FR2814179A1 - Creep-resistant silver-based alloy for production of neutron absorption elements includes indium, cadmium and, optionally, hafnium, and their oxides in the form of particles distributed in the alloy - Google Patents

Abstract

Silver-based alloy having improved creep resistance and used in the production of neutron absorption elements includes indium, cadmium, optionally hafnium, and oxygen present a dissolved form and in the form of indium, cadmium and hafnium oxides distributed throughout the alloy as particles of size 0.02-2 microns. Preferred Features: The silver-based alloy comprises (in weight %): indium 10-16; cadmium 4-6; oxygen 0.003-1.25 oxygen; optionally hafnium up to 1.5; and silver and unavoidable impurities the remainder. The amount of oxygen present in the alloy is preferably less than 1 weight %, more preferably less than around 0.8 weight %. The oxides in the form of particles mainly comprise indium oxide (In2O3) particles. The preferred size of the particles is 20-300 nm. Independent claims are given for first and second processes for manufacturing a rod from the above-described alloy. The first process for manufacturing a rod from the above-described alloy comprises: (a) producing a silver-indium-cadmium alloy powder by water atomization; (b) compacting the powder at very high pressure to produce a billet; (c) sintering the billet in nitrogen at around 700 degrees C for about an hour; (d) preheating the billet at around 700 degrees C in air or argon for 20-40 minutes; and (e) extruding the billet into the form of a rod at around 700 degrees at a die temperature of around 500 degrees C. The second process for manufacturing a rod from the above-described alloy comprises: (a') producing a silver-indium-cadmium alloy powder by water atomization; (b') controlled oxidation of the powder at high temperature in an oxygen atmosphere, preferably in an autoclave at around 400 degrees C under a pressure of oxygen of 1-10 bars; (c') filling a container with the oxidized powder; (d') closing the container under vacuum; (e') preheating the container for around two hours at around 400 degrees C; (f') extruding the container containing the powder; and (g') removing the container wall to obtain a rod of a silver-indium-cadmium alloy containing oxide particles.

Description

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 The invention relates to a silver-based alloy with improved creep resistance for the production of neutron absorbing elements.

 Nuclear reactors and in particular nuclear reactors cooled by pressurized water have a core consisting of fuel assemblies of prismatic shape, arranged vertically and juxtaposed. Each of the fuel assemblies is constituted by a bundle of fuel rods parallel to each other maintained by a framework constituted by grids-spacers and guide tubes rigidly connected to the grids-spacers which are distributed along the length of the fuel assembly.

 During the operation of the nuclear reactor, it is necessary to have means to regulate the power supplied by the core in which the fission reactions giving off energy in the form of heat take place. It is also necessary to have means to stop the nuclear reactor at the end of an operating cycle or in the event of an incident requiring an emergency stop.

 The power of the core is adjusted and the reactor is shut down using neutron absorbing elements and in particular absorbing elements which can be more or less inserted into the core depending on the conditions of piloting or stopping the nuclear reactor.

 In particular, absorbent elements are used which are produced in the form of rod clusters which can be introduced and displaced inside the guide tubes of certain fuel assemblies.

 The rods of a cluster are connected, at one of their ends to a support comprising radiating arms and a knob for connecting the cluster to an extension, this support for the cluster being generally designated by the term "spider" .

 The absorbent rods of the control and shutdown clusters of a nuclear reactor comprise a tubular sheath in which an absorbent material is placed.

 Among the most widely used absorbent materials are boron carbide 840 and ternary silver, indium, cadmium (AIC) alloys.

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 Depending on the use of the absorbent clusters for the operation or shutdown of the nuclear reactor, the constitution of the absorbent rods can be variable and these rods can only contain pellets of 846 or AIC, both pellets of B4C and d 'AIC, or pellets of highly absorbent materials (B4C or AIC for example), and absorbent materials more weakly neutrons.

 One of the objectives pursued when designing the rods of the absorbent clusters of a nuclear reactor is to extend the life of the clusters as much as possible, so as to reduce the operating costs of the nuclear reactor.

