Nuclear reactor subsystem

Abstract

A nuclear reactor subsystem 100 includes a reactor pressure vessel 10 comprising: a main chamber region 20 containing a reactor core 60; and an inlet plenum region 30 housing a recuperator 50. Coolant enters the main chamber via inlet 12 and flows along the length of the pressure vessel so as to cool the core structure 40 by flowing over an external surface thereof. The coolant then enters the inlet plenum where it is heated by the recuperator before subsequently entering the core structure. The recuperator receives a fluid at a first temperature via recuperator inlet 54 and expels said fluid at a second, cooler temperature via recuperator outlet 52 by transferring heat energy from the fluid to the coolant. The nuclear reactor subsystem may form part of a system having either a direct (see Fig. 3) or indirect (Fig. 2) Brayton cycle architecture.

Description

CROSS REFERENCE TO RELATED APPLICATIONS This represents the first application directed towards the subject-matter. FIELD This disclosure relates to nuclear reactor systems, and more specifically to a nuclear reactor subsystem for cooling a reactor pressure vessel. BACKGROUND Common designs of nuclear reactor work on the principle of using a working fluid to extract heat energy produced by radioactive materials as they undergo fission events. The radioactive fuel used to produce the heat energy is located within the reactor core. The reactor core can reach a very high temperature, which presents a challenge in terms of ensuring the materials used in the construction of other parts of the nuclear reactor do not degrade at an unacceptable rate. To preserve the lifetime of the materials used in and around the core, various cooling arrangements are used. Improvements in these cooling arrangements can lead to increased efficiency and lifetime of the nuclear reactor. SUMMARY The present disclosure provides a nuclear reactor subsystem for a nuclear reactor system as set out in claim 1, a nuclear reactor system as set out in claim 2, a nuclear microreactor as set out in claim 5, and a method of operating a nuclear reactor subsystem for a nuclear reactor system as set out in claim 7. Optional features are included in the dependent claims. According to a first aspect there is provided a nuclear reactor subsystem for a nuclear reactor system, the nuclear reactor subsystem comprising a recuperator and a reactor pressure vessel, the reactor pressure vessel comprising a main chamber region containing a core structure for containing a reactor core, an inlet plenum region for housing a recuperator, a coolant inlet, a coolant outlet, a recuperator inlet and a recuperator outlet, the nuclear reactor subsystem being configured such that coolant can enter the main chamber via the coolant inlet to make thermal contact with the external surface of the core structure so as to cool the core structure, then enter the inlet plenum and be heated by the recuperator, before entering the core structure after it has passed through the inlet plenum, wherein the reactor pressure vessel houses the recuperator within the inlet plenum, such that the recuperator can receive a fluid at a first temperature via the recuperator inlet, and expel the fluid at a second temperature which is cooler than the first temperature via the recuperator outlet by transferring heat energy from the fluid to the coolant after the coolant has cooled the reactor pressure vessel, and before the coolant enters the core structure. According to a second aspect there is provided a nuclear reactor system comprising the nuclear reactor subsystem of the first aspect. The nuclear reactor system may comprise a first heat exchanger configured to transfer heat energy between the coolant and the fluid, such that the nuclear reactor system has an indirect Brayton cycle architecture. The working fluid received by the recuperator in the nuclear reactor system may be the coolant that cools the core structure, such that the nuclear reactor system has a direct Brayton cycle architecture. According to a third aspect, there is provided a nuclear microreactor comprising the nuclear reactor subsystem of the first aspect. According to a fourth aspect, there is provided a method of operation of a nuclear reactor subsystem for a nuclear reactor system, the nuclear reactor subsystem comprising a recuperator and a reactor pressure vessel, the reactor pressure vessel comprising a main chamber region containing a core structure for containing a reactor core, an inlet plenum region for housing a recuperator, a coolant inlet, a coolant outlet, a recuperator inlet, and a recuperator outlet, the method comprising flowing a coolant into the main chamber of the interior of the reactor pressure vessel so as to cool the core structure, flowing the coolant from the main chamber to the inlet plenum of the reactor pressure vessel, the inlet plenum being adjacent to the main chamber of the reactor pressure vessel, heating the coolant by bringing it into thermal contact with a fluid passing through the recuperator, flowing the coolant into the core structure and through the reactor core within the core structure, so as to absorb heat energy from the reactor core, and flowing the coolant out of the core structure and out of the reactor pressure vessel. 