Passive heat removal systems for nuclear reactors.

Abstract

The nuclear reactor has an intermediate coolant loop for transferring thermal energy from the reactor core for beneficial purposes, the intermediate coolant loop including a bypass flow path with an air heat exchanger for rejecting reactor heat during startup and / or shutdown, and a fluidic diode along the bypass flow path asymmetrically restricting flow across the bypass flow path, inhibiting flow in a first flow direction during full-power operating conditions and allowing relatively uninhibited flow in a second direction during lower-power operating conditions during startup and / or shutdown.

Description

Detailed Description of the Invention

[0001] CROSS-REFERENCE TO RELATED APPLICATIONS This application claims the benefit of U.S. Patent Application No. 17 / 159,734, entitled "PASSIVE HEAT REMOVAL SYSTEM FOR NUCLEAR REACTORS," filed January 27, 2021, and claims the benefit of U.S. Provisional Patent Application No. 63 / 066,785, entitled "CARTRIDGE CORE BARREL FOR NUCLEAR REACTOR," filed August 17, 2020, the entire contents of which are incorporated herein by reference.

[0002] 〔background〕 Most nuclear reactors have a core in which fuel elements and control elements are supported in various interrelated arrangements to support critical reactivity and control the power output of the reactor. Coolant is typically pumped through passages between the fuel elements and the control elements so that heat generated by the nuclear fission fuel elements is transferred to heat exchangers where it is used for beneficial purposes.

[0003] When a reactor is shut down, such as by inserting control elements into the core to reduce reactivity, the fission products continue to produce heat as they undergo radioactive decay. In some cases, the decay heat power level is initially about 6-7% of the core power before shutdown. This can be a significant amount of heat that must be dealt with during reactor shutdown.

[0004] Residual heat removal systems are standard safety systems designed to deal with decay heat from a shut down nuclear reactor. Residual heat removal systems may include emergency generators that drive circulating pumps, passive heat removal systems, and other types of heat removal systems. Without a properly functioning decay heat removal system, the residual heat could cause catastrophic failures within the reactor systems.

[0005] To deal with residual heat, some reactors utilize Direct Reactor Auxiliary Cooling Systems (DRACS) or Reactor Vessel Auxiliary Cooling Systems (RVACS), which are primarily safety systems relied upon to release decay heat after the reactor is shut down.

[0006] It would be an improvement over the use of passive heat removal systems in the art to function to remove heat from the reactor during start-up procedures, rather than just helping to remove residual decay heat in passive systems. It would be an additional advantage if such a system were not classified as a safety system, i.e., were a commercial first line of defense in addition to the safety system, rather than having a requirement to be designed and constructed to safety rating standards.

[0007] 〔overview〕 According to some embodiments, the passive heat removal system functions to enable decay heat removal during both reactor startup and shutdown by enabling both forced and natural circulation between the hot and cold legs of heat exchangers located in the intermediate fluid loop. Natural circulation of fluid is facilitated by relying on gravity to cause the denser cold fluid to descend and the less dense heated fluid to ascend. This causes natural circulation through the intermediate coolant loop through the heat exchangers. In some cases, selective flow regulators cause a high pressure drop in a first fluid direction and a low pressure drop in a second fluid direction.

[0008] According to some embodiments, a sodium-cooled nuclear reactor includes a reactor vessel, a reactor core within the reactor vessel, a primary heat exchanger within the reactor vessel, an intermediate coolant loop that circulates an intermediate coolant through the primary heat exchanger and has a hot leg and a cold leg through which the intermediate coolant flows, a pump in fluid communication with the intermediate coolant loop to circulate the intermediate coolant through the intermediate coolant loop, an air heat exchanger disposed along a bypass flow path between the hot leg and the cold leg of the intermediate coolant loop, and a fluid diode disposed along the bypass flow path to asymmetrically restrict fluid flow along the bypass flow path.

