Solid interface joint with open pores for nuclear control rod

Abstract

A new interface between the cladding and the stack of pellets in a nuclear control rod. According to the invention, an interface joint made of a material transparent to neutrons, in the form of a structure with a high thermal conductivity and open pores, adapted to deform by compression across its thickness, is inserted between the cladding and the stack of pellets made of B.sub.4C neutron absorber material over at least the height of the stack. The invention also relates to associated production methods.

Claims

1. A nuclear control rod extending along a longitudinal direction (XX), comprising a plurality of pellets, made of boron carbide B.sub.4C neutron absorber material, stacked on each other in the form of a column and a cladding surrounding the column of pellets, in which, the cladding and the pellets have a circular cross-section transverse to the longitudinal direction (XX), and in which an interface joint, also with a circular cross-section transverse to the longitudinal direction (XX), made of a material transparent to neutrons and is inserted between the cladding and the column of stacked pellets, at least over the height of the column, in which the interface joint is a structure, mechanically decoupled from the cladding and from the column of pellets, with a high thermal conductivity and open pores, adapted to deform by compression across its thickness so as to be compressed under the effect of the three-dimensional swelling of the pellets under irradiation, the initial thickness of the joint and its compression ratio being such that the mechanical load transmitted to the cladding by the pellets under irradiation remains less than a predetermined threshold value characterized in that the interface joint is made from a braid comprising a plurality of carbon fibre layers and a plurality of layers comprising silicon carbide fibres superposed on the carbon fibre layers.

2. The nuclear control rod according to claim 1, in which the open pores of the interface joint have a volume equal to at least 30% of the total volume of the interface joint as produced in fabrication.

3. The nuclear control rod according to claim 2, in which the open pores of the interface joint have a volume between 30% and 95% of the total volume of the interface joint as produced in fabrication.

4. The nuclear control rod according to claim 3, in which the open pores of the interface joint have a volume between 50% and 85% of the total volume of the interface joint as produced in fabrication.

5. The nuclear control rod according to claim 1, in which the thickness of the interface joint in its section transverse to the (XX) direction is more than at least 10% of the radius of the pellets.

6. The nuclear control rod according to claim 1, in which the interface joint has a volume percentage of fibres between 15 and 50% and the open pores of the interface joint have a volume between 50% and 85% of the total volume of the interface joint as produced in fabrication.

7. The nuclear control rod according to claim 1, for a gas-cooled fast reactor (GFR), in which the basic material of the cladding is a refractory ceramic matrix composite (CMC) and the absorber pellets are made of B.sub.4C.

8. The nuclear control rod according to claim 1, for a sodium-cooled fast reactor (SFR), in which the cladding is made of a metallic material, and the absorber pellets are made of B.sub.4C.

9. The nuclear control rod according to claim 1, for Pressurized Water Reactors (PWR), or Boiling Water Reactors (BWR) in which the cladding comprises a refractory ceramic matrix composite CMC material and the absorber pellets are made of B.sub.4C.

10. Nuclear absorber assembly comprising a plurality of the nuclear control rods according to claim 1, and arranged to form a lattice.

11. A control rod according to claim 7, wherein the refractory ceramic matrix composite is SiCSiC.sub.F.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) Other advantages and characteristics of the invention will become clear after reading the detailed description of a nuclear control rod according to the invention with reference to FIGS. 1 and 1A below among which:

(2) FIG. 1 is a partial longitudinal cross-sectional view of a nuclear control rod according to the invention;

(3) FIG. 1A is a cross-sectional view of the nuclear control rod according to FIG. 1;

(4) FIG. 2 shows cyclic compression tests of an interface joint according to the invention in the form of curves, this load mode being representative of operation under irradiation in a nuclear reactor (non-stationary due to power variations).

DETAILED DESCRIPTION OF PARTICULAR EMBODIMENTS

(5) Note that the element shown is a nuclear control rod. This element is shown cold, in other words once the final control rod has been fabricated and before use in a nuclear reactor.

(6) The control rod according to the invention comprises the following from the outside to the inside:

(7) cladding 1 made of a metallic or CMC (ceramic matrix composite) material(s), possibly coated with a liner on its internal wall,

(8) a first assembly set 2 (optional), to the extent that it may possibly be eliminated during fabrication following the binder evaporation process described above),

(9) a solid joint 3 with open pores according to the invention;

(10) a second assembly set 4 (optional, to the extent that it can possibly be eliminated during fabrication following the binder evaporation process described above);

(11) a stack of pellets 5 of neutron absorbing boron carbide B.sub.4C material forming a column.

