Interdiffusion behaviors in doped molybdenum uranium and aluminum or aluminum silicon dispersion fuels: Effects of the microstructure

Journal of Nuclear Materials 416 (2011) 205–210

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Interdiffusion behaviors in doped molybdenum uranium and aluminum or aluminum silicon dispersion fuels: Effects of the microstructure J. Allenou a,b,⇑, O. Tougait b, M. Pasturel b, X. Iltis a, F. Charollais a, M.C. Anselmet a, P. Lemoine c a

CEA, DEN, DEC, F-13108 St. Paul Lez Durance Cedex, France Université de Rennes 1, UMR-CNRS 6226, Campus de Beaulieu, 35042 Rennes Cedex, France c CEA, DEN, DISN, 91191 GIF SUR YVETTE, France b

a r t i c l e

i n f o

Article history: Available online 24 February 2011

a b s t r a c t Si addition to Al is considered as a promising route to reduce (U,Mo)–Al interaction kinetics, due to its accumulation in the interaction layer, yielding the formation of silicide phases. The (U,Mo) alloy microstructure, and especially its homogenization state, could play a role on this accumulation process. The addition of a third element in c(U,Mo) could also influence diffusion mechanisms of Al and Si. These two parameters were studied by means of diffusion couple experiments by joining cU based alloys with Al and (Al,Si) alloy. Chemical elements X added into c(U,Mo) were thoroughly chosen on the following criteria: (i) the potential solubility of the alloying element into the c(U,Mo) matrix, (ii) its capability to form the ternary aluminides based on the CeCr2Al20 and Ho6Mo4Al43 – types, and (iii) the feasibility to control the microstructure of the alloys. On this basis, a test matrix is defined. It concerns c(U80,Mo15,X5) alloys (in at.%) with X = Y, Cu, Zr, Ti or Cr. These alloys were homogenized and coupled with Al or (Al,Si) alloy. Results evidenced, first, the importance of the state of homogenization of the c(U,Mo) binary alloy on interaction processes with (Al,Si) alloy, and the benefit on the diffusion of Si through the interaction layer, as observed on the elementary concentration profiles, when the third element X has some solubility into c(U,Mo) alloy. Ó 2011 Elsevier B.V. All rights reserved.

1. Introduction (U,Mo) alloy dispersion fuel is being developed as a high uranium density fuel for research and test reactor cores due to its excellent stability under irradiation [1]. However, irradiation tests performed on (U,Mo) dispersion with pure Al as matrix indicated that an extensive porosity is formed in the interaction layer (IL) between (U,Mo) and Al, which leads, in certain cases, in pillowing and eventual failure of fuel plates [2]. Si additions to the Al matrix may constitute a possible solution to this problem since Si permits to reduce significantly the IL volume and to promote the formation of silicon-rich phase(s) that exhibit(s) better irradiation performance [3–9]. In fact, experiments performed by different authors on (U,Mo)– (Al,Si) diffusion couples showed that Si in enough content in (Al,Si) alloy, modifies the nature of (U,Mo)–Al interaction phases by a Si accumulation process in IL, suppressing the formation of the UAl4 brittle compound and also of the ternary phase U6Mo4Al43 [5,6].

⇑ Corresponding author at: CEA, DEN, DEC, F-13108 St. Paul Lez Durance Cedex, France. Tel.: +33 4 42 25 52 40; fax: +33 4 42 25 48 86. E-mail address: [email protected] (J. Allenou). 0022-3115/$ - see front matter Ó 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.jnucmat.2011.01.130

