WO2015089665A1

WO2015089665A1 - Nuclear reactor shutdown system

Info

Publication number: WO2015089665A1
Application number: PCT/CA2014/051229
Authority: WO
Inventors: John Andrew Ferguson Shaw; Maciej Urban JASTRZEBSKI; Bert Orland Wasmund; William Mark WESTGATE
Original assignee: Hatch Ltd.
Priority date: 2013-12-17
Filing date: 2014-12-17
Publication date: 2015-06-25

Abstract

According to an aspect of the present disclosure, there is provided a nuclear reactor system which includes a core and containment structure. The interior of the containment structure is filled with both a liquid fuel and a gas. The reactor core is located inside the containment structure above the maximum resting level of the liquid fuel. In this condition, the core is filled by the gas, and a sustained fission chain reaction will not take place. A pump is provided draw the liquid fuel from the lower portion of the containment structure and circulates it through the reactor core, where the fuel reacts to produce heat. The reactor sustains a fission chain reaction only when the pump is on, and thus the reactor can be shut-down simply by turning off the pump, and allowing gravity to drain the core of the fuel salt.

Description

NUCLEAR REACTOR SHUTDOWN SYSTEM CROSS-REFERENCE TO RELATED APPLICATION

[0001] This application claims priority to and the benefit of United States Provisional Patent Application No. 61/917,023 filed December 17, 2013, the contents of which are incorporated herein by reference.

FIELD

[0002] The present application relates generally to nuclear reactors and, more specifically, to a reactor which can be shut down by draining the liquid fuel out of the core by gravity and/or a gravity induced pressure gradient.

BACKGROUND

[0003] Nuclear reactors contain fissile fuel which reacts to produce heat. This heat is removed from the reactor and used, typically to produce electricity. The most common type of nuclear reactor contains a core of solid fuel that is cooled and moderated by a circulating fluid, typically water.

[0004] Alternate nuclear reactor designs exist where the fuel is dissolved into the circulating liquid coolant, typically a molten salt. In these reactors a core is provided with a sufficient volume for the liquid fuel to reach criticality. If the reactor is intended to operate in the thermal neutron spectrum, a moderator is also incorporated into the core.

[0005] Liquid fuelled reactors can be shut down by draining the fuel out of the core into a series of dump tanks external to the reactor vessel. Typically, a cooled plug of frozen material is provided in the piping connecting the core to the dump tanks, usually located close to the core. In an emergency situation, if the liquid fuel heats above a specified temperature, this freeze plug would melt and allow the liquid fuel to drain into the dump tanks. This system is considered to be very reliable because it relies only on natural forces (gravity and phase change) to shut-down the nuclear reaction. [0006] While reliable, this draining system adds complexity to the reactor design, and can be slow to engage because the frozen plug must first melt before draining of the reactor begins. In addition, the auxiliary containment building must be larger to enclose not only the reactor, but also the dump tanks.

[0007] There exists a need for a simplified method of draining a reactor core of liquid fuel.

SUMMARY

[0008] In an embodiment, there is provided a nuclear reactor system, comprising : (a) a containment structure having an upper portion and a lower portion; (b) a reactor core located in the upper portion of the containment structure, wherein the reactor core has a first end defining an inlet area for a liquid fuel, and a second end defining an outlet area for the liquid fuel; (c) a liquid fuel reservoir located in the lower portion of the containment structure; (d) a drain passage for flow of the liquid fuel between the reactor core and the reservoir; (e) a recirculation passage connecting the reservoir to the inlet area of the reactor core; and (f) a pump for circulating the liquid fuel through the recirculation passage.

[0009] In one aspect, the containment structure is filled with both the liquid fuel and with a gas.

[0010] In another aspect, the reactor core has a plurality of flow channels for flow of the liquid fuel from the inlet area to the outlet area.

[0011] In yet another aspect, the nuclear reactor system further comprises : (g) a heat exchanger to transfer heat from the liquid fuel to a coolant, wherein the heat exchanger includes at least one coolant flow passage.

[0012] In yet another aspect, the heat exchanger is located within the reservoir or is connected to the reservoir by a conduit.

[0013] In yet another aspect, the heat exchanger further comprises a liquid fuel inlet and a liquid fuel outlet, wherein the liquid fuel inlet is arranged to receive the liquid fuel from the reactor core, and the liquid fuel outlet is arranged to supply the liquid fuel to the recirculation passage.

