THE STABILITY OF FISSION-PRODUCTS IN URANIUM-DIOXIDE

A review of experimental data concerning the behaviour of fission products in nuclear fuels is used to illustrate the significant variation in solubility exhibited by the different species. To understand the reasons for this variation, it is necessary to obtain a reliable estimate of the solution energies and thus to determine the most stable solution site. This we suggest will be critical in predicting the behaviour of nuclear fuels in both accident and normal operating conditions. We have therefore used the Mott-Littleton simulation technique to calculate solution energies for the fission products Br, Kr, Rb, Sr, Y, Zr, Te, I, Xe, Cs, Ba, La and Ce in UO2. We considered solution at both uranium and oxygen vacancies, the interstitial site and at the di-, tri- and tetra-vacancy complexes. Non-stoichiometry and variable charge state are important components of the model. From these results we conclude that the solubility is significantly affected by non-stoichiometry. In UO2 and UO2-x, products such as Cs, Rb and Ba are thermodynamically more stable as binary oxide precipitates. Conversely, Y, La and Sr are soluble in UO2 and UO2+x,while Cs, Rb, Sr and Ba are only soluble in UO2+x. The behaviour of I, Br and Te is complicated by the fact that these species are most stable as anions in UO2 and UO2-x but as cations in UO2+x. In our model, Zr and the inert gas species Xe and Kr are always predicted to be insoluble, while CeO2 will form a solid solution with UO2.