Nuclear fuel assembly with a lower fissile zone with enlarged needles, topped with a liquid metal plenum and a neutron-absorbing plate, associated liquid metal-cooled RNR reactor.

Abstract

Nuclear fuel assembly with a lower fissile zone with enlarged needles surmounted by a plenum of heat transfer fluid and a neutron-absorbing plate, associated RNR-Na reactor. The invention relates to a nuclear fuel assembly (1) comprising: - a bundle of nuclear fuel needles (100), each needle comprising a sheath housing an exclusively fissile column (14), the height of the column being less than or equal to 65 cm, the external diameter of the sheath of the needles being greater than or equal to 9 mm, - an assembly body comprising a casing (10) in the form of a closed hexagonal tube sealed against a heat transfer liquid intended to pass through the bundle, the central portion (12) of the casing enveloping the bundle, while the upper portion (11) forming the assembly head houses an upper neutron protection device (PNS) filled with neutron-absorbing material,the housing comprising an intermediate portion defining a plenum volume, - a lower portion forming the foot of the assembly, in the extension of the housing, the foot being adapted to allow the heat transfer liquid passing through the assembly. Figure for the abstract: Fig. 5A,

Description

Title of the invention: Nuclear fuel assembly with a lower fissile zone with enlarged needles, topped with a liquid metal plenum and a neutron-absorbing plate, associated liquid metal-cooled RNR reactor. Technical field

[0001] The present invention relates to a fuel assembly for a fast neutron nuclear reactor cooled with liquid metal, in particular liquid sodium known as RNR-Na or SFR (English acronym for “Sodium Fast Reactor”) and which is part of the family of so-called fourth generation reactors.

[0002] The fuel assemblies targeted by the invention can be used both in an integrated type nuclear reactor, that is to say for which the primary sodium circuit with pumping means is completely contained in a vessel also containing heat exchangers, and in a loop type reactor, that is to say for which the intermediate heat exchangers and the primary sodium pumping means are located outside the vessel.

[0003] By “fuel assembly” is meant an assembly comprising fuel elements and loaded and / or unloaded into / from a nuclear reactor.

[0004] By “RNR-Na or SFR type fuel assembly” is meant a fuel assembly suitable for being irradiated in a fast neutron nuclear reactor cooled with liquid sodium called RNR-Na or SFR.

[0005] By "hexagonal tube" is meant a tube whose cross-section is regular hexagonal.

[0006] By "homogeneous core" is meant a core of a fast neutron reactor into which only fissile fuel assemblies are introduced. Conversely, a "heterogeneous core" is a core into which one (or more) fully or partially fertile fuel assemblies are introduced.

[0007] Although described with reference to the main application in question, namely an RNR-Na reactor (sodium heat transfer fluid), the invention applies to any type of RNR cooled by liquid metal (lead, lead-bismuth, etc.). Prior art

[0008] Fuel assemblies intended for use in liquid sodium-cooled fast neutron reactors (RNR-Na) have a particular mechanical structure in order in particular to allow liquid sodium to pass through them.

[0009] Figures 1 to 2B show a conventionally used fuel assembly 1. in at least part of an RNR-Na nuclear reactor.

[0010] Such an assembly 1 of elongated shape along a longitudinal axis X firstly comprises a tube or casing 10 of hexagonal section, closed and sealed around the perimeter, the upper portion 11 of which forms the gripping head of the assembly and can house an upper neutron protection device (PNS), and the central portion 12 of which encloses fuel needles 100.

[0011] The portions 11, 12 form the same tubular envelope 10 or housing of identical hexagonal section over its entire height. The head 11 of the assembly has a central opening 110 opening into it and used for its handling.

[0012] The central portion 12 of an assembly comprises a plurality of nuclear fuel pins. Each pin 100 is in the form of a sealed cylindrical steel sheath tube closed at both ends by a welded cap inside which is stacked a column 14 of fissile fuel pellets within which the nuclear reactions that release heat occur. All the columns 14 define what is usually called the fissile zone which is approximately located halfway up an assembly 1. This fissile zone 12 may have a height H equal to 1 m. The external diameter <e>needles can be of the order of 9.5mm. The sheath of the 100 needles thus constitutes the first containment barrier, the integrity of which must be preserved by protecting it from external aggressions such as shocks / mechanical constraints or excessive temperatures.

