20 are arranged within the containment 15. Fluid flowing in the first fluid circuit 18 is referred to as a first fluid herein to distinguish it from fluid flowing in other circuits. It should be understood that although second and third fluids etc., may be described herein, these fluids may be the same fluid. For example first and second fluids may both be water; they are merely distinguished by being present in different circuits.

The first fluid circuit comprises a first section 18a, which may be referred to as a first hot line 18a. The first hot line 18a extends from an upper region of the reactor vessel 12 to the heat exchanger 20. In use, fluid from the reactor vessel 12 travels away from the reactor 10 towards the first heat exchanger 20 along the first hot line 18a. In other arrangements, the first hot line 18a may extend from another position in the reactor circuit, such as from the hot leg 17 to the steam generator 14.

The first fluid circuit 18 also comprises a second section 18b which may be referred to as a first cool line 18b. The first cool line 18b extends from the first heat exchanger 20 back to the reactor vessel 12. In use, fluid flows back from the first heat exchanger 20 to the reactor vessel 12 via the cool line. In this case, the first cool line 18b connects with the steam generator cool leg 13 which leads back to the reactor vessel 12, but it will be understood that the first cool line 18b may in other examples return directly to the reactor vessel 12. Thus, the first circuit 18 forms a circuit which flows from the reactor vessel 12, through the first heat exchanger 20, and back to the pressure vessel 12. It should be understood that the hot and cool lines 18a,b are so described, as the fluid within the hot line 18a (which has not yet passed through the first heat exchanger 20) will generally be hotter than the fluid in the cool line 18b (which has passed through the first heat exchanger 20). The fluid in both the hot and cool lines 18a, b may nevertheless be hotter or cooler than fluid in other lines or the ambient temperature.

The first fluid circuit 18 also includes a valve 22. The valve 22 is, in normal operation of the reactor 10, closed to prevent fluid flow along the first fluid circuit 18. In some examples, a pump 24 may be provided to pump water along the first circuit 18 from the reactor vessel 12 although, as will be described in the next paragraph, this is not always required.

[7]

It should be noted that the first heat exchanger 20 is elevated with respect to the reactor vessel 12. Thus, hot fluid leaving the reactor vessel along the first circuit 18 will be predisposed to rise up along the first hot line 18a to the first heat exchanger 20. Furthermore, the fluid in the first circuit 18, after having been cooled in the first heat exchanger 20, will be predisposed to sink back down to the reactor vessel 12. Accordingly, when the first circuit 18 is open and in use, fluid will automatically flow along the first circuit 18 without the need for a pump. However, a pump 24 may be provided when greater flow rates are required than would naturally occur.

The cooling system 16 further comprises a second fluid circuit 26, which connects the first heat exchanger 20 with second heat exchangers 28. In this example, two second heat exchangers 28 are provided in parallel, but in other examples only one or more than two may be provided, either in parallel or in series in the fluid circuit 26. The second heat exchangers 28 are arranged at an elevated position with respect to the first heat exchanger. In this case, only part of the second heat exchangers 28 is elevated with respect to the first heat exchanger 28, but in other cases the whole of the second heat exchangers 28 may be elevated.

Similarly to the first fluid circuit 18, the second fluid circuit 26 comprises a second hot line 26a and a second cool line 26b. In use, the second hot line 26a extends away from the first heat exchanger 20 to an upper part of the second heat exchangers 28. In use, a second fluid within the second circuit 26 is heated within the first heat exchanger 20 by the first fluid in the first circuit 18. Many different types of heat exchanger are available, but it will be understood that the first heat exchanger 20 is configured to transfer heat between the first and second fluids without mixing the two fluids. The heated second fluid then flows up out of the first heat exchanger 20 via the second hot line 26a to the second heat exchangers 28 via natural convection. The second heat exchangers 28 comprise a plurality of heat exchanger fins 30 through which the second fluid can flow. The fins 30 may alternatively be tubes or the like which increase the surface area of the heat exchanger to air compared to a single large-bore pipe.

