NUCLEAR REACTOR COOLED BY LIQUID METAL INCORPORATING A PASSIVE DECAY HEAT REMOVAL SYSTEM WITH A PHASE CHANGE MATERIAL THERMAL RESERVOIR AND A REMOVABLE THERMALLY-INSULATING LAYER AROUND THE PHASE CHANGE MATERIAL RESERVOIR

Abstract

A nuclear reactor cooled by liquid metal incorporating a passive system for evacuation of the decay heat with a phase change material thermal reservoir and a removable thermally-insulating layer around the phase change material reservoir. A nuclear reactor incorporates an integral system that guarantees: totally passive evacuation of decay heat from the initial moment of the accident; evacuation of power via the primary containment vessel; the presence of a final cold source with a reservoir incorporating an integral exchanger divided into a plurality of parallel tubes between which a phase change material is inserted, the reservoir being surrounded by a thermally-insulating layer that can be detached in a passive manner in the event of reaching a predetermined threshold temperature.

Claims

1. A fast neutron nuclear reactor cooled by liquid metal, comprising: a so-called primary containment vessel filled with a liquid metal as a heat-exchange fluid of the primary circuit of the reactor; a containment vessel sink located around the primary containment vessel and defining an inter-vessel space; a closer slab to enclose the liquid metal inside the primary containment vessel; a system for evacuation of both at least some of the nominal power and of the decay heat of the reactor in an accident situation, the system including: a closed circuit filled with a heat-exchange liquid, including: a layer of a plurality of U-shape pipes located in the inter-vessel space and distributed around the primary containment vessel and each extending along the primary containment vessel with the bottom of the U-shapes facing the bottom of the latter, a first collector termed the cold collector to which each of the pipes of the layer is welded via one branch of the U-shape, termed the cold branch, the cold collector being located outside and above the closer slab, a second collector termed the hot collector to which each of the pipes of the layer is welded by the other branch of the U-shape, termed the hot branch, the hot collector being located outside and above the closer slab, an exchanger one end of which is connected to the cold collector and the other end of which is connected to the hot collector, the circuit being configured so that the heat-exchange liquid circulates therein by natural convection and remains in the liquid state in operation in an accident situation releasing the decay heat; a cold source including: at least one reservoir located at a distance from the primary containment vessel and above the closer slab, the reservoir containing a phase change material of solid-liquid type wherein the exchanger is inserted, the phase change material being adapted, during the exchange with the liquid metal of the exchanger, to be in the solid state in normal operation of the nuclear reactor and to go to the liquid state in an accident situation releasing the decay heat; a thermally-insulating layer adapted to be fixed in a removable manner to at least part of the external wall of the reservoir, covering the latter, and to be passively detached therefrom if the temperature of said wall reaches a predetermined threshold value.

2. The nuclear reactor according to claim 1, the thermally-insulating layer being configured to fall by gravity when it is detached from the external wall of the reservoir.

3. The nuclear reactor according to claim 1, the insulating layer comprising a plurality of contiguous thermally-insulating panels.

4. The nuclear reactor according to claim 1, comprising at least one passive device for removably fixing the thermally-insulating layer configured to fix the thermally-insulating layer up to the predetermined threshold temperature and to detach it passively above the threshold temperature.

5. The nuclear reactor according to claim 4, comprising at least one passively removable fixing device per thermally-insulating panel.

6. The nuclear reactor according to claim 5, the reservoir being made of a magnetic material, the passively removable fixing device comprising at least one permanent magnet fixed to each thermally-insulating panel, the magnet being magnetically attached to the external wall of the reservoir below the threshold temperature, the Curie temperature from which the magnet loses its magnetic properties being determined as a function of the threshold temperature.

7. The nuclear reactor according to claim 6, the permanent magnet being made of Fe—Ni alloy.

8. The nuclear reactor according to claim 1, the external wall of the reservoir comprising a plurality of fins covered by the thermally-insulating layer when the latter covers said wall.

9. The nuclear reactor according to claim 8, at least one of the plurality of fins being inserted in each thermally-insulating panel.

10. The nuclear reactor according to claim 1, further comprising at least one active device for removably fixing the thermally-insulating layer configured to fix the thermally-insulating layer and to be activated on command by a user to detach the latter from the external wall of the reservoir whatever the temperature of the latter.

11. The nuclear reactor according to claim 10, further comprising at least one passively removable fixing device per thermally-insulating panel.

