A lower height fissile zone nuclear fuel assembly with enlarged pins surmounted by a liquid metal plenum and neutron absorbing plate, and an associated liquid metal cooled FNR (fast neutron reactor) reactor

Abstract

The invention relates to a nuclear fuel assembly (1) comprising: a bundle (100) of nuclear fuel pins, each pin comprising a cladding housing an exclusive fissile column (14), the height of the column being not more than 65 cm and the outer diameter of the cladding of the pins being not less than 9 mm; an assembly body comprising a housing (10) in the form of a hexagonal tube that is closed and sealed against the heat transfer liquid intended to pass through the bundle, the central part (12) of the housing surrounding the bundle, while the upper part (11) forming the assembly head houses an upper neutron shielding device filled with neutron absorbing material, the housing comprising a middle part defining a plenum volume; and a lower part that is an extension of the housing and forms the base of the assembly, the lower part being suitable for allowing the passage of the heat transfer liquid flowing through the assembly.

Description

[Technical Field]

[0001] The present invention relates to a fuel assembly for a liquid metal cooled fast neutron reactor, known as RNR-Na or SFR (Sodium Fast Reactor), which is cooled in particular with liquid sodium and which is part of the Generation IV reactor family.

[0002] The fuel assemblies covered by the present invention can be used equally well in integral reactors (i.e., the primary sodium circuit with pumping means is contained entirely within a vessel which also contains the heat exchangers) and in loop reactors (i.e., the intermediate heat exchangers and primary sodium pumping means are located outside the vessel).

[0003] The term "fuel assembly" is understood to mean an assembly that contains fuel elements and that is loaded and / or extracted into / from a nuclear reactor.

[0004] The term "fuel assembly of the RNR-Na or SFR type" is understood to mean a fuel assembly that is intended to be irradiated in a liquid sodium-cooled fast neutron reactor, designated RNR-Na or SFR.

[0005] The term "hexagonal tube" is understood to mean a tube having a cross section that is a regular hexagon.

[0006] The term "homogeneous core" is understood to mean a core of a fast neutron reactor into which only fissile fuel assemblies are introduced, whereas a "heterogeneous core" is a core into which one (or more) full or partial fertile fuel assemblies are introduced.

[0007] Although described with reference to the primary intended application, i.e., RNR-Na (sodium-cooled) reactors, the invention applies to any type of RNR cooled by liquid metal (lead, lead-bismuth, etc.). [Background technology]

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

[0009] 1 through 2B show a fuel assembly 1 conventionally used in at least some RNR-Na nuclear reactors.

[0010] Such an assembly 1, of elongated shape along a longitudinal axis X, firstly comprises a tube or housing 10 with closed and sealed hexagonal sections at its ends, an upper part 11 forming the retaining head of the assembly and capable of housing an upper neutron shielding (UNS) device, and a central part 12 surrounding the fuel rods 100.

[0011] The sections 11, 12 form the same tubular enclosure 10 or housing having the same hexagonal section over its entire height. The assembly head 11 includes a central opening 110 emerging therein and used for its handling.

[0012] The central part 12 of the assembly contains a number of nuclear fuel rods. Each rod 100 is in the form of a sealed cylindrical cladding tube made of steel and closed at both ends by a welded plug. Fissile columns 14 of fuel pellets are stacked within the rod, with nuclear reactions occurring within the pellets that release heat. All columns 14 define what is commonly called a fissile zone, located approximately halfway up the assembly 1. The height H of this fissile zone 12 can be equal to 1 m. The outer diameter Φ of the rods can be approximately 9.5 mm. The rod cladding 100 thus forms the first containment barrier, the integrity of which must be strictly preserved by protecting it from external threats such as mechanical shocks / stresses or excessive temperatures.

[0013] The assembly 1 finally comprises a lower part 13 forming the base of the assembly in the extension of the housing 10. The assembly base 13 has a distal end 15 of conical or rounded form so that it can be inserted vertically into the columns of the (supporting) lattice structure of the core. The assembly base 13 comprises, at its periphery, openings 16 emerging therein.

