Low height fission zone nuclear fuel assembly with widened fuel rods covered by liquid metal plenums and neutron absorbing plates and related liquid metal cooled FNR reactors

Abstract

The invention relates to a nuclear fuel assembly (1) comprising:-a bundle of nuclear fuel rods (100), each rod comprising a cladding housing a pure fission column (14), the fission column having a height of less than or equal to 65 cm, the cladding of the rods having an outer diameter of greater than or equal to 9 mm; -an assembly body comprising a hexagonal tubular housing (10) closed and sealed with respect to the heat transfer fluid for flowing through the rod bundle, a central portion (12) of the housing surrounding the rod bundle while an upper portion (11) forming the assembly head housing an upper neutron shielding (UNS) device filled with neutron absorbing material, the housing includes a middle portion defining a pressurized volume; forming a lower portion of a base of the assembly in the extension of the housing, the base being adapted to allow the passage of the heat transfer fluid flowing through the assembly.

Description

Technical Field

[0001] The present invention relates to a fuel assembly for a fast neutron nuclear reactor cooled by liquid metal (referred to as RNR-Na or SFR (sodium fast reactor)), which is cooled by liquid sodium in particular and is part of the fourth generation reactor series.

[0002] The fuel assembly targeted by the present invention can also be used in an integral nuclear reactor (i.e., the primary sodium circuit with pumping devices is completely housed in a container that also houses the heat exchanger), and can also be used in a loop-type nuclear reactor (i.e., the intermediate heat exchanger and the primary sodium pumping device are located outside the container).

[0003] "Fuel assembly" is understood to mean an assembly that includes fuel elements and is loaded into and / or unloaded from a nuclear reactor.

[0004] "Fuel assembly of RNR-Na or SFR type" should be understood to mean a fuel assembly suitable for being irradiated in a fast neutron nuclear reactor cooled by liquid sodium (referred to as RNR-Na or SFR).

[0005] "Hexagonal tube" should be understood to mean a tube with a regular hexagonal cross-section.

[0006] "Homogeneous core" should be understood to mean the core of a fast neutron reactor that only loads fissile fuel assemblies. In contrast, "heterogeneous core" means a core that loads one (or more) fully or partially fertile fuel assemblies.

[0007] Although described with reference to the main target application, namely the RNR-Na (sodium coolant) reactor, the present invention is applicable to any type of RNR cooled by liquid metal (lead, lead-bismuth, etc.). Background Art

[0008] Fuel assemblies intended for fast neutron reactors cooled by liquid sodium (RNR-Na) have a specific mechanical structure, especially to allow liquid sodium to pass through them.

[0009] Figures 1 to Figure 2 B show a fuel assembly 1 that is typically used for at least a part of an RNR-Na nuclear reactor.

[0010] This assembly 1 with an elongated shape along the longitudinal axis X first includes a tube or housing 10, which has a closed and sealed hexagonal cross-section on the periphery. Its upper part 11 forms the gripping head of the assembly and can accommodate an upper neutron shielding (UNS) device, and its central part 12 surrounds the fuel rods 100.

[0011] Parts 11 and 12 form the same tubular outer shell 10 or housing having the same hexagonal cross-section over its entire height. The assembly head 11 includes a central opening 110 that appears in the assembly head 11 and is used to operate the assembly head.

[0012] The central part 12 of the assembly includes a plurality of nuclear fuel rods. Each fuel rod 100 is in the form of a sealed cylindrical cladding tube made of steel and closed at both ends by welded plugs. Fission columns 14 composed of fuel pellets are stacked inside the fuel rod, and nuclear reactions that release heat occur inside the pellets. All columns 14 define a so-called fission zone, which is approximately located in the middle of the assembly 1. The height H of this fission zone 12 can be equal to 1 m. The outer diameter Φ of the rod can be about 9.5 mm. Therefore, the rod cladding 100 forms the first containment barrier, and its integrity must be strictly maintained by protecting it from external threats such as mechanical shock / stress or excessive temperature.