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 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. BRIEF DESCRIPTION OF THE DRAWINGS Embodiments will now be described by way of example only with reference to the accompanying drawings, in which: FIG. 1 shows a schematic arrangement of an example nuclear reactor subsystem; FIG. 2 shows a schematic of an example nuclear reactor system; FIG. 3 shows a schematic of another example nuclear reactor system; FIG. 4 shows a graphic representation of a method of operating the nuclear reactor subsystem of FIG. 1; and FIG. 5 shows a graphic representation of a nuclear microreactor comprising the nuclear reactor subsystem of FIG. 1. DETAILED DESCRIPTION FIG. 1 shows a schematic arrangement of an example nuclear reactor subsystem 100 according to the present disclosure. The nuclear reactor subsystem comprises a reactor pressure vessel 10. A core structure 40 for holding the reactor core 60 is within the reactor pressure vessel 10. The reactor pressure vessel 10 provides passage for a coolant. Where the nuclear reactor is a high temperature gas reactor, typical coolants could include nitrogen, helium, supercritical C02, and helium-xenon. The purpose of the reactor pressure vessel 10 is twofold: to maintain a pressure boundary around the coolant at the location where the coolant is hottest, and prevent the escape of radioactive material in the event of an accident scenario. In the example of FIG. 1, the reactor pressure vessel 10 is shown having two main sections: the main chamber 20 (indicated by the dotted region in FIG. 1), which is the part of the reactor pressure vessel 10 which houses the core structure 40; and an inlet plenum 30 (indicated by the vertical striped region in FIG. 1), which is the part of the reactor pressure vessel 10 which houses a recuperator 50. A layer of neutron-reflecting material can surround the core structure 40 such that coolant can flow between the layer of neutron-reflecting material and the reactor pressure vessel 10, but for clarity the layer of neutron-reflecting material has been omitted from FIG. 1. Coolant can enter the reactor pressure vessel 10 through a coolant inlet 12, which in the example of FIG. 1 is situated at the opposite end of the reactor pressure vessel to the inlet plenum. After entering the reactor pressure vessel, the coolant is directed to flow along the length of the reactor pressure vessel 10, around and along the inside surface(s) of the reactor pressure vessel 10 and the external surface of the core structure 40, as indicated by the block arrows. When installed as part of a nuclear reactor system having an indirect Brayton cycle architecture (see FIG. 2 for example), the coolant entering the pressure vessel will come from a heat exchanger 210 which will have extracted the heat energy from the coolant. When installed as part of a nuclear reactor system having a direct Brayton cycle architecture (see FIG. 3 for example), the coolant entering the pressure vessel will come from a compressor system 260. Advantageously, the coolant will be at or near the coolest temperature it will reach within the nuclear reactor system 200, 300, when it is brought into contact with the core container, and will therefore have a maximal cooling effect on the material(s) of the reactor pressure vessel 10 and core structure 40. In the case where a layer of neutron-reflecting material is present, the layer of neutron-reflecting material will be in thermal contact with both the coolant and the core structure 40, effectively forming the outermost layer, or external surface, of the core structure. Cooling the reactor pressure vessel and core structure to lower temperatures is beneficial for the lifetime of both the reactor pressure vessel and core structure, helping them to endure the extreme environment of the nuclear reactor. After flowing around and along the sides of the reactor pressure vessel 10 and the external surface of the core structure 40, the coolant arrives at the inlet plenum 30, where it will come into contact with the recuperator 50. The recuperator acts as a heat exchanger between the coolant in the reactor pressure vessel 10, and a working fluid flowing through the recuperator. The working fluid flowing through the recuperator will come from a turbine system 220 (see examples of FIG. 2 and FIG. 3), and as such will carry residual heat energy not used by the turbine. As a result, the working fluid flowing through the recuperator will be at a higher temperature than the coolant flowing through the reactor pressure vessel 10, and so heat energy will be transferred between the working fluid and the coolant, warming the coolant, and cooling the working fluid flowing through the recuperator. Re-using residual heat energy will improve the efficiency of a nuclear reactor system using the nuclear reactor subsystem, as more of the heat energy absorbed by the coolant will be recycled within the nuclear reactor system, rather than just being wasted (i.e. ejected into the surrounding environment). Within the inlet plenum 30, in addition to being heated by the recuperator, the coolant will be redirected so as to flow back towards the core structure 40 to enter the core structure 40 via one or more core structure inlets 42 (only a single core structure inlet 42 is shown in FIG. 1 for clarity). Here, within the core structure 40, the coolant will come into contact with the components of the reactor core 60. As the coolant has been warmed prior to entering the core structure 40 not only by the inside surface(s) of the reactor pressure vessel 10 and external surface of the core structure 40, but also by the recuperator 50, the temperature of the coolant as it enters the reactor core 60 is closer to the temperature of the reactor core components than it would be had it arrived from the turbine or heat exchanger only via the inside surface(s) of the reactor pressure vessel 10 and external surface of the core structure 40, without the additional heating of the recuperator 50. As such, the temperature gradient across the reactor core, which is to say the difference in temperature between those parts of the core components closest to the core structure inlet 42 (where the cooling gas is at its coolest whilst within the core structure), and those parts of the core components closest to a core structure outlet 44 (where the cooling gas is at its warmest whilst within the core structure), is reduced. Reducing the temperature gradient across the reactor core (i.e. the difference in temperature between one end of the reactor core and the opposite end of the reactor core) is beneficial for at least the following reasons. Firstly, it reduces the thermal