[0014] Thus, when the fuel assembly is in the installed configuration, i.e., in its loaded position in the core, the base 13 of the male assembly 1 is inserted into an opening in the reactor lattice structure, thereby holding the assembly 1 therein with its longitudinal axis X in a vertical line. It should be noted that the lattice structure is a caisson forming a reservoir of pressurized primary sodium that it distributes to all the assemblies via the openings in the base 13.

[0015] The primary sodium can circulate within the housing 10 of the assembly 1, thereby carrying the heat released by the fuel rods by thermal conduction. The sodium is thus introduced through the opening 16 in the base 13, passes through the bundle of combustible rods, and then exits through the central opening 110 in the head 11. In other words, as shown by the arrows in FIG. 2A, the flow of sodium coolant enters the assembly base 13 through the opening 16 and passes through the bundle of rods 100 and the UNS before exiting through the assembly head 11. The base 13 incorporates a system called a differential pressure system, formed by more or less porous elements 17, which creates a pressure drop and allows the sodium flow through the assembly to be regulated.

[0016] All the assemblies of the same reactor are arranged vertically on a lattice structure to form a compact array of cores with a hexagonal mesh.

[0017] The assemblies arranged on the grid structure are spaced apart from one another across their bodies, typically by 2-3 mm between the opposing faces of two adjacent hexagonal cross-section housings.

[0018] It should be noted that all RNR-Na reactors under investigation, under construction, or in operation worldwide use the closed hexagonal tube (TH) assembly as their housing, as previously mentioned, which has formed the benchmark since the inception of this nuclear industry in the 1960s.

[0019] The safety issue relates to the behavior of high-power RNR-Na cores, typically greater than 1,000 MW, when they can be assumed to be in conditions to prevent and mitigate severe accidents, i.e., the induction of partial or complete meltdown of the core.

[0020] Furthermore, severe accidents are studied from the reactor design stage onwards to ensure acceptable radioactive releases.

[0021] Since the RNR-Na reactor is not in its most reactive configuration during normal operation, in the event of a severe accident, power output could increase dramatically. Melting of the core and cladding or rearrangement of the fuel could indeed lead to the release of radioactive materials into the environment.

[0022] As a result, the safety demonstration of the RNR-Na reactor must prove that the reactor will behave properly in the event of a severe accident, and that after such an accident the reactor can be returned to and maintained in a safe state.

[0023] To examine the severe accident at RNR-Na in detail, the progression of the accident sequence is generally divided roughly into several stages, as follows: - the initiation phase, which begins when an initiation event occurs while the reactor is in its normal operating condition and ends when the fuel rods begin to deteriorate; - the primary stage, which begins when rod degradation begins and ends with the rupture of the first hexagonal tube (TH) of the assembly in the core. This stage is characterized by a predominantly axial movement of molten material within the degraded assembly, while the core retains its overall geometric shape; - a transitional phase corresponding to the loss of integrity of the THs or resulting from their melting or loss of mechanical properties. This phase is in fact the origin of the transition between the axial relocation of molten material within each assemblage and the radial propagation of degraded material between the various assemblages; - a secondary stage, during which one or more large molten baths of material form in the depleted core, which can become the origin of repeated critical states; - A relocation and cooling phase, during which part of the list of materials in the core, which must be cooled, is relocated to the molten material recuperator.

[0024] Severe accidents can occur due to various initiating events. For large (high power) cores, detailed investigations have led to sequences involving loss of primary flow (ULOF, Unprotected Loss Of Flow) without dropping the emergency barrier rod used as the reference initiator.

[0025] The objective set by the inventors is to design an RNR-Na core that, under severe accident conditions, i.e., generalized melting of the fuel, does not lead to mechanical energy deposition or release at levels that could compromise or damage the integrity of the radioactive containment barriers, especially with regard to the second barrier (main vessel).