[0013] Finally, the assembly 1 includes a lower part 13 that forms the base of the assembly in the extension of the housing 10. The base 13 of the assembly has a distal end 15 in the form of a cone or a circle so that it can be vertically inserted into the struts of the (support) grid structure of the reactor core. The base 13 of the assembly includes openings 16 that appear on its periphery.

[0014] Therefore, when the fuel assembly is in the installed configuration, that is, in the loading position in the reactor core, the base 13 of the convex assembly 1 is inserted into the opening of the grid structure of the reactor, thereby holding the assembly 1 therein, where its longitudinal axis X is vertical. It should be noted that the grid structure is a caisson that forms a reservoir for pressurized primary sodium, and it distributes the pressurized primary sodium to all assemblies through the openings in the base 13.

[0015] The primary sodium can circulate inside the housing 10 of the assembly 1, thereby transferring the heat released by the fuel rods through heat conduction. Therefore, sodium is introduced through the opening 16 of the base 13 and flows out through the central opening 110 of the assembly head 11 after passing through the combustible rod bundle. In other words, as Figure 2 shown by the arrow in A, the flow rate of the sodium coolant enters the base 13 of the assembly through the opening 16, passes through the rod bundle 100 and UNS, and then flows out through the assembly head 11. The base 13 contains a system called a differential pressure system, which is formed by elements 17 that are more or less porous. These elements 17 generate a pressure drop and allow the regulation of the sodium flow through the assembly.

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

[0017] Components located on the grid structure are spaced apart from each other on their bodies, typically a few millimeters (mm) between the opposite faces of two adjacent hexagonal cross-section shells.

[0018] It should be noted that, as mentioned above, all RNR-Na reactors being studied, under construction, or in operation around the world use closed hexagonal tubes (TH) as components of the shell, and since the beginning of the nuclear industry in the 1960s, this type of component has become an industry standard.

[0019] Safety issues are related to the behavior that may occur in the core of high-power (usually greater than 1000 MW) RNR-Na reactors in the case of preventing and mitigating severe accidents (i.e., inducing partial or complete melting of the core).

[0020] In addition, starting from the design stage of the reactor, severe accidents are studied to ensure acceptable radioactive emissions.

[0021] Since RNR-Na reactors are not in the most reactive configuration during normal operation, in the case of a severe accident, the power will increase sharply. The melting of the core and the migration of the cladding or fuel may actually lead to the release of radioactive substances into the environment.

[0022] Therefore, the safety demonstration of RNR-Na reactors must prove that the reactor can operate normally in the event of a severe accident, and after such an accident, the reactor can recover and remain in a safe state.

[0023] To study severe accidents in RNR Na, the process of the accident is usually schematically divided into several stages as follows:

[0024] - Initiation stage, which starts when the triggering event occurs while the reactor is in normal operation and ends when the fuel rods start to deteriorate;

[0025] - Main stage, which starts when the fuel rods start to deteriorate and ends with the rupture of the first hexagonal tube (TH) of the components in the core. The characteristic of this stage is that the molten material in the deteriorated components mainly moves axially, where the core maintains its overall geometry;

[0026] - Transition stage, which corresponds to the loss of integrity of the TH, either due to the melting of the TH or due to the loss of their mechanical properties. This stage is actually the source point of the transition stage between the axial migration of the molten material in each component and the radial propagation of the deteriorated material between various components;

[0027] - Secondary stage, during which one or more large molten pools are formed in the deteriorated core, which may be the source point of a repeated critical state;

[0028] - Transfer and cooling phase, during which a part of the core material inventory is transferred to the molten material recycler, which has to be cooled.

[0029] The occurrence of a severe accident can be caused by different triggering events. For large (high-power) cores, studies have shown that, in the case of not dropping the emergency barrier rods (ULOF (Unprotected Loss of Flow)), the sequence involving the loss of primary coolant flow will be used as the reference triggering event.

[0030] The aim set by the inventors is to design an RNR-Na reactor core which, in the case of a severe accident (i.e., large-scale melting of the fuel), does not lead to mechanical energy deposition, i.e., does not release mechanical energy that could damage or impair the integrity of the radioactive containment barrier, especially for the second barrier (the primary vessel).