 For this, it is necessary to limit the swelling under irradiation of the neutron absorbing material and the creep of the material in the atmosphere of the nuclear reactor, that is to say at a temperature of the order of 320OC, under a load a few MPa.

 In the case of AIC ternary alloys used hitherto in the usual way to constitute neutron absorbing elements in the core of a nuclear reactor, these alloys containing by mass from 14 to 16% of indium and from 4 to 6% of cadmium, the rest of the alloy being made up of silver, with the exception of unavoidable impurities in a proportion by weight of less than 0.25%, it has been proposed to improve the creep resistance by increasing the size of grain of the alloy, in its initial state, before its use as an absorbent material in the core of the nuclear reactor.

This mode of action on the creep resistance of the alloy is quite limited.

 In PCT / FR-95/01356, it has been proposed to limit the swelling under irradiation of a ternary alloy AIC used as absorbent in the core of a nuclear reactor, by adjusting the composition of the ternary alloy and in particular by significantly lowering the indium content, between 9 and 12 atomic%. This adjustment of the composition, however, has no significant effect on the creep resistance. Taking into account these alloys with modified composition, the technical alloys silver, indium, cadmium generally contain, 10 to 16% by mass of indium, 4 to 6% by mass of cadmium, the rest of the alloy, except for impurities, being made up of silver.

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 Alloys reinforced by a dispersion of oxides or dispersoid alloys are known, for example based on precious metals and used for applications in the electrical field requiring both good electrical conductivity and high hardness, the characteristics of which are improved by a fine dispersion of oxides in the matrix of the alloy, the dispersoids generally having a size of the order of a few tens of nanometers.

 At high temperatures, the oxide dispersoids limit the deformation of the thermally activated alloy by interaction with the dislocations.

 Although this technique of reinforcing alloys and improving creep resistance has been used in the case of alloys based on precious metals, efforts to improve the creep resistance of AIC technical alloys have never been successful so far. by dispersoids made up of oxides in the form of very fine particles.

 It has been proposed to use, for the sintering manufacture of control rods of nuclear reactors in alloy A! C, powders obtained by atomization with water which can contain a certain proportion of oxygen. However, the oxygen content is limited and the size of the oxide particles is not controlled. The object of the invention is therefore to propose an alloy based on silver with improved creep resistance, for the production of neutron absorbing elements, containing by mass from 10 to 16% of indium, from 4 to 6%. of cadmium and from 0.003 to 1.25% of oxygen and optionally up to 1.5% of hafnium, the oxygen being present in the alloy in dissolved form and in the form of oxides of indium and cadmium and possibly hafnium, distributed in the mass of the alloy in the form of particles having a size of between 0.02 and 2 μm, the rest of the alloy, with the exception of unavoidable impurities being constituted by money.

 In order to clearly understand the invention, a description will now be given, by way of example, with reference to the attached figures, of a neutron absorbing element of a control cluster of a pressurized water nuclear reactor and two embodiments of a method of manufacturing the absorbent element, so that it is constituted by an AIC alloy reinforced by a dispersion of oxides.

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 Figure 1 is an exploded perspective view of a fuel assembly into which a control cluster is introduced consisting of absorbent rods.

 FIG. 2 is an elevational view with partial axial section of an absorbent pencil of the cluster shown in FIG. 1.

 FIG. 3 is a diagram showing the steps for manufacturing an absorbent pencil according to the invention and according to a first embodiment.

 FIG. 4 is a diagram showing the steps for manufacturing an absorbent element according to the invention and according to a second embodiment.

 In Figure 1, we see a fuel assembly generally designated by the reference 1 which includes a bundle of rods 2 parallel to each other held in a frame formed by spacer grids 3 for transversely holding the rods, guide tubes 4 are substituting some of the rods 2 of the bundle of the fuel assembly and the end caps 5a and 5b.