stress on the components within the reactor core, which can increase their operable lifetime. Secondly, because parameters relating to neutronics vary with temperature, reducing the temperature gradient reduces variations in the neutronics profile across the core, allowing for simpler core design. As the coolant flows through the core structure 40 and around and through the components of the reactor core 60, heat energy will be transferred from the components of the reactor core 60 to the coolant. This process serves to cool the components of the reactor core, and heat the coolant. The coolant then exits the core structure 40 via the core structure outlet 44, and the reactor pressure vessel 10 via a coolant outlet 14 connected to the core structure outlet 44. FIG. 2 shows a schematic of an example nuclear reactor system 200 of the indirect Brayton cycle type comprising the nuclear reactor subsystem 100. In this example nuclear reactor, the circulation of the coolant around the nuclear reactor subsystem is aided by the inclusion of a pump 80. The coolant exits the nuclear reactor subsystem via the coolant outlet 14, and passes into a first heat exchanger 210. Within the first heat exchanger 210, the heat energy absorbed by the coolant as it passes through the reactor core 60 is transferred to a working fluid of the Brayton cycle architecture. The working fluid, thus heated, passes through a turbine system 220. The turbine system can comprise a single turbine or multiple turbines arranged in series (for example, as a high pressure turbine and a low pressure turbine) or parallel. As the heated working fluid passes through the turbine system, the heat energy is converted into kinetic energy, as the one or more turbines within the turbine system 220 are rotated. The turbine system is connected to a generator system 230, which converts the kinetic energy into electrical energy which can be supplied to a load or stored in a battery, for example. After leaving the turbine system 220, the working fluid travels to the recuperator 50 (not shown in FIG. 2) via a recuperator inlet 52, which passes through the reactor pressure vessel 10 to enter the recuperator. Within the recuperator, residual heat in the working fluid is used to heat the coolant after the coolant has been used to cool the reactor pressure vessel and core structure. The working fluid then leaves the recuperator and the reactor pressure vessel 10 via a recuperator outlet 54, and travels to a second heat exchanger 240, which is connected to the fluid circuit of a heat rejection system 250. Here, the working fluid transfers any remaining heat energy into the heat rejection system working fluid. The heat rejection system working fluid flows round the heat rejection system fluid circuit to the heat rejection system 250, where residual heat energy is ejected into the surrounding environment. For example, the heat rejection system 250 may take the form of a radiator system, whereby the heat energy is transferred from the heat rejection system working fluid into the radiator system, from where it can be radiatively dissipated into the surrounding environment. Alternatively, if a suitable heatsink is available, the heat energy can be conductively extracted from the heat rejection system. The working fluid, having transferred any residual heat energy to the heat rejection system 250 via the second heat exchanger 240, continues around the fluid circuit to the compressor system 260. The compressor system 260 can comprise a single compressor or multiple compressors arranged in series or parallel. The compressor system is connected to the turbine system 220, such that rotation of the one or more turbines within the turbine system will cause rotation of one or more compressors within the compressor system 260. As the working fluid passes through the one or more compressors of the compressor system, it is compressed. After passing through the compressor system 260, the working fluid then returns to the first heat exchanger 210 to complete a loop of the Brayton cycle architecture. FIG. 3 shows a schematic of another example nuclear reactor system 300 of the direct Brayton cycle type comprising the nuclear reactor subsystem 100. In this example nuclear reactor, there is no first heat exchanger 210, meaning that when the coolant exits the nuclear reactor subsystem via the coolant outlet 14, it enters the gas turbine system to become the working fluid of the direct Brayton cycle architecture. This means that when the coolant, having been heated by the reactor core, exits the nuclear reactor subsystem via the coolant outlet 14, it then travels to the turbine system 220, where the heat energy carried by the coolant can be converted into kinetic energy by the turbine system, and ultimately electrical energy at the generator 230. In other words, the working fluid and the coolant of the example nuclear reactor system 300 of the direct Brayton cycle type are one in the same. The rest of the working fluid circuit for the nuclear reactor system 300 of FIG. 3 works in the same way as that of FIG.2. So, the coolant, after leaving the turbine system 220, will travel to the recuperator 50 (not shown in FIG. 3), entering the reactor pressure vessel 10 via the recuperator inlet 52. Within the recuperator, residual heat energy still present in the coolant after leaving the turbine system is used to heat coolant within the nuclear reactor subsystem that has just been used to cool the reactor pressure vessel and core structure. The coolant then leaves the recuperator and reactor pressure vessel via the recuperator outlet 54, and travels to a second heat exchanger 240 (the only heat exchanger in the example nuclear reactor system of FIG. 3), which is connected to the fluid circuit of a heat rejection system 250. Here, the coolant transfers any remaining heat energy into the heat rejection system working fluid. As before, the heat rejection system working fluid flows round the heat rejection system fluid circuit to