In this case, the reactor apparatus 100 is housed within a housing 32. Openings 34 are provided in the housing adjacent the upper and lower ends of the second heat exchangers 28 to permit air to flow freely past the fins 30. In other examples, the second heat exchangers 28 may be arranged outside of any housing 32. The fins 30 [δ] extend substantially vertically such that air can rise through the heat exchangers past the fins 30 to be heated by the second fluid flowing through the fins 30. Accordingly, the air increases in temperature as it flow up between the fins 30, while the second fluid decreases in temperature as it falls down through the fins 30. Once the second fluid reaches the lower end of the second heat exchanger 28, it flows back to the first heat exchanger 20 via the second cool line 26b. Conversely, once the heated air reaches the upper end of the heat exchangers 28, it exits to the outside via the opening 34. Thus, the temperature variations within the second circuit 26 naturally create flow within the second circuit 26 to drive heat transfer from the second fluid to the ambient air.

Accordingly, when the valve 22 is opened, the heated first fluid from the reactor vessel 12 naturally and automatically establishes flow through both the first circuit 18 and the second circuit 26 to transfer heat from within the reactor vessel 12 to the ambient air.

In this example, in use, the temperature of water within the first circuit 18 (i.e. the first fluid) is at a temperature of around 300-350 degrees centigrade and at a pressure of around 100-200 bar, or preferably around 150 bar. Although these are specific values, it should be understood that generally the water in the first circuit 18 is maintained in liquid phase despite its high temperature regardless of its specific temperature and pressure values. Furthermore, in use, the water in the second circuit 26 (i.e. the second fluid) is at a temperature of around 150-200 degrees centigrade and a pressure of around 3-5 bar. Similarly to the first circuit 18, the water in the second circuit should be maintained in liquid phase. Maintaining the first and second fluids in liquid phase improves the heat transfer efficiency in the first and second heat exchangers 20, 28 thus reducing the heat transfer areas required.

Furthermore, maintaining the second fluid at a high temperature relative to the ambient air temperature (typically around 20 degrees centigrade) creates a high temperature difference (deltaT) across the second heat exchangers 28. Thus, a smaller heat transfer area is required at the second heat exchanger.

As previously mentioned, the cooling system 16 may be an emergency cooling system. The valve 22 may be configured to open when it is measured or detected that the temperature within the reactor vessel 12 is above a threshold temperature, or when conditions within the reactor vessel 12 are indicative of an emergency incident, such as [9] a critical incident. A control system may be provided (not shown) to monitor temperature, pressure, or other variables of the reactor or reactor vessel and open or close the valve according to whether the cooling system 16 is required to cool the reactor 10.

In other examples, the valve 22 may be a ‘fail-safe’ valve. For example, the valve may be maintained in the open position by a solenoid in an electrical circuit powered by the reactor 12 itself. Thus, if power fails, for example due to an emergency incident, then the valve may automatically open to activate the cooling system 16. Alternatively, the 10 valve 22 may be arranged to open, such as with a soluble or frangible part when the temperature or pressure within the reactor vessel reaches a predetermined threshold.

It will be understood that the invention is not limited to the embodiments abovedescribed and various modifications and improvements can be made without departing 15 from the concepts described herein. Except where mutually exclusive, any of the features may be employed separately or in combination with any other features and the disclosure extends to and includes all combinations and sub-combinations of one or more features described herein.

Claims

1. A cooling system (16) for a nuclear reactor (10), the cooling system comprising first (18) and second (26) fluid circuits and first (20) and second (28) heat exchangers, wherein the first fluid circuit is arranged to flow a first fluid from a reactor vessel (12) to the first heat exchanger;

the second fluid circuit is arranged to flow a second fluid from the first heat exchanger to the second heat exchanger;

the first heat exchanger is arranged within a containment (15) surrounding the nuclear reactor and configured to transfer heat from the first fluid in the first fluid circuit to the second fluid in the second fluid circuit; and the second heat exchanger is arranged outside of the containment and configured to transfer heat out of the fluid in the second fluid circuit.