 The guide tubes 4, the length of which is greater than the length of the fuel assembly rods, are fixed at their ends to the end pieces 5a and 5b, the end piece 5a disposed at the top of the fairly fuel in its operating position allowing passage of the absorbent rods 7 of a cluster 6 for reactivity control in the core of the nuclear reactor.

 The upper end piece 5a comprises an adapter plate 8 comprising through openings for fixing the guide tubes 4 of the fuel assembly and water passage openings of elongated shape.

 The absorbent rods 7 of the control cluster 6 are fixed to a pommel 6a of the cluster via a spider and can each be moved inside a guide tube 4 of the fuel assembly, through the adapter plate 8.

 As can be seen in FIG. 2, each of the absorbent rods 7 comprises a tubular envelope 7a closed at its ends by plugs 9a and 9b and enclosing a column of neutron absorbing material 10 which can be constituted for example by one or more bars silver-indium-cadmium alloy.

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 The bar 10 made of silver-indium-cadmium alloy bears at its lower end on the lower plug 9b of the absorbent rod 7 and is held by a helical spring 11 interposed between the upper plug 9a and the upper end of the bar 10.

 The upper plug 9a of the pencil 7 makes it possible to fix the pencil on the spider of the control cluster 6.

 The bar of absorbent material 10 can be made according to the invention in a silver-based alloy reinforced with oxide particles.

 The silver-indium-cadmium alloy used for the manufacture of control cluster absorbent pencils or industrial AIC usually contains by mass, 79.7% silver, 15.14% indium, 5.16% cadmium and 30 ppm oxygen maximum. Before its use as an absorbent, at the end of manufacture, it has a grain size with an ASTM 7 index, ie approximately 30 μm.

 For temperatures between 300 and 350OC and constraints of a few MPa, the behavior of plasticity of industrial AIC can be both governed by the HARPËR-DORN mechanism and class 1 dislocations. The grain size n ' has practically no influence on these mechanisms.

 Increasing the grain size of the material would therefore not be an effective solution to reinforce its resistance to creep.

 In general, the industrial AIC contains, by mass, 80 0.5% of silver, 15 0.25% of indium, 5 0.25% of cadmium and a total impurity content of less than 2500 ppm, the contents residuals of Pb and Bi are both less than 300 ppm.

 However, the inventors have been able to determine that an oxygen content of less than or equal to 1% by mass could be accepted in the alloy, while retaining satisfactory neutron absorption characteristics.

 The usual method of manufacturing bars in industrial AIC to obtain absorbent rods will be described below.

 A billet is produced with the composition of the industrial AIC alloy. Machining is then carried out around the billet and then extruded by non-lubricated reverse spinning in a press. We get a thread

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 which is drawn, possibly peeled and then stretched before being cut into bars and annealed. The bars are straightened and then checked.

 By modifying the conditions of the final annealing of the bars, it is possible to obtain a large grain industrial AIC whose grain size is between 350 and 500 μm, instead of 30 μm in the case of the small grain industrial AIC.

 It was not possible to obtain a significant improvement in the creep resistance by the enlargement of the grains of the AIC.

 According to the invention, an increase in the creep resistance of an AIC type alloy is obtained by a dispersion of oxides inside the alloy.

 The production of AIC alloy bars containing an oxide dispersion can be carried out in two different ways which will be described below.

 The work relating to AIC alloys containing an oxide dispersion was carried out with the assistance of the Laboratory of the Ecole des Mines de Paris (Armines).

First production process
The first production process is shown diagrammatically in FIG. 3. The starting product is an AIC powder obtained by atomization with water of a liquid metal having the usual composition of an industrial AIC, ie that is to say by mass 80 0.5% silver, 15 0.25% indium and 5.05% cadmium, with an impurity level of less than 2500 ppm, of which less than 300 ppm lead and less than 300 ppm bismuth. The powder 12 obtained by atomization with water is then compacted in the form of a billet 13 and then sintered. The sintered billet 13 is subjected to preheating in air or in argon and then extruded in the form of a bar 14.