[0014] In yet another aspect, the nuclear reactor system further comprises : (h) a controller to control the operation of the pump.

[0015] In yet another aspect, a maximum liquid level in the containment structure when the pump is off is defined as a level of liquid fuel in the containment structure when the reactor core contains insufficient liquid fuel to enable a fission chain reaction to occur within the reactor core.

[0016] In yet another aspect, the maximum liquid level when the pump is off is defined as a level of liquid fuel in the reservoir when the reactor core contains substantially none of the liquid fuel.

[0017] In yet another aspect, the maximum liquid level when the pump is off is defined as a level of liquid fuel in the reactor core when the pump is off and all or substantially all of the liquid fuel has been drained from the reactor core.

[0018] In yet another aspect, the maximum liquid level when the pump is off is spaced below the outlet area of the reactor core.

[0019] In yet another aspect, the maximum liquid level when the pump is off is located in the drain passage.

[0020] In yet another aspect, the reservoir has a volume sufficient to hold all or substantially all of the liquid fuel present in the reactor system, and wherein the maximum liquid level when the pump is off is located in the reservoir.

[0021] In yet another aspect, the flow channels are arranged to permit the liquid fuel to flow from the inlet area to the outlet area via gravity.

[0022] In yet another aspect, at least some of the flow channels are substantially vertically disposed.

[0023] In yet another aspect, the core contains a neutron moderator to enable a fission chain reaction . [0024] In yet another aspect, the liquid fuel is comprised of a fissile material dissolved in a liquid medium, and may comprise a molten salt with a fissile material as a constituent element.

[0025] In yet another aspect, the reactor core is enclosed within a reactor vessel, and the nuclear reactor system further comprises: (j) an opening in the upper portion of the reactor vessel through which gas may enter or be expelled from the reactor vessel.

[0026] In yet another aspect, the lower portion of the containment structure comprises sufficient neutron absorbing material to prevent a fission chain reaction from occurring outside the reactor core.

[0027] In yet another aspect, there is provided a method of operating a nuclear reactor system, comprising : (a) providing a nuclear reactor system as defined herein; (b) providing a liquid fuel in said reactor vessel, the liquid fuel comprising a fissile material in a liquid medium; (c) operating the pump to circulate the liquid fuel through the recirculation passage, the reactor core, the drain passage and the reservoir, wherein the fissile material in the liquid fuel circulating through the flow channels of the reactor core undergoes a fission chain reaction, thereby heating the liquid medium;and (d) ceasing operating of the pump, such that the flow of liquid fuel to the reactor core ceases, and the liquid fuel partly or completely drains out of the reactor core through the drain passage and into the reservoir, such that an insufficient amount of the liquid fuel remains in the reactor core to sustain the fission chain reaction.

[0028] In yet another aspect, the method further comprises the step of extracting heat energy from the liquid fuel with a heat exchanger.

[0029] In yet another aspect, gas is allowed to enter the flow channels of the reactor core to replace the liquid fuel draining out of the reactor core after the operation of the pump ceases.

[0030] In yet another aspect, the liquid fuel which is drained from the moderating core causes a level of the liquid fuel in the reservoir to rise. BRIEF DESCRIPTION OF THE DRAWINGS

[0031] Reference will now be made, by way of example, to the

accompanying drawings which show example implementations; and in which :

[0032] Figure 1 shows a nuclear reactor according to a first embodiment in an operating state;

[0033] Figure 2 shows the nuclear reactor of Figure 1 in an idle state;

[0034] Figure 3 shows a nuclear reactor according to a second embodiment in an idle state;

[0035] Figure 4 shows the nuclear reactor of Figure 3 in an operating state;

[0036] Figure 5 shows a nuclear reactor according to a third embodiment in an operating state;

[0037] Figure 6 shows the nuclear reactor of Figure 5 in an idle state;

[0038] Figure 7 shows a nuclear reactor according to a fourth embodiment in an operating state; and

[0039] Figure 8 shows the nuclear reactor of Figure 7 in an idle state.

DETAILED DESCRIPTION

[0040] The embodiments described herein relate to nuclear reactors in which a liquid fuel is circulated through a reactor core and undergoes a fission chain reaction in the core. The liquid fuel comprises a fissile material dissolved in a liquid medium such as a molten salt, with the fissile material contained therein as a constituent element thereof.