[0026] This mechanical energy is - The energetic interaction between the molten fuel and the liquid primary sodium (a phenomenon called FCI (Fuel-Coolant Interaction)), which is the origin of the sudden vaporization of the liquid sodium and the propagation of pressure waves in the primary circuit of the reactor. - Vaporization of molten fuel associated with a sudden increase in reactor power due to a runaway nuclear chain reaction, for example due to sodium drainage, steel drainage, or axial compression of the core. can arise from

[0027] In either case, the generation of pressure waves can adversely affect the integrity of the primary vessel and the plates overlying this vessel, which are essential components of the second containment barrier for the radioactive product.

[0028] This objective can be achieved by implementing a heterogeneous core, which by design will limit the level of mechanical energy released, known as a Low Void Effect (CFV) core or as described in patent application FR 2 961 337, which may be cited herein.

[0029] A key specific feature of a heterogeneous core is that, unlike a homogeneous core, the neutron counter-reaction associated with sodium expansion or drainage within the core is generally very small or even negative. - The initiation phase is much longer in a heterogeneous core than in a homogeneous core. In the case of boiling, this does not cause a primary power excursion or the release of mechanical energy associated with the expansion of the fuel due to this excursion, but instead reduces the total power of the core. Depending on the power conditions, this boiling can further stabilize in the superstructure, in which case the accident sequence is stopped even before the meltdown of the core; - If a core meltdown occurs, the power is much lower than in a homogeneous core and this meltdown occurs much later. Therefore, if a fuel meltdown occurs, unlike what happens in a homogeneous core, the sodium has already been vaporized, and the possibility of interaction between the molten fuel and liquid sodium (FCI) is significantly reduced.

[0030] Regardless of the type of core, management of molten fuel (controlling reactivity, cooling) can be envisaged.

[0031] Figures 3 and 3A show a CFV-type heterogeneous core CHe being considered in the ASTRID reactor project.

[0032] The heterogeneous core CHe basically comprises three parts: an inner core part CI surrounded by an outer core part CE which is itself surrounded by a neutron reflector RE.

[0033] The inner core section CI includes a fuel assembly 1' in a fissile zone above which is a fertile zone and above another fertile zone.

[0034] As shown in FIGS. 1 through 2B, the outer core section CE includes fuel assemblies 1 only within the fissile zone 12.

[0035] Furthermore, safety bars 2 and control bars 3 are installed in the inner core section CI.

[0036] All fuel assemblies 1, 1' define fissile columns that are exclusively fissile columns ZFi in the outer core section CE, as well as fissile columns ZFi and fertile columns CFe, ZFe in the layers. Typically, the height of the fissile columns ZFi in the outer core section CE is 90 cm, while the cumulative height of the two fissile columns ZFi and the fertile column ZFe between them is 80 cm.

[0037] As shown, the heterogeneous core includes a reflector RE surrounding and underlying both sections CE and CI, a neutron absorbing zone ZA and a liquid sodium plenum PLE both underlying sections CE and CI, with a portion of the plenum located inside section CE.

[0038] The heterogeneous core CHe creates new problems, in particular due to the requirement for intermediate fertile plates in each assembly 1', which would define the fertile columns ZFe. These problems relate in particular to the manufacturability of the fuel columns (fissile, fertile), their thermomechanical strength during accident sequences, etc.

[0039] Instead of considering a heterogeneous core, supplementary systems can be added to achieve the stated objectives. Thus, passively triggered safety bars can be used that allow neutron absorbers to be inserted after a threshold is exceeded. For example, the threshold can be a flow rate of less than 40% for hydraulically triggered bars or a temperature greater than 650°C for thermal fuses. Also, devices that rely on core inlet pressure for operation allow free concentrations of absorber liquids, such as lithium, to be displaced. However, adding such systems inevitably makes the reactor as a whole more complex.

[0040] 4 and 4A show a core called the homogeneous core CHo.

[0041] Like the heterogeneous core CHe, the homogeneous core CHo also includes an outer core portion CE and an inner core portion CI that surrounds the outer core portion CE and is surrounded by a neutron reflector RE that lies below and above the two portions CE, CI.

[0042] As shown in FIGS. 1 to 2B, both parts, the inner core CI and the outer core CE, contain only fuel assemblies 1 with only fissile zones 12 .