[0013] The assembly 1 finally comprises a lower portion 13 forming the foot of the assembly, in the extension of the housing 10. The foot 13 of the assembly has a distal end 15 in the shape of a cone or rounded so that it can be inserted vertically into the candlesticks of the base (support) of a reactor core. The foot 13 of the assembly has at its periphery openings 16 opening into it.

[0014] Thus, in the installed configuration of a fuel assembly, that is to say in the loaded position in a reactor core, the foot 13 of an assembly 1, of male shape, is inserted into an opening in the reactor base, thus maintaining the assembly 1 in the latter with its longitudinal axis X vertical. It is specified that the base is a box forming a reservoir of primary sodium under pressure which it distributes to all of the assemblies via the openings in the foot 13.

[0015] The primary sodium can circulate inside the housing 10 of the assembly 1 and thus convey by thermal conduction the heat released by the fuel pins. The sodium is thus introduced through the openings 16 of the foot 13 and exits through the central opening 110 of the head 11, after having passed through the bundle of fuel pins. In other words, as symbolized by the arrows in [Fig.2A], the flow of sodium heat transfer fluid enters the assembly foot 13 through the openings 16, passes through the bundle of needles 100 and the PNS before exiting through the assembly head 11. The foot 16 incorporates within it a so-called depressogenic system, consisting of a more or less porous element 17 which generates pressure losses and makes it possible to adjust the flow rate of sodium passing through the assembly.

[0016] All the assemblies of the same reactor are arranged vertically on a base to form a compact hexagonal mesh network core.

[0017] The assemblies in position on the bed base are spaced from each other at the level of their body, typically a few mm between the facing faces of two adjacent hexagonal section boxes.

[0018] It should be noted that all RNR-Na reactors under study, construction or operation in the world use assemblies having a closed hexagonal tube (TH) as a casing, as described above, which has been the reference since the origin of this nuclear sector in the 1960s.

[0019] A safety issue is linked to a behavior that a core of RNR-Na reactor with high power, typically greater than 1000 MW thermal, could have a situation of prevention and mitigation of serious accidents, i.e. inducing a partial or total meltdown of the core.

[0020] Serious accidents are also studied in order to guarantee acceptable radiological releases, starting from the design phase of a reactor.

[0021] Since Na-RNRs are not in their most reactive configuration during normal operation, the power can increase drastically in the event of a serious accident. Core meltdown and relocation of cladding or fuel can in fact lead to radioactive releases into the environment.

[0022] Therefore, the safety demonstration of an RNR-Na reactor must prove the good behavior of the reactor in the event of a serious accident and that following such an accident, the reactor can be returned to and maintained in a safe state.

[0023] To study a serious accident in an RNR-Na, the course of the accident sequence is generally divided schematically into several phases as follows:

[0024] - the initiation phase, which begins at the instant of occurrence of the initiating event, while the reactor is in normal operating mode, and which ends at the start of the degradation of the fuel needles;

[0025] - the primary phase, which begins at the initiation of needle degradation, and which ends with the rupture of the first hexagonal tube (TH) of an assembly within the core. This phase is characterized by predominantly axial movements of molten materials in the degraded assemblies, the core retaining its overall geometry;

[0026] - the transition phase, which corresponds to the loss of integrity of the THs, resulting either from their fusion, or their loss of mechanical properties. This phase is in fact the seat of a transition between the axial relocation of the molten materials in each as assembly, and the radial propagation of degraded materials between the different assemblies;

[0027] - the secondary phase, during which one or more molten baths of large dimension in the degraded core, which could be the site of repeated criticalities;

[0028] - the relocation and cooling phase during which part of the inventory of core materials is relocated to the molten material collector, which must be cooled.

