Further insight into mechanisms of solid-state interactions in UMo/Al system

Abstract

In this paper, the solid-state interactions between metastable γ-UMo alloys (containing 5, 7 and 10 wt%Mo) and Al, at temperatures ranging from 440 to 600 °C and for ageing times up to 10 h, are studied using the diffusion couple technique and nuclear fuel plate annealing. The reaction product consists of three main zones, two of them presenting a periodic layered morphology. The growth kinetics is limited by solid-state diffusion and Al is the most mobile species. Both growth kinetics and its global energy of activation are similar to that found for the U/Al binary system. The diffusion path is determined and phase equilibrium relations are deduced for the Mo-poor part of the U–Mo–Al metastable ternary phase diagram.

Introduction

The UMo dispersion fuel is being developed to convert the Materials Testing Reactors cores currently working with UAl_x and U₃Si₂, with a more dense fuel capable to meet the requirements of the nuclear non-proliferation treaty, with low or no modification in initial design. This treaty promotes peaceful nuclear issues and gives a value of 20% as the maximum permissible limit of ²³⁵U enrichment (low enriched uranium (LEU)) [1]. U–Mo alloys are considered as one of the most promising uranium alloys for a high uranium density dispersion fuel due to the good irradiation performance of the cubic γ-uranium phase. Understanding the interaction between UMo and its Al matrix is a key stage for the research and development of a UMo-based LEU fuel, behaving in a satisfactory manner under irradiation [2], [3], [4].

In spite of the great interest in this system, there are very few relevant studies on the mechanisms of interdiffusion in the U–Mo–Al system. The main reasons for this lack of investigation are as follows: (i) At temperatures lower than 565 °C, the cubic metastable γ-UMo phase undergoes the metastable transformation γ-UMo → α-U + γ-UMo(Mo enriched) or the eutectoid decomposition γ-UMo → α-U + U₂Mo(γ′) (see Fig. 1 [5], [6]) thus limiting the annealing time of interaction in γ phase in UMo/Al diffusion couples. (ii) The investigated T range (440–600 °C) is close to the Al melting point and may lead to a plastic deformation of Al during diffusion couple experiments. (iii) Thermodynamics of the U–Mo–Al ternary system is not well known.

Many studies on binary U/Al diffusion couples were performed in the sixties [7], [8], [9]. In this system, UAl₃ is the only phase formed at the diffusion temperature [10] and Al is the mobile species [7]. To the author’s knowledge, only three relevant studies on UMo/Al diffusion couple experiments [11], [12], [13] have been carried out:

• Mirandou et al. [11] studied the interaction between homogenised γ-U–7 wt%Mo (noted U7Mo) and Al at 580 °C under a high-purity Ar atmosphere. Annealing times, of less than 4 h, are chosen to prevent the decomposition of γ-UMo phase. They reported that the interaction zone (thickness e = 175 μm after 4 h of annealing) is composed of three layers: from UMo to Al, a first layer L1 (containing 3.4 at.%Mo and 19.6 at.%U), a second layer L2 (containing 2.6 at.%Mo and 15.1 at.%U) and a very thin layer L3 (e ≈ 1 μm), close to Al, the composition of which was not determined. The compounds constituting both layers L1 and L2 were identified as (U,Mo)Al₃, (U,Mo)Al₄ and UMo₂Al₂₀ but no details concerning the constitution of each layer are given. When U7Mo alloy is used as-cast (not homogeneous in composition), the decomposition of the γ-UMo phase occurred and the total thickness of the reaction layer increased considerably (e ≈ 700 μm after 4 h of annealing). The interaction layer, which is neither regular nor layered, is made of an undetermined mixture of (U,Mo)Al₃ and U₆Mo₄Al₄₃ phases. Although the presence of (U,Mo)Al₃ and (U,Mo)Al₄ phases is often assumed in UMo/Al couples, the solubility of Mo in UAl₃ and UAl₄ phases has never been demonstrated. This means that if UAl₃ and UAl₄ are stoichiometric compounds then layers L1 and L2 can be composed by more than one phase.

Thus, the question is to know whether the layers L1 and L2 are single-phase or not.

• In a recent study, Park et al. [12] reported U7Mo/Al diffusion couple experiments performed in vacuum at 580 and 600 °C. The reaction layers are regular and the total thickness at 580 °C (145 μm after 5 h of annealing) is lower than that obtained by Mirandou et al. [11]. The ratio Al/(U + Mo) through the reaction layer, increases from γ-UMo to Al, but no details on the nature of the phases contained in this layer are given.

• Ryu et al. [13] performed U10Mo/Al diffusion couple experiments at 550 °C for 5 and 40 h in vacuum. The authors reported nearly the same conclusions concerning the three-layered morphology previously described. However, the total thickness of the interaction zone after 5 h of annealing (e ≈ 540 μm) is much higher than that obtained by Mirandou et al. (e ≈ 175 μm) [11] and Park et al. (145 μm) [12] at higher temperature (580 °C).

Most probably, a change in UMo alloy composition from 7 to 10 wt%Mo cannot lead to such a dramatic increase in the growth rate of the interaction layer.

Ryu et al. [13] also studied the thermal ageing of dispersed UMo fuel plates (U10Mo/Al) between 500 and 550 °C. For T = 525 °C and 550 °C, the reaction layer is divided into two layers, an internal layer (close to UMo) similar to (U,Mo)Al₃ and an external layer labelled (U,Mo)Al_4,4. By assuming solid-state diffusion control of the growth kinetics they calculated a global activation energy of about 300 kJ mol⁻¹. This activation energy is substantially higher than that of the interdiffusion process in the U/Al binary system (60–80 kJ mol⁻¹ [7]). Despite the fact that the overall composition of the interaction zone is almost comparable to that obtained in diffusion couple experiments, the growth kinetics of the interaction layer for dispersed UMo fuel plates is substantially decreased compared to diffusion couple case (e ≈ 15 μm compared to 540 μm after 5 h at 550 °C). This is well confirmed by a recent study performed by Park et al. [12] on the thermal ageing of dispersed U7Mo fuel plates at 550 and 580 °C for ageing times up to 50 h.

In all these studies neither the fine structure of the interaction zone (spatial distribution), nor the dynamics of the system and the diffusion path are described.

The aim of this paper is to study the role of Mo in solid-state interaction between UMo alloys and Al and to describe the fine structure of the interaction zone and the diffusion path through this system.