This last compound is suggested in recent studies to have a poor gas retention capability [10]. Moreover, several parameters driving the growth and the Si enrichment of the IL developed on (U,Mo)–(Al,Si) diffusion couples were identified: temperature and time of annealing, Si content in the (Al,Si) alloys, microstructure of these alloys (i.e. localization of the Si precipitates), stability of c(U,Mo) phase [5–8]. However, the influence of the microstructure of the (U,Mo) alloys on the interdiffusion processes, in particular their homogenization state, remains undetermined, at our knowledge. The first objective of this study consists to evaluate the effect of the homogenization of the (U,Mo) alloys on the Si diffusion process, in (U,Mo)–(Al,Si) diffusion couples, compared with (U,Mo)– Al ones. For this purpose, diffusion-couple tests between as-cast and heat treated c(U,Mo) alloys and Al or (Al,Si) alloys were performed. To enhance the effect of Si addition in the matrix, chemical alloying of c(U,Mo) fuel by addition of a third element was also proposed by several teams [3,11–18]. The ability of various metals to reduce the interdiffusion processes was evaluated in out of pile [11–15] as well as in pile studies [16–18], indicating promising results in the cases of Ti [12,13,17] and Zr [14,15]. In addition, diffusion couple experiments have demonstrated that adding small

J. Allenou et al. / Journal of Nuclear Materials 416 (2011) 205–210

amounts of Ti or Zr into c(U,Mo) can not only reduce the interaction layer growth with Al or (Al,Si) alloy, but also induces a strong accumulation of Si within the interaction layer developed with (Al,Si) alloys, especially in the case of Ti [12,13]. The second objective of this study is to determine the influence on interdiffusion processes when a third element is added to c(U,Mo) coupled with Al and (Al,Si) counterparts. For this purpose, criteria of choice of X addition elements to (U,Mo) alloy were defined and, diffusion-couple tests between c(U,Mo,X) alloys and Al or (Al,Si) alloys were performed.

2.2. Experimental procedure Binary alloys of c(U82,Mo18) (composition in at.%) were melted in an arc-furnace using a non-consumable tungsten electrode and a cooled copper crucible under partial pressure of argon. High purity metals, uranium piece (99.8 wt.%) and molybdenium chips (99.99 wt.%) were used. The ingots were cut into two pieces, half of the samples were employed as-cast, the second half were introduced into silica tubes, which were sealed under argon cleaned vacuum, for homogenization. The heat-treatment was performed at 1175 K for 3 weeks and subsequently the reaction tubes were water-quenched to stabilize the high temperature c(U,Mo) phase. Diffusion-couple tests were prepared by mechanically pressing plates of uranium alloy, with pure Al (99.99 wt.%) or the commercial Al 4343 alloy provided by ALCAN (nominal composition 7.1 wt.% Si, i.e. 6.9 at.% Si), using stainless steel clamps. These couples were isothermally treated at 875 K for 2 or 4 h under Ar atmosphere and quenched into water. X-ray diffraction (XRD – D8 Advanced Brucker) and scanning electron microscopy coupled with energy dispersive spectroscopy (SEM + EDS – JEOL JSM 6400 + OXFORD EDS system) techniques were used to characterize the as-cast and heat-treated (U,Mo) alloys. Microstructure and concentration profiles of interaction layers (ILs) in the different diffusion couple test specimens were determined by SEM + EDS.

2. Influence of the c(U,Mo) homogenization states on c(U,Mo)– Al and c(U,Mo)–(Al,Si) interdiffusion processes 2.1. Previous work on c(U,Mo)–(Al,Si) diffusion couples