[0009] In some cases, the fluidic diode is located outside the reactor vessel and may provide a flow resistance to the pump circulating the intermediate coolant across the bypass flow path, i.e., when the pump is operating, the fluidic diode provides a resistance to the flow of intermediate coolant along the bypass flow path from the cold leg to the hot leg.

[0010] The fluidic diode may allow relatively unrestricted fluid flow across the bypass flow path when the pump is not operating, in other words, when the pump is not operating, natural circulation allows intermediate fluid to flow through the fluidic diode from the hot leg to the cold leg with very little flow resistance.

[0011] In some examples, the air heat exchanger discharges reactor power heat during low power operation startup conditions. For example, the air heat exchanger may discharge reactor power heat during low power shutdown conditions (e.g., when reactor power falls below about 20% power, 15% power, 10% power, or less power).

[0012] In some examples, the intermediate coolant is liquid sodium. In other examples, the intermediate coolant is a molten salt. Of course, other intermediate coolants are contemplated and may be used with the disclosed systems and configurations.

[0013] In some embodiments, a bypass flow pump may be configured to force the intermediate coolant along the bypass flow path. In some cases, the bypass flow pump is omitted and natural circulation forces the intermediate coolant along the bypass flow path.

[0014] According to some embodiments, a method of operating a nuclear reactor having an intermediate coolant loop includes generating heat in a reactor core, flowing primary coolant into the reactor through a primary heat exchanger, flowing the intermediate coolant through an intermediate coolant loop and the primary heat exchanger, flowing the intermediate coolant through a bypass flowpath, and rejecting heat from an air heat exchanger located along the bypass flowpath in the intermediate coolant loop during periods of low reactor power.

[0015] In some examples, the method includes continuing to reject heat from the air heat exchanger disposed along the bypass flowpath until the reactor reaches a target threshold power output, or in other words, diverting a portion of the intermediate coolant away from the bypass flowpath when the reactor exceeds the target threshold power output.

[0016] In some embodiments, flowing the intermediate coolant through a bypass flow path further comprises flowing the intermediate coolant through a fluidic diode in fluid communication with the bypass flow path.

[0017] In some examples, the periods of low reactor power are less than about 5% reactor power, less than about 10% reactor power, or less than about 20% reactor power. In some examples, the intermediate coolant is caused to reject heat through an air heat exchanger during reactor startup. In some examples, the intermediate coolant is caused to reject heat through an air heat exchanger during reactor shutdown.

[0018] In some embodiments, flowing the intermediate coolant includes flowing sodium or salt through the intermediate cooling loop. Other suitable coolants may be utilized as alternatives, depending on reactor and intermediate loop configurations.

[0019] BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a schematic diagram of a nuclear reactor intermediate coolant loop in steady-state operation, according to some embodiments.

[0020] FIG. 2 illustrates a schematic diagram of a nuclear reactor intermediate coolant loop in natural circulation operating conditions, according to some embodiments.

[0021] FIG. 3 illustrates a schematic diagram of a nuclear reactor intermediate coolant loop incorporating fluidic diodes in steady-state operating conditions, according to some embodiments.

[0022] FIG. 4 illustrates a schematic diagram of a nuclear reactor intermediate coolant loop incorporating fluidic diodes in natural circulation operating conditions, according to some embodiments.

[0023] Detailed Description The present disclosure relates generally to passive devices and arrangements for decay heat removal from a nuclear reactor during shutdown and / or startup, including passive systems that may operate through the principles of natural circulation.

[0024] 1 illustrates a nuclear reactor intermediate coolant loop during steady-state operation. A nuclear reactor 100, of any suitable design, includes a reactor core, typically within a reactor vessel 104. The core typically contains fuel that generates heat through the process of nuclear fission. A primary coolant, which may be any suitable coolant, is forced through the core 102 to extract heat from the nuclear fission fuel. After passing through the core, the primary coolant is directed to a primary heat exchanger 106 where it transfers thermal energy to an intermediate coolant flowing through an intermediate coolant loop 108.