[0031] This mechanical energy can originate from:

[0032] - The energy interaction between the molten fuel and the liquid primary sodium (this is a phenomenon called FCI (Fuel-Coolant Interaction)), which is the source point of the sudden vaporization of the liquid sodium and the propagation of pressure waves in the primary circuit of the reactor;

[0033] - The vaporization of the molten fuel, which is related to the sudden increase in reactor power caused by the runaway of the nuclear chain reaction (e.g., due to sodium ejection, steel ejection, or axial compaction of the reactor core).

[0034] In both cases, the resulting pressure waves can have an adverse effect on the integrity of the primary vessel and the plate covering the vessel, which are important components of the second containment barrier for radioactive products.

[0035] This aim can be achieved by implementing a heterogeneous core, which, by design, will limit the level of mechanical energy released. This document can refer to the heterogeneous core called the low void effect (CFV) core, or even to the heterogeneous core described in patent application FR 2961337.

[0036] The main specific feature of the heterogeneous core is that, unlike the homogeneous core, the neutron inverse reaction associated with the expansion or ejection of sodium in the core is usually very low, or even negative. Therefore:

[0037] - The startup phase of the heterogeneous core is much longer than that of the homogeneous core. In the case of boiling, this does not lead to a surge in the primary power or the release of mechanical energy associated with the fuel expansion caused by this surge, but rather to a decrease in the total power of the core. Depending on the power conditions, this boiling can even stabilize in the upper structure, in which case the accident sequence stops even before the core melts;

[0038] - If the core melts, its power is much lower than that of a homogeneous core, and this melting occurs much later. Therefore, the possibility of interaction between the molten fuel and liquid sodium (FCI) is greatly reduced because, unlike what happens in a homogeneous core, when the fuel melts, the sodium has already vaporized.

[0039] Regardless of the type of core, the management of molten fuel (control of reactivity, cooling) can be expected.

[0040] Figure 3 and Figure 3A shows the heterogeneous core CHe of the CFV type envisaged in the ASTRID reactor project.

[0041] The heterogeneous core CHe basically consists of three parts: an inner core part CI, which is surrounded by an outer core part CE, and the outer core part CE itself is surrounded by a neutron reflector RE.

[0042] The inner core part CI includes fuel assemblies 1' in the fission zone, which is covered by a breeding zone and covers another breeding zone.

[0043] The outer core part CE only includes the fuel assemblies 1 as shown in Figures 1 to Figure 2 B in the fission zone 12.

[0044] In addition, safety rods 2 and control rods 3 are installed inside the inner core part CI.

[0045] All fuel assemblies 1, 1' define a fission column ZFi that is pure fission in the outer core part CE, as well as a stratified ZFi fission column and CFe, ZFe breeding columns. Generally, the height of the fission column ZFi in the outer core part CE is 90 cm, while the cumulative height of the two fission columns ZFi and the breeding column Zfe between the two is 80 cm.

[0046] As shown in the figure, the heterogeneous core includes a reflector RE that exists around and below parts CE and CI, a neutron absorption zone ZA that exists below both parts CE and CI, and a liquid sodium pressurization chamber PLE, where a part of the pressurization chamber is on the inner side of part CE.

[0047] The heterogeneous core CH poses new problems, especially due to the need for intermediate breeding plates in each assembly 1' to define the breeding column ZFe. These problems particularly involve the manufacturability of the fuel columns (fission columns and breeding columns), the thermo-mechanical strength during the accident sequence, etc.

[0048] To achieve the above object, instead of considering a heterogeneous core, additional systems can be added. Thus, passively triggered safety rods can be used to allow the insertion of neutron absorbers after a threshold is exceeded. For example, for a hydraulically triggered rod, the threshold can be a flow rate of less than 40%, and for a thermal fuse, the threshold can be a temperature greater than 650 °C. Devices activated depending on the inlet pressure of the core also allow the free level of the absorber liquid material (such as lithium) to move. However, adding such systems will necessarily make the entire reactor more complex.