[0029] The occurrence of a serious accident can result from different initiating events. For large cores (high power), studies have led to the selection as the reference initiator of the sequence of loss of primary flow without fall of the emergency stop rods (ULOF for “Unprotected Loss Of Flow”).

[0030] An objective that the inventors have set themselves is to design an RNR-Na reactor core which, in a serious accident situation, i.e. generalized meltdown of the fuel, does not lead to a deposit of mechanical energy, i.e. the release of mechanical energy at levels likely to damage or harm the integrity of the radiological containment barriers, in particular with regard to the second barrier (the main vessel).

[0031] This mechanical energy can come from: - the energetic interaction between the molten fuel and the liquid primary sodium (a phenomenon called FCI, an English acronym for “Fuel-Coolant Interaction”), an interaction which causes sudden vaporization of the liquid sodium and the propagation of a pressure wave in the primary circuit of the reactor; - vaporization of molten fuel, linked to a sudden increase in reactor power due to runaway nuclear chain reaction, for example due to sodium depletion, steel depletion or axial compaction of the reactor core.

[0032] In both cases, the pressure wave caused is likely to harm the integrity of the main tank and the slab above this tank, which are components constituting the bulk of the second containment barrier for radiological products.

[0033] This objective can be solved by implementing a heterogeneous core which will, by design, limit the level of mechanical energy released. Here, we can cite heterogeneous cores known under the name “Low Drainage Core” (LVC) or that described in patent application FR2961337.

[0034] The main specificity of heterogeneous cores is that the neutron counter-reaction linked to the expansion or emptying of sodium in the core is generally very weak, or even negative, unlike homogeneous cores. As a result: - the initiation phase is much longer for heterogeneous cores than for homogeneous cores. In the event of boiling, this does not cause a primary power excursion, nor a release of mechanical energy associated with the expansion of the fuel resulting from this excursion, but on the contrary causes the total power of the core to drop. Depending on the power conditions, this boiling can even stabilize in the upper structures, in which case the accidental sequence stops even before the core melts; - if core meltdown occurs, the power is much lower than for a homogeneous core, and this meltdown is much later. The possibility of interaction between molten fuel and liquid sodium (FCI) is therefore greatly reduced, since the sodium is already vaporized at the time of fuel meltdown, unlike what happens in a homogeneous core.

[0035] Whatever the type of core, management of the molten fuel (reactivity control, cooling) may be required.

[0036] Figures 3 and 3A show a heterogeneous CHe core, of CFV type as envisaged in the ASTRID reactor project.

[0037] The heterogeneous core CHe essentially comprises three parts: an internal core part CI, surrounded by an external core part CE, itself surrounded by a neutron reflector RE.

[0038] The internal core part CI comprises fuel assemblies 1' with a fissile zone surmounted by a fertile zone and surmounting another fertile zone.

[0039] The external core part CE comprises fuel assemblies 1 with a fissile-only zone 12 like those illustrated in FIGS. 1 to 2B.

[0040] Furthermore, safety bars 2 and control bars 3 are implanted within the internal core part CI.

[0041] The entirety of the fuel assemblies 1, 1' defines exclusively fissile fissile columns ZFi in the part of the external core CE and fissile columns ZFi and fertile CFe, ZFe in layers. Typically, the height of the fissile column ZFi of the external core part CE is equal to 90 cm, while the cumulative height of the two fissile columns ZFi and the fertile column Zfe between the two is equal to 80 cm.

[0042] As shown, the heterogeneous core comprises a reflector RE present around and on the underside of both the CE and CI portions, a neutron absorber zone ZA and a liquid sodium plenum PLE present on the underside of both the CE and CI portions, with a portion of the plenum on the inner side of the CE portion.

[0043] Heterogeneous CH cores generate new problems, in particular made of the need for a median fertile plate in each 1' assembly which will define the ZFe fertile column. These issues are in particular the manufacturability of the fuel column (fissile and fertile), the thermomechanical resistance during the accident sequence...