[0041] Figures 1 and 2 show a nuclear reactor system 10 according to a first embodiment, with Figure 1 illustrating the reactor system 10 in operation, and Figure 2 illustrating the reactor system 10 in an idle state. It will be appreciated that these drawings are highly simplified and only show components of the reactor system 10 which are necessary for an explanation of the present embodiment.

[0042] The reactor system 10 comprises a containment structure 12 which, in the present embodiment, is also the reactor vessel, and the terms "reactor vessel" and "containment structure" are interchangeably used in the description of the present embodiment. However, it will be appreciated that the reactor system 10 may comprise a reactor vessel 12 which is surrounded by a separate containment structure, similar to the arrangement provided in other

embodiments which will be described below, or it may be a constituent element of the containment structure.

[0043] The reactor vessel 12 has a upper portion 14 and a lower portion 16, wherein a reactor core 18 is located in the upper portion 14 of the reactor vessel 12. The reactor core 18 has an upper end 20 (sometimes referred to herein as the "first end") defining an inlet area 22 for the liquid fuel and a lower end 24 (sometimes referred to herein as the "second end") defining an outlet area 26 for the liquid fuel. The reactor core 18 further comprises a plurality of flow channels 28 for flow of the liquid fuel from the inlet area 22 to the outlet area 26. The inlet area 22 may take the form of an inlet manifold space through which the liquid fuel enters all the flow channels 28. Similarly, the outlet area 26 may take the form of an outlet manifold space which receives the liquid fuel from all the flow channels 28.

[0044] The flow channels 28 may be arranged to permit the liquid fuel to flow from the inlet area 22 to the outlet area 26 via gravity. For example, as shown in Figures 1 and 2, at least some of the flow channels 28 are substantially vertically disposed. However, it will be appreciated that the liquid fuel may instead flow out of the core under the influence of a gravity induced pressure gradient. For example, the liquid fuel may be siphoned out of the core 18, in which case it may be withdrawn through the inlet area 22 rather than through the outlet area 26. According to this variation, the flow channels 28 are not necessarily vertically disposed. This variation applies to all the embodiments described herein. [0045] The reactor core 18 contains a neutron moderator, and is of sufficient volume, to enable the liquid fuel to undergo a fission chain reaction as it flows through the core 18. This reaction produces heat which increases the

temperature of the liquid fuel. Thus, the liquid fuel is at a higher temperature at the outlet area 26 of the reactor core 18 than at the inlet area 22.

[0046] The reactor system 10 further comprises a liquid fuel reservoir 30 located in the lower portion 16 of the reactor vessel 12. The reservoir 30 provides a space for collection of the liquid fuel after it has passed through the reactor core 18. The features of reservoir 30 will be further described below.

[0047] The reactor system 10 may further comprise a drain passage 32 connecting the outlet area 26 of the reactor core 18 to the reservoir 30, in order to permit circulation of the liquid fuel from the reactor core 18 to the reservoir 30. In the embodiment of Figures 1 and 2, the drain passage 32 is shown as being integral with the reservoir 30, and is shown as comprising a space through which the liquid fuel exiting the lower end 24 of the reactor core 18 falls downwardly via gravity into the reservoir 30. However, it will be appreciated that the drain passage 32 may have various configurations or may not be present at all. For example, in some embodiments the outlet area 26 of reactor core 18 may communicate directly with the reservoir 30 in the idle and/or the active state of the reactor. As further discussed below, the space defined by drain passage may be filled with a gas.

[0048] The reactor system 10 further comprises a recirculation passage 34 connecting the reservoir 30 to the inlet area 22 of the reactor core 18, in order to permit circulation of the liquid fuel upwardly from the reservoir 30 to the reactor core 18. The recirculation passage 34 may be located either inside or outside the reactor vessel 12 and, in the embodiment of Figures 1 and 2, is shown as being located outside the reactor vessel 12.