(12) The solid joint with open pores 3 according to the invention has a height greater than the height of the column of stacked pellets 5. The difference in height between the porous solid joint 3 and the column of stacked pellets is chosen to assure that this column remains axially facing the joint throughout the irradiation phase during operation of the nuclear reactor during which its length increases due to swelling under irradiation. Thus according to [8], the absorber in the SUPERPHENIX reactor targets functioning with 10.sup.22 captures per cm.sup.3 of absorber and per year, and the elongation rate due to swelling of B.sub.4C is of the order of 0.05% for 10.sup.20 captures per cm.sup.3 of absorber, giving an elongation of the order of 5% per year of irradiation.

(13) Several types of materials may be suitable for fabrication of the porous solid joint 3 according to the invention, and advantageously fibrous structures possibly with a matrix deposited in these structures, or honeycomb materials with open pores.

(14) Fibrous structures that may be suitable include braids, felts, webs, fabrics or knits, or a combination of them, comprising a volume percentage of fibres equal to at least 15%, or possibly at least 5% in the case of felts, before densification. The fibres may be made of ceramic compounds (carbon, carbides, nitrides or oxides) or metallic compounds (such as W, WRe alloys, MoSi.sub.2, etc.). One way of making fibrous structures suitable for a porous joint 3 according to the invention may be to use conventional braiding, felt forming techniques or webbing, needlebonding, weaving or knitting [4].

(15) It is possible to envisage increasing the thermal conductivity of the material or protecting the fibres by depositing chemical compounds that are also refractory (ceramic or metallic compounds) on the fibres. These depositions then represent a volume percentage such that the open porosity of the final material, fibrous structure reinforced by a deposit, is between 30% and 85%, or even up to 95% in the case of felts. These depositions on fibrous structures may be made using conventional chemical vapour deposition (CVD) techniques [1] or other techniques such as impregnation of ceramic polymer precursor, pyrolysis, etc.

(16) The joint 3 may be placed either by positioning it around the pellets 5 and then inserting the joint 3/pellets 5 assembly into the cladding 1, or by inserting it into the cladding 1, the pellets then being inserted later.

(17) Physical contact firstly between the cladding 1 and the joint 3 and secondly between the joint 3 and the pellets 5 may be formed during the temperature rise in the nuclear reactor by differential thermal expansion, since joint 3 expands more. Another way of achieving this physical contact is radial compression of the joint 3, and then the joint 3 can be released after placement of the cladding 1-joint 3-pellets 5 assembly, before the assembly is put into service in the nuclear reactor for which the control rod is to be used.

(18) Honeycomb materials or foams that might be suitable are open pore materials with between 30% and 85% of porosity, with cell diameters preferably less than 100 m to prevent movement of macro-fragments of pellets, but sufficiently high for interconnection of the pores. The composition of these materials may be based on ceramic or metallic compounds. It would be possible to make honeycomb materials suitable for porous joints 3 according to the invention using conventional techniques for the injection of gas bubbles or compounds generating bubbles in the molten material or a precursor compound (organic resin for carbon), powder metallurgy with porogenic compounds or particles, deposition of a compound on a foam acting as a substrate [2],[7]. The basic foam can then be reinforced by deposition of a compound (among ceramic or metallic compounds) with a nature that may be identical to or different from the foam compound. This deposition may for example be obtained by chemical vapour deposition (CVD) [1].

(19) Three examples of nuclear control rods according to the invention are given below, with the characteristics of the main control system (SCP) for the SUPERPHENIX reactor [8]: in all these examples, the control rod comprises a stack of cylindrical boron carbide neutron absorber pellets 5 with a diameter of 17.4 mm and cladding 1 surrounding the column of stacked pellets with an inside diameter of 19.8 mm, namely a radial pellet/cladding clearance of 1.2 mm (cold).