 To carry out the compaction of the powder to obtain the billet 13, the powder 12 is placed inside a latex sheath which is closed by a plug. After degassing for one hour under primary vacuum, the assembly is subjected to a pressure of 2000 or 3000 bars for 7 seconds. The compacted billet 13 is obtained after removal of the sheath.

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The sintering of the billet 13 is carried out at approximately 700 ° C. under nitrogen for a period of approximately one hour.

 The bars are preheated for about 20 or 40 minutes to about 700OC in an air or argon muffle oven. The extrusion is carried out by direct spinning in a press whose tools are at 500OC.

The bars are obtained at their final diameter, of the order of 8 mm, under a length which can be that of the absorbent bar which is produced or a submultiple of this length.

 Analysis of the metal of the bar 4 obtained at the end of the last phase of the process has shown that the total oxygen content of the bar is approximately 0.1% by mass, this content depending in particular on the preheating carried out before l extrusion, preheating in air resulting in an increase in the oxygen content of the bars.

 Part of the oxygen contained in the rod was absorbed by the powder, at the time of atomization with water and another part of the oxygen was possibly absorbed during the preheating of the billet before extrusion, in an oxidizing atmosphere.

 A metallurgical study has shown that the oxides formed in the alloy are mainly indium oxides, to a lesser extent, cadmium oxides. The oxides are generally in the form of particles rather than platelets or needles, which is more favorable for improving the resistance to sintering.

 These observations are consistent with the fact that the free enthalpy of formation of indium oxide In203 at the production temperature is negative and significantly lower than the free enthalpy of formation of cadmium oxide CdO which is also negative. The free enthalpy of formation of silver oxide Ag20 is positive at the production temperature.

 The majority of the oxides have diameters between 250 nm and 2 μm. The majority of the oxide particles are located in the vicinity of the grain boundaries.

 Tensile tests have shown a reinforcement of the AIC produced by the process according to the invention compared to a conventional industrial AIC. This strengthening in traction is probably due to a blocking effect on the waist

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 grains by oxides, the material prepared by the process according to the invention having a grain size of the order of 10 μm, against a grain size of 30 μm for conventional industrial AIC. Creep tests were carried out on bars prepared by the process according to the invention containing approximately 0.1% by weight of oxygen, some of the bars having been obtained by extrusion after preheating in air and other bars having been extruded after preheating under argon.

 Comparatively, creep tests are carried out on a conventional industrial AIC.

 Compression creep tests are carried out at 350OC under a load of 7.5 MPa.

 The lifetime of the bars obtained, according to the method of the invention, is on average thirty times higher than the lifetime of the bars in conventional industrial AIC.

 It should be noted that the conditions of the creep test are more severe than the conditions of creep in compression in the reactor in service.

 It turned out that the first process for the production of internally oxidized AIC bars according to the invention did not guarantee a sufficient oxygen content and perfectly determined properties.

 The method does not make it possible to optimize the distribution of the oxides inside the alloy and in particular to obtain fine oxides which can lead to a very significant increase in the creep resistance in compression.

Second production process
A second method was therefore used in which a controlled oxidation of the AIC is carried out.

 We first had to determine whether it was preferable to obtain a controlled oxidation on a solid material or on a powder. The main advantage of the internal oxidation carried out on a powder is that the oxidation times are reduced compared to an oxidation carried out on a solid alloy due to a much larger specific surface.

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 In addition, the dispersion of oxides formed is finer and more homogeneous in the consolidated material obtained from the powder. One can in particular avoid a needle morphology of the oxides.

 A powder atomized with water is used which can be identical to the powder used in the case of the first method of producing a product according to the invention.

 A typical composition of the powder is as follows: silver 79.47%, indium 14.98% and cadmium 5.27% (by mass). The balance of the alloy consists of oxygen and impurities.

 Preferably, a controlled oxidation of the AIC powder is carried out, inside an autoclave, in an atmosphere of pure oxygen.

 The internal oxidation of the powder is carried out so that the proportion of oxygen in the powder is not more than 1% by mass and is preferably of the order of 0.8% by mass.

