Leaching behaviour of unirradiated high temperature reactor (HTR) UO$_2$–ThO$_2$ mixed oxides fuel particles

The dissolution of different mixed oxide (U, Th)O$_2$ particles under reducing conditions has been studied using a continuous flow-through reactor. The U/Th ratio seems to have no or little influence on the normalised leaching rate of thorium or uranium, The release rate of uranium from the outer surface of a Th rich matrix seems to follow the behaviour of pure UO2 even though U is a minor component in these phases and the dissolution rate of Th is much lower. After long time U concentrations will become depleted at the solids surface and it will be expected that U release rates will become controlled by the release rates of thorium (rates at neutral pH < 10$^{−6}$ g m^{−2}$ d$^{−1}$). Under reducing conditions, the matrix of HTR fuel particles presents significant intrinsic radionuclide confinement properties