[0043] In homogeneous core CHo, as they have been considered to date, it is impossible to prove the absence of any mechanical energy deposition during accident sequences. In fact, the deposited mechanical energy is calculated under "best estimate" conditions (i.e., by following a method considered to be the standard in the field of studying accidents involving generalized core meltdowns, which does not include taking into account harmful uncertainties regarding transient learning) and then compared with a dimensioning threshold, for example, equal to 800 MJ for the Superphenix reactor, evaluated within the context of a specific comprehensive safety approach. In this approach, harmful uncertainties are taken into account to ensure that the possible variability of such transient learning overall can be encompassed.

[0044] Homogeneous cores currently under consideration cannot achieve the above-mentioned objective of no mechanical energy deposition throughout the accident sequence.

[0045] Therefore, there is a need to further improve liquid metal cooled RNR reactors, in particular to achieve the objective of not having any mechanical energy deposition throughout the entire accident sequence (primary and secondary stages) that could damage structures, and to do so while avoiding any problems of a heterogeneous core and without the need to use supplementary systems. [Prior art documents] [Patent documents]

[0046] [Patent Document 1] French Patent Application Publication No. 2961337 Summary of the Invention [Problem to be solved by the invention]

[0047] The purpose of the present invention is to address this need. [Means for solving the problem]

[0048] To this end, the present invention provides a nuclear fuel assembly having a longitudinal axis, - a bundle of nuclear fuel rods, each rod including a cladding containing an exclusive fissile column of stacked fuel pellets, the height of the fissile column being 65 cm or less, and the outer diameter of the rod cladding being 9 mm or more; - an assembly body comprising a housing in the form of a hexagonal tube closed and sealed with respect to the heat transfer fluid intended to pass through said bundle of rods, the central part of the housing surrounding the bundle of rods, while the upper part forming the assembly head contains an upper neutron shielding (UNS) device filled with a neutron absorbing material, the housing further comprising an intermediate part defining a plenum volume intended to be filled with the heat transfer fluid; a lower portion that is an extension of the housing and forms a base of the assembly, the base being adapted to allow a heat transfer fluid to flow through the assembly; and The present invention relates to a nuclear fuel assembly, including

[0049] Each rod contains a single exclusive fissile column of fuel pellets.

[0050] The height of the fissile column can be up to 60 cm.

[0051] The cladding diameter of the rod can be greater than 10 mm.

[0052] Several rod sizes are contemplated within the scope of the present invention.

[0053] Thus, for a cladding outer diameter D1 of 10 mm, it is possible to have a rod with a fissile column height H1 of 60 cm.

[0054] It is also possible to have a larger rod with a fissile column height H1 of 65 cm for a cladding outer diameter D1 of 13 mm.

[0055] Conversely, smaller rods with a fissile column height H1 of 55 cm can be considered for a cladding outer diameter D1 of 9 mm. These smaller rods may be intended for lower power reactor cores. For example, they may be for reactors with a thermal power of about 100 MW.

[0056] According to an advantageous embodiment, the upper neutron shielding (UNS) device is more or less 10 It is formed by a plate made of at least one neutron-absorbing material selected from among boron carbide enriched with B (B4C), metallic hafnium, refractory boride-based materials such as HfB2 and TiB2, europium hexaboride EuB6 or Eu2O3.

[0057] Thus, the present invention essentially provides four new features over known assemblies: - the presence of a sodium plenum above the rod bundle; - installation of a neutron absorbing device, preferably in the form of an absorbing plate; - a lower fissile column height, which is advantageously less than 60 cm; - a rod with a wider diameter, advantageously of about 1 cm or more and fabricating fuel assemblies for liquid metal cooled RNR reactors.

[0058] These four cumulative features thus enable the main deteriorating phenomena caused in the event of a severe accident, namely sodium drainage, steel drainage, and axial compression of the core, respectively, to be overcome.

[0059] So far, none of the proposed solutions have addressed the absence of any mechanical energy deposition throughout the accident sequence of an RNR reactor.

[0060] Some homogeneous cores have been proposed with assemblies having only three of the four stated characteristics. In particular, the height of the fissile column has always been maintained at excessively high values, usually above 70 cm, due to performance requirements (Pu content, reactivity conservation, control, etc.), and for the same reason, a reduction in this height is not associated with a reduction in power density due to the use of larger diameter rods.

