Safety assessment of the used fuel disposal concept requires a study of the integrity of used fuel in contact with groundwater. In this context, the initial release of 137Cs and I29I from used Canada deuterium uranium fuel segments, exposed to water at 25°C, has been studied as a function of fuel irradiation history. Percentages of inventories released after 5 days of leaching are compared to stable xenon fuel-sheath gap inventories. Cesium-137/xenon and 129I/xenon release ratios average ∼0.2 