[0025] In the illustrated embodiment, the primary coolant in the reactor vessel may be any suitable coolant and in some examples is a liquid metal such as sodium. The intermediate coolant in the intermediate coolant loop 108 may likewise be any suitable coolant and in some examples is sodium. In some examples, the intermediate coolant in the intermediate coolant loop 108 is a salt.

[0026] The intermediate coolant loop 108 is often a closed fluid loop that includes a hot leg 110, a cold leg 112, and a pump 114. In normal use, the pump 114 pressurizes the intermediate coolant, which causes the coolant in the cold leg 108 to flow into the primary heat exchanger 106 and receive thermal energy from the primary coolant in the reactor 100. The intermediate coolant exits the primary heat exchanger 106 through the hot leg 110. The intermediate coolant is transported to another system where the thermal energy is used for beneficial purposes. In some embodiments, the thermal energy transferred by the intermediate coolant loop 108 is used to store thermal energy, to generate steam, or to provide other purposes.

[0027] In some embodiments, an air heat exchanger 116 is provided along a bypass flowpath 118 between the hot leg 110 and the cold leg 112. In some examples, the air heat exchanger 116 is elevated such that gravity inhibits direct bypass flow from the cold leg 112 to the hot leg 110. The bypass flow tends to reduce the temperature of the intermediate coolant in the hot leg 110, thus reducing the thermal energy available for beneficial purposes downstream of the bypass flowpath 118.

[0028] During normal operation of the reactor 100, the pump 114 pressurizes the fluid in the intermediate coolant loop 108 to flow the intermediate coolant to the primary heat exchanger 106. As the intermediate coolant loop is pressurized, the intermediate coolant may flow along a bypass fluid path 118 that is parallel to the flow path through the primary heat exchanger 106. In some cases, this is an undesirable situation because the bypass flow path results in mixing of the cold and hot legs, thus reducing the thermal energy of the intermediate coolant in the hot leg. By raising the air heat exchanger 116, some of the bypass flow may be alleviated due to gravity providing flow resistance along the bypass flow path 118.

[0029] FIG. 2 illustrates the reactor intermediate coolant loop 108 in a reactor shutdown configuration. As shown, the pump 114 lacks power and therefore does not pressurize the intermediate coolant loop 108 and circulate the intermediate coolant. The intermediate coolant may continue to flow through natural circulation (e.g., in the direction indicated by arrow 202). During reactor shutdown, decay heat from the core must be addressed and is typically addressed by a residual heat removal system. The residual heat removal system may be any number of suitable safety systems configured to bleed residual heat from the core. As shown, if the pump 114 is not energized and does not provide pumping power, the fluid in the hot leg 110 and cold leg 112 of the intermediate coolant loop 108 may stagnate and therefore not transport thermal energy from the core as intended. The air heat exchanger 116 may continue to enable natural circulation flow, which provides a fluid path for the intermediate coolant. It allows fluid in the hot leg 110 to circulate through the air heat exchanger 116 to the cold leg 112 so that it can continue to provide heat removal from the core 102 even in the absence of pumping power. Of course, a pump (e.g., along a bypass flow path 118) may be provided to provide forced circulation of the intercooling fluid from the primary heat exchanger 106 to the air heat exchanger 116 to dump decay heat into the ambient air. The illustrated embodiment is represented diagrammatically, and in many cases the air heat exchanger 116 may be elevated, thus providing natural circulation by relying on gravity to move the denser cold fluid down and the less dense heated fluid up. This causes natural circulation through the intercoolant loop through the air heat exchanger 116.

[0030] In many cases, the bypass flow path 118 is automatically accessed even in the absence of electrical power and does not require any change in the fluid path to flow the intermediate coolant through the air heat exchanger 116. For example, the hot leg 110 and the cold leg 112 may provide sufficient flow friction such that the bypass flow path 118 is the path of least resistance to fluid flow. In this case, the intermediate coolant flows from the primary heat exchanger 106, thus picking up heat from the core 102, flows along the hot leg 110, flows toward the air heat exchanger 116, flows toward the cold leg 112, and returns to the primary heat exchanger 106. Thus, the intermediate coolant receives thermal energy from the core 102 and rejects it to the ambient air at the air heat exchanger 116.

