Molecular Dynamics Study on Grain Boundary Diffusion of Actinides and Oxygen in Oxide Fuels

Abstract

Diffusion phenomena of actinides and oxygen are relating to various properties of oxide fuels, e.g. actinide and oxygen redistribution, pore migration, sintering behavior. Although lots of experimental data have been obtained for U and O diffusions, these seem to be a little scattered among experiments. In addition, there are few studies of transuranium elements. Recently, we experimentally showed that diffusion coefficients of Am and Pu were almost comparable with that of U and the contribution of grain boundaries (GB) were relatively large in oxide fuels. On the other hand, the molecular dynamics (MD) calculation showed that U ions migrated via its vacancies and the O diffusion mechanism changed with temperature in bulk. In this study, the GB diffusion coefficients and the mechanism were systematically evaluated for U, Am and O in oxide fuels by MD calculations. For actinides and oxygen, their GB diffusion coefficients are significantly larger than bulk ones, and the diffusion mechanisms in GBs are almost independent on temperature. Furthermore, the GB diffusion is strongly dependent on its structure and the interfacial potential energy. The lower oxygen-to-metal (O/M) ratio gives the larger GB diffusion coefficients for actinides and oxygen.

Application of the UMACS process to highly dense U1−xAmxO2±δ MABB fuel fabrication for the DIAMINO irradiation

“…At a microscopic scale, comparing to a reactive sintering, the intermediate milling step of the UMACS process allows the removal of several microstructural defects generated during solid solution formation. Indeed, the limitations of U 1Àx Am x O 2±d reactive sintering are presumably mainly due to the Kirkendall effect [20][21][22][23], since the rare data available on Am +III interdiffusion in U 0.8 Am 0.2 O 1.9 [24] or in UO 2 [25] and on self-diffusion of U in UO 2 and UO 2+d [26][27][28][29][30][31] suggest quite different diffusion kinetics within and between the two oxides. As a result, anchored closed porosity is created by the Kirkendall effect during the first thermal treatment and is eliminated by the grinding step.…”