WO2015089662A1

WO2015089662A1 - Nuclear reactor safety system

Info

Publication number: WO2015089662A1
Application number: PCT/CA2014/051224
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

A nuclear reactor system is disclosed which provides an auxiliary cooling system and improved safety in the event of corrosion failure. The reactor comprises an inner reactor vessel containing a reactor core and a liquid fuel, and an outer containment vessel, with an intermediate space between the vessels being filled with a protective material. The protective material is at least partly in a solid state during normal operation of the reactor, and is liquefied by increasing temperatures within the reactor vessel due to abnormal operating conditions and provides enhanced cooling in the liquid state. The protective material and/or the coolant of a heat exchanger may dilute the liquid fuel during corrosive failure, and may optionally be provided with neutron absorbing material to prevent a fission chain reaction from occurring outside the reactor vessel.

Description

NUCLEAR REACTOR SAFETY SYSTEM CROSS-REFERENCE TO RELATED APPLICATION

[0001] This application claims priority to and the benefit of United States Provisional Patent Application No. 61/917,035 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 system which mitigates the risk of reactor failure from corrosion of its components and/or overheating of its components.

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] Many 4th generation reactor concepts utilize high temperature coolants, for example sodium, lead or a molten salt, which have correspondingly high boiling points. In some reactors utilizing molten salt, the fuel is dissolved into the circulating liquid coolant.

[0005] Many of the high temperature coolants are also corrosive to reactor components. Exhaustive corrosion testing is required before certifying a reactor when corrosive failure of its components could pose a safety risk, for example allowing radioactive materials to be released into the environment. Corrosion testing can be both expensive and time consuming, prolonging the

commercialization of a new reactor by years. In some cases, for example where the fuel is dissolved into the liquid coolant, it is unclear how the corrosion resistance of the reactor materials could be proven, since the coolant composition can change over time as fission products accumulate.

[0006] There exists a need for a reactor design which is safe in the event that the components of the reactor in contact with the coolant are corroded.

[0007] Nuclear reactors continue to produce decay heat for a long time after the reactor has been shut-down. This decay heat must be removed to prevent failure of reactor components from overheating. Reactors incorporate auxiliary cooling systems to remove this decay heat.

[0008] In one type of auxiliary cooling system, heat is transmitted directly through an inner and an outer containment vessel to a cooling medium, for example water or air, at the exterior of the outer containment vessel. Heat is transmitted between the inner and outer containment vessel by convection of a heat transfer liquid located between the vessels.

[0009] There exists a need for a robust auxiliary cooling system to remove decay heat which is resistant to corrosion and also unaffected by corrosion of any components in direct contact with the primary coolant or heat transfer liquid.

SUMMARY

[0010] In an embodiment there is provided a nuclear reactor system, comprising : (a) an inner reactor vessel having a wall surrounding an interior space for circulation of a liquid coolant; (b) a reactor core located in said interior space; (c) an outer containment vessel surrounding the inner reactor vessel, wherein the outer containment vessel has a wall which is separated from the wall of the inner reactor vessel by an intermediate space; and (d) a protective material provided in the intermediate space.

[0011] In one aspect, the protective material has a melting temperature which is greater than a normal operating temperature of the reactor core, such that the protective material is in a solid state at said normal operating temperature. [0012] In another aspect, the melting temperature of the protective material is less than an abnormal operating temperature of the reactor core, such that the protective material is in a liquid state at said abnormal operating temperature.

[0013] In yet another aspect, the protective material is less corrosive to the wall of the outer containment vessel when it is in the solid state than when it is in the liquid state.

[0014] In yet another aspect, the protective material has a greater thermal insulation value when it is in the solid state than when it is in the liquid state.

[0015] In yet another aspect, the wall of the inner reactor vessel is at a first temperature during normal operation of the reactor and the wall of the outer containment vessel is at a second temperature during normal operation of the reactor core, wherein the first temperature is higher than the first temperature; and wherein the protective material has a melting temperature which is less than the first temperature and greater than the second temperature, such that the

protective material is in a liquid state at the wall of the inner reactor vessel and is in a solid state at the wall of the outer containment vessel.