[0049] Figure 4 and Figure 4A shows a reactor core, called a homogeneous reactor core CHo.

[0050] Similar to the heterogeneous core CH, the homogeneous core CHo also includes an inner core part CI surrounded by an outer core part CE and a neutron reflector RE surrounding the part CE, and the neutron reflector RE also exists below and above the two parts CE and CI.

[0051] These two parts, namely the inner core IC and the outer core CE, only include the fuel assemblies 1 having only the fission zones 12 as shown in FIGS. 1 to Figure 2 B.

[0052] For the homogeneous core CHo envisaged so far, it has not been possible to demonstrate that there is no mechanical energy deposition during the accident sequence. In fact, the deposited mechanical energy is calculated under "best estimate" conditions (i.e., by following a reference method considered to be in the field of studying accidents involving large-area melting of the core, which does not consider adverse uncertainties in transient studies), and then it is compared with the design threshold (for example, equal to 800 MJ for the Superphénix reactor), which is evaluated in the context of a specific and comprehensive safety method. In this method, adverse uncertainty factors are considered to ensure that all possible variations of such transient studies can be covered.

[0053] The homogeneous core currently envisaged cannot achieve the above object of having no mechanical energy deposition during the entire accident sequence.

[0054] Therefore, there is a need to further improve the liquid metal cooled RNR nuclear reactor, especially in order to achieve the object of having no mechanical energy deposition that may damage the structure during the entire accident sequence (primary and secondary stages), and to achieve this while avoiding any problems that occur in heterogeneous cores, without having to use additional systems.

[0055] The object of the present invention is to meet this need. Summary of the Invention

[0056] To this end, the present invention relates to a nuclear fuel assembly having a longitudinal axis, comprising:

[0057] - a bundle of nuclear fuel rods, each rod comprising a cladding that houses a pure fission column made up of stacked fuel pellets, the height of the fission column being less than or equal to 65 cm, and the outer diameter of the cladding of the rod being greater than or equal to 9 mm;

[0058] - a component body, the component body comprising a hexagonal tubular housing that is closed and sealed with respect to a heat transfer fluid for flowing through the rod bundle, a central portion of the housing surrounding the rod bundle, and an upper portion forming a component head accommodating an upper neutron shielding device filled with a neutron absorbing material, the housing further comprising an intermediate portion defining a pressurized volume for being filled with the heat transfer fluid;

[0059] - a lower portion forming a base of the component in an extension of the housing, the base being adapted to allow the heat transfer fluid flowing through the component to pass through.

[0060] Each rod houses a single pure fission column made up of fuel pellets.

[0061] The height of the fission column may be less than or equal to 60 cm.

[0062] The cladding diameter of the rod may be greater than 10 mm.

[0063] Multiple rod sizes may be considered within the scope of the present invention.

[0064] Thus, for a cladding outer diameter D1 of 10 mm, rods with a fission column height H1 of 60 cm may be provided.

[0065] For a cladding outer diameter D1 of 13 mm, larger-sized rods with a fission column height H1 of 65 cm may also be provided.

[0066] In contrast, for a cladding outer diameter D1 of 9 mm, smaller-sized rods with a fission column height H1 of 55 cm may be considered. These smaller-sized rods may be used in the core of a lower-power reactor. For example, they may be reactors with a thermal power of approximately 100 MW.

[0067] According to an advantageous embodiment, the upper neutron shielding (UNS) device is formed by a plate made of at least one neutron absorbing material selected from: boron carbide (B 4 C) (more or less enriched 10 B), hafnium metal, refractory boride-type materials (such as HfB 2 and TiB 2 ), europium hexaboride EuB 6 or Eu 2 0 3 .

[0068] Accordingly, the present invention mainly relates to the production of fuel assemblies for RNR reactors cooled by liquid metal, which assemblies have a combination of four new features with respect to known assemblies, namely:

[0069] - the presence of a sodium plenum above the rod bundle;

[0070] - the installation of a neutron absorption device, preferably in the form of an absorption plate;

[0071] - a lower fissile column height, advantageously less than or equal to 60 cm;

[0072] - rods with a wider diameter, advantageously about 1 cm or greater.