[0031] In some embodiments, a pump may be provided along the bypass flow path 118 to further facilitate the release of decay heat. While the air heat exchanger 116 may be elevated to provide resistance to bypass flow of the intermediate fluid from the cold leg 112 to the hot leg 110, in some embodiments, additional resistance to bypass flow may be introduced to further reduce the likelihood of bypass flow.

[0032] 3 illustrates a reactor intermediate coolant loop 300 having a fluidic diode 302 incorporated in the bypass flowpath 118 to provide selective flow resistance. During normal full power operation of the reactor 100, the pump 114 circulates an intermediate fluid through the intermediate coolant loop 108. The fluidic diode 302 may be provided in the bypass flowpath 118 (e.g., between the cold leg 112 and the air heat exchanger 116). The fluidic diode 302 may provide an increased flow resistance to prevent intermediate coolant from entering the air heat exchanger 116 from the cold leg 112.

[0033] The fluidic diode 302 is a device that provides increased flow resistance and therefore provides lower resistance to fluid flow in a second direction while restricting fluid flow in a first direction. A check valve may be used in such a configuration. A check valve typically has a movable component (e.g., a sealing member) that is movable between an open position and a closed position, where the sealing member is biased to a closed position in response to fluid flow in a first direction and is biased to an open position in response to fluid flow in a second direction. However, check valves can be complex, unreliable, and in some cases can become tangled or stuck in either the open or closed configuration. Thus, the fluidic diode 302 can provide many of the same benefits without the complexity or potential for failure.

[0034] According to some embodiments, the fluidic diode has no moving parts, but rather a fluid friction differential due to the shape of the internal fluid path and / or the size of the inlet and outlet. The result is a valve with no moving parts that provides high resistance to flow in a first direction and low resistance to flow in a second direction. According to some embodiments, the pressure drop in the first direction caused by the fluidic diode is about 1.5 times, 2 times, 5 times, 10 times, or more than the pressure drop in the second direction caused by the fluidic diode. In some embodiments, the fluidic diode resists flow from the cold leg 112 to the hot leg 110 across the bypass flow path 118. The fluidic diode 302 may allow fluid flow from the hot leg 110 to the cold leg 112 through the air heat exchanger 116 with significantly less resistance than fluid flow in the opposite direction.

[0035] 4 illustrates a reactor intermediate coolant loop 300 with fluidic diodes 302 incorporated to provide selective flow resistance. As illustrated, when the intermediate pumps 114 are not pumping to pressurize the intermediate coolant loop 300, the intermediate coolant fluid may flow by natural circulation in the direction indicated by arrows 202. In this configuration, fluid from the hot leg 110 naturally flows through the air heat exchanger 116, through the fluidic diodes 302, and into the cold leg 112.

[0036] There are situations in which it is desirable to release heat generated in the reactor rather than sending the thermal energy to a destination such as a power generation cycle or thermal storage. For example, during reactor startup, the air heat exchanger 116 may be utilized to release thermal energy generated until the reactor reaches a threshold operating power. For example, during reactor startup, intermediate coolant may flow through the air heat exchanger 116 as the reactor power increases. In some cases, the air heat exchanger is utilized to release heat generated in the reactor during low power operating conditions (e.g., until the reactor reaches 3% power, 4% power, 5% power, 7% power, 8% power, 10% power, or more). As the reactor power gradually increases after startup, heat may be gradually transferred to the primary heat transport structure. Once the reactor reaches a threshold power, the air heat exchanger 116 may be substantially idle.