(20) For comparison with the joint solution that will be presented below, for an SCP control rod for the SUPERPHENIX reactor [8], the absorber pellets column is surrounded by a 200 m thick liner confining pellet fragments formed under irradiation, and the residual pellet/cladding space is filled with liquid sodium to provide efficient heat transfer. The end of life of such a control rod is associated particularly with the occurrence of a mechanical interaction between pellets and the cladding situation, when the three-dimensional expansion of B.sub.4C pellets eventually fills in the free radial space that initially separated the column of pellets from the cladding, leading to a mechanical load that quickly makes the cladding unusable. The thickness of the liner (200 m) should be naturally subtracted from the initial value of the pellet/cladding clearance (1.2 mm), therefore the allowable future expansion of the pellets is of the order of 1 mm for a pellet radius of 8.7 mm, which gives an allowable expansion ratio of the order of 11.5% before the mechanical interaction between pellets and the cladding is reached. These characteristics are usually sufficient to achieve neutron capture ratios of the order of 200*10.sup.20 per cm.sup.3 of absorber.

(21) With a porous solid joint according to the invention, and considering the end of life reached for complete disappearance of the joint porosity (by compression under three-dimensional expansion of B.sub.4C pellets), the gain on the neutron absorption ratio that could be envisaged from the design fabrication porosity for the joint according to the invention can be evaluated. For changing from a 200 m thick liner to a 1.2 mm thick joint, the required value of the joint porosity is typically a value equal to a ratio of 1/1.2, namely of the order of 83% (joint with 17% of the theoretical density of the material of which it is composed), to achieve the capture ratio obtained with a sleeve type solution and also to benefit from the advantage of centring the pellets in the cladding. Note that the thermal effect induced by the joint is neglected (calculations show that this is a second order effect concerning the swelling ratio of the absorber).

Example 1

Braid with SiC Layers/C Layers

(22) A first series of three layers of superposed braids is made with carbon fibres (trade name Thornel P-100 each containing 2000 filaments and that are cracked to reduce the thread diameter) on a mandrel with the following characteristics:

(23) inside diameter: 17.5 mm

(24) outside diameter: 19.0 mm

(25) braiding type: 2D

(26) braiding angle: 45

(27) A second series of three braid layers is made on the previous series of braid layers with silicon carbide fibres (trade name HI-NICALON type S, each containing 500 filaments), with the following characteristics:

(28) inside diameter: 19.0 mm,

(29) outside diameter: 21.2 mm

(30) braiding type: 2D

(31) braiding angle 45

(32) The multi-layer braid 3 thus formed is compressed in a cylindrical mould with an inside diameter of 19.7 mm. An eliminable soluble binder, in this case a polyvinyl alcohol, is then added into the braid and the solvent is then evaporated.

(33) The braid 3 is then stripped and inserted into a metal cladding 1 with inside diameter of 19.8 mm. The central mandrel is then removed, and a column of 17.4 mm diameter boron carbide B.sub.4C neutron absorber pellets 5 is then inserted into the braid. The binder is eliminated by heat treatment of the assembly under a vacuum. The braid 3 then expands and comes into physical contact with the pellets 5 and the cladding 1.

(34) Therefore, the fabricated thickness of the braid 3 is equal to the total assembly clearance between the cladding 1 and the pellets 5, namely 1.2 mm.

(35) The cladding 1 may then be closed at its ends, for example by welding. Even if not shown, before the final closing step is performed, a helical compression spring is housed in the expansion chamber or vessel 6 with its lower end bearing in contact with the stack of pellets 5 and its other end bearing in contact with the upper plug. The main functions of this spring are to hold the stack of pellets 5 along the direction of the longitudinal axis XX and to absorb the elongation of the fuel column with time under the effect of longitudinal swelling of the pellets 5.

(36) The nuclear control rod thus made with a porous solid joint 3 according to the invention can then be used for application in a fast neutron nuclear reactor.

Example 2

Carbon Needlebonded Structure

(37) Carbon fibre layers (trade name Thornel P-25) are needlebonded in the form of a tube with inside diameter 17.5 mm and outside diameter 21.2 mm, on a graphite mandrel.

(38) A heat treatment is then applied on the assembly at 3200 C. under Argon. The tube thus formed is compressed in a cylindrical mould with an inside diameter of 19.7 mm. An eliminable soluble binder, in this case a polyvinyl alcohol, is then added into the structure and the solvent is then evaporated.

(39) The porous solid joint 3 thus obtained is then stripped and inserted into a cladding 1 with inside diameter of 19.8 mm. The central mandrel is then removed, and a column of 17.4 mm diameter boron carbide B.sub.4C neutron absorber pellets 5 is then inserted into the mixed joint 3/cladding 1 structure.