 The oxidation is preferably carried out at a temperature of approximately 400 ° C. under a pressure of pure oxygen of 2 bars and for a period of 90 minutes.

 Of course, the duration of oxidation depends on the characteristics of the autoclave enclosure and on the distribution of the powder inside the enclosure.

 After internal oxidation, the various powder samples which have been treated have an oxygen content which is between 0.8 and 1.05% by mass.

 Other operations for the oxidation of powder samples were carried out in an autoclave at approximately 400 ° C., with treatment times and oxygen pressures in the autoclave enclosure different from those indicated above.

 In general, the powder can be oxidized at the temperature of about 400 ° C. at an oxygen pressure of between 1 and 10 bars.

 For example, oxidation was carried out under an oxygen pressure of 10 bars for 80 minutes and under an oxygen pressure of 2 bars for 4 hours 35.

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The powders are covered with a very thin film of indium oxide and are internally oxidized.

 The powder particles contain both oxides and oxygen in dissolved form in a proportion greater than the proportion of dissolved oxygen in industrial AIC (30 ppm).

 Bars were produced from the oxidized powders in a controlled manner according to the process shown diagrammatically in FIG. 4.

 The powder 15, oxidized in a controlled manner, is introduced into a copper container 16 of cylindrical shape closed by a bottom.

 The powder is placed under vacuum inside the container which is closed by a plug 17 welded by electronic bombardment in a vacuum enclosure.

 A preheating is then carried out for approximately two hours, at approximately 400 ° C. under argon, of the container containing the powder.

 The container is then extruded at a speed of one meter per second in a die having a 12 mm diameter.

 The useful diameter of the bar obtained 18 is close to 8 mm.

 The bar can be separated from its copper sheath by peeling and machining.

 Various metallographic examinations are carried out on the material of the extruded bars.

 Depending on the samples, the oxide particles inside the bars which are distributed homogeneously in the cross section and in the longitudinal direction of the bar have a size of between 20 and 300 nm. Some oxide particles have a larger size of up to 600 nm.

 The oxides are mainly indium oxides In203 and some rare cadmium oxides.

 Tensile tests were carried out on the bars at temperatures of 300 ° C. and 350 ° C. until the bars broke (for a deformation of approximately 6%).

 At 300OC, the elastic limit at 0.2% of the material of the bars is of the order of 97 MPa, the flow stress being 100 MPa.

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At 350OC, the same parameters have corresponding values of 68 and 70 MPa.

 Comparative tests with industrial AIC bars and with bars prepared according to the first and second embodiments show that the flow stress of the bars obtained by the second embodiment is significantly higher than the stress of corresponding flow of the bars obtained by the first embodiment which is itself higher than the flow stress of the industrial AIC bars.

 This increase in the flow stress can be attributed to the presence of the oxide particles which are more effective than in the case of the first embodiment, because they are smaller and more numerous.

 Creep tests in tension and compression at 350OC were carried out on test specimens taken from the bars.

 Comparative tests are carried out on AIC bars prepared by the method according to the first and second embodiments and of small grain industrial AIC.

 It has been shown that the material of the AIC bars prepared according to the second production method deforms much less quickly than the industrial small grain AIC and that the AIC prepared by the first production method.

 The AIC prepared by the second production method does not deform in creep at 350oC under a stress of 10 MPa while the industrial small grain AIC and the AIC prepared by the process according to the first embodiment are deformed sensibly for a stress of 7.5 MPa.

 In addition, the AIC prepared by the process of the second production method is reinforced by work hardening.

 Tests were carried out to determine the lifetime of samples drawn from the AIC bars prepared by the method according to the second mode of production, in comparison with AIC bars prepared by the method according to the first mode of production. production and industrial AIC.

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 The gain in lifespan obtained with the AIC developed by the method of the second production mode increases when the stress decreases. For a constraint of 10 MPa at 350, the lifetime is multiplied by 2250 compared to the industrial AIC and by 4800 compared to the AIC prepared by the method according to the first embodiment. This factor is even more important for stresses lower than 10 MPa.