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Previous work summarized on the following was carried out to achieve a better knowledge of the microstructural characteristics of ILs developed in the (U,Mo)–(Al,Si) system, depending on the silicon content in aluminum alloys [6]. For this purpose, a set of (U,Mo)–(Al,Si) diffusion couples, with silicon content going from 2 wt.% to 10 wt.% was obtained by thermal annealing. These couples were prepared with homogenized arc melted ingots of c(U,Mo) supplied by AREVA-CERCA (a subsidiary of AREVA NP) fuel manufacturer and (Al,Si) alloys plates supplied by ALCAN. The thermal annealing conditions used for (U,Mo) alloys homogenization were 1275 K for 24 h. Based on SEM + EDS and l-XRD characterisations, our study showed that two main types of ILs existed, depending on the Si content in the (Al,Si) alloy (threshold value: found at about 5 wt.% in the tested conditions). In this work, we will focus on the type corresponding to the highest Si accumulation in IL, assuming that it could lead to the better irradiation behavior. This type of IL is typically obtained when coupling an homogenized (U,Mo) alloy with an Al + 7 wt.% Si alloy. It is characterized by two sub-layers which mainly differ in term of Si content: a high Si enrichment (about 53 at.%) is measured close to the (U,Mo) side and a lower one (about 20 at.%) characterizes the sub-layer located close to the (Al,Si) alloy. l-XRD measurements allowed to identify, close to the (U,Mo) side, the U3Si5 phase coexisting with a highly Si enriched U(Al,Si)3 phase. In order to complete this previous study, and since industrial fuels are not always fabricated with homogenized (U,Mo) alloys, it appears interesting to determine the eventual influence of the homogenization state of the (U,Mo) alloy on diffusion processes with (Al,Si) alloys. The two following sections deal with the experiments performed with this aim.

2.3. Results 2.3.1. Microstructural analysis of IL The diffusion-couple tests performed with as-cast c(U,Mo) alloys provided interfaces with irregular shapes, whereas the IL obtained with c(U,Mo) homogenized samples exhibited much regular and rather parallel interfaces. Typical dimensions of the corresponding IL’s widths were about 350 lm, for (U,Mo)–Al, and about 70 lm, for (U,Mo)–(Al,Si). Metallographic observations of the IL microstructure on the (U,Mo)–Al diffusion couples were in accordance with the literature [19], with no significant change depending on the (U,Mo) homogenization state. Electronic micrographs obtained on the (U,Mo)–(Al,Si) diffusion couples clearly showed that the IL stratification depends of the c(U,Mo) homogenization state: the IL on the (U,Mo)–(Al,Si6.9) diffusion couples annealed at 875 K for 4 h is bi-layered for the c(U,Mo) homogenized alloy (Fig. 1b) whereas it is not for the as-cast c(U,Mo) alloy (Fig. 1c). 2.3.2. Compositional analysis of IL Elementary compositions of IL formed with the c(U,Mo) alloys, in homogenized and as-cast states, coupled with Al or (Al,Si) alloy

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Fig. 1. Compositional profiles (at.%) of IL from (U,Mo)–(Al,Si6.9) diffusion couples with c(U,Mo) homogenized alloys annealed at 875 K for 2 h (a), for 4 h (b) and with as-cast c(U,Mo) alloy annealed at 875 K for 4 h (c).

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and annealed at 875 K for 2 and 4 h were measured using X-ray micro-analysis (EDS). In presence of pure Al, the IL show similar compositional profiles for diffusion couples fabricated with the as-cast alloys and homogenized c(U,Mo) alloys. The Al-to-(U + Mo) atomic ratio increases from the (U,Mo) side to the Al side of IL, which has a composition equivalent to UAl4. Thus, the heat treatment of the c(U,Mo) alloy does not seem to induce any change in IL composition, in (U,Mo)–Al diffusion couples. In presence of (Al,Si) alloy, the profiles of the elemental composition are affected by the homogenization state of the (U,Mo) alloys, as shown by Fig. 1: a preferential accumulation of Si in the interaction products occurred in the (U,Mo)–(Al,Si) diffusion couples prepared with c(U,Mo) homogenized alloy (see Fig. 1a and b), whereas no peak indicating an accumulation of Si in IL was detected in the case of (U,Mo)–(Al,Si) diffusion couples performed with as-cast c(U,Mo) alloy (Fig. 1c). It is also worth noting that the position of the Si peak showing its accumulation in IL (as high as 45 at.% Si) moved towards the (U,Mo) side with the increase of the duration of annealing treatment (Fig. 1b). Regarding the shape of the IL, measurements of the thickness were only performed on couples made with homogenized cU based alloys in the following.

