Fission gas release and volume diffusion enthalpy in UO2 irradiated at low and high burnup

Samples of UO2, irradiated in LWRs at burnups from 25 000 to 95 000 MWd/t at in-pile temperatures below 800 K, were submitted to Knudsen-effusion experiments. In addition to the equilibrium vapour pressure of non-volatile species, release of fission gas was measured as a function of temperature and time up to approximately 2700 K, where complete vaporisation of the sample was eventually achieved. The fractional release curves of the main isotopes of Kr and Xe were measured and analysed with an algorithm adequate to discriminate between the relevant release stages, one of which, controlled by atomic diffusion, is analysed in this paper. The obtained enthalpy of diffusion – the same for all the examined nuclides – decreases from 4.6 eV/atom, at low burnup, to 2.95 eV/atom, at the highest burnup. Furthermore, the fraction of fission gas released via atomic diffusion markedly increases with increasing burnup, indicating a dramatic attenuation of the intragranular sink strength.