[0073] Thus, these four cumulative features allow the main aggravating phenomena encountered in severe accident situations to be overcome, namely sodium loss, steel loss, and axial compaction of the reactor core, respectively.

[0074] To date, none of the solutions proposed have addressed the problem of there being no mechanical energy deposition throughout the accident sequence of RNR reactors.

[0075] Some homogeneous cores have been proposed, the assemblies of which have only three of the four said features. In particular, the height of the fissile column has always been kept at too high a value, usually above 70 cm; due to performance requirements (Pu content, reactivity reserve, control, etc.), and for the same reason, the reduction of the fissile column height has never been achieved by using rods with a large diameter, and thus the power density has not been reduced.

[0076] In addition, although the implementation of the sodium plenum has been combined with neutron absorption plates, it does not allow the problem of the ULOF sequence of industrial power cores to be fully solved. Thus, to date, the choices of those skilled in the art have necessarily involved adding additional and specific safety systems, which complicate the design and operation of the reactor.

[0077] The reduction of the fissile column height is known per se to improve the accident performance in the ULOF sequence, but this method has never been adopted for high-power cores.

[0078] The choice to install rods with a wider diameter, thus reducing the proportion of sodium in the assembly, is still contrary to the conventional choices of those skilled in the art who focus on performance, which complicates other aspects (control, container size design, etc.).

[0079] Thus, to date, for high-power cores with a plenum and neutron absorption plates, increasing the fissile column height and reducing the rod diameter have never been used.

[0080] Thus, by focusing on safety, which has become the main objective of the current RNR reactor core, the inventors have overcome the prejudice by proposing a combination of four features that would not have been conceivable by a person skilled in the art before.

[0081] Another advantage of the present invention is that it simplifies the safety method, thereby simplifying the sizing of the main components, in particular the reactor vessel.

[0082] Finally, the present invention relates to a nuclear power plant that includes a fast neutron nuclear reactor cooled by liquid metal, in particular by liquid sodium, called RNR-Na (or SFR), and includes a homogeneous core that houses a plurality of nuclear fuel assemblies as described above.

[0083] The present invention avoids the use of additional safety systems (rods, fuses, etc. triggered passively using pressure drop) that would increase the complexity of the project.

[0084] Thus, according to the present invention, the nuclear core of the reactor of a nuclear power plant may not be provided with additional safety devices for mitigating the consequences of ULOF-type accidents.

[0085] It is conceivable to apply the present invention to cores with high power and thus large volume (usually greater than 1 m 3 ).

[0086] Furthermore, the lower fission height allows for the provision of smaller components that are easier to manufacture, transport, and operate because of their lower power.

[0087] Further advantages and features will become more apparent after reading the detailed description provided by way of non-limiting illustration with reference to the following drawings. BRIEF DESCRIPTION OF THE DRAWINGS

[0088] [FIG. 1] FIG. 1 is an external perspective view of a fuel assembly according to the prior art, which has been used in a sodium-cooled RNR-Na nuclear reactor.

[0089] Figure 2 Figure 2 is a perspective view of a fuel assembly according to the prior art, which is commonly used in an RNR-Na nuclear reactor.

[0090] Figure 2 A] Figure 2 A is a longitudinal sectional view of a fuel assembly according to Figure 2 .

[0091] Figure 2 B] Figure 2 B is a cross-sectional view of a rod bundle of a fuel assembly according to Figure 2 .

[0092] Figure 3 Figure 3 ​​​​​​It is a top-down schematic view of the heterogeneous core of the RNR-Na reactor envisioned within the scope of the ASTRID project.

[0093] Figure 3A Figure 3A is a schematic longitudinal half-section view of the elements of the heterogeneous core according to Figure 3 represented in "R-Z" coordinates.

[0094] Figure 4 Figure 4 It is a top-down schematic view of the homogeneous core of the RNR-Na reactor.