12. The nuclear reactor according to claim 1, the cold source comprising two distinct reservoirs.

13. The nuclear reactor according to claim 12, one of the two exchangers of the two distinct reservoirs being connected to an end of the collector that is opposite that to which the other of the exchangers is connected.

14. The nuclear reactor according to claim 1, the exchanger(s) being divided into multiple tubes arranged in parallel in each reservoir and surrounded by the phase change material.

15. The nuclear reactor according to claim 1, comprising a circulation loop including at least one hydraulic branch connecting the cold collector to the end of the monotube exchanger and at least one hydraulic branch connecting the cold collector to the end of the exchanger, and where appropriate one or more other fluidic components.

16. The nuclear reactor according to claim 1, comprising at least one confinement building for confining each reservoir of the evacuation system.

17. The nuclear reactor according to claim 1, the heat-exchange liquid of the decay heat removal circuit being a liquid metal chosen from a binary lead-bismuth alloy, a binary sodium-potassium alloy, such as NaK, or other ternary alloys of the liquid metals.

18. The nuclear reactor according to claim 1, the phase change material filling the reservoir(s) being chosen from lead, cadmium, zinc or a zamak-type zinc alloy, tin and its alloys with lead or a ternary Li—Na—K carbonate mixture.

19. The nuclear reactor according to claim 1, the reservoir(s) of the evacuation system being made of Hastelloy® or of ferritic stainless steel, based on nickel, comprising of Hastelloy® and Inconel®.

Description

BRIEF DESCRIPTION OF THE FIGURES

[0168] FIG. 1 is a diagrammatic view in perspective of a nuclear reactor (SFR) cooled by liquid sodium with a decay heat removal system according to the invention.

[0169] FIG. 2 is a view in partial section of part of FIG. 1.

[0170] FIG. 3 is a view in partial longitudinal section showing the primary containment vessel and part of the fuel assemblies of an SFR type nuclear reactor and part of the layer of pipes of a decay heat removal system according to the invention.

[0171] FIG. 4 repeats FIG. 3 but without the presence of a layer of thermally-insulating material.

[0172] FIG. 5 is a diagrammatic view in perspective of the interior of a thermal storage reservoir containing a phase change material and a monotube exchanger of the closed circuit of a decay heat removal system according to the patent application FR3104311A1.

[0173] FIG. 6 is a view in section illustrating one possible embodiment of a prior art thermal storage reservoir containing a phase change material and an exchanger divided into a plurality of tubes.

[0174] FIGS. 7A, 7B are partial views in cross-section at the level of a finned external wall of a thermal storage reservoir with the thermal-insulating panels covering said wall in accordance with the invention, respectively before and after their passive detachment from the wall.

[0175] FIG. 8 illustrates in the form of a curve the Curie temperature for Fe—Ni alloys as a function of the proportion of nickel.

[0176] FIGS. 9A, 9B are partial views in cross-section at the level of an external wall of a thermal storage reservoir with the thermally-insulating panels covering said wall in accordance with a first variant of the invention in which the wall is without fins, respectively before and after their passive detachment from the wall.

[0177] FIG. 10 illustrates in the form of respective curves an example of thermal power to be evacuated imposed in a nuclear reactor, the thermal losses to the outside of the thermal storage reservoir incorporating an integral exchanger divided into multitubes with passive detachment at 240° C. of the thermally-insulating panels with a reservoir external wall with fins, without fins and for comparison a prior art configuration with a permanent thermally-insulating layer (FIG. 6).

[0178] FIG. 11 illustrates in the form of respective curves an example of temperatures of the heat-exchange fluid at the entry and at the exit the thermal storage reservoir incorporating an exchanger divided into multitubes with passive detachment at 240° C. of the thermally-insulating panels with a reservoir external wall with fins, without fins and for comparison a configuration with a prior art permanent thermally-insulating layer (FIG. 6).

[0179] FIG. 12 illustrates in the form of respective curves an example of temperatures of the heat-exchange fluid at the entry and at the exit of the thermal storage reservoir incorporating an integral multitube exchanger with passive detachment at 240° C. of the thermally-insulating panels with a reservoir external wall with fins.

[0180] FIGS. 13A, 13B, 13C are partial views in cross-section and detail views at the level of an external wall of a thermal storage reservoir with the thermally-insulating panels covering said wall in accordance with a second variant of the invention including active devices for detaching the panels, respectively before and after their passive detachment from the wall.

