In situ high temperature X-ray diffraction study of the kinetics of phase separation in the uranium-plutonium mixed oxid
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In situ high temperature X-ray diffraction study of the kinetics of phase separation in the uranium-plutonium mixed oxide (U0.55Pu0.45)O2-x Romain VAUCHYa,b, Renaud.C. BELINa*, Anne-Charlotte ROBISSONa, Fiqiri HODAJb a
CEA Cadarache, CEA, DEN, DEC, SPUA, Cadarache F-13108 Saint-Paul-Lez-Durance, France b
Science et Ingénierie des Matériaux et Procédés (SIMaP, associé au CNRS UMR 5266 – UJF/INP-Grenoble), Domaine Universitaire, 1130 rue de la piscine, BP 75, F-38402 Saint Martin d’Hères, France *Corresponding author at CEA, DEN, DEC, SPUA, Cadarache F-13108 Saint-Paul-LezDurance, France. Phone: +334 42 25 49 54, Fax: +334 42 25 47 17, E-mail: [email protected] ABSTRACT Uranium-plutonium mixed oxides incorporating high amounts of plutonium are considered for future nuclear reactors. For plutonium content higher than 20%, a phase separation occurs, depending on the temperature and on the oxygen stoichiometry. This phase separation phenomenon is still not precisely described, especially at high plutonium content. Here, using an original in situ fast X-ray diffraction device dedicated to radioactive materials, we evidenced a phase separation occurring during rapid cooling from 1773 K to room temperature at the rate of 0.05 and 2 K per second for a (U0.55Pu0.45)O2-x compound under a reducing atmosphere. The results show that the cooling rate does not impact the lattice parameters of the obtained phases at room temperature but their fraction. In addition to their obvious fundamental interest, these results are of utmost importance in the prospect of using uranium-plutonium mixed oxides with high plutonium content as nuclear fuels. INTRODUCTION For plutonium content above 20%, uranium-plutonium mixed oxides are polyphasic at room temperature, the UO2-PuO2-Pu2O3 sub-system exhibiting a miscibility gap [1-9]. Based on various experimental studies, several representations are available in the literature [3,5,8,9]. Guéneau et al. [10] have modeled the U-Pu-O system with the Calphad method [11]. Several domains are observed in the UO2-PuO2-Pu2O3 subsystem. Between 20% ≤ %Pu ≤ 45%, the miscibility gap is composed of a slightly hypostoichiometric fcc phase (high-oxygen phase) in equilibrium with a more reduced fcc phase (low-oxygen phase). Above ~45% Pu, the highoxygen fcc phase is in equilibrium with a α-Pu2O3 -type body centred cubic bcc phase [5-9]. However, the phase separation not only depends on the plutonium content but also on the O/M ratio and on the temperature. At high temperature, the miscibility gap boundary is shifted to higher Pu contents and disappears at 1073 K [3]. For a given Pu content, the critical temperature is the highest temperature at which the phase separation is likely to occur and corresponds to a specific value of O/M ratio. Thus, if sintered fuel is processed in reducing conditions, phase separation is likely to occur during cooling. This is a critical point because this may induce the formation of cracks [8,9]. Furthermore, the fuel properties may vary and lead to important in-pile
safety prob