[0016] In yet another aspect, the wall of the outer containment vessel has an outer surface which is in contact with at least one cooling medium.

[0017] In yet another aspect, the cooling medium is water, air, sodium and/or molten salt.

[0018] In yet another aspect, the wall of the outer containment vessel has an inner surface which is in direct contact with a solid layer of said protective material in said solid state, and wherein the inner surface is provided with a plurality of protrusions to which the solid layer of protective material is attached.

[0019] In yet another aspect, the protective material is a salt. [0020] In yet another aspect, the protective material contains sufficient neutron absorbing material to prevent a fission chain reaction from occurring outside the inner reactor vessel.

[0021] In yet another aspect, the nuclear reactor system further comprises: (e) a heat exchanger to transfer heat from the liquid coolant to a secondary coolant, wherein the heat exchanger includes at least one flow passage for the secondary coolant and is located in said interior space.

[0022] In yet another aspect, the secondary coolant comprises a neutron absorbing poison, and may comprise a molten salt.

[0023] In yet another aspect, a fissile fuel is dissolved in the liquid coolant circulating in the interior space.

[0024] In yet another aspect, the core contains a neutron moderator to enable a fission chain reaction.

[0025] In yet another aspect, the intermediate space is pressurized to force the protective material into the inner reactor vessel in the event that the inner reactor vessel is compromised by corrosion.

[0026] In yet another aspect, the protective material is miscible with the liquid coolant.

BRIEF DESCRIPTION OF THE DRAWINGS

[0027] Reference will now be made, by way of example, to the accompanying drawings which show example implementations, and in which :

[0028] Figure 1 shows a nuclear reactor according to a first embodiment; and

[0029] Figure 2 shows a nuclear reactor according to a second embodiment. DETAILED DESCRIPTION

[0030] The embodiments described herein relate to nuclear reactors in which a fuel undergoes a fission chain reaction in the core to produce heat. The fuel can be located in the core in a solid form and cooled by a circulating fluid, referred to herein as a "liquid coolant". Alternatively, the fuel may be in liquid form, and is referred to herein as a "liquid fuel". 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. The embodiments described herein specifically relate to nuclear reactors in which the fuel is a liquid fuel .

However, it will be appreciated that the nuclear reactors described herein may instead comprise reactors in which the fuel is in solid form and is cooled by a liquid coolant. For this reason, the terms "liquid fuel" and "liquid coolant" are used interchangeably herein.

[0031] Figure 1 shows a nuclear reactor 10 according to a first embodiment. It will be appreciated that Figure 1 is highly simplified and only shows components of the reactor 10 which are necessary for an explanation of the present embodiment.

[0032] Reactor 10 comprises an inner reactor vessel 12, which is sometimes referred to herein as the inner containment vessel 12. The inner reactor vessel 12 has a wall 14 having an inner surface 16 and an outer surface 18. In the present embodiment, the wall 14 has a substantially cylindrical sidewall 20 and a bottom wall 22. The top of the inner reactor vessel 12 is shown as being closed by a top wall 24.

[0033] The wall 14 of inner reactor vessel 12 surrounds an interior space 26 for circulation of a liquid fuel 28, sometimes referred to herein as "liquid coolant".

[0034] . In the illustrated embodiment, the interior space 26 is almost completely filled with liquid fuel 28, such that the level of liquid fuel 28 in the interior space 26 is close to the top of the wall 14.

[0035] The reactor 10 further comprises a reactor core 30 which is located in the interior space 26. The reactor core 30 comprises a plurality of flow channels 32 through which the liquid fuel 28 flows. The reactor core 30 contains a neutron moderator, and is of sufficient volume to enable the liquid fuel 28 to undergo a fission chain reaction as it flows through the flow channels 32. This reaction produces heat which increases the temperature of the liquid fuel 28. Thus, the liquid fuel 28 is at a higher temperature at the outlet of the reactor core 30 (lower end of reactor core) than at its inlet (upper end of reactor core).