the heat rejection system 250, where residual heat energy is ejected into the surrounding environment. The coolant, having transferred any residual heat energy to the heat rejection system 250 via the second heat exchanger 240, continues around the fluid circuit to the compressor system 260. The compressor system 260 can comprise a single compressor or multiple compressors arranged in series or parallel. The compressor system is connected to the turbine system 220, such that rotation of the one or more turbines within the turbine system will cause rotation of one or more compressors within the compressor system 260. As the coolant passes through the one or more compressors of the compressor system, the coolant is compressed. After passing through the compressor system 260, the coolant then re-enters the nuclear reactor subsystem via the coolant inlet 12, circulating around the nuclear reactor subsystem to complete a loop of the direct Brayton cycle fluid circuit. Removing the first heat exchanger can reduce the mass and the part count of the nuclear reactor system. The nuclear reactor subsystem described herein is an improvement on known systems as it provides cooling of the reactor pressure vessel and core structure at the lowest temperature of the coolant, whilst also reducing the temperature gradient across the reactor core. This is achieved by using the recuperator positioned within the inlet plenum to recycle residual heat energy between cooling of the reactor pressure vessel 10 and core structure 40, and cooling of the reactor core 60. In this way both the lifetime and efficiency a nuclear reactor system fitted with the nuclear reactor subsystem can be improved. FIG. 4 is a graphic representation of a method 400 of operating the nuclear reactor subsystem of FIG. 1. In a first step 410, coolant is flowed into a main chamber of the interior of the reactor pressure vessel, which houses the reactor core within the core structure, so as to cool the reactor pressure vessel and core structure. In a second step 420, the coolant flows from the main chamber to the inlet plenum of the reactor pressure vessel, which is adjacent to the main chamber of the reactor pressure vessel, and houses the recuperator. In a third step 430, the coolant is heated by being brought into thermal contact with a fluid passing through the recuperator. This is achieved as a result of the fluid passing through the recuperator being at a higher temperature than the coolant as the fluid and coolant flow through the recuperator. In a fourth step 440, the coolant is flowed into the core structure and through the reactor core within the core structure, so as to absorb heat energy from the reactor core. In a fifth step 450, the coolant flows out of the core structure and out of the reactor pressure vessel. Using this method achieves the advantages of cooling the reactor pressure vessel and core structure to a lower temperature, thus prolonging the life of the reactor pressure vessel, but also reduces the temperature gradient along the length of the reactor core by providing a higher temperature coolant, which is as a result of the coolant being heated by the recuperator after having cooled the reactor pressure vessel and core structure, but before the coolant enters the reactor core. The nuclear reactor system 200, 300, may be used in a nuclear microreactor or advanced modular reactor. The nuclear reactor subsystem and nuclear reactor systems described herein may be particularly suited for use in smaller nuclear reactor systems, such as microreactors, which can have applications in areas such as disaster relief, offshore facilities, and extraterrestrial environments such as satellites, lunar bases, and spaceships. FIG. 5 shows a graphic representation of a nuclear microreactor 500 comprising the nuclear reactor subsystem 100 described herein. The nuclear microreactor 500 may comprise the nuclear reactor subsystem as part of a nuclear reactor system 200, 300, such as those described herein. Various examples have been described, each of which comprise one or more combinations of features. It will be appreciated by those skilled in the art that, except where clearly mutually exclusive, any of the features may be employed separately or in combination with any other features and the invention extends to and includes all combinations and sub-combinations of one or more features described herein. Further modifications within the scope of the nuclear reactor subsystem and nuclear reactor systems described herein will be apparent to the skilled person. For example, whilst in the example nuclear reactor subsystem of FIG. 1 a single coolant inlet 12 is shown at the opposite end of the reactor pressure vessel to the inlet plenum, it is to be understood that this is not an essential feature of the design. For example, a plurality of coolant inlets may be present to allow coolant to enter the reactor pressure vessel at a number of different locations. One or more coolant inlets may for example be present on the sides of the reactor pressure vessel, with the reactor pressure vessel having interior walls or pipes directing the coolant to one end of the reactor core so that the coolant can cool the length of the reactor pressure vessel prior to passing through the inlet plenum. Similarly, whilst only a single recuperator inlet 52 and recuperator outlet 54 are shown in FIG. 1, it is to be understood that this is not an essential feature of the design. For example, a plurality of recuperator inlets may be present to allow coolant or a working fluid to enter the recuperator 50 at a number of different locations, and similarly, a plurality of recuperator outlets may be present to allow coolant or a working fluid to exit the recuperator 50 at a number of different locations. In addition, it is to be understood that the absence of a pump in the example nuclear reactor system shown in FIG. 3 does not preclude the addition of a pump to the fluid circuit should it be deemed advantageous.