[0061] Furthermore, although the implementation of sodium plenums has already been combined with neutron absorbing plates, it does not allow the problem of ULOF sequences in industrial power cores to be fully addressed. Thus far, the choice of those skilled in the art has necessarily involved the addition of supplementary and specific safety systems, which complicate the design and operation of the reactor.

[0062] A reduction in the fissile column height is known per se to improve accident behavior in ULOF sequences, but this has not been maintained for high power cores.

[0063] The choice to install rods with a wider diameter, and thus to reduce the sodium fraction in the assembly, still goes against the natural choice of a person skilled in the art who is performance-oriented, complicating other aspects (control, vessel sizing, etc.).

[0064] Therefore, increasing the fissile column height and reducing the rod diameter have not been used in high power cores with plenums and neutron absorbing plates until now.

[0065] Therefore, with an emphasis on safety, which has become the primary objective of current RNR cores, the inventors have overcome preconceptions by proposing a combination of four features not previously available to those skilled in the art.

[0066] Another advantage of the present invention is the simplification of the safety measures and therefore the sizing of the main components, in particular the reactor vessel.

[0067] Finally, the present invention relates to a nuclear power plant comprising a liquid metal-cooled fast neutron reactor, designated RNR-Na (or SFR), which, as previously mentioned, comprises a homogeneous core cooled in particular by liquid sodium and containing a plurality of nuclear fuel assemblies.

[0068] The present invention avoids the use of supplemental safety systems (passively triggered bars using pressure drops, fuses, etc.) which add to the complexity of the project.

[0069] Thus, according to the invention, the core of the reactor of the nuclear power plant may be free of supplementary safety devices aimed at mitigating the consequences of a ULOF type accident.

[0070] The present invention is applied to high power and therefore high capacity (typically 1m 3 It is possible to consider applying this to cores (larger than

[0071] Furthermore, the lower fissile height allows smaller assemblies to be provided which are easier to manufacture, transport and handle as they are supplied with less power.

[0072] Further advantages and features will become more apparent on reading the detailed description given by way of non-limiting example with reference to the following figures. [Brief explanation of the drawings]

[0073] [Figure 1] FIG. 1 is a perspective view of a prior art fuel assembly already in use in a sodium-cooled RNR-Na nuclear reactor. [Figure 2] FIG. 1 is a perspective view of a prior art fuel assembly conventionally used in an RNR-Na reactor. [Figure 2A] FIG. 3 is a longitudinal sectional view of the fuel assembly according to FIG. 2. [Figure 2B] FIG. 3 is a cross-sectional view of a bundle of rods of the fuel assembly according to FIG. 2. [Figure 3] Schematic top view of the heterogeneous core of the RNR-Na reactor, which is being considered within the scope of the ASTRID project. [Figure 3A] 4 is a schematic longitudinal half-section of an element as an "RZ" depiction of the heterogeneous core according to FIG. 3. [Figure 4] FIG. 1 is a top view schematic of the homogeneous core of the RNR-Na reactor. [Figure 4A] 5 is a schematic longitudinal half-section of the homogeneous core according to FIG. 4 as an "RZ" depiction. [Figure 5A] 1 is a schematic longitudinal cross-sectional view of a fuel assembly for an RNR-Na reactor according to the present invention; [Figure 5B] FIG. 5B is a cross-sectional view of a bundle of rods of a fuel assembly according to FIG. 5A. DETAILED DESCRIPTION OF THE INVENTION

[0074] For clarity, identical elements have been designated using identical numerical references according to the prior art and according to the present invention.

[0075] It should be noted that throughout this application, the terms "vertical," "lower," "upper," "low," "high," "below," and "above" should be understood with respect to the fuel assembly when it is in a vertical configuration within a nuclear reactor.

[0076] 1 through 4A, which relate to the prior art, have already been discussed above, and therefore will not be discussed below.