[0044] To achieve the desired objective, instead of considering heterogeneous cores, additional systems can be added. It is thus possible to use passively triggered safety rods, which allow neutron absorbers to be inserted following the exceeding of a threshold. For example, the threshold may be a flow rate lower than 40% for a hydraulically triggered rod, a temperature higher than 650°C for a thermal fuse. Devices whose actuation is dependent on the pressure at the core inlet also allow free levels of absorbent liquid materials, such as lithium, to be displaced. However, the addition of such systems necessarily brings complexity to the reactor as a whole.

[0045] Figures 4 and 4A show a so-called homogeneous CHo reactor core.

[0046] Like a heterogeneous core CH, a homogeneous core CHo also comprises an inner core part CI, surrounded by an outer core part CE and a neutron reflector RE which surrounds the part CE and is also present below and above the two parts CE, CI.

[0047] The two parts, internal core CI and external core CE, comprise exclusively fuel assemblies 1 with a solely fissile zone 12 like those illustrated in figures 1 to 2B.

[0048] For homogeneous CHo cores, as they are currently envisaged, the absence of mechanical energy deposition during an accident sequence cannot be demonstrated. Indeed, the mechanical energy deposited is calculated under so-called "best-estimate" conditions (i.e. by following a methodology considered to be the reference in the field of studying accidents involving generalized core meltdown, which does not require considering penalizing uncertainties for the study of the transient), then compared to a design threshold value, for example equal to 800 MJ for the Superphénix reactor, evaluated within the framework of a specific and envelope safety approach. In this approach, penalizing uncertainties are considered in order to ensure that all of the possible variability of such a transient is included.

[0049] Homogeneous cores as currently envisaged cannot achieve the above-mentioned objective of absence of mechanical energy deposition during the entire accident sequence.

[0050] There is therefore a need to further improve liquid metal-cooled RNR type nuclear reactors, in particular in order to achieve the objective of no deposition of mechanical energy damaging to the structures during an entire ac sequence. incidental (primary and secondary phase) and this, by freeing itself from the problems of heterogeneous cores, and by not having recourse to additional systems.

[0051] The aim of the invention is to meet this need. Statement of the invention

[0052] To do this, the invention relates to a nuclear fuel assembly with a longitudinal axis comprising:

[0053] - a bundle of nuclear fuel needles, each needle comprising a cladding housing a column of stacked pellets of exclusively fissile fuel, the height of the fissile column being less than or equal to 65 cm, the external diameter of the cladding of the needles being greater than or equal to 9 mm,

[0054] - an assembly body comprising a housing in the form of a hexagonal tube closed and sealed against a heat transfer liquid intended to pass through the bundle of needles, the central portion of the housing enveloping the bundle of needles, while the upper portion forming the assembly head houses an upper neutron protection device filled with neutron absorbing material, the housing further comprising an intermediate portion defining a plenum volume intended to be filled with the heat transfer liquid,

[0055] - a lower portion forming the foot of the assembly, in the extension of the housing, the foot being adapted to allow the heat transfer fluid to pass through the assembly.

[0056] The height of the fissile column may be less than or equal to 60 cm.

[0057] The sheath diameter of the needles may be greater than 10 mm.

[0058] Several needle dimensions can be envisaged within the framework of the invention.

[0059] We can thus have needles with a height H1 of the fissile column equal to 60 cm for an external diameter DI of the sheath equal to 10 mm.

[0060] We can also have larger needles with a height H1 of the fissile column equal to 65 cm for an external diameter DI of the sheath equal to 13 mm.

[0061] On the contrary, we can consider smaller needles with a height H1 of the fissile column equal to 55 cm for an external diameter DI of the cladding equal to 9 mm. These smaller needles can be dedicated to lower power reactor cores. For example, these can be reactors with a power of the order of 100 MW thermal.

[0062] According to an advantageous embodiment, the upper neutron protection device (PNS) consists of a plate made of at least one neutron absorption material chosen from boron carbide (B4C), more or less enriched in 10B, metallic hafnium, a refractory boride type material, for example HfB2 and TiB2, europium hexaboride EuB6 or Eu203.

