Gap and grain boundaries inventories from pressurized water reactor spent fuels

To assess the long term behaviour of spent fuel in a French nuclear waste repository, the chemical reactions between the fuel and possible intruding water must be understood and the resulting radionuclide release must be quantified. The instant release fraction source term, assumed to be instantaneously accessible to water after the failure of the waste container and the saturation of the engineered barrier, includes the soluble radionuclides (RN) located within the gap, the fractures and grain-boundary areas. At first, to provide an estimate of the potential RN releases from French PWR spent fuels, Cs and Sr inventories from the gap region have been measured on different UOX and MOX fuel samples (20 mm segments of clad fuel rods) as a function of the Burnup (from 22 to 60 GW d t−1) and the fission gas releases (FGR). Moreover and because of a lack of knowledge on high Burnup fuels, a specific measurement of grain boundary inventories has been carried out on a spent fuel with a Burnup of 60 GW d t−1 and 2.8% of FGR. Leaching experiments have been made under pseudo dynamic conditions in a presence of carbonates on a powder 20–50 μm in size. The chemical and radiochemical analysis of the different samplings show that the Cs grain boundary inventory is less than 0.4% of the total Cs inventory whereas the Cs gap release is about 1%. The estimated Sr grain boundary inventory is less than 0.2% of the total Sr inventory. Theses results are also compared with the literature data.