[0181] FIGS. 14A, 14B are partial views in cross-section at the level of an external wall of a thermal storage reservoir with the thermally-insulating panels covering said wall according to a variant of the invention in which the panels have a shape adapted to falling thereof by gravity when the reservoir is provided with fins.

DETAILED DESCRIPTION

[0182] Throughout the present application the terms “vertical”, “lower”, “upper”, bottom”, “top”, “below” and “above” are to be understood with reference to a primary containment vessel filled with liquid sodium and a thermal storage reservoir in a vertical operating configuration.

[0183] For the sake of clarity, the same references designating the same elements according to the prior art and according to the invention are used for all FIGS. 1 to 13C.

[0184] FIGS. 1 to 6 have already been commented on in the preamble and will therefore not be further commented on hereinafter.

[0185] Nor is there described the sodium-cooled (SFR type) nuclear reactor 1 with a loop type architecture, with a system 2 for evacuation of at least some of the nominal power and of the decay heat from the reactor represented in FIGS. 1 to 4, to which the cold source 5 according to the invention is applied instead and in place of that according to the prior art from FIG. 5.

[0186] The cold source 5 includes at least one thermal storage reservoir 50 accommodating multiple tubes 430 located in parallel and embedded in the phase change material 51, as in FIG. 6.

[0187] According to the invention, a thermally-insulating layer 6 is located to be removably fixed to the lateral envelope of the external wall 500 of the reservoir, covering the latter and being detached therefrom in a passive manner if the temperature of said wall reaches a predetermined threshold value.

[0188] As shown in FIGS. 7A and 7B the layer 6 consists of a plurality of contiguous thermally-insulating panels 60. These may be panels based on glass wool or rock wool.

[0189] For the removable fixing of the isolating panels 60 at least one passive device 7 is configured to fix the thermally-insulating layer up to the predetermined threshold temperature and to detach it in passive manner above the threshold temperature.

[0190] The passive device 7 preferably consists of a permanent magnet fixed to each thermally-insulating panel 60. In this case the reservoir 50 is made of a magnetic material.

[0191] For example, Fe—Ni type alloys having a Curie temperature, the temperature above which the alloy loses its magnetic properties, that depends on the proportions of the constituents, as shown in FIG. 8. The proportion of constituents can therefore be chosen to define the Curie temperature as a function of the threshold temperature from which a magnet 7 will no longer be functional and from which the panels 60 will therefore be detached from the external wall 500 of the reservoir 50.

[0192] To increase the area of exchange with the outside air, necessary only when the thermal-insulating panels 60 are detached from the wall 500 of the reservoir 50, fins 501, which are straight in the example illustrated, are produced on the wall 500. The fins 501 extend radially with respect to the cylindrical reservoir 50 and are covered by each thermally-insulating panel 60 in its fixed configuration and therefore have no effective role in thermal exchanges with the outside when the insulating panels are in place. In the example illustrated two fins 501 are inserted in a panel 60. The fins 501 may be fixed to the reservoir 50 or form an integral part thereof.

[0193] The magnet 7 is therefore magnetically attached to the external wall 500 (FIG. 7A) when the temperature of the external wall 500 of the reservoir 50 is below the threshold temperature.

[0194] As soon as the external wall temperature 500 is above the threshold temperature the fixing of the thermally-insulating panels 60 is no longer effective because of the loss of the magnetic properties of the magnet 7, as explained hereinafter. The panels 60 are therefore detached from the wall 500 and drop through gravity. The external wall 500 is therefore bare and via its fins 501 exchanges heat directly with the surrounding air (FIG. 7B).

[0195] A variant may consist in there being no fins on the external wall. This variant is illustrated in FIGS. 9A and 9B respectively showing the fixed configuration of the insulating panels 60 and the latter once detached from the external wall 500. This variant can facilitate the production of the external wall 500 and that of the insulating layer 6 and fixing thereof to the wall 500. However, the absence of fins limits the efficacy of thermal-exchange with the outside, as corroborated by the simulations described hereinafter the results of which are shown in FIGS. 10 and 11.

[0196] To illustrate the benefit of the removable thermally-insulating panel solution according to the invention, the inventors have carried out numerical simulations with by way of hypotheses a threshold temperature for detachment of the panels equal to 240° C. and multiplication by a factor of 3 of the area of exchange with the air surrounding the wall 500 via the fins 501.