[0077] Unlike assemblies 1 having a longitudinal axis (X), such as those of the prior art described with reference to Figures 1 to 2B, the assembly 1 according to the invention also includes, as shown in Figures 5A and 5B, an intermediate section 18 between the fissile column 12 and the head of the assembly, which contains plates made of a neutron-absorbing material 17, such as B4C, and which defines a plenum volume intended to be filled with liquefied sodium.

[0078] Furthermore, each fuel rod 100 has an outer diameter Φ1 that is wider than the outer diameter Φ of the assembly of Figures 1 to 2B, which diameter Φ1 may be 10 mm or greater.

[0079] Furthermore, the height H1 of the fissile column 12 is smaller than the height H of the assembly of Figures 1 to 2B. This height H1 can be 60 mm or less.

[0080] Using the combination of these four features, a homogeneous RNR-Na core can be fabricated with multiple fuel assemblies 1 that result in no mechanical energy deposition during the accident sequence (primary and secondary stages).

[0081] Other variations and modifications are contemplated without departing from the scope of the present invention. (References) [1] Annual Report 2016, GEN IV International Forum pages 52-56. [Explanation of symbols]

[0082] 1 assembly, male assembly, fuel assembly, nuclear fuel assembly 1' fuel assembly 2 safety bars 3 Control Bar 10 Tubular enclosure, housing 11 (Housing / Assembly) Top, Assembly Head 12 (Housing / Collection) Center, Fissile Zone, Fissile Column 13 Lower part, base (of a housing / assembly) 14. ZFi Exclusive Fissure Column 15 (proximal) distal end 16 (Base) Opening 17 More or less porous elements, neutron absorbing materials 18 Middle section 100 rod bundles, fuel rods, rod cladding, nuclear fuel rod bundles, nuclear fuel pin bundles 110 (of the assembly head) central opening CE outer core part CFe, ZFe fertile column CHe heterogeneous core CHo homogeneous core CI inner core section D1 Cladding outer diameter FCI Liquefied Sodium H (Fissile Zone) Height H1 fissile column height RE Neutron Reflector PLE Liquefied Sodium Plenum X Longitudinal Axis ZA Neutron absorption zone Φ (Outer diameter of the assembly of Figures 1 to 2B) Φ1 (of the assembly in Figures 5A and 5B)

Claims

1. A nuclear fuel assembly (1) having a longitudinal axis (X), a bundle (100) of nuclear fuel rods, each rod comprising a cladding containing an exclusive fissile column (14) of stacked fuel pellets, the height of said fissile column being equal to or less than 65 cm, and the outer diameter of said cladding of said rods being equal to or greater than 9 mm; an assembly body comprising a housing (10) in the form of a hexagonal tube closed and sealed with respect to the heat transfer fluid intended to pass through said bundle of rods, the central part (12) of said housing surrounding said bundle of rods, while the upper part (11) forming the assembly head contains an upper neutron shielding (UNS) device filled with neutron absorbing material, said housing further comprising an intermediate part defining a plenum volume intended to be filled with said heat transfer fluid; a lower part which is an extension of the housing and forms the base of the assembly, the base being adapted to allow the heat transfer fluid to pass through the assembly; A nuclear fuel assembly (1).

2. 2. The assembly of claim 1, wherein the height of the fissile column is 60 cm or less.

3. The assembly of claim 1 , wherein the diameter of the cladding of the rod is greater than 10 mm.

4. the upper neutron shielding (UNS) device, １０ Boron carbide enriched with B (B ４ C) Metal hafnium, a refractory boride-based material, such as HfB ２ and TiB ２ , europium hexaboride EuB ６ or EU ２ O ３ 2. The assembly of claim 1, wherein the assembly is formed by plates made of at least one neutron absorbing material selected from the group consisting of:

5. A nuclear power plant comprising a liquid metal cooled fast neutron reactor, designated RNR-Na or SFR, in particular cooled with liquid sodium and comprising a homogeneous core containing a plurality of nuclear fuel assemblies according to any one of claims 1 to 4.

6. 6. The nuclear power plant of claim 5, wherein the core is free of supplemental safety devices intended to mitigate the consequences of an unprotected loss of flow (ULOF) accident.

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