(40) The binder is then eliminated by heat treatment of the assembly under a vacuum. The joint 3 then expands and comes into contact with the stacked pellets 5 and the cladding 1.

(41) The cladding 1 may then be closed at its ends, for example by welding. Even if not shown, before the final closing step is performed, a helical compression spring is housed in the expansion chamber or vessel 6 with its lower end bearing in contact with the stack of pellets 5 and its other end bearing in contact with the upper plug. The main functions of this spring are to hold the stack of pellets 5 along the direction of the longitudinal axis XX and to absorb the elongation of the fuel column with time under the effect of longitudinal swelling of the pellets 5. The nuclear control rod thus made with a porous solid joint 3 according to the invention can then be used for application in a fast neutron nuclear reactor.

Example 3

Carbon Foam Coated with a WRe 5% Alloy

(42) A tube with an inside diameter of 17.4 mm and outside diameter of 19.8 mm made of carbon foam composed of 40 m diameter open honeycombs is placed in a chemical vapour deposition (CVD) furnace.

(43) An approximately 7 m thick deposition of WRe 5% alloy obtained from the decomposition of a mix of tungsten and rhenium halide compounds is applied on the ligaments forming the foam.

(44) This foam tube is then inserted into the cladding 1 with inside diameter 19.8 mm, and the column of 17.4 mm diameter boron carbide B.sub.4C neutron absorber pellets 5 is in turn inserted into the foam tube.

(45) The cladding 1 may then be closed at its ends, for example by welding. Even if not shown, before the final closing step is performed, a helical compression spring is housed in the expansion chamber or vessel 6 with its lower end bearing in contact with the stack of pellets 5 and its other end bearing in contact with the upper plug. The main functions of this spring are to hold the stack of pellets 5 along the direction of the longitudinal axis XX and to absorb the elongation of the fuel column with time under the effect of longitudinal swelling of the pellets 5. The nuclear control rod thus made with a porous solid joint 3 according to the invention can then be used for application in a fast neutron nuclear reactor.

(46) Other improvements would be possible without going outside the scope of the invention. Thus, in all examples 1 to 3 mentioned above, the fabrication thickness of the porous solid joint 3, in other words the thickness after the cladding 1 has been closed and the control rod is ready for application, is equal to the total design assembly clearance between the cladding 1 and the column of pellets 5 made of B.sub.4C neutron absorber material.

(47) Obviously, clearances could be provided (see references 2, 4 in FIG. 1) that are maintained once the control rod is ready, provided that the fabrication methods and properties (particularly differential thermal expansion firstly of the cladding 1 and the porous solid joint 3, and secondly of the joint 3 and the pellets 5) make it possible.

(48) These clearances as shown in references 2, 4 in FIG. 1 are a priori filled with a gas or a liquid metal that then naturally occupies the open pores of the porous solid joint 3 according to the invention, and the open pores of the B.sub.4C neutron absorber pellets 5.

(49) But according to the invention and unlike solutions according to the state of the art, and more particularly the solution according to U.S. Pat. No. 4,235,673, assembly clearances are not essential and therefore are not functional clearances provided to accommodate the three-dimensional swelling of the pellets under irradiation.

(50) Furthermore, the mandrel used to form the porous solid joint as in the examples described may be made of different materials compatible with the materials used in the joint, such as graphite and quartz.

(51) Similarly, for the final step in the process before the cladding is closed, examples 1 to 3 describe placement of a helical compression spring. More generally, during this final step before the actual closing step of the cladding, it would be possible to use what is currently referred to as an internals system in the nuclear domain, in other words an assembly of components such as springs, packing, etc., the function of which is to position the column of pellets axially within the cladding.

(52) FIG. 2 shows the compression behaviour of interface joints according to the invention with high open porosity and based on braids or based on felt made of a SiC material.

(53) More precisely, as shown, these are tests in cycled compression, with each cycle alternating a load and an unload, which in FIG. 2 is illustrated by loading loops in the strain-stress plane.

(54) The abscissa indicates the values of the compression ratio (strain in %) of the joint across its thickness.

(55) The ordinate indicates values of mechanical loads (stress in MPa) transferred by the joint under the effect of its compression.

