Xenon migration and trapping in doped ThO 2

Abstract

The effect of small amounts of additives of different valence (Y 2O 3 or Nb 2O 5) on xenon migration and on radiation-induced trapping in ThO 2 has been studied to improve the understanding of the mechanism of rare gas diffusion in UO 2 and ThO 2. ThO 2 was chosen as it has the same lattice structure as UO 2 and the valence of Th 4+ is unlikely to change to compensate for the different valence of the impurity. The diffusion of 233U was measured at 1400° C and 1550° C in both doped and undoped ThO 2. More rapid diffusion in Nb 2O 5-doped ThO 2 and less rapid diffusion in Y 2O 3-doped ThO 2 were found. This indicated that doping was effective in producing extrinsic behaviour. Xenon was introduced into the specimens by ion bombardment. At low bombardment dose, doping did not appreciably affect the release indicating that xenon does not diffuse via single vacancies on either sublattice. As alternatives, diffusion via interstitial sites or in small vacancy clusters of constant size remained. Temporary trapping (retarded gas release) was observed at higher doses. Additions of Nb 2O 5 decreased and additions of Y 2O 3 increased this retardation, thus showing that impurities can affect trapping phenomena.