CeMo2±yAl20±y and Ho6Mo4+xAl43 x structure types [22]). It should be noted that no data about the formations of these compounds with Zr and Cu can be found in the open literature. In the case of the ternary Al-rich compounds, it is worth mentioning that, substitutions could take place either on the site A or B, depending on the nature of the X element. Such possibilities of substitution could lead to a modification of the physico-chemical characteristics of the AB2Al20 and A6B4Al43 phases found in the (U,Mo)–Al diffusion couples. The composition of all ternary alloys used in this study is (U80,Mo15,X5) (in at.%). It was chosen on an experimental work basis about the solubility limit of Ti into c(U80,Mo20) alloy. 3.2. Experimental procedures Ternary alloys of c(U80,Mo15,X5) with X = Y, Cu, Zr, Ti or Cr were melted in the same way as the binary alloys (see Section 2.2). High purity metal X chips: Y, Cu, Zr, Ti and Cr (all 99.99 wt.%) were used. Other experimental details related to the homogenization heat treatment, the diffusion couples with pure Al or (Al,Si6.9) alloy and the characterization methods of the samples are also the same as those described in Section 2.2. 3.3. Results

3. Influence of a third element addition into c(U,Mo) on c(U,Mo,X)–Al and c(U,Mo,X)–(Al,Si) interdiffusion processes 3.1. Choice of chemical elements to be added into c(U,Mo) Alloying X elements that will be added to (U,Mo) alloy were chosen according to the following criteria of selection: – Potential solubility of the third element into the c(U,Mo) matrix. – Capability to form the ternary aluminides based on CeCr2Al20 and Ho6Mo4Al43 structure–types, written as AB2Al20 and A6B4Al43. – Feasibility to control the microstructure of the alloys (single phase or not). Regarding these criteria, five elements were selected, namely Y, Cu, Zr, Ti and Cr. As shown in Table 1, these elements mainly differ in terms of solubility in c(U,Mo) and capability to form the ternary Al-rich compounds. According to the binary diagrams (U,X) and (Mo,X), yttrium and copper have no solubility domain with U or Mo elements [20,21]. Thus, these elements should be present in the c(U,Mo) matrix only as precipitates. Conversely, c(U,Mo) alloys can accept in solid solution some amounts of Zr, Ti and Cr (taking into account their respective limits of solubility) [20,21]. These differences will induce different microstructures for the (U,Mo,X) alloys that are expected to impact the interdiffusion processes between these alloys and Al or (Al,Si). Table 1 indicates also the capability of the third element to form or not ternary aluminides AB2Al20 and A6B4Al43 (with respectively

3.3.1. Microstructure of c(U,Mo,X) ternary alloys Metallographic observations of the microstructure of all the ternary alloys, homogenized or not, revealed precipitates dispersed in an (U,Mo) matrix. The homogenization by heat treatment leads to a more regular distribution of the precipitates (in terms of shape and size). The solubility of the X element into (U,Mo) alloy was evaluated by composition measurements using EDS method. In agreement with the literature [20,21], the solubility of Y and Cu is almost null in the homogenized U-based alloys. The solubility of Zr, Ti and Cr was assessed using this semi-quantitative method, revealing that the elemental composition of the final alloys is roughly in agreement with the target composition, for the heat treated samples with X = Zr and Ti only, and for both as-cast and homogenized samples for X = Cr. For all ternary alloys, the XRD characterisations showed diffraction peaks which correspond to the metastable cU phase. The width of the diffraction peaks appears narrower for the homogenized alloys than for the as-cast ones. The influence of the addition of the third element X in solid solution or not into c(U,Mo,X) alloy on the interdiffusion process was only studied on homogenized c(U,Mo,X). 3.3.2. Microstructural analysis of IL The interdiffusion experiments yield the formation of an IL between the U-based alloys and the Al or (Al,Si) alloy. Results concerning the IL thickness measured on c(U,Mo,X) homogenized vs. Al or (Al,Si) diffusion couples, compared with c(U,Mo) (U82,Mo18 in at.%) reference diffusion couples, annealed at 875 K for 4 h, are recapped in Fig. 2.

