FR3140980A1 - Nuclear fuel assembly with fissile zone of lower height with enlarged needles, topped with a liquid metal plenum and a neutron absorbing plate, FNR reactor cooled by associated liquid metal. - Google Patents

Abstract

Nuclear fuel assembly with fissile zone of lower height with enlarged needles surmounted by a heat transfer fluid plenum and a neutron absorbing plate, associated FNR-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 at 65 cm, the external diameter of the needle sheath being greater than or equal to 9 mm, - an assembly body comprising a housing (10) in the form of a closed hexagonal tube and sealed to a heat transfer liquid intended to pass through the beam, the central portion (12) of the housing enveloping the beam, while the upper portion (11) forming the assembly head houses a superior 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 abstract: Fig. 5A

Description

Nuclear fuel assembly with fissile zone of lower height with enlarged needles, topped with a liquid metal plenum and a neutron absorbing plate, FNR reactor cooled by associated liquid metal.

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

The fuel assemblies covered by the invention can also be used in a nuclear reactor of the integrated type, that is to say for which the primary sodium circuit with pumping means is completely contained in a tank also containing exchangers of heat, than in a loop type reactor, that is to say for which the intermediate heat exchangers and the means for pumping the primary sodium are located outside the tank.

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

By “RNR-Na or SFR type fuel assembly” is meant a fuel assembly adapted to be irradiated in a fast neutron nuclear reactor cooled with liquid sodium called RNR-Na or SFR.

By “hexagonal tube” we mean a tube whose cross section is regular hexagonal.

By “homogeneous core”, we mean 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.

Although described with reference to the main intended application, namely an FNR-Na reactor (sodium coolant), the invention applies to any type of FNR cooled by liquid metal (lead, lead-bismuth, etc.) .

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

We show in Figures 1 to 2B, a fuel assembly 1 conventionally used in at least part of an FNR-Na nuclear reactor.

Such an assembly 1 of elongated shape along a longitudinal axis a superior neutron protection device (PNS), the central portion of which 12 envelops fuel needles 100.

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.

The central portion 12 of an assembly comprises a plurality of nuclear fuel needles. Each needle 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 reactions take place. nuclear weapons that release heat. All columns 14 define what is usually called the fissile zone which is approximately located halfway up an assembly 1. This fissile zone 12 can have a height H equal to 1 m. The external diameter Φ of the needles can be around 9.5mm. The sheath of the needles 100 thus constitutes the first containment barrier whose integrity it is very important to preserve by protecting it from external attacks such as mechanical shocks/constraints or excessive temperatures.

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 in the candles of the bed base (support) of a reactor core. The foot 13 of the assembly has at its periphery openings 16 opening into it.

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 base of the reactor in thus maintaining the assembly 1 in the latter with its longitudinal axis X vertical. It should be noted that the bed base is a box forming a primary sodium reservoir under pressure which it distributes to all of the assemblies via the openings in foot 13.

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 needles. 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 passing through the bundle of fuel needles. In other words, as symbolized by the arrows in , the flow of sodium heat transfer 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 integrates within it a so-called deprimogenic system, constituted by a more or less porous element 17 which generates pressure losses and makes it possible to adjust the flow of sodium passing through the assembly.

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

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.

It should be noted that all FNR-Na reactors under study, construction or operation throughout the world use assemblies having a closed hexagonal tube (TH) for casing, as described above, which has constituted the reference since the beginning. of this nuclear sector in the 1960s.

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

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

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

Consequently, the safety demonstration of an FNR-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 brought back and maintained in a safe state.

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

- 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 pins;

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

- the transition phase, which corresponds to the loss of integrity of the TH, resulting either from their fusion or from their loss of mechanical properties. This phase is in fact the site of a transition between the axial relocation of the melted materials in each assembly, and the radial propagation of the degraded materials between the different assemblies;

- the secondary phase, during which one or more large molten pools form in the degraded core, which may be the site of repeated criticalities;

- the relocation and cooling phase during which part of the inventory of core materials is relocated to the molten material recoverer, the cooling of which must be ensured.

The occurrence of a serious accident can result from different initiating events. For large cores (high power), studies have led to retaining as a reference initiator the sequence of loss of primary flow without falling emergency stop bars (Anglo-Saxon acronym ULOF for “Unprotected Loss Of Flow). ").

An objective that the inventors have set for themselves is to design an FNR-Na reactor core which, in the event of a serious accident, that is to say widespread melting 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 tank).

• l’interaction énergétique entre le combustible fondu et le sodium primaire liquide (phénomène appelé FCI, acronyme anglo-saxon pour « Fuel-Coolant Interaction »), interaction à l’origine d’une vaporisation soudaine du sodium liquide et de la propagation d’une onde de pression dans le circuit primaire du réacteur ;
• la vaporisation du combustible fondu, liée à une augmentation soudaine de la puissance du réacteur pour des raisons d’emballement de la réaction nucléaire en chaîne, par exemple du fait de la vidange du sodium, la vidange d’acier ou la compaction axiale du cœur de réacteur.