[0197] In these simulations the material assuring the magnet role may be an Fe—Ni alloy having a Curie temperature higher than the threshold temperature for detachment of the thermally-insulating panels. The difference between the Curie temperature of a permanent magnet 7 and the threshold temperature may typically be of the order of 50° C. This temperature difference is explained by the fact that the reduction of magnetisation commences before reaching the Curie temperature, to zero at the Curie temperature. An insulating panel 60 will therefore be detached from the external wall 500 by virtue of its own weight before the Curie temperature is actually reached at the level of the external wall, whence the difference, typically of the order of 50° C.

[0198] In the simulations the exchanger 43 is treated as a plurality of parallel vertical tubes 430 that offers better performance for exchanging the thermal power of the heat-exchange fluid with the storage phase change material, as schematically represented in FIG. 6.

[0199] Account is also taken of a power profile to be evacuated imposed at the level of the heat-exchange fluid and of an objective of maintaining the temperature of the heat-exchange fluid below 550° C. for a period of 7 days. This imposed power profile comprises two phases, namely a first phase of 5 days under continuous conditions and a second phase with a peak of power to be evacuated that commences at the start of the 6.sup.th day and takes a characteristic form with rapid growth for 12 h followed by a slow decrease (curve in solid line in FIG. 10).

[0200] Three thermal storage configurations are compared: [0201] prior art reference storage using multitubes 430 sized to conform to the constraints on the temperature rise of the heat-exchange fluid (550° C.) for 7 days; [0202] storage with the same dimensions as the reference storage, with thermally-insulating panels that are detached from the wall 500 in a completely passive manner at a threshold temperature of 240° C. by virtue of the demagnetisation of the permanent magnets 7 and of the fins 501 being sized to increase by a factor of 3 the area of exchange with the surrounding air; [0203] with the same dimensions as the reference configuration, with thermally-insulating panels that are detached from the wall 500 with no fins in a completely passive manner at a threshold temperature of 240° C. by virtue of the demagnetisation of the permanent magnets 7.

[0204] It emerges from the simulations that the thermal exchanges of the storage with the outside are increased with the insulating panels 60 that are detached in a passive manner and even further increased when the storage reservoir 50 has fins 501 at the periphery of its wall 500, as shown in FIG. 10 and described hereinafter.

[0205] This increased evacuation of heat to the outside of the reservoir 50 enables limitation of its rise in temperature: this therefore makes it possible to maintain the temperature of the heat-exchange fluid below the limit temperature for longer, as illustrated in FIG. 11.

[0206] In the reference configuration, i.e. with insulating panels 60 that are not detachable, the temperature of the heat-exchange fluid reaches the limit value of 550° C. after 7 days from the commencement of the total loss of electricity because the dimensions of the reservoir 50, of the tubes 430 and of the phase change material 51 have been defined to enable this constraint to be complied with.

[0207] Using the thermally-insulating panels 60 according to the invention, the heat-exchange fluid remains at all times below 470° C. with, moreover, a temperature that is decreasing 2.5 days after the start of the peak (FIG. 11). The invention therefore enables storage to be considerably more efficacious for evacuation of the power of the heat-exchange fluid.

[0208] This improved storage performance using the panels 60 according to the invention enables reduction of the dimensions of the storage reservoir 50 and of the material 51 necessary to comply with the temperature limit of 550° C. for 7 days.

[0209] Table 1 below summarises the various dimensions for the three configurations stated.

TABLE-US-00001 TABLE 1 Configuration Storage with Storage with Reference panels 60 panels 60 Dimensions storage without fins 501 with fins 501 Diameter of 11.7 10.6 7.2 cylindrical reservoir 50 (m) Height of cylindrical 2 .Math. 77 reservoir 50 (m) Volume of phase 298 242 113 change material per reservoir 50 (m.sup.3)

[0210] It emerges from this table 1 that, compared to the reference configuration: [0211] the reduction is 10% in terms of the diameter of the reservoir 50 and 19% in terms of the volume of phase change material with the configuration of panels 60 without fins in accordance with the invention; [0212] the reduction is 38% in terms of the diameter of the reservoir 50 and 62% in terms of the volume of phase change material with the configuration of panels 60 with fins 501 in accordance with the invention;

[0213] With dimensions reduced thanks to the invention, the maximum temperature of 550° C. is reached after 3 days and the temperature then decreases (FIG. 11). This means that using the invention it is possible to continue to comply with the temperature constraint beyond the target 7 days (FIG. 10), which is not the case for the reference storage configuration, i.e. without detachable panels 60.

[0214] FIGS. 13A to 13C show another possible variant embodiment.