Table 1 Solubility into c(U,Mo) and ternary aluminides formation capability, for different elements [20–22]. Chemical elements X

Solubility into the c(U–Mo) matrix (at least 5 at.%)

Ternary compounds formation (AB2Al20 and A6B4Al43)

Y Cu Zr Ti Cr

No Low (<1 at.%) Yes Yes Yes

Yes No No Yes Yes

(A,B)–Al type phase Substitution to A sites

Substitution to B sites

Yes No Partial No No

No Partial No Yes Yes

J. Allenou et al. / Journal of Nuclear Materials 416 (2011) 205–210

IL thickness (µm)

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450 400 350 300 250 200 150 100 50 0 References

(U,Mo,X)-Al (U,Mo,X)-(Al,Si)

Y

Cu

Zr

Ti

Cr

Doping elements X Fig. 2. IL thickness measured for reference diffusion couples with c(U,Mo) and homogenized c(U,Mo,X) vs. Al or (Al,Si). The heat-treatment of the interdiffusion experiments was carried out at 875 K for 4 h.

in U(Mo,Y)–(Al,Si6.9) diffusion couple annealed at 875 K for 2 and 4 h, performed with homogenized cU based alloy, show a preferential localization of Si on the (Al,Si) alloy side, with a content as high as 20 at.% Si (Fig. 3a and b). The Y precipitates in the initial U alloy remain in form of precipitates in the c(U,Mo) alloy and in the IL. Unlike the reference (U,Mo) alloy, which leads to Si accumulation in the vicinity of the (U,Mo) side (cf. Fig. 3b), the homogenization of c(U,Mo,Y) alloy does not seem to affect the diffusion of Si and does not promote the accumulation of this element in the IL, close to (U,Mo). Similar concentration profiles were recorded when the third element was copper.

IL widths were measured by drawing parallel lines on each side of the IL, observed in cross-section by optical microscopy. An estimated error corresponding to the minimum and maximum of the IL thickness is also given for each IL width. Alloying (U,Mo) with a third element did not lead to a sharp decrease of the IL thickness developed either with Al or with (Al,Si) alloy, compared to the results of the reference experiments. The reduction of 25% of the IL thickness observed in the case of (U,Mo,Y)–Al and (U,Mo,Ti)–Al couples cannot be considered as valuable, when it is compared to the factor 5 gain induced by Si addition to the Al-matrix. Moreover, all the diffusion couples fabricated by joining with c(U,Mo,X) and (Al,Si) blocks exhibit ILs width of the same order of magnitude.

3.3.3.2. X element in solid solution into c(U,Mo): case of (U,Mo,Cr)– (Al,Si) diffusion couples. The IL of the (U,Mo,Cr)–(Al,Si6.9) diffusion couple, obtained with homogenized c(U,Mo,Cr) alloy annealed at 875 K for 2 h (Fig. 4a), shows a Si concentration profile similar to the one recorded in the (U,Mo)–(Al,Si6.9) reference couple annealed at 875 K for 4 h: a peak indicating a Si accumulation close to the c(U,Mo,Cr) side, up to 40 at.% Si, is clearly evidenced. After 4 h of annealing, a constant amount of Si throughout the IL width is evidenced (Fig. 4b). This amount is about 20 at.% Si, without any preferential localization. The presence of Cr is detected as trace in IL (Fig. 4b). Regarding the effect of the length of the heat treatment on the kinetic of the Si diffusion, it seems than shorter annealing promote the formation of a thin interdiffusion layer with high accumulation in Si. Diffusion couples made with c(U,Mo,X) alloys, where X = Ti and Zr were characterized by the same global interaction behavior, especially in term of Si diffusion process.