This mechanical energy can come from:

• the energetic interaction between the molten fuel and the liquid primary sodium (phenomenon called FCI, Anglo-Saxon acronym for “Fuel-Coolant Interaction”), interaction causing a sudden vaporization of the liquid sodium and the propagation of a pressure wave in the primary circuit of the reactor;
• the vaporization of molten fuel, linked to a sudden increase in reactor power for reasons of runaway nuclear chain reaction, for example due to sodium draining, steel draining or axial compaction of the core reactor.

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 essential part of the second containment barrier for radiological products.

This objective can be solved by implementing a heterogeneous core which will, by design, limit the level of mechanical energy released. We can cite here the heterogeneous cores known under the name “Low Emptying Effect Core” (CFV) or the one described in patent application FR2961337.

• la phase d’initiation est beaucoup plus longue pour les coeurs hétérogènes que pour les coeurs homogènes. En cas d’ébullition, celle-ci n’entraîne pas d’excursion de puissance primaire,ni de libération de l’énergie mécanique associée à la détente du combustible qui découle de cette excursion, mais fait au contraire chuter la puissance totale du cœur. Selon les conditions de puissance, cette ébullition peut même se stabiliser dans les structures supérieures, auquel cas la séquence accidentelle s’arrête avant même la fusion du cœur ;
• si la fusion du cœur se produit, la puissance est largement plus faible que pour un cœur homogène, et cette fusion est beaucoup plus tardive. La possibilité d’interaction entre combustible fondu et sodium liquide (FCI) est donc largement diminuée, puisque le sodium est déjà vaporisé au moment de la fusion du combustible, contrairement à ce qui se passe dans un cœur homogène.

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. Consequently :

• the initiation phase is much longer for heterogeneous hearts than for homogeneous hearts. In the event of boiling, this does not cause a primary power excursion, nor a release of the mechanical energy associated with the expansion of the fuel which results 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 higher structures, in which case the accidental sequence stops even before the core melts;
• if core fusion occurs, the power is much lower than for a homogeneous core, and this fusion takes place 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 fusion, unlike what happens in a homogeneous core.

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

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

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.

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

The external core part CE comprises fuel assemblies 1 with only fissile zone 12 like those illustrated in Figures 1 to 2B.

Furthermore, safety bars 2 and control bars 3 are installed within the internal core part CI.

The entirety of the fuel assemblies 1, 1' defines exclusively 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 outer core part CE is equal to 90 cm, while the cumulative height of the two fissile columns ZFi and of the fertile column Zfe between the two is equal to 80 cm.

As shown, the heterogeneous core includes a reflector RE present around and on the underside of both the CE and CI parts, a neutron absorber zone ZA and a liquid sodium plenum PLE present on the underside of both the parts CE and CI, with a portion of the plenum on the internal side of the part CE.

Heterogeneous CH cores generate new problems, in particular due to 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, etc.

To achieve the desired objective, instead of considering heterogeneous cores, additional systems can be added. It is thus possible to use passively triggered safety bars, which allow neutron absorbers to be inserted following a threshold being exceeded. For example, the threshold can be a flow rate less than 40% for a hydraulically triggered bar, a temperature greater than 650°C for a thermal fuse. Devices whose actuation depends on the inlet pressure of the heart also make it possible to move free levels of absorbent liquid materials, such as lithium. However, the addition of such systems necessarily brings complexity to the reactor as a whole.

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

Like a heterogeneous core CH, a homogeneous core CHo also includes an internal core part CI, surrounded by an external 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.

The two parts, internal core CI and external core CE exclusively comprise fuel assemblies 1 with only fissile zone 12 like those illustrated in Figures 1 to 2B.

For homogeneous CHo cores, as they are currently considered, 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. following a methodology considered to be the reference in the field of the study of 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 as part of a specific safety approach and envelope. In this approach, we consider penalizing uncertainties in order to ensure that we encompass all of the possible variability of such a transient.

Homogeneous cores as currently envisaged cannot achieve the aforementioned objective of absence of mechanical energy deposition during the entire accident sequence.

There is therefore a need to further improve nuclear reactors of the FNR type cooled by liquid metal, in particular in order to achieve the objective of no deposit of mechanical energy damaging to the structures during an entire accident sequence (primary and secondary phase). ) and this, by overcoming the problems of heterogeneous cores, and by not having to resort to additional systems.

The aim of the invention is to meet this need.

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

- a bundle of nuclear fuel needles, each needle comprising a sheath 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 sheath of the needles being greater than or equal to 9 mm,

- an assembly body comprising a housing 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 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 absorbent 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.

The height of the fissile column can be less than or equal to 60cm.

The needle sheath diameter can be greater than 10mm.

Several dimensions of needles can be considered in the context of the invention.