[0036] The reactor 10 further comprises an outer containment vessel 34 having a wall 36 having an inner surface 38 and an outer surface 40. In the present embodiment, the wall 36 has a substantially cylindrical sidewall 42 and a bottom wall 44. The wall 36 of the outer containment vessel 34 surrounds the wall 14 of the inner reactor vessel 12 and is separated therefrom by an intermediate space 46. In the illustrated embodiment the spacing between walls 14 and 36 is substantially constant, and therefore the intermediate space 46 is of substantially constant thickness.

[0037] The top of the outer containment vessel 34 is shown as being closed by a top wall 48 which encloses an upper chamber 50 which contains neutron- absorbing control rods 52 and the motor 54 of pump 56 for circulation of the liquid fuel 28 within the interior space 26 of the inner reactor vessel 12.

[0038] The reactor system 10 further comprises a heat exchanger 58 for transferring heat from the liquid fuel to a secondary coolant, wherein the secondary coolant carries the heat away from the inner reactor vessel 12. The secondary coolant may comprise a liquid coolant, such as a molten salt. The heat exchanger 58 of the first embodiment is a shell-and-tube heat exchanger enclosed by an outer shell 60 which receives hot liquid fuel 28 from the outlet of the reactor core 30 through a drain passage 62.

[0039] The heat exchanger 58 comprises a plurality of U-shaped tubes 64, with the opposite ends of tubes 64 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 64 and the liquid fuel 28 flows around the outsides of tubes 64. To maximize heat recovery, the heat exchanger 58 may be provided with baffles 72 to cause the liquid fuel 28 to follow a tortuous path through the heat exchanger 58. As shown, the baffles 72 may be connected to the wall of the shell 60 to prevent short-circuit flow of the liquid fuel 28. The shell 60 is shown as having an open bottom 74 which provides an outlet for the cooled liquid fuel 28, which is then recirculated back to the inlet of the reactor core 30 by the action of pump 56, through an annular gap between the shell 60 and the inner surface 16 of wall 14.

[0040] It will be appreciated that the heat exchanger 58 may have a variety of different configurations other than that shown in Figure 1. Also, it will be

appreciated that the heat exchanger 58 may be located outside the inner reactor vessel 12 and the reactor 10 may include supply and return conduits (not shown) for supplying the hot liquid fuel to the externally located heat exchanger and for returning the cooled liquid fuel to the interior space 26 of the inner reactor vessel 12.

[0041] As shown in Figure 1, portions of the inner reactor vessel 12 outside of reactor core 30, which are in contact with the liquid fuel 28, are provided with sufficient neutron absorbing material to prevent a fission chain reaction from occurring outside the reactor core 30. In the illustrated embodiment, these elements comprise pins, rods or other structures comprised of said neutron absorbing materials, these elements being located on the inner surface 16 of wall 14, and being identified in Figure 1 by reference numeral 76.

[0042] The intermediate space 46 between the outer surface 18 of wall 14 and the inner surface 28 of wall 36 is filled or substantially filled with a protective material 78. The outer surface 40 of wall 36 is in contact with a cooling medium 80. The cooling medium surrounds wall 36 and is in thermal contact with the protective material 78 through wall 36. In the first embodiment shown in Figure 1, the cooling medium is water, and is contained in a reservoir 81 surrounding the wall 36 of the outer containment vessel 34.

[0043] The protective material 78 in the intermediate space 46 functions to insulate the inner reactor vessel 12 under normal operating conditions of the reactor 10, and to transmit heat out of the reactor 10 where there is an excessive increase in temperature within the interior space 26.

[0044] For example, where reactor 10 has been shut down, the reactor core 30 may continue to produce decay heat, which will result in a temperature increase within the liquid fuel 28 in the interior space 26. Failure of the heat exchanger 58 will also result in a temperature increase within the liquid fuel 28 in the interior space 26.