(56) Thus, the indicated stresses actually correspond to the radial mechanical load .sub.r applied to the cladding of a nuclear control rod under the effect of the three-dimensional swelling of B.sub.4C neutron absorber pellets stacked on each other, the stresses being transmitted to the cladding directly by compression of the joint between the pellets and the cladding. This radial load introduces a controlling circumferential load .sub., the intensity of which corresponds to the intensity of the radial load to which a multiplication factor is applied, which is approximately equal to the ratio of the average radius r.sub.G of the cladding to its thickness e.sub.G, which is typically equal to 5 to 10: .sub.(r.sub.G/e.sub.G).sub.r.

(57) FIG. 2 thus illustrates the fact that an interface joint according to the invention is adapted to function like a stress absorber: the transmitted load only becomes significant for a sufficiently high compression ratio beyond which the transmitted load increases progressively with the compression ratio, until it reaches the threshold value of the allowable limiting load (without any sudden changes). Thus, for a load .sub.r considered to be significant starting from 1 MPa, the compression ratio is of the order of 40% and 70% respectively for the braid and felt type joints considered in FIG. 2.

(58) In a situation of operation under irradiation in a reactor, the cladding of a nuclear control rod cannot resist a mechanical load from B.sub.4C neutron absorbers unless it remains below a limit guaranteeing that there is no cladding failure. Thus, for example if the threshold value of the allowable circumferential load .sub. is fixed at 100 MPa (which is a reasonable value considering usually allowed loads), namely a radial load .sub.r of the order of 10 MPa (for a ratio r.sub.G/e.sub.G of the order of 10), FIG. 2 shows that braid and felt type joints considered will give a compression ratio of the order of 60% and 95% respectively, below which the mechanical load transmitted to the cladding remains acceptable.

(59) Note that the tests done according to FIG. 2 showed that the interface joint according to the invention based on braids and the joint based on felt maintained their integrity; thus, the braid/felt structure is preserved without any formation of fragments that could move into a reopened gap between pellets and the cladding in a control rod in a fast neutron reactor FNR.

(60) A control rod must be kept for as long as possible in a fast neutron reactor if economic performances are to be optimised. These performances are usually limited by various operating constraints so as to satisfy safety objectives. One of the most severe constraints is imposed by the need to guarantee mechanical integrity of the control rod cladding under all circumstances. This leads to the definition of an allowable limiting load on the cladding (stress and/or strain beyond which the integrity of the cladding can no longer be guaranteed). However under irradiation, the B.sub.4C neutron absorber pellets are affected by a continuous three-dimensional swelling that leads to a pellet/cladding mechanical interaction (PCMI) that could eventually lead to an unacceptable load on the cladding. Therefore, the operating life for a nuclear control rod with B.sub.4C nuclear absorbers is strongly dependent on the time for such an excessive interaction to occur. The interface joint according to the invention as defined above provides a satisfactory response because it enables long term expansion or three-dimensional swelling of the pellets. For a fixed three-dimensional swelling of the pellets, the durability depends on the initial thickness of the joint and the compression ratio that it can accommodate before its compression state causes the transmission of an unacceptable mechanical load to the cladding; the initial thickness of the joint to be installed reduces as the allowable compression ratio increases.

(61) FIG. 2 illustrates the fact that very high compression ratios are necessary to reach the compression limit of the proposed braid or felt type joints, which means that increased irradiation times can be reached if a reasonably thick joint is installed. The large joint thicknesses characteristic of control rods for a fast spectrum reactor, support the installation of high porosity joints that can easily reach and probably exceed the performances of the sleeve type solution used in SUPERPHENIX.

(62) Furthermore, shear tests were carried out by imposing forces on an approximately 1 cm thick fibrous structure according to the invention, corresponding to cyclic displacements of the order of 100 m at temperatures of the order of 400 C. For these elongations of 1%, the fibrous structure remained perfectly intact. In the case of control rods for fast spectrum reactors, the large thicknesses of the joints also enable the use of joints according to the invention comprising several layers of superposed braids and/or felts. Concerning the axial shear to which the joint under irradiation is submitted, due to the elongation of the column of pellets (effect of swelling) that is more pronounced than the elongation of the cladding, this multi-layer structure reduces the mechanical load on the joint by enabling relative sliding of layers on each other, and consequently limits the risk that the joint would be damaged by shear.

REFERENCES MENTIONED

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