[0049] The recirculation passage 34 has a lower end 36 which is in flow communication with the reservoir 30, and which may be located at or proximate to the bottom of reservoir 30. The recirculation passage 34 also has an upper end 38 which is in flow communication with the inlet area 22 at the upper end 20 of reactor core 18. [0050] The reactor system 10 further comprises a pump 40 for circulating the liquid fuel through the reactor system 10. Because the liquid fuel circulates from the reactor core 18 to the reservoir under the influence of gravity, the function of pump 40 may be restricted to circulating the liquid fuel upwardly from the reservoir 30 to the inlet area 22 of reactor core 18. For this reason the pump 40 is shown in Figures 1 and 2 as being located in the recirculation passage 34. However, it will be appreciated that it is not necessary to locate the pump 40 in the recirculation passage 34. For example, it may be possible to locate pump 40 in the reservoir 30 or in the inlet area 22 of the reactor core 18.

[0051] The reactor system 10 may further comprise a pump controller 42 to control the operation of pump 40. The controller 42 is adapted to turn the pump on and off. For example, the controller 42 may be connected to one or more sensors (not shown) which send an electronic signal to the controller 42 to indicate the presence of an abnormal condition in the reactor system 10. When such a signal is received by the controller 42, the controller 42 shuts off the pump 40 to stop the circulation of liquid fuel from the reservoir 30 to the reactor core 18. This is further discussed below.

[0052] The reactor system 10 further comprises a heat exchanger 44 for transferring heat from the liquid fuel to a coolant, wherein the coolant carries the heat away from the reactor vessel 12. The coolant may comprise a liquid coolant, such as a molten salt. In the embodiment shown in Figures 1 and 2, the heat exchanger 44 has a tubular structure, and comprises one or more heat exchanger tubes 46 through which the coolant circulates. The heat exchanger 44 further comprises a liquid fuel inlet 48 located at the top of the heat exchanger 44 to receive hot liquid fuel from the reactor core 18, and a liquid fuel outlet 50 located at the bottom of the heat exchanger 44, which is arranged to supply the liquid fuel cooled by the heat exchanger 44 to the recirculation passage 34.

[0053] The liquid fuel flows through the heat exchanger 44 in contact with the outer surfaces of the one or more tubes 46, such that heat energy is transferred from the liquid fuel to the coolant through the tube walls. It will be appreciated that the heat exchanger may have a variety of different configurations other than that shown in Figures 1 and 2, some of which are discussed below.

[0054] In the first embodiment, the heat exchanger 44 is shown as being located inside the reactor vessel 12, and more specifically inside the reservoir 30 where it is submerged in the liquid fuel. However, it will be appreciated that the heat exchanger may be located outside the reactor system 10 and may include supply and return conduits (not shown) for supplying the hot liquid fuel to the heat exchanger and for returning the cooled liquid fuel to the interior of the reactor vessel 12. In another embodiment, the heat exchanger may be provided in the recirculation passage 34 between the lower and upper ends 36, 38.

[0055] It can be seen that the level of liquid fuel in the reservoir 30 is lower in Figure 1 than it is in Figure 2. Figure 1 shows the active state of the reactor system 10 in which a portion of the liquid fuel is circulating through the reactor core 18. In this active state, the level of liquid fuel in the reservoir 30 is at a minimum, and this minimum liquid level is indicated by reference numeral 52 in Figure 1. Conversely, Figure 2 shows the idle or inactive state of the reactor system 10, in which the pump 40 has been shut off and some or all of the liquid fuel has been drained from the core 18. In this inactive state, the level of liquid fuel in the reservoir 30 is at a maximum, and this maximum liquid level is indicated by reference numeral 54 in Figure 2. In this idle state, the reactor core 18 contains either no liquid fuel or insufficient liquid fuel to enable a fission chain reaction to occur within the reactor core 18.

[0056] In the first embodiment both the minimum and maximum liquid levels 52, 54 are spaced below the outlet area 26 at the lower end 24 of the reactor core 18. Therefore, in the present embodiment, there is a gas space between the core 18 and the liquid fuel in reservoir 30 when the reactor is active and when it is idle, this gas space defining the drain passage 32. However, it will be appreciated that there is not necessarily a gas space or drain passage 32 between the core 18 and reservoir 30, in either the active or idle state.

[0057] In the present embodiment, the idle state is defined as the state in which the pump 40 is shut off and all or substantially all the liquid fuel has drained from the reactor core 18 by gravity, and in which a gas space separates the reactor core 18 and the maximum liquid level 54 in reservoir 30. However, it will be appreciated that the core 18 may be only partly drained in the idle state, such that the maximum liquid level 54 may be located within the core 18. In this idle state, insufficient liquid fuel remains in the reactor core 18 to sustain the fissile chain reaction, and therefore no reaction takes place in the reactor core 18 in the idle state.