We can thus have needles with a height H1 of fissile column equal to 60 cm for an external diameter D1 of sheath equal to 10mm.

We can also have larger needles with a height H1 of fissile column equal to 65 cm for an external diameter D1 of sheath equal to 13mm.

Conversely, we can consider smaller needles with a height H1 of fissile column equal to 55 cm for an external diameter D1 of cladding equal to 9mm. These less important needles can be dedicated to lower power reactor cores. For example, these may be reactors with a power of around 100 MW thermal.

According to an advantageous embodiment, the superior neutron protection device (PNS) consists of a plate made of at least one neutron absorption material chosen from boron carbide (B ₄ C), more or less enriched in 10B, metallic hafnium, a refractory boride type material, for example HfB ₂ and TiB ₂ , europium hexaboride EuB ₆ or Eu ₂ 0 ₃ .

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

- the presence of a sodium plenum above the needle bundle;

- the installation of a neutron absorption device, preferably in the form of an absorbent plate;

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

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

These four cumulative characteristics thus make it possible to oppose the main aggravating phenomena encountered in serious accidents, namely respectively the draining of sodium, the draining of steel and the axial compaction of the reactor core.

Until now, no solution proposed could address the absence of mechanical energy deposition during the entire accident sequence of an FNR reactor.

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 maintained 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 power density via the use of needles large diameter.

Furthermore, if 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 has therefore necessarily focused on the addition of complementary and specific safety systems which complicate the design and operation of the reactor.

Reducing the height of the fissile column is known in itself to allow an improvement in accidental behavior in ULOF sequences, but this has never been used for high-power cores.

The choice to install needles with an enlarged diameter and therefore to reduce the sodium fraction 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.).

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

Thus, by focusing on safety, which has become the priority objective of current FNR reactor cores, the inventors have overcome a prejudice by proposing a combination of the four characteristics that a person skilled in the art had until then prohibited.

Another advantage of the invention is the simplification of the safety approach and therefore of the sizing of primary components, in particular of the reactor vessel.

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

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

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.

We can envisage applying the invention to a core of high power and therefore of large volume, typically greater than 1 m ³ .

In addition, a lower fissile height makes it possible to have smaller assemblies, easy to manufacture, transport and handle because of lower power.

Other advantages and characteristics will become clearer on reading the detailed description, given for illustrative and non-limiting purposes, with reference to the following figures.

there is an external perspective view of a fuel assembly according to the state of the art, already used in a sodium-cooled nuclear reactor FNR-Na.

there is a perspective view of a fuel assembly according to the state of the art, conventionally used in an FNR-Na nuclear reactor.

there is a longitudinal sectional view of the fuel assembly according to the .

there is a cross-sectional view at the level of the needle bundle of the fuel assembly according to the .

there is a schematic top view of a heterogeneous core of an FNR-Na reactor, as it was envisaged within the framework of the ASTRID project.

there is a schematic view in longitudinal half-section of elements, in so-called “RZ” representation, of the heterogeneous core according to the .

there is a schematic top view of a homogeneous core of an FNR-Na reactor.

there is a schematic view in longitudinal half-section, in RZ representation, of the homogeneous heart according to the .

there is a schematic view in longitudinal section of a fuel assembly for an FNR-Na reactor according to the invention.

there is a cross-sectional view at the level of the needle bundle of the fuel assembly according to the .

detailed description

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.

It is specified that throughout the application, the terms “vertical”, “lower”, “upper”, “lower”, “high”, “below” and “above” are to be understood with reference to a fuel assembly such that it is in a vertical configuration in a nuclear reactor.

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

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

In addition, each fuel needle 100 has an external diameter Φ1 enlarged compared to that Φ of an assembly of Figures 1 to 2B. This diameter Φ1 can be equal to 10 mm or more.

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

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

Other variants and improvements can be considered without departing from the scope of the invention.

List of cited references

[1] Annual Report 2016, GEN IV International Forum pp 52-56.

Claims (6)

Nuclear fuel assembly (1) with longitudinal axis (X) comprising:
- a bundle of needles (100) of nuclear fuel, 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 needle sheath 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 a superior neutron protection device (PNS) filled with neutron absorbent material, the housing further comprising an intermediate portion defining a plenum volume intended to be filled with the liquid heat carrier,
- 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. Assembly according to claim 1, the height of the fissile column being less than or equal to 60cm. Assembly according to claim 1 or 2, the needle sheath diameter being greater than 10mm. Assembly according to one of the preceding claims, the superior neutron protection device (PNS) consisting of a plate made of at least one neutron absorption material chosen from boron carbide (B ₄ C), enriched in ¹⁰ B, metallic hafnium, a refractory boride type material, for example HfB ₂ and TiB ₂ , europium hexaboride EuB ₆ or Eu ₂ 0 ₃ . 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. Nuclear installation according to claim 5, the nuclear core being free of additional safety devices dedicated to mitigating the consequences of a ULOF accident.