[0095] Figure 4A Figure 4A is a schematic longitudinal half-section view of the homogeneous core according to Figure 4 represented in R-Z coordinates.

[0096] Figure 5A Figure 5A is a schematic longitudinal sectional view of the fuel assembly of the RNR-Na reactor according to the present invention.

[0097] Figure 5B Figure 5B is a cross-sectional view of the rod bundle of the fuel assembly according to Figure 5A Specific embodiments For clarity, the same numerical reference signs are used to denote the same elements according to the prior art and according to the present invention.

[0098] It should be noted that throughout the application, the terms "vertical", "lower", "upper", "low", "high", "below" and "above" should be understood with reference to the fuel assembly in a vertical configuration within the nuclear reactor.

[0099] The figures 1 to

[0100] related to the prior art have been discussed in the background art. Figure 4A Therefore, they will not be discussed further below.

[0101] Unlike the assembly 1 having a longitudinal axis (X) (such as the assembly 1 of the prior art described with reference to FIGS. 1 to Figure 2 B), as Figure 5A and Figure 5B shown, the assembly 1 according to the present invention further houses a plate made of neutron-absorbing material 17 (such as B4C) between the fission column 12 and the assembly head, and includes an intermediate portion 18 that defines a pressurized volume for filling with liquid sodium.

[0102] Furthermore, unlike FIGS. 1 to Figure 2 ​​​​​​​​​​Compared with the outer diameter Φ of the components of B, the outer diameter Φ1 of each fuel rod 100 is wider. This diameter Φ1 can be 10 mm or more.

[0103] In addition, the height H1 of the fission column 12 is less than the height H of the components of B. This height H1 can be 60 mm or less. Figure 2 By combining these four features, a homogeneous RNR-Na reactor core can be produced using multiple fuel assemblies 1, which has no mechanical energy deposition during the accident sequence (primary and secondary phases).

[0104] Other variations and improvements can be envisaged without departing from the scope of the present invention.

[0105] List of cited references

[0106] Annual Report 2016, GEN IV International Forum pages 52 - 56 (2016).

[0107] [1]Annual Report 2016, GEN IV International Forum pages 52 - 56 (Annual Report 2016, Fourth Generation International Forum, pages 52 - 56).

Claims

1. A nuclear fuel assembly (1) having a longitudinal axis (X), comprising: - a bundle of nuclear fuel rods (100), each rod including a cladding that houses a pure fission column (14) formed by stacked fuel pellets, the height of the fission column being less than or equal to 65 cm, and the outer diameter of the cladding of the rod being greater than or equal to 9 mm; - an assembly body that includes a hexagonal tubular housing (10) that is closed and sealed with respect to a heat transfer fluid for flowing through the rod bundle, a central portion (12) of the housing surrounding the rod bundle, and an upper portion (11) forming the head of the assembly accommodating an upper neutron shield (UNS) device filled with a neutron absorbing material, the housing further including an intermediate portion that defines a pressurized volume to be filled with the heat transfer fluid; - a lower portion forming the base of the assembly in the extension of the housing, the base being adapted to allow the heat transfer fluid flowing through the assembly to pass therethrough.

2. The assembly according to claim 1, wherein, the height of the fission column is less than or equal to 60 cm.

3. The assembly according to claim 1 or 2, wherein, the diameter of the cladding of the rod is greater than 10 mm.

4. The assembly according to any one of the preceding claims, wherein, The upper neutron shield (UNS) device is formed of plates made of at least one neutron absorbing material selected from: enriched 10 boron carbide (B 4 C) enriched with ; hafnium metal; refractory boride type materials such as HfB 2 and TiB 2 ; europium hexaboride EuB 6 or Eu 2 0 3 .

5. A nuclear power plant that includes a fast neutron nuclear reactor cooled by a liquid metal, especially liquid sodium, called RNR-Na or SFR, and includes a homogeneous core that houses a plurality of nuclear fuel assemblies according to any one of the preceding claims.

6. The nuclear power plant according to claim 5, wherein, the nuclear core is not configured with additional safety devices for mitigating the consequences of an unprotected loss of flow (ULOF) accident.

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