 The gain in service life at 350OC under a constraint of 7.5 MPa is 30 with respect to the AIC prepared by the method of the first production method compared to the industrial AIC.

 In all cases, this gain is at least 2000 under a constraint of 10 MPa, when comparing the AIC prepared by the method of the second embodiment and the industrial AIC.

 A dispersion of indium oxide and in a lower proportion of cadmium in the AIC alloy therefore makes it possible to significantly improve the creep resistance of the alloy.

 It is also possible to produce alloys reinforced by a dispersion of oxides containing, in addition to silver, indium and cadmium, a proportion of hafnium which can range up to 1 atomic% or 1.5%. en masse.

 During the internal oxidation of the pre-alloyed silver-indiumcadmium-hafnium powder, an internal oxidation of hafnium is carried out, because the standard free enthalpy of formation of hafnium oxide is more negative than that of the oxides indium, cadmium and silver. Oxidation at the heart of powder particles containing hafnium is then possible, without exceeding a proportion of oxygen of the order of 1% by mass. One can then obtain a more homogeneous and finer dispersion of hafnium oxide, thanks to the addition of hafnium.

 The invention is not strictly limited to the embodiments which have been described.

 Thus, the basic composition of the silver, indium and cadmium alloy may be different from those which have been described in the examples, the different contents of additives being given by the intervals of claim 1.

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 The methods for preparing products such as bars of alloy reinforced with oxides can be different from the methods described.

 In general, the invention applies in the case of the production of numerous neutron absorbing elements which can be used in nuclear reactors or in fuel storage pools, for example lining plates for storage racks.

Claims (7)

 1. - Silver-based alloy with improved creep resistance, for the production of elements (10) absorbing neutrons, containing by mass from 10 to 16% of indium, from 4 to 6% of cadmium from 0.003 to 1.25% oxygen and possibly up to 1.5% hafnium, the oxygen being present in the alloy in dissolved form and in the form of indium and cadmium oxides and optionally hafnium, distributed in the mass of the alloy in the form of particles having a size of between 0.02 and 2 μm, the rest of the alloy, with the exception of unavoidable impurities, being constituted by silver.  2.-An alloy according to claim 1, characterized in that it contains less than 1% oxygen by mass and preferably about 0.8%.  3.-Alloy according to any one of claims 1 and 2, characterized in that the oxides in the form of particles consist mainly of indium oxide In203.  4.-Alloy according to any one of claims 1 to 3, characterized in that the particles have a size between 20 and 300 nm.  5.-A method of developing a bar of an alloy according to any one of claims 1 to 4, characterized in that it develops, by atomization with water, a pre-alloyed powder in a silver-indiumcadmium alloy (AIC), which is produced by compacting under very high pressure a billet (13) from the powder (12), that a sintering of the billet is carried out under nitrogen at a temperature of the order of 700OC for approximately one hour, then preheating the billet to approximately 700 ° C. in an atmosphere of air or argon, for a period of the order of 20 or 40 minutes and finally an extrusion of the billet in the form of a bar ( 14) at around 700OC, in press tools at 500OC.  6.-A method of developing an alloy bar according to any one of claims 1 to 4, characterized in that a powder is produced in silver-indium-cadmium alloy (AIC) by atomization with water of the liquid alloy, which performs a controlled oxidation of the powder at high temperature in an oxygen atmosphere, which is carried out <Desc / Clms Page number 15>  pleating of a container (16) with powder (15) oxidized in a controlled manner, closing the container under vacuum, preheating the container (17) for a period of the order of two hours to approximately 400OC, then that the container containing the powder is extruded, and finally that the bar peeling is carried out to remove the wall of the container, to obtain an AIC alloy bar (18) containing oxide particles.  7.-A method according to claim 6, characterized in that one carries out the oxidation of the powder in an autoclave at a temperature of about 400OC under an oxygen pressure between 1 and 10 bars.