3.3.3. Compositional analysis of IL Elementary compositions of IL formed with the c(U,Mo,X) alloys, in homogenized and as-cast states, vs. Al or (Al,Si) alloy diffusion couples, annealed at 875 K for 2 and 4 h, were measured by using X-ray micro-analysis (EDS). In presence of pure Al, the IL show similar compositional profiles for both types of diffusion couples, those fabricated with the c(U,Mo,X) alloys and those made with (U,Mo) alloy. Note that the concentration of the X element remains constant and negligible throughout the IL even if the alloying metal was in solid solution into the cU based alloy. The effect of the solubility of the third element X into c(U,Mo) alloy does not seem to produce any change in IL composition compared to the (U,Mo)–Al references. In presence of (Al,Si) alloy, the profiles of the elemental composition is affected by the solubility (or not) of the third element X into c(U,Mo) alloys. Results concerning the elementary composition obtained on (U,Mo,X)–(Al,Si) diffusion couples in presence or not of a X element in solid solution into c(U,Mo) alloy are presented below.

4. Discussion This discussion is divided in two parts corresponding to the aims of the study: the first is devoted to the influence of the

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3.3.3.1. X element in form of precipitates into c(U,Mo): case of (U,Mo,Y)–(Al,Si) diffusion couples. The concentration profiles of IL

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Fig. 3. Compositional profiles (at.%) of IL for (U,Mo,Y)–(Al,Si6.9) diffusion couples, prepared with c(U,Mo,Y) homogenized alloy annealed at 875 K for 2 h (a), for 4 h (b).

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Fig. 4. Compositional profiles (at.%) of ILs for (U,Mo,Cr)–(Al,Si6.9) diffusion couples, prepared with c(U,Mo,Cr) homogenized alloys annealed at 875 K for 2 h (a), for 4 h (b).

(U,Mo) homogenization state on the interdiffusion processes with Al and (Al,Si) alloys and, the second, to the influence on these processes of a third element added to homogenized c(U,Mo) coupled with Al or (Al,Si) counterparts. 4.1. Influence of the c(U,Mo) homogenization state The first factor identified in our study to have an impact on the Si diffusion is the homogenization state of c(U,Mo) alloy. In fact, an (U,Mo)–(Al,Si) diffusion couple performed with c(U,Mo) homogenized alloy exhibits a high Si accumulation on the (U,Mo) alloy side, whereas no accumulation is obtained with an as-cast (U,Mo) alloy. In an out of pile study performed on diffusion couples, Perez also reported that Mo and Si content could affect the interdiffusion behavior of the (U,Mo)–Al system [7]. This process is very likely linked to the homogenization of the Mo concentration within the grains and to the impact of the Mo local concentration on the Si diffusion. Indeed, using atomistic modelisation, Garcès evidenced a strong interaction between Si and Mo atoms, at the reacting interface, which could modify the Si diffusion processes [9]. In order to precise the role of Mo on the Si diffusion process, it would be interesting to get information about the local environment of this element in the IL, thanks to micro X-ray Absorption Spectroscopy (l-XAS). 4.2. Influence of a third element added to c(U,Mo) The influence of a third element added to c(U,Mo) on the interdiffusion processes in (U,Mo)–Al and (U,Mo)–(Al,Si) systems was investigated using diffusion couple experiments between pure Al or (Al,Si) alloy and ternary c(U,Mo,X) based alloys, with X = Y, Cu, Zr, Ti, Cr. The interaction layers (IL) developed on diffusion couples were carefully examined in terms of kinetics of the IL growth and elementary composition. From the IL growth point of view, the alloying with a ternary element such as Cu, Zr, Cr does not significantly change the growth kinetics of IL between c(U,Mo,X) and pure Al, compared to the reference (U,Mo)–Al couples. When the X element is Ti or Y, a reduction of about 25% of the IL thickness is observed. Our results confirm the potential efficiency of Ti to reduce the interdiffusion between c(U,Mo,Ti) and pure Al, as already reported in the literature [12,13]. Conversely, the capability of Zr to reduce the interdiffusion, claimed in some studies [14,15] was not confirmed. The diffusion couples between c(U,Mo,X) and (Al,Si) display similar thickness range of IL when compared to the (U,Mo)–(Al,Si) references. Only a slight reduction of the interdiffusion thickness is observed in the case of alloying with Ti or Zr, in accordance with the results obtained by Park et al. [13,14]. So, in agreement with literature [12–15], no significant change (at least with the characterization means used in this study) of