[0058] Cooling of the liquid fuel in the reservoir 30 by heat exchanger 34 may continue after the pump 40 has been shut off, so as to cool the contents of the reactor vessel.

[0059] Having described the features of the reactor system 10, its operation is now described below.

[0060] With the reactor system 10 in its active state as shown in Figure 1, the pump 40 operates to circulate the liquid fuel upwardly through the recirculating passage 34, from the reservoir 30 to the inlet area 22 at the upper end 20 of the reactor core 18. The liquid fuel then enters the flow channels 28 of the reactor core 18 and the fissile material of the fuel undergoes a fission reaction within the reactor core, which causes the liquid medium of the fuel to become heated. As it is heated, the liquid fuel flows downwardly through the flow channels 28 to the outlet area 26 at the lower end of the reactor core 24. From there, the heated liquid fuel flows downwardly through the drain passage 32 and enters the reservoir 30. The liquid fuel within reservoir 30 may be cooled by a heat exchanger 44 located in the reservoir or externally of the reactor vessel, and the cooled liquid fuel is then again drawn up into the recirculation passage 34 and pumped back to the reactor core 18. Thus, the liquid fuel follows a continuous circulation path with the reactor system in the active state, and heat is generated by the fission reaction occurring in the core 18.

[0061] When an abnormal condition is detected, the operation of the pump 40 is stopped, for example by controller 42. Once the pump 40 is shut off, the liquid fuel is no longer moved upwardly through the recirculation passage, and therefore no liquid fuel enters the inlet area 22 of the reactor core 18. Any remaining liquid fuel within the reactor core 18 drains out through the outlet area 26 and the drain passage 32 and into the reservoir 30, causing the level of the liquid fuel in the reservoir 30 to rise. Therefore, in the inactive state the liquid fuel is removed from contact with the reactor core 18, eliminating the possibility that the fission reaction will continue and thereby safely shutting down the reactor system until the abnormal condition can be corrected.

[0062] Figures 3 and 4 show a nuclear reactor system 100 according to a second embodiment, with Figure 3 showing the reactor system 100 in an idle state, and Figure 4 showing the reactor system 100 in an operating state. It will be appreciated that these drawings are highly simplified and only show components of the reactor system 100 which are necessary for an explanation of the present embodiment. Also, a number of the elements of reactor system 100 are similar or identical to elements of reactor system 10 described above. These like elements are identified by like reference numerals in Figures 3 and 4, and the above description of these elements in connection with the first embodiment applies equally to the second embodiment. Therefore, the following description will focus on the differences between the first and second embodiments.

[0063] The reactor system 100 has a reactor vessel 12 similar to that described above, with an upper portion 14 and a lower portion 16, with the upper portion 14 containing the reactor core 18. In this embodiment, the reactor vessel 12 and the recirculation passage 34 are surrounded by an outer containment structure 56. The reactor vessel 12 has a bottom opening 58 through which the lower portion 16 of the reactor vessel 12 is in communication with the interior of the containment structure 56. Therefore, in the present embodiment, the reservoir 30 for the liquid fuel is enclosed within the

containment structure 56 and the reactor vessel 12.

[0064] Instead of taking up the liquid fuel from inside the reactor vessel 12, the lower end 36 of the recirculation passage 34 may be open to the interior of the containment structure 56. Thus, as shown in the operating state of Figure 4, the drawing of liquid fuel from the containment structure 56 by pump 40 causes the level of liquid fuel within the containment structure 56 to decrease and the level of liquid fuel within the reactor vessel 12 to increase. In the present embodiment, the inside of reactor vessel 12 becomes substantially filled with the liquid fuel during operation of reactor system 100, such that there is no empty space below the reactor core 18 in the operating condition.

[0065] On the other hand, in the idle state shown in Figure 3, the pump 40 has stopped pumping the liquid fuel from the reservoir 30 to the reactor core 18. Therefore, all or substantially all of the liquid fuel drains out of the reactor core 18, causing the liquid fuel level in the reactor vessel 12 to drop below the reactor core 18, while the liquid fuel level in the surrounding containment structure 56 rises until the liquid fuel levels in the reactor vessel 12 and the containment vessel 56 are at the same or substantially the same level, as shown in Figure 3. It will be appreciated, however, that the liquid fuel may only partly be drained from the reactor core 18 in the idle state, such that the reactor core contains insufficient liquid to sustain the fissile chain reaction .

