Irradiation effects of synthetic coffinite (USiO 4 ) studied by in-situ TEM
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Irradiation effects of synthetic coffinite (USiO4) studied by in-situ TEM J. M. Zhang,1 F. Y. Lu,2 V. Pointeau,1 F. X. Zhang,1 M. Lang,1 C. Poinssot,3 J. Lian,2 and R. C. Ewing1* 1
Department of Geological Sciences, University of Michigan, Ann Arbor, MI 48109 Department of Mechanical, Aerospace & Nuclear Engineering, Rensselaer Polytechnic Institute, Troy, NY 12180 3 Nuclear Energy Division, Department of Radiochemistry and Processes, Commissariat a l’Energie Atomique – CEA Marcoule, 30200 Bagnols-Sur-Ceze Cedex 2
*Corresponding author. Emails: [email protected] ABSTRACT Coffinite (USiO4, I41/amd, Z=4) is the major alteration phase of uraninite, UO2+x, under reducing conditions in natural uranium deposits. Thus, it is important to understand the radiation response of coffinite because it is an expected alteration product of the UO2 in spent nuclear fuel. In the present study, we conducted in-situ transmission electron microscopy (TEM) investigation of synthetic coffinite under 1 MeV-Kr2+ ion beam irradiation. The radiation-induced crystallineto-amorphous transformation was observed in the synthetic nanocrystalline USiO4, with a critical dose of ~ 0.27 displacements per atoms (dpa) for which full amorphization occurred at room temperature. The critical dose increases as rising irradiation temperature, and above the critical temperature (Tc), ~ 608 K, coffinite cannot be amorphized. These results are compared with previous studies on the isostructural zircon (ZrSiO4, Tc=1000K) and thorite (ThSiO4, Tc>1100K), which indicates that synthetic coffinite is more stable to ion beam irradiation at elevated temperature. INTRODUCTION Coffinite, USiO4.nH2O, is the important phase as the alteration product of the spent nuclear fuel (SNF) (~95% of UO2) in Si-rich groundwaters under reducing condition [1-9]. Consequently, it is important to understand the radiation damage in coffinite, as it is inevitably exposed to significant radiation dose due to alpha-decay events over geological periods. Coffinite is an orthosilicate, with general formula of ABO4, and the space group is I41/amd , Z=4 [10]. The A-site, which contains uranium, is surrounded by 8 oxygen atoms: 4 at the distance of 0.238 nm and 4 at the distance 0.236 nm, distances comparable to those in UO2 (0.236 nm) [11]. The coffinite structure consists of chains of alternating, edge-sharing SiO4 tetrahedra and UO8 triangular dodecahedra extending parallel to the c-axis [12]. The phase stability and irradiation response of the isostructural orthosilicates, including zircon (ZrSiO4), hafnon (HfSiO4), and thorite (ThSiO4), have been studied extensively by ion beam irradiations combined with in-situ TEM observation [13-15]. All of these orthosilicates are sensitive to the radiation-induced, crystalline-to-amorphous transformation at room temperature at relatively low doses (0.17-0.22 dpa) and can be amorphized even at temperatures up to 1000 K. However, no irradiation stability data were available for the uranium end-member, coffinite,
Table I. The ratio of electronic to nuc
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