2. The cooling system for a nuclear reactor as claimed in Claim 1, wherein the first fluid circuit and first heat exchanger are arranged such that temperature variations in the first fluid about the first fluid circuit drive fluid flow along the first fluid circuit.

3. The cooling system for a nuclear reactor as claimed in Claim 2, wherein the first heat exchanger is elevated from the reactor vessel.

4. The cooling system for a nuclear reactor as claimed in any preceding claim, wherein the first fluid circuit comprises a valve (22) for selectively preventing or permitting fluid flow along the first fluid circuit.

5. The cooling system for a nuclear reactor as claimed in Caim 4, wherein the valve is configured to open when a temperature of the reactor vessel is above a predetermined threshold, or when a measured parameter indicates a complete or partial loss of normal cooling.

6. The cooling system for a nuclear reactor as claimed in any preceding claim, wherein the second fluid circuit and second heat exchanger are arranged such that temperature variations in the second fluid about the second fluid circuit drive fluid flow along the second fluid circuit.

[11]

7. The cooling system for a nuclear reactor as claimed in Claim 6, wherein the second heat exchanger is elevated from the first heat exchanger.

8. The cooling system for a nuclear reactor as claimed in any preceding claim, wherein the second heat exchanger is arranged to transfer heat from the second fluid in the second fluid circuit to air.

9. The cooling system for a nuclear reactor as claimed in Claim 8, wherein the second heat exchanger is configured such that, in use, convective airflow occurs through the second heat exchanger.

10. The cooling system for a nuclear reactor as claimed in any one of Claims 1 to 7, wherein the second heat exchanger is arranged to transfer heat from the second fluid in the second fluid circuit to a third fluid in a third fluid circuit.

11. The cooling system for a nuclear reactor as claimed in any preceding claim, wherein both of the first and second fluids are liquids in use.

12. The cooling system for a nuclear reactor as claimed in Claim 11, wherein both of the first and second fluids are water.

13. The cooling system for a nuclear reactor as claimed in any preceding claim, wherein, in use, the first fluid has a maximum temperature of around 300-340 degrees centigrade and a maximum pressure of around 100-200 bar.

14. The cooling system for a nuclear reactor as claimed in any preceding claim, wherein, in use, the second fluid has a maximum temperature of around 150200 degrees centigrade and a maximum pressure of around 1-10 bar.

15. The cooling system for a nuclear reactor as claimed in any preceding claim, further comprising one or more of:

a pump (24) for pumping the first fluid along the first fluid circuit; and a pump for pumping the second fluid along the second fluid circuit.

16. The cooling system for a nuclear reactor as claimed in any preceding claim, wherein, in use, a temperature difference between the second fluid in the second heat exchanger and a third fluid in the second heat exchanger is around 100-200 degrees centigrade.

[12]

17. The cooling system for a nuclear reactor as claimed in any preceding claim, wherein, in use, a temperature difference between the first fluid in the first heat exchanger and the second fluid in the first heat exchanger is around 80-200 degrees centigrade.

18. The cooling system for a nuclear reactor as claimed in any preceding claim, wherein the cooling system is an emergency cooling system, a secondary cooling system, a supplementary cooling system, or a contingency cooling system.

19. A nuclear reactor apparatus (100) comprising a nuclear reactor (10) arranged within a reactor vessel (12), a containment (15), and a cooling system (16) as claimed in any preceding claim.

20. A method of cooling a nuclear reactor (10) comprising:

flowing a first fluid in a first fluid circuit (18) from a reactor vessel (12) to a first heat exchanger (20);

exchanging heat from the first fluid to a second fluid in a second fluid circuit (26) via the first heat exchanger;

flowing the second fluid from the first heat exchanger to a second heat exchanger (28) via the second fluid circuit; and exchanging heat out of the second fluid via the second heat exchanger.

21. An emergency method of cooling a nuclear reactor comprising: maintaining a valve (22) preventing fluid flow in the first fluid circuit in a closed position while the nuclear reactor is in a normal condition;

opening the valve if an emergency condition occurs at the nuclear reactor to allow fluid flow in the first fluid circuit; and then performing the method of Claim 20.