[0066] It can be seen that the embodiment of Figures 3 and 4 minimizes wasted space within the reactor vessel 12 in the operating state, thereby maximizing the volume of the reactor vessel 12 which may be occupied by the reactor core 18 and/or the heat exchanger 44, while maximizing the volume of the reservoir 30 in the idle state, so as to drop the level of the liquid fuel below the lower end 24 of the reactor core 18.

[0067] As shown in Figures 3 and 4, an opening 60 may be provided in the upper portion 14 of the reactor vessel 12 through which gas may enter or be expelled from the reactor vessel 12. This gas opening 60 helps the liquid fuel to drain from the reactor core 18 by allowing gas to enter the flow channels 28 of the reactor core to replace the liquid fuel draining out of the reactor core 18, and by equalizing the pressure within the reactor vessel 12 and the containment structure 56 in the idle state. The opening 60 may optionally be provided with a conduit 62 as shown in Figures 3 and 4. In the active state, where the reactor vessel 12 is substantially filled with liquid fuel, the opening 60 may either be closed by a valve (not shown) or may be allowed to bleed small amounts of liquid fuel into the reservoir 30. Although not shown in Figures 1 and 2, a similar gas opening 60 may be provided in the first embodiment.

[0068] As also shown in Figures 3 and 4, the lower portion of the reactor vessel 12 is provided with sufficient neutron absorbing material to prevent a fission chain reaction from occurring outside the reactor core 18, and these elements may also be provided in the surrounding containment structure 56. In the illustrated embodiment, these elements comprise pins, rods or other structures comprised of said neutron absorbing materials, and are identified in the drawings by reference numeral 64. Although not shown in Figures 1 and 2, similar neutron absorbing elements may be provided in the first embodiment.

[0069] Figures 5 and 6 show a nuclear reactor system 200 according to a third embodiment, with Figure 5 showing the reactor system 200 in an operating state, and Figure 6 showing the reactor system 200 in an idle state. It will be appreciated that these drawings are highly simplified and only show components of the reactor system 200 which are necessary for an explanation of the present embodiment. Also, a number of the elements of reactor system 200 are similar or identical to elements of reactor systems 10 and 100 described above. These like elements are identified by like reference numerals in Figures 5 and 6, and the above description of these elements in connection with the first and/or second embodiments applies equally to the third embodiment. Therefore, the following description will focus on the differences between the third embodiment and the first and second embodiments described above.

[0070] The third embodiment is conceptually similar to the second embodiment, in that the reactor vessel 12 and the recirculation passage 34 are surrounded by an outer containment structure 56, and the reactor vessel 12 has a bottom opening 58 through which the lower portion 16 of the reactor vessel 12 is in communication with the interior of the containment structure 56.

[0071] In the third embodiment, the heat exchanger 44 comprises a plurality of U-shaped tubes 46, with the opposite ends of tubes 46

communicating with an inlet manifold 66 and an outlet manifold 68 separated by a barrier 70. As in the first two embodiments, a coolant flows through the tubes 46 and the liquid fuel flows around the outsides of tubes 46. To maximize heat recovery, the heat exchanger 44 may be provided with baffles 72 to cause the liquid fuel to follow a tortuous path through the heat exchanger 44. As shown, the baffles 72 may be connected to the wall of the reactor vessel 12, which forms a shell of the heat exchanger 44 to prevent short-circuit flow of the liquid fuel. [0072] In the third embodiment the drain passage 32 between the core 18 and the reservoir 30 is in the form of a connecting pipe having a diameter which is smaller than that of the reactor core 18 and reservoir 30.

[0073] The reactor system 200 further comprises an upper chamber 74 which contains neutron-absorbing control rods 76 and the motor 78 of pump 40. A layer of thermal insulation 80 is provided between the chamber 74 and the reactor vessel 12.

[0074] As in the second embodiment, empty space 82 is provided within the containment structure 56 for compressible gas, which will enter the reactor vessel through gas opening 60 when the pump motor 78 is shut off. When the pump 40 is operating, the top surface of the liquid fuel in the containment structure 56 lowers as the liquid fuel is pumped to the inlet area 22 of the reactor core 18.