[0063] Thus, the invention essentially consists of producing a fuel assembly for a liquid metal-cooled RNR reactor, with a combination of four new characteristics compared to known assemblies, namely:

[0064] - the presence of a sodium plenum above the needle bundle;

[0065] - the implantation of a neutron absorption device preferably in the form of an absorbent plate;

[0066] - a reduced fissile column height, advantageously less than or equal to 60 cm ;

[0067] - needles of enlarged diameter, advantageously of the order of 1 cm or more.

[0068] These four cumulative characteristics thus make it possible to oppose the main aggravating phenomena encountered in a serious accident, namely respectively the emptying of sodium, the emptying of steel and the axial compaction of the reactor core.

[0069] Until now, no proposed solution has made it possible to respond to an absence of mechanical energy deposition during the entire accident sequence of an RNR reactor.

[0070] Some homogeneous cores have been proposed with assemblies with only three of the four stated characteristics. In particular, the height of the fissile column has always been kept at too high a value, typically above 70 cm; for performance needs (Pu content, reactivity reserve, control, etc.) and, for the same reasons, a reduction in this height has never been associated with a reduction in the power density via the use of large diameter needles.

[0071] Furthermore, although the implementation of a sodium plenum has already been combined with a neutron absorber plate, it does not fully address the problem of the ULOF sequence for industrial power cores. Until now, the choice of a person skilled in the art therefore necessarily focused on the addition of complementary and specific safety systems which complicate the design and operation of the reactor.

[0072] Reducing the height of the fissile column is known in itself to allow an improvement in accidental behavior in ULOF sequence, but this has never been retained for high power cores.

[0073] The choice to implant needles of enlarged diameter and therefore to reduce the fraction of sodium in the assembly still goes against the natural choices of a person skilled in the art oriented towards performance, which complicates other aspects (control, tank sizing, etc.).

[0074] Increasing the height of the fissile column and reducing the diameter of the needles has therefore never been considered until now for a high-power core with plenum and neutron absorber plate.

[0075] Thus, by focusing on safety, which has become the priority objective of the cores of In the current RNR reactor, the inventors overcame a prejudice by proposing a combination of the four characteristics that a person skilled in the art had previously prohibited.

[0076] Another advantage of the invention is the simplification of the safety approach and therefore of the dimensioning of primary components, in particular of the reactor vessel.

[0077] The invention finally relates to a nuclear installation comprising a fast neutron nuclear reactor cooled with liquid metal, in particular liquid sodium known as RNR-Na (or SFR) and comprising a homogeneous core housing a plurality of nuclear fuel assemblies as described above.

[0078] The invention makes it possible to avoid the use of additional safety systems (passively triggered bars triggered by a drop in pressure, fuses, etc.) which add complexity to the project.

[0079] Thus, according to the invention, the nuclear core of the reactor of a nuclear installation can be free of additional safety devices dedicated to mitigating the consequences of a ULOF accident.

[0080] It is possible to envisage applying the invention to a high-power core and therefore a large volume, typically greater than 1 m3.

[0081] In addition, a lower fissile height allows for smaller assemblies, which are easy to manufacture, transport and handle because they have lower power.

[0082] Other advantages and characteristics will become more apparent upon reading the detailed description, given for illustrative and non-limiting purposes, with reference to the following figures. Brief description of the drawings

[0083] [Fig.l] [Fig.l] is an external perspective view of a fuel assembly according to the state of the art, already used in a sodium-cooled nuclear reactor RNR-Na.

[0084] [Fig.2] [Fig.2] is a perspective view of a fuel assembly according to the state of the art, conventionally used in an RNR-Na nuclear reactor.

[0085] [Fig.2A] [Fig.2A] is a longitudinal sectional view of the fuel assembly according to [Fig.2].

[0086] [Fig.2B] [Fig.2B] is a cross-sectional view at the level of the needle bundle of the fuel assembly according to [Fig.2].

[0087] [Fig.3] [Fig.3] is a schematic top view of a heterogeneous core of an RNR-Na reactor, as envisaged within the framework of the ASTRID project.

[0088] [Fig.3A] [Fig.3A] is a schematic view in longitudinal half-section of elements, in so-called “RZ” representation, of the heterogeneous core according to [Fig.3].