Claims

1. A nuclear reactor subsystem for a nuclear reactor system, the nuclear reactor subsystem comprising:a recuperator;a reactor pressure vessel, the reactor pressure vessel comprising:a main chamber region containing a core structure for containing a reactor core;an inlet plenum region for housing a recuperator;a coolant inlet;a coolant outlet;a recuperator inlet; anda recuperator outlet;the nuclear reactor subsystem being configured such that coolant can enter the main chamber via the coolant inlet to make thermal contact with the external surface of the core structure so as to cool the core structure, then enter the inlet plenum and be heated by the recuperator, before exiting the inlet plenum and entering the core structure;wherein the reactor pressure vessel houses the recuperator within the inlet plenum, such that the recuperator can receive a fluid at a first temperature via the recuperator inlet, and expel the fluid at a second temperature which is cooler than the first temperature via the recuperator outlet by transferring heat energy from the fluid to the coolant after the coolant has cooled the reactor pressure vessel, and before the coolant enters the core structure.

2. A nuclear reactor system comprising the nuclear reactor subsystem of claim 1.

3. The nuclear reactor system of claim 2, further comprising a first heat exchanger configured to transfer heat energy between the coolant and the fluid, such that the nuclear reactor system has an indirect Brayton cycle architecture.

4. The nuclear reactor system of claim 2, wherein the working fluid received by the recuperator is the coolant that cools the core structure, such that the nuclear reactor system has a direct Brayton cycle architecture.

5. A nuclear microreactor comprising the nuclear reactor subsystem of claim 1.

6. A nuclear microreactor comprising the nuclear reactor system of any of claims 2, 3, or 4.

7. A method of operation of a nuclear reactor subsystem for a nuclear reactor 5 system, the nuclear reactor subsystem comprising a recuperator and a reactor pressure vessel, the reactor pressure vessel comprising a main chamber region containing a core structure for containing a reactor core, an inlet plenum region for housing a recuperator, a coolant inlet, a coolant outlet, a recuperator inlet, and a recuperator outlet, the method comprising:io flowing a coolant into the main chamber of the interior of the reactor pressure vesselso as to cool the core structure;flowing the coolant from the main chamber to the inlet plenum of the reactor pressure vessel, the inlet plenum being adjacent to the main chamber of the reactor pressure vessel;heating the coolant by bringing it into thermal contact with a fluid passing through is the recuperator;flowing the coolant into the core structure and through the reactor core within the core structure, so as to absorb heat energy from the reactor core;flowing the coolant out of the core structure and out of the reactor pressure vessel.

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