[0075] In the idle state of reactor system 200 shown in Figure 6, the pump motor 78 is off and the liquid fuel is resting in the lower portion of the reactor vessel 12 outside of the reactor core 18. In the reservoir 30 the liquid fuel is prevented from reacting by a combination of an unfavorable geometry, lack of a moderator, and neutron absorbing elements 64. In this configuration, the reactor 200 can be re-started or be allowed to cool to freezing by removal of heat by the heat exchanger tubes 46 or by natural convection from the exterior of the containment structure 56. The reactor 200 may be restarted from the solidified state by re-melting the solidified salt of the liquid fuel using high- temperature salt pumped through the heat exchanger tubes 46.

[0076] Figures 7 and 8 show a nuclear reactor system 300 according to a fourth embodiment, with Figure 7 showing the reactor system 300 in an operating state, and Figure 8 showing the reactor system 300 in an idle state. It will be appreciated that these drawings are highly simplified and only show components of the reactor system 300 which are necessary for an explanation of the present embodiment. Also, a number of the elements of reactor system 300 are similar or identical to elements of reactor systems 10, 100 and 200 described above. These like elements are identified by like reference numerals in Figures 7 and 8, and the above description of these elements in connection with the first to third embodiments applies equally to the fourth embodiment. Therefore, the following description will focus on the differences between the fourth embodiment and the first and second embodiments described above.

[0077] The reactor system 300 comprises a containment structure 56 in which each of the reactor vessel 12, the reservoir 30 and the heat exchanger 44 are provided in discrete vessels which are connected together with piping to form a recirculation loop. According to this embodiment, the reactor core 18 s contained in the reactor vessel 12 and is connected to a separate reservoir vessel 84 by a conduit comprising drain passage 32, wherein the reservoir 30 is contained in the reservoir vessel 84. The reservoir vessel 84 is shown in these drawings as being connected to a heat exchanger 44 through a conduit 86, which feeds the liquid fuel from reservoir 30 to the liquid fuel inlet 48 of heat exchanger 44. The heat exchanger 44 is enclosed in a vessel 88 which may comprise a shell or housing enclosing at least one heat exchanger tube 46, although the structure of heat exchanger 44 is variable.

[0078] A recirculation passage 34 receives the liquid fuel from the liquid fuel outlet 50 of heat exchanger 44, and pump 40 recirculates the liquid fuel to the inlet area 22 of the reactor core 18.

[0079] As shown in Figure 7, a minimum liquid level 52 is defined in reservoir 30 in the operating state, and as shown in Figure 8 a maximum liquid level 54 is defined in Figure 8 in the idle state, where some or all of the liquid fuel has drained from the reactor core 18 by gravity, and insufficient liquid fuel remains in the core 18 to sustain the fissile chain reaction.

[0080] Other than the separation of the core 18, reservoir 30 and heat exchanger 44 into separate vessels, the embodiment of Figures 7 and 8 is conceptually similar to the embodiment of Figures 1 and 2 described above.

[0081] Other aspects and features of the present disclosure will become apparent to those of ordinary skill in the art upon review of the following description of specific implementations of the disclosure in conjunction with the accompanying figures. [0082] The above-described implementations of the present application are intended to be examples only. Alterations, modifications and variations may be effected to the particular implementations by those skilled in the art without departing from the scope of the application, which is defined by the claims appended hereto.

Claims

What is claimed is :

1. A nuclear reactor system, comprising :

(a) a containment structure having an upper portion and a lower portion;

(b) a reactor core located in the upper portion of the containment structure, wherein the reactor core has a first end defining an inlet area for a liquid fuel, and a second end defining an outlet area for the liquid fuel;

(c) a liquid fuel reservoir located in the lower portion of the containment structure;

(d) a drain passage for flow of the liquid fuel between the reactor core and the reservoir;

(e) a recirculation passage connecting the reservoir to the inlet area of the reactor core; and

(f) a pump for circulating the liquid fuel through the recirculation passage.

2. The nuclear reactor system of claim 1, wherein the containment structure is filled with both the liquid fuel and with a gas.

3. The nuclear reactor system of claim 1 or 2, wherein the reactor core has a plurality of flow channels for flow of the liquid fuel from the inlet area to the outlet area.

4. The nuclear reactor system of any one of claims 1 to 3, further

comprising :

(g) a heat exchanger to transfer heat from the liquid fuel to a coolant, wherein the heat exchanger includes at least one coolant flow passage.