[0037] Similarly, during reactor shutdown, the air heat exchanger 116 may be used to dump thermal energy to cool the reactor when it is shut down during low power operating conditions. For example, when the reactor ramps down to about 20% power, the intermediate coolant loop may begin flowing intermediate coolant through the air heat exchanger 116 to dump excess heat. When the reactor continues ramping down below about 20% power, heat transfer duty may be handed over to the air heat exchanger 116 accordingly. As a result, when the reactor reaches about 5% power during shutdown, the air heat exchanger 116 is handling 100% of the decay heat. The air heat exchanger 116 can safely handle decay heat removal from a shut down reactor in a passive mode with no moving parts and no intervention required to operate systems, valves, or pumps.

[0038] In some embodiments, the air heat exchanger 116 receives thermal energy in response to the reactor power. For example, during start-up, the air heat exchanger 116 initially receives 100% of the thermal energy generated by the reactor until the reactor reaches a threshold power (e.g., about 5% power). At this point, the air heat exchanger 116 gradually begins to pass thermal energy to the normal thermal management system, such as by activating the intermediate coolant pump 114. Then, the reactor reaches another threshold power and the entirety of the thermal energy is passed to the thermal management system. In many cases, the fluidic diode 302 restricts fluid flow asymmetrically between the first and second flow directions. Thus, the fluidic diode 302 inhibits flow in the first direction when the intermediate coolant loop is pressurized and the reactor is operating at full power, and allows flow in the second direction during low reactor power scenarios, such as start-up and / or shutdown. In some cases, the fluid flow in the second direction is not substantially restricted compared to the fluid flow in the first direction.

[0039] In some examples, the intermediate coolant is sodium and the primary heat exchanger is a sodium / sodium heat exchanger. In some examples, the intermediate coolant is salt and the primary heat exchanger is a sodium / salt heat exchanger. Of course, these configurations are dependent upon the reactor utilizing sodium as the primary coolant, and other primary coolants are equally applicable to the disclosed systems and configurations.

[0040] This disclosure presents exemplary embodiments. Therefore, this disclosure is not intended to limit the scope of the embodiments of the present disclosure and the appended claims in any respect. The embodiments have been described above with the aid of functional building blocks showing implementation of specified functions and their relationships. In this specification, the boundaries of these functional components are arbitrarily defined for the convenience of description. Alternative boundaries may be defined to the extent that the specified functions and their relationships are appropriately implemented.

[0041] The general nature of the embodiments of the present disclosure will be sufficiently clear from the above description of the specific embodiments that others may easily modify and / or adapt the specific embodiments for various applications without undue experimentation by applying the knowledge of those skilled in the art without departing from the general spirit of the embodiments of the present disclosure. Such adaptations and modifications are therefore intended to be within the meaning and range of equivalents of the disclosed embodiments, based on the teaching and guidance presented herein. The phraseology or terminology in this specification is intended to be illustrative, not limiting, as the term or terminology in this specification would be interpreted by one skilled in the art in light of the teaching and guidance presented herein.

[0042] The breadth and scope of the presently disclosed embodiments should not be limited by any of the above-described exemplary embodiments, but should be defined only in accordance with the following claims and their equivalents.

[0043] Throughout this specification, the term "substantially" referring to a given parameter, property, or condition will mean and include the extent to which a person skilled in the art would understand that the given parameter, property, or condition satisfies a small variation (e.g., tolerance tolerance). By way of example, depending on the particular parameter, property, or condition that is substantially met, the parameter, property, or condition may be at least about 90% met, at least about 95% met, or at least about 99% met.

[0044] Unless otherwise specified or understood otherwise within the context in which they are used, conditional words such as "may," "could," "may," or "may," among others, are generally intended to convey that a particular implementation may include a particular feature, element, and / or operation, while other implementations do not include the particular feature, element, and / or operation. As such, such conditional words generally do not intend that the features, elements, and / or operations are in any way required by one or more implementations, or that one or more implementations necessarily include logic for determining whether those features, elements, and / or operations are included in any particular implementation, or whether those features, elements, and / or operations should be performed in any particular implementation, with or without user input or prompting.

