Alpha particle damage in oxide nuclear fuel

A model for the ingrowth of Frenkel defects in alpha particle irradiated uranium dioxide is proposed. It is assumed that interstitials and vacancies spontaneously recombine at room temperature if they are separated by a single cation jump. A statistical calculation using large computer arrays to simulate the geometry of the lattice enables the defect concentration to be obtained as a function of atomic displacements. Comparison of the model to experimental lattice parameter data for UO2 enables the number of stable cation Frenkel pairs produced per alpha particle determined to be around 25. This figure is supported by independent evidence. The differing damage behaviour of single and polycrystalline material is discussed and is attributed to the difference in original microstructures. A computer simulation of a damaged lattice is used to deduce the annealing behaviour of point defects during post-irradiation heating. It is deduced that the observed lattice dilation is primarily due to cation defects and that up to three annealing stages will be observed. The stages are, with increasing temperature, 'correlated' recombination, vacancy migration and finally dissociation of clustered vacancies and/or migration of cation interstitials. (author)