[0089] [Fig.4] [Fig.4] is a schematic top view of a homogeneous core of an RNR-Na reactor.

[0090] [Fig.4A] [Fig.4A] is a schematic view in longitudinal half-section, representing- RZ sensation, of the homogeneous heart according to [Fig.4].

[0091] [Fig.5A] [Fig.5A] is a schematic view in longitudinal section of a fuel assembly for an RNR-Na reactor according to the invention.

[0092] [Fig.5B] [Fig.5B] is a cross-sectional view at the level of the needle bundle of the fuel assembly according to [Fig.5A]. Detailed description

[0093] For the sake of clarity, the same elements are designated by the same numerical references according to the state of the art and according to the invention.

[0094] It is specified that throughout the application, the terms “vertical”, “lower”, “upper”, “bottom”, “top”, “below” and “above” are to be understood by reference to a fuel assembly as it is in vertical configuration in a nuclear reactor.

[0095] Figures 1 to 4A relating to the state of the art have already been commented on in the preamble. They will therefore not be commented on below.

[0096] Unlike an assembly 1 with a longitudinal axis (X) such as that of the state of the art described with reference to FIGS. 1 to 2B, the housing 10 assembly 1 according to the invention as illustrated in FIGS. 5A and 5B, further houses between the fissile column 12 and the head of the assembly, a plate made of neutron absorbing material 17, such as B4C, and comprises an intermediate portion 18 defining a plenum volume intended to be filled with liquid sodium.

[0097] In addition, each fuel needle 100 has an external diameter ¢1 enlarged compared to that <e>of an assembly of figures 1 to 2B. This diameter ¢1 can be equal to 10 mm or more.

[0098] Also, the height H1 of the fissile column 12 has a lower height H1 compared to that H of an assembly of FIGS. 1 to 2B. This height H1 may be equal to 60 mm or less.

[0099] With the combination of these four characteristics, it is possible to produce with a plurality of fuel assemblies 1 a homogeneous RNR-Na reactor core which induces an absence of mechanical energy deposition during the accident sequence (primary and secondary phase).

[0100] Other variants and improvements can be envisaged without departing from the scope of the invention. List of cited references

[0101] [1] Annual Report 2016, GEN IV International Forum pp 52-56.< / e> < / e>

Claims

Claims

1. Nuclear fuel assembly (1) with a longitudinal axis (X) comprising: - a bundle of nuclear fuel needles (100), each needle comprising a sheath housing a column (14) of stacked pellets of exclusively fissile fuel, the height of the fissile column being less than or equal to 65 cm, the external diameter of the sheath of the needles being greater than or equal to 9 mm, - an assembly body comprising a housing (10) in the form of a closed hexagonal tube sealed against a heat transfer liquid intended to pass through the bundle of needles, the central portion (12) of the housing enveloping the bundle of needles, while the upper portion (11) forming the assembly head houses an upper neutron protection device (PNS) filled with neutron absorbing material, the housing further comprising an intermediate portion defining a plenum volume intended to be filled with the heat transfer liquid,- a lower portion forming the foot of the assembly, in the extension of the housing, the foot being adapted to allow the heat transfer liquid passing through the assembly.,

2. Assembly according to claim 1, the height of the fissile column being less than or equal to 60cm.

3. Assembly according to claim 1 or 2, the sheath diameter of the needles being greater than 10 mm.

4. Assembly according to one of the preceding claims, the upper neutron protection device (PNS) being made of a plate made of at least one neutron absorption material chosen from boron carbide (B4C), enriched in 10B, metallic hafnium, a refractory boride type material, for example HfB2 and TiB2, europium hexaboride EuB6 or Eu203.

5. Nuclear installation comprising a fast neutron nuclear reactor cooled with liquid metal, in particular liquid sodium known as RNR-Na or SFR and comprising a homogeneous core housing a plurality of nuclear fuel assemblies according to one of the preceding claims.

6. Nuclear installation according to claim 5, the nuclear core being free of additional safety devices dedicated to mitigating the consequences of a ULOF accident.

Images