The Effect of Fuel Power on the Leaching of Cs and I From Used Fuel

Abstract

The safety assessment of the concept of geological disposal of used fuel requires a source term for the instantaneous release of long-lived radionuclides from used fuel. Preferential release of the gap inventories of Cs-137 and I-129 from used fuel with a variety of linear power ratings (LPR) was studied. A one-to-one and a ten-to-one correlation exists between measured gap inventories of stable xenon and the amount of Cs-137 and I-129 in the gap, for high-LPR and low-LPR fuels, respectively, as obtained from 5-day leaching experiments. These differences in release patterns for high- and low-LPR fuels can be explained by differences in the microstructure, and disappear when longer leaching times (i.e., 3 months) are used. These results imply that, on a geological time scale, the entire inventory of the long-lived isotopes of cesium and iodine must be considered as part of the instantaneous release source term. Attempts to quantify inventories of cesium and technetium at grain boundaries by comparing the short-term leaching behaviour of oxidized and non-oxidized fuel indicated that either very insoluble cesium-uranium compounds may exist at the grain boundaries, or that cesium and stable xenon grain boundary inventories are not similar. More research is needed before a firm conclusion can be reached as to whether the entire grain boundary inventories of Cs-137 (and Tc-99), as estimated from power histories, should be part of the instantaneous release source term.