[0045] The specification and drawings disclose examples of systems, apparatus, devices and techniques that allow reactor modules to be manufactured in a manufacturing facility and transported to a manufacturing site where the modules are assembled, thereby significantly reducing the complexity and expense of on-site manufacturing. Additionally, reactor systems are simplified, further facilitating factory manufacturing instead of on-site manufacturing.

[0046] Those skilled in the art will recognize that any process or method disclosed herein can be modified in many ways. The process parameters and sequences of the processes described and / or illustrated herein are given by way of example only and can be changed as desired. For example, although the processes illustrated and / or described herein may be shown or described in a particular order, these processes do not necessarily have to be performed in the order illustrated or described.

[0047] The various exemplary methods described and / or illustrated herein may omit one or more of the steps described or illustrated herein or may include additional steps in addition to those disclosed. Furthermore, the steps of any method disclosed herein may be combined with any one or more steps of any other method disclosed herein.

[0048] Of course, for purposes of describing the various features of the present disclosure, it is not possible to describe every conceivable combination of elements and / or methods. However, those skilled in the art will recognize that many additional combinations and permutations of the disclosed features are possible. Accordingly, various modifications can be made to the present disclosure without departing from the scope or spirit of the present disclosure. Moreover, other embodiments of the present disclosure may become apparent from consideration of the specification and the accompanying drawings, and from the practice of the disclosed embodiments presented herein. The examples presented in this specification and the accompanying drawings are to be considered in all respects as illustrative and not restrictive. Although specific terms have been employed herein, they are used in a generic and descriptive sense only and not for purposes of limitation.

[0049] Unless otherwise specified, the terms "connected" and "coupled" (and their derivatives) used in the specification should be interpreted as allowing both direct and indirect (i.e., via other elements or components) connections. Additionally, the terms "a" or "an" used in the specification should be interpreted as meaning "at least one of." Finally, for ease of use, the terms "including" and "having" (and their derivatives) used in the specification are interchangeable with the word "comprising" and shall have the same meaning.

[0050] From the above and the accompanying drawings, it will be understood that although certain implementations are described herein for illustrative purposes, various modifications may be made without departing from the spirit and scope of the appended claims and the requirements recited therein. In addition, although certain aspects are presented below in certain claim forms, the inventors contemplate various aspects in any available claim form. For example, while only some aspects may be currently described as embodied in a specific configuration, other aspects may be embodied in the same manner. Various modifications and changes may be made as would be apparent to those skilled in the art having the benefit of this disclosure. It is intended to cover all such modifications and changes. Thus, the above description should be viewed in an illustrative rather than a limiting sense. [Brief description of the drawings]

[0051] [Figure 1] 1 is a schematic diagram of a nuclear reactor intermediate coolant loop in steady-state operation according to some embodiments. [Diagram 2] 1 illustrates a schematic diagram of a nuclear reactor intermediate coolant loop in natural circulation operating conditions, according to some embodiments. [Diagram 3] 1 illustrates a schematic diagram of a nuclear reactor intermediate coolant loop incorporating fluidic diodes in a steady-state operating condition, according to some embodiments. [Figure 4] 1 illustrates a schematic diagram of a nuclear reactor intermediate coolant loop incorporating fluidic diodes in natural circulation operating conditions according to some embodiments.

Claims

1. A reactor vessel; a reactor core within the reactor vessel; a primary heat exchanger within the reactor vessel; an intermediate coolant loop circulating the intermediate coolant through the primary heat exchanger and having a hot leg and a cold leg through which the intermediate coolant flows; a pump in fluid communication with the intermediate coolant loop for circulating the intermediate coolant through the intermediate coolant loop; an air heat exchanger disposed along a bypass flowpath between the hot leg and the cold leg of the intermediate coolant loop; a fluidic diode disposed along the bypass flowpath to asymmetrically restrict fluid flow along the bypass flowpath.

2. The sodium-cooled nuclear reactor of claim 1 , wherein the fluid diode is located outside the reactor vessel.

3. 2. The sodium-cooled nuclear reactor of claim 1, wherein the fluidic diode provides a flow resistance to the pump circulating the intermediate coolant across the bypass flow path.