[0045] In order to insulate the inner reactor vessel 12 under some conditions, and to conduct heat away from the inner reactor vessel 12 under other conditions, at least some of the protective material 78 is in a solid state under normal operating conditions of the reactor 10 and at normal operating temperatures of the inner reactor vessel 12, the reactor core 30 and/or the liquid fuel 28 contained in the interior space 26, referred to herein as the "normal operating temperature".

[0046] For example, in some embodiments, the protective material 78 has a melting temperature which is greater than the normal operating temperature, such that the protective material 78 is in a solid state at the normal operating

temperature, and forms a substantially completely solid mass in the intermediate space 46. In its solid state, the protective material 78 will tend to have a greater thermal insulation value than when it is in the liquid state, and will help to prevent excessive heat loss through the wall 14 of the inner reactor vessel 12.

[0047] Another benefit of maintaining the protective material 78 in its solid state during normal operating conditions and at the normal operating temperature is that it will tend to be less corrosive to the wall 36 of the outer containment vessel 34 and/or to the wall 14 of the inner reactor vessel 12 when it is in the solid state, than when it is in the liquid state.

[0048] Under abnormal operating conditions of the reactor 10, excessively high operating temperatures may exist in the inner reactor vessel 12, the reactor core 30 and/or the liquid fuel 28 contained in the interior space 26, referred to herein as the "abnormal operating temperature". Under these conditions, it is beneficial for the protective material 78 to be in a liquid state so as to conduct heat away from the inner reactor vessel 12, at least partly by convection. Therefore, in some embodiments, the melting temperature of the protective material 78 is less than the abnormal operating temperature, such that all or substantially all of the protective material 78 will be in a liquid state at the abnormal operating

temperature.

[0049] Figure 1 shows the protective material 78 in its solid state, with the reactor 10 operating under normal conditions and at the normal operating temperature. Under the conditions shown in Figure 1, the protective material 78 is substantially completely solid and serves to insulate the inner reactor vessel 12, thereby preventing excessive heat loss through the wall 14.

[0050] The protective material 78 may be a salt and, in the embodiment of Figure 1, may be a salt with a higher melting temperature than the molten salt which comprises the liquid medium of the liquid fuel 28. The solid protective material 78 may be provided by pouring or casting the protective material 78 into the intermediate space 46 in its liquid state, and permitting it to cool and solidify. Alternatively, the protective material 78 may be pre-formed as blocks or similar structures which are placed into the intermediate space 46.

[0051] In addition to its dual functions as an insulating and heat transfer material, the protective material also protects the reactor 10 from the effects of a corrosive failure of the wall 14 of the inner reactor vessel 12. In the event of such a corrosive failure, the liquid fuel 28 will come into contact with the protective material 78 through the wall 14. Depending at least partly on the temperature of the liquid fuel 28 and the melting temperature of the protective material 78, there may be some mixing of the liquid fuel 28 and the protective material 78, with some dilution of the liquid fuel 28. This dilution may be sufficient to decrease or stop the fission chain reaction in the reactor core 30.

[0052] A similar dilution effect may be provided by the secondary liquid coolant circulating in the heat exchanger 58. In this regard, where there is a corrosive failure of one or more of the coolant-carrying components of the heat exchanger 58, such as one of the tubes 64, there will be mixing of the coolant with the liquid fuel 28 with some dilution of the liquid fuel 28. This dilution may be sufficient to decrease or stop the fission chain reaction in the reactor core 30. The coolant may be selected to enhance this effect, and may optionally comprise the same salt as the liquid medium of the liquid fuel 28.

[0053] In addition to the dilution effect, either or both of the coolant and the protective material may contain one or more neutron absorbing materials (also referred to herein as "neutron absorbing poisons") in sufficient concentrations to decrease and/or prevent a fission chain reaction from occurring outside the reactor core 30. In the case of the coolant, the concentration of the one or more neutron absorbing materials will be sufficient to decrease or stop the fission chain reaction within the reactor core 30 and/or the remainder of interior space 26. In the case of the protective material 78, the concentration of the one or more neutron absorbing materials will be sufficient to prevent a fission chain reaction from occurring outside the inner reactor vessel 12.