[0215] In accordance with this variant there is added to each passive device 7 for removably fixing the thermally-insulating panels 60 an active device 8 that enables each insulating panel 60 to be detached on command by a user.

[0216] Several types of active devices 8 may be envisaged.

[0217] First of all, an electromagnetic sucker may be used. An electromagnetic sucker comprises a sucker as such and a magnetic backing plate. For example, here there may be provision for fixing the sucker to a removable panel 60 and the backing plate to a support, which is itself fixed to the wall 500 of the reservoir by a permanent magnet by way of a passive device, which loses its magnetic capacity beyond the threshold temperature. If the electrical power supply is interrupted the two parts of the sucker separate and the panel is therefore released.

[0218] Another active device 8 may consist in a device actuated by an electrical pulse, for example producing a magnet effect that moves a strike that would retain the panel in place in normal operation of the reactor. Relative to an electromagnetic sucker device 8, an electrical pulse actuating device can make it possible not to be dependent on the smallest possible interruption of current, even of minimum duration, that could lead to unwanted detachment of all the panels when the situation of the reactor core does not necessarily require it.

[0219] Another active device 8 may consist in a retaining structure movable by a motor, movement thereof causing detachment of the thermally-insulating panels and therefore causing them to fall by gravity.

[0220] Another active device 8 solution that may be envisaged consists in arranging the permanent magnet 7 only at the edge of each thermally-insulating panel 60, which is placed on rails fixed to the reservoir 50, and an active element 8 descends the rails by gravity and tips over the permanent magnets 7.

[0221] More generally, other active devices 8 may be envisaged, for example a device with human intervention, such as a rolling structure to be moved to which the thermally-insulating panels would be fixed.

[0222] Any such active device 7 may be controlled for example via a control wire 80.

[0223] With an active device 8 an operative can detach the panels 60 even before the passive detachment threshold temperature criterion is reached. For example, in the event of going to the decay heat removal mode noted at the level of the core of the reactor, an operator may wish to detach the panels 60 as a preventive measure even before the external wall 500 reaches the threshold temperature. This makes it possible to save time for the effect of thermal exchanges with the surrounding air and therefore to limit the rise in temperature. If the operator does not make use of this possibility of detachment by the active device, passive detachment nevertheless occurs automatically when the threshold temperature criterion is reached.

[0224] In the configuration with fins 501, as a function of the design of the latter and of the thermally-insulating panels 60, at least some of the fins could be retained on at least one of the thermal-insulating panels 60 whereas the latter has just been detached either in passive manner or in active manner. This fin (these fins) could be retained up to the point of blocking the falling of the panel(s).

[0225] To eliminate this risk the inventors have conceived a geometric configuration that, despite the presence of the fins, causes the thermally-insulating panel or panels to fall by gravity whatever happens when passive or active triggering, for example demagnetisation of the permanent magnet, occurs. A panel geometric configuration must enable movement in rotation of the latter enabling it to tilt from the bottom. An example of this configuration before and during detachment is shown in FIG. 14A: the thermally-insulating panels 60 has a tapered shape 600 in its lower part which enables it to tilt from the top when it is detached from the wall 500 of the reservoir 50.

[0226] FIG. 14B shows grooves 601 produced in the panel 60 in each of which a fin 500 can be inserted to be covered by said panel 60.

[0227] The invention is not limited to the examples that have just been described; the features of the examples illustrated found in variants that are not illustrated may in particular be combined with one another.

[0228] Other variants and embodiments may be envisaged without departing from the scope of the invention.

[0229] The decay heat removal system that has just been described with reference to a loop-type nuclear reactor may be employed in a nuclear reactor of integrated type for generating electricity or heat.

[0230] In one integrated reactor design the layer 40 of pipes surrounds all of the primary containment vessel 10 in a homogeneous manner.

[0231] In some loop-type reactors the pipes 400 that are located alongside the primary circuit are able to join in a micro-collector at the level of the branch in order to prevent possible hot spots for the U-shape pipes 400 involved.

[0232] In the examples illustrated the fins 501 are straight and extend radially with respect to the cylindrical storage reservoir 50. There may be any other type of fins, in particular corrugated plates, metal braids.

[0233] The invention may also entirely be employed in a nuclear reactor of heat-generating type.

REFERENCE CITED

[0234] [1]: HOURCADE E. et al., “ASTRID Nuclear Island design: update in French-Japanese joint team development of decay heat removal system”, 2018, ICAPP.