4. 2. The sodium-cooled nuclear reactor of claim 1, wherein the fluidic diode allows relatively unrestricted fluid flow across the bypass flow path when the pump is not operating.

5. 10. The sodium-cooled nuclear reactor of claim 1, wherein the air heat exchanger rejects reactor power heat during low power operational start-up conditions.

6. 10. The sodium-cooled nuclear reactor of claim 1, wherein the air heat exchanger rejects reactor power heat during low power shutdown conditions.

7. 2. The sodium-cooled nuclear reactor of claim 1, wherein the intermediate coolant is sodium.

8. 2. The sodium-cooled nuclear reactor of claim 1, wherein the intermediate coolant is salt.

9. 10. The sodium-cooled nuclear reactor of claim 1 further comprising a bypass flow pump for flowing intermediate coolant along the bypass flow path.

10. 2. The sodium-cooled nuclear reactor of claim 1, wherein the fluidic diode provides a flow restriction for fluid flowing in a first direction that is greater than five times greater than a flow restriction for fluid flowing in a second direction.

11. A method comprising the steps of generating heat in a nuclear reactor core; flowing primary coolant into the reactor through a primary heat exchanger; flowing an intermediate coolant through an intermediate coolant loop and the primary heat exchanger; flowing the intermediate coolant through a bypass flow path; rejecting heat from an air heat exchanger disposed along the bypass flowpath in the intermediate coolant loop during periods of low reactor power, the method comprising:

11. The method of claim 10, further comprising continuing to reject heat from the air heat exchanger disposed along the bypass flowpath until the reactor reaches a target threshold power.

12. 12. The method of operating a nuclear reactor as recited in claim 11, further comprising diverting a portion of the intermediate coolant from the bypass flow path when the reactor exceeds the target threshold power.

13. A method comprising: generating heat in a nuclear reactor core; flowing primary coolant into the reactor through a primary heat exchanger; flowing an intermediate coolant through an intermediate coolant loop and the primary heat exchanger; flowing the intermediate coolant through a bypass flow path; rejecting heat from an air heat exchanger disposed along the bypass flowpath in the intermediate coolant loop during periods of low reactor power, the method comprising: The method of operating a nuclear reactor, wherein flowing the intermediate coolant through a bypass flowpath further comprises flowing the intermediate coolant through a fluid diode in fluid communication with the bypass flowpath.

14. The method of claim 1, comprising the steps of: generating heat in a nuclear reactor core; flowing primary coolant into the reactor through a primary heat exchanger; flowing an intermediate coolant through an intermediate coolant loop and the primary heat exchanger; flowing the intermediate coolant through a bypass flow path; rejecting heat from an air heat exchanger disposed along the bypass flowpath in the intermediate coolant loop during periods of low reactor power, the method comprising: The method of operating a nuclear reactor, wherein the periods of low reactor power are less than 5% reactor power.

15. A method comprising: generating heat in a nuclear reactor core; flowing primary coolant into the reactor through a primary heat exchanger; flowing an intermediate coolant through an intermediate coolant loop and the primary heat exchanger; flowing the intermediate coolant through a bypass flow path; rejecting heat from an air heat exchanger disposed along the bypass flowpath in the intermediate coolant loop during periods of low reactor power, the method comprising: The method of operating a nuclear reactor, wherein the period of low reactor power is less than 20% reactor power.

16. 16. The method of operating a nuclear reactor according to any one of claims 11 to 15, wherein the periods of low reactor power occur when the reactor is shut down.

17. 20. The method of claim 16, wherein the step of releasing heat comprises releasing decay heat from the reactor during a reactor shutdown cycle.

18. 16. The method of claim 11, wherein the step of flowing the intermediate coolant includes the step of flowing sodium through the intermediate coolant loop.

19. 16. The method of operating a nuclear reactor according to any one of claims 11 to 15, wherein the step of flowing the intermediate coolant includes the step of flowing salt through the intermediate coolant loop.

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