[0054] The dilution effect provided by the protective material 78 may be enhanced by pressurizing the intermediate space to force the protective material 78 into the inner reactor vessel 12 in the event that the inner reactor vessel 12 is compromised by corrosion.

[0055] To further enhance the dilution effect, the protective material 78 may be miscible with the liquid coolant 28.

[0056] Figure 2 shows a nuclear reactor 100 according to a second

embodiment. It will be appreciated that Figure 2 is highly simplified and only shows components of the reactor 100 which are necessary for an explanation of the present embodiment. Also, a number of the elements of reactor 100 are similar or identical to elements of reactor system 10 described above. These like elements are identified by like reference numerals in Figure 2, 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. [0057] The reactor 100 of Figure 2 includes an inner reactor vessel 12 having an interior space 26 containing a reactor core 30, a heat exchanger 58 and a liquid fuel 28 which is circulated through reactor vessel 12 by a pump 56. The wall 14 of the inner reactor vessel 12 is surrounded by an outer containment vessel 34 having a wall 36 with an inner surface 38 and an outer surface 40, wherein the outer surface 40 is in contact with a cooling medium 80.

[0058] During normal operation of the reactor 100 the wall 14 of the inner reactor vessel 12 is at a first temperature and the wall 36 of the outer containment vessel 34 is at a second temperature. The first temperature will be higher than the second temperature since wall 36 is in contact with the cooling medium 80, whereas the wall 14 is in contact with the liquid fuel 28.

[0059] In the present embodiment the protective material 78 is selected such that it has a melting temperature which is less than the first temperature and greater than the second temperature. Thus, the protective material 78 is in a liquid state at the wall 14 of the inner reactor vessel 12 and is in a solid (frozen) state at the wall 36 of the outer containment vessel 34. As shown in Figure 2, this results in the protective material 78 comprising an inner liquid layer 82 and an outer solid layer 84.

[0060] As in the first embodiment described above, the maintenance of a solid layer 84 of protective material 78 against wall 36 during normal operating

conditions and at the normal operating temperature will tend to result in less corrosion of wall 36, as compared to when the protective material 78 is in its liquid state.

[0061] Under abnormal operating conditions of the reactor 100 and the

abnormal operating temperature, the protective material 78 will increase in temperature and will cause the outer solid layer 84 of protective material 78 to melt, such that all or substantially all of the protective material 78 will be in its liquid state. Under these conditions, the protective material 78 will more efficiently conduct heat away from the inner reactor vessel 12, at least partly by convection. [0062] The protective material 78 in the second embodiment may be a salt and may comprise a salt with the same or a similar melting temperature as the molten salt which comprises the liquid medium of the liquid fuel 28. In some

embodiments, the protective material 78 may comprise the same salt as the liquid medium of the liquid fuel. The protective material 78 of the second embodiment may also contain one or more neutron absorbing materials, as described above with reference to reactor 10.

[0063] It can be seen that the protective material will protect the reactor 100 from the effects of a corrosive failure of the wall 14 of the inner reactor vessel 12. In the event of such a corrosive failure, the liquid fuel 28 will come into contact with the liquid layer of the protective material 78 through the wall 14. The contact between these two liquids will result in mixing of the liquid fuel 28 and the protective material 78, and dilution of the liquid fuel 28. This dilution may be sufficient to decrease or stop the fission chain reaction in the reactor core 30.

[0064] In order to retain the outer solid layer 84 of protective material 78, the inner surface 38 of wall 36 may be provided with protrusions 86 to which the outer solid layer 84 of protective material 78 is attached. These protrusions 86 may take a variety of different forms, such as inwardly extending pins, rods, plates or the like. These protrusions 86 may be provided along all or substantially all of the portions of sidewall 42 and bottom wall 44 which are in contact with the protective material 78.