(U,Mo)–Al interaction mechanisms seems to be observed when Ti or Zr are added to c(U,Mo): the thickness of the IL remains of the same order of magnitude and the solubility of the third element seems to be negligible in the interdiffusion zone (it is close to the detection limits of EDS analytical method). Similar behaviors are observed when X = Cr and also with elements which are precipitated (X = Y, Cu). In contrast, the diffusion behavior between (U,Mo,X) and (Al,Si) strongly depends on the nature of the X alloying element, more especially its solubility into the c(U,Mo) matrix. Indeed, when the third element X can be solubilized into c(U,Mo) alloy, the kinetics of diffusion of Si through the IL seems to be favoured. This is the case of (U,Mo) alloying with elements such as Zr, Ti, Cr, which provoke a preferential localization of the Si at the vicinity of the c(U,Mo,X) side. The Si accumulation is more or less strong, depending on the annealing treatment conditions. On the opposite, when the third element X does not have a detectable solubility into c(U,Mo) alloy (case of Y and Cu, in this study), only low-Si enriched IL are developed. In such cases, the Si remains on the (Al,Si) alloy side. To summarize, the Si diffusion through the IL seems to be strongly dependant on the microstructure of the (U,Mo,X) alloys, which can be described as single phase or multiphases with the presence of precipitates of the X element. Our results corroborate the out of pile studies performed on (U,Mo,X)–(Al,Si) diffusion couples with X = Zr and Ti in solid solution into c(U,Mo) alloy, which evidenced an enhancement of the diffusion of Si through the IL [13–15]. Nevertheless some questions need to be solved on (U,Mo,X)– (Al,Si) interaction system, in the case of X elements being in solid solution into c(U,Mo) alloy. In particular, the nature of the phases formed in the interaction layer and the atomic local environment of X atoms are to be determined, For this purpose, l-XRD and l-XAS measurements are in progress on (U,Mo,X)–(Al,Si) samples, in order to try to determine the role played by X elements on the (U,Mo)–(Al,Si) interdiffusion processes.

5. Conclusion The influence on (U,Mo)–Al and (U,Mo)–(Al,Si) interdiffusion processes of (i) the homogenization state of the c(U,Mo) alloy, (ii) the addition of a third element X into c(U,Mo) were studied thanks to interdiffusion experiments. Results evidenced first the importance of the state of homogenization of the c(U,Mo) alloy, on the diffusion of Si in (U,Mo)–(Al,Si) diffusion couples to form Si rich IL. Concerning the addition of a third element X into c(U,Mo) alloy, a kinetics approach revealed that, in the testing conditions used in this study, c(U,Mo,X) vs. Al or (Al,Si) alloy diffusion couples present

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IL thicknesses similar to those obtained on (U,Mo)–Al and (U,Mo)– (Al,Si) reference couples. Nevertheless, elementary composition profiles showed that, when the third element X has some solubility into c(U,Mo) alloy, diffusion and accumulation of Si in the IL seems to be favoured. Further works are in progress to understand this effect.

Acknowledgments C. Jarousse and M. Grasse, from AREVA-CERCA (Romans, France) are gratefully acknowledged for supplying the (U,Mo) samples used in this study. H. El Bekkachi and members of CMEBA (Université de Rennes 1) are warmly thanked for diffusion couples preparation and their characterizations.

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