Segregation of Fission Products to Surfaces of UO 2
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Segregation of Fission Products to Surfaces of UO2 C.R. Stanek, Robin W. Grimes1 and Mark R. Bradford2 Dept. of Materials, Imperial College of Science, Technology and Medicine London, SW7 2BP UNITED KINGDOM 1 Los Alamos National Laboratory MS-K765 Los Alamos, NM 87545 2 British Energy Generation Ltd. Barnwood, Gloucestershire, GL4 3RS UNITED KINGDOM ABSTRACT In order to predict the release rates of fission products from UO2 nuclear fuel, it is necessary to determine the energetics associated with their segregation from the bulk to surfaces. Here segregation is determined by calculating the total energy of a simulation repeat unit which includes a fission product, as a function of the distance of the fission product from the surface. We have investigated the possibility that specific fission products segregate preferentially to certain surfaces. In particular, Ce4+ and Zr4+ segregate to the (100) surface, while Ba2+ and Sr2+ segregate to the (111) surface. Two issues make these calculations more complex. First, charged defects need to be compensated by oxygen vacancies. For example, divalent Ba2+ and Sr2+ substituting on uranium sites are compensated by a single oxygen vacancy. Second, certain surfaces, such as the (100), are inherently unstable, but can be stabilized by oxygen vacancies. Arrangements of these surface defects lead to complex surface structures that affect segregation energetics. INTRODUCTION Our motivation for this study is to better predict the release of fission products from nuclear fuel, if the fuel cladding is breached. There have been extensive studies, both theoretical and experimental, which have attempted to initially categorize fission products based upon their chemistry within the fuel matrix [1,2] and later studies which concerned themselves with the diffusion of fission products and the distribution of fission products within a single grain [3-5]. An important aspect of this problem is the segregation of fission products to specific surfaces of UO2. Such calculations require an atomistic approach which, to our knowledge, has not been applied to this problem previously. More specifically, we calculate the segregation of fission products to the (111) and (100) surfaces. It will be shown that the segregation of fission products is surface dependent. Other aspects important to fission product segregation (such as defect cluster orientation) will also become evident. METHODOLOGY Fission product segregation is determined using atomistic simulation techniques based on energy minimisation using pair potentials. In this work, the computational codes CASCADE [6] and MARVIN [7] are employed. CASCADE is used to determine bulk perfect lattice energies, which are subsequently incorporated into MARVIN, which simulates surfaces and interfaces. AA3.32.1
The energies calculated by the different codes are comparable because they use the same shortrange potential description of forces and both codes use energy minimisation techniques. MARVIN constructs a surface by periodically repeating a characteristic
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