[0065] As mentioned above, the outer surface 40 of wall 36 is in contact with a cooling medium 80. In the present embodiment the cooling medium 80 is air which is circulated through an air circulation channel 88 enclosed within a shell 90 which surrounds wall 36 or at least the sidewall 42 thereof. To enhance cooling, protruding structures 92 may be provided within the air circulation channel 88 to increase the surface area available for cooling, and these structures may comprise thermally conductive plates, fins, pins, etc., which may optionally be rigidly secured to either the outer surface 40 of wall 36 or the inner surface of shell 90. [0066] 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.

[0067] The above-described embodiments 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) an inner reactor vessel having a wall surrounding an interior space for circulation of a liquid coolant;

(b) a reactor core located in said interior space;

(c) an outer containment vessel surrounding the inner reactor vessel, wherein the outer containment vessel has a wall which is separated from the wall of the inner reactor vessel by an intermediate space; and

(d) a protective material provided in the intermediate space.

2. The nuclear reactor system of claim 1, wherein the protective material has a melting temperature which is greater than a normal operating temperature of the reactor core, such that the protective material is in a solid state at said normal operating temperature.

3. The nuclear reactor system of claim 1 or 2, wherein the melting temperature of the protective material is less than an abnormal operating temperature of the reactor core, such that the protective material is in a liquid state at said abnormal operating temperature.

4. The nuclear reactor system of claim 3, wherein the protective material is less corrosive to the wall of the outer containment vessel when it is in the solid state than when it is in the liquid state.

5. The nuclear reactor system of claim 3 or 4, wherein the protective material has a greater thermal insulation value when it is in the solid state than when it is in the liquid state.

6. The nuclear reactor system of claim 1, wherein the wall of the inner reactor vessel is at a first temperature during normal operation of the reactor and the wall of the outer containment vessel is at a second temperature during normal operation of the reactor core, wherein the first temperature is higher than the second temperature; and

wherein the protective material has a melting temperature which is less than the first temperature and greater than the second temperature, such that the protective material is in a liquid state at the wall of the inner reactor vessel and is in a solid state at the wall of the outer containment vessel.

7. The nuclear reactor system of claim 6, wherein the wall of the outer containment vessel has an outer surface which is in contact with at least one cooling medium.

8. The nuclear reactor system of claim 7, wherein the cooling medium is water, air, sodium and/or molten salt.

9. The nuclear reactor system of any one of claims 6 to 8, wherein the wall of the outer containment vessel has an inner surface which is in direct contact with a solid layer of said protective material in said solid state, and wherein the inner surface is provided with a plurality of protrusions to which the solid layer of protective material is attached.

10. The nuclear reactor system of any one of claims 1 to 9, wherein the protective material is a salt.

11. The nuclear reactor system of any one of claims 1 to 10, wherein the protective material contains sufficient neutron absorbing material to prevent a fission chain reaction from occurring outside the inner reactor vessel.

12. The nuclear reactor system of any one of claims 1 to 11, further comprising :

(e) a heat exchanger to transfer heat from the liquid coolant to a secondary coolant, wherein the heat exchanger includes at least one flow passage for the secondary coolant and is located in said interior space.

13. The nuclear reactor system of claim 12, wherein the secondary coolant comprises a neutron absorbing poison.

14. The nuclear reactor system of claim 12 or 13, wherein the secondary coolant comprises a molten salt.

15. The nuclear reactor system of any one of claims 1 to 14, wherein a fissile fuel is dissolved in the liquid coolant circulating in the interior space.

16. The nuclear reactor system of any one of claims 1 to 14, wherein a fissile fuel is in a solid form in the core and cooled by liquid coolant circulating in the interior space.

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

18. The nuclear reactor system of any one of claims 1 to 17, wherein the intermediate space is pressurized to force the protective material into the inner reactor vessel in the event that the inner reactor vessel is compromised by corrosion.

19. The nuclear reactor system of claims 1 to 18, wherein the protective material is miscible with the liquid coolant.