5. The nuclear reactor system of claim 4, wherein the heat exchanger is located within the reservoir or is connected to the reservoir by a conduit.

6. The nuclear reactor system of claim 4 or 5, wherein the heat exchanger further comprises a liquid fuel inlet and a liquid fuel outlet, wherein the liquid fuel inlet is arranged to receive the liquid fuel from the reactor core, and the liquid fuel outlet is arranged to supply the liquid fuel to the recirculation passage.

7. The nuclear reactor system of any one of claims 1 to 6, further

comprising :

(h) a controller to control the operation of the pump.

8. The nuclear reactor system of any one of claims 1 to 7, wherein a maximum liquid level in the containment structure when the pump is off is defined as a level of liquid fuel in the containment structure when the reactor core contains insufficient liquid fuel to enable a fission chain reaction to occur within the reactor core.

9. The nuclear reactor system of claim 8, wherein the maximum liquid level when the pump is off is defined as a level of liquid fuel in the reservoir when the reactor core contains substantially none of the liquid fuel.

10. The nuclear reactor system of claim 8 or 9, wherein the maximum liquid level is defined as a level of liquid fuel in the reactor core when the pump is off and all or substantially all of the liquid fuel has been drained from the reactor core.

11. The nuclear reactor system of any one of claims 8 to 10, wherein the maximum liquid level when the pump is off is spaced below the outlet area of the reactor core.

12. The nuclear reactor system of claim 11, wherein the maximum liquid level when the pump is off is located in the drain passage.

13. The nuclear reactor system of claim 11, wherein the reservoir has a volume sufficient to hold all or substantially all of the liquid fuel present in the reactor system, and wherein the maximum liquid level is located in the reservoir.

14. The nuclear reactor system of any one of claims 1 to 13, wherein the flow channels are arranged to permit the liquid fuel to flow from the inlet area to the outlet area via gravity.

15. The nuclear reactor system of claim 14, wherein at least some of the flow channels are substantially vertically disposed.

16. The nuclear reactor system of any one of claims 1 to 13, wherein the core contains a neutron moderator to enable a fission chain reaction.

17. The nuclear reactor system according to any one of claims 1 to 16, wherein the liquid fuel is comprised of a fissile material dissolved in a liquid medium.

18. The nuclear reactor system according to any one of claims 1 to 16, wherein the liquid fuel is a molten salt with a fissile material as a constituent element.

19. The nuclear reactor system according to any one of claims 1 to 18, wherein the reactor core is enclosed within a reactor vessel, and wherein the nuclear reactor system further comprises:

(j) an opening in the upper portion of the reactor vessel through which gas may enter or be expelled from the reactor vessel into the containment structure.

20. The nuclear reactor system of any one of claims 1 to 19, wherein the flow channels are arranged to permit the liquid fuel to flow out of the reactor via gravity and/or gravity induced pressure gradient.

21. The nuclear reactor system of any one of claims 1 to 20, wherein the lower portion of the containment structure comprises sufficient neutron absorbing material to prevent a fission chain reaction from occurring outside the reactor core.

22. A method of operating a nuclear reactor system, comprising :

(a) providing a nuclear reactor system as defined in any one of claims 1 to 21 ;

(b) providing a liquid fuel in said reactor vessel, the liquid fuel comprising a fissile material in a liquid medium; (c) operating the pump to circulate the liquid fuel through the

recirculation passage, the reactor core, the drain passage and the reservoir, wherein the fissile material in the liquid fuel circulating through the flow channels of the reactor core undergoes a fission chain reaction, thereby heating the liquid medium;and

(d) ceasing operating of the pump, such that the flow of liquid fuel to the reactor core ceases, and the liquid fuel partly or completely drains out of the reactor core through the drain passage and into the reservoir, such that an insufficient amount of the liquid fuel remains in the reactor core to sustain the fission chain reaction.

23. The method of claim 22, further comprising the step of extracting heat energy from the liquid fuel with a heat exchanger.

24. The method of claim 22 or 23, wherein gas is allowed to enter the flow channels of the reactor core to replace the liquid fuel draining out of the reactor core after the operation of the pump ceases.

25. The method of any one of claims 22 to 24, wherein the liquid fuel which is drained from the moderating core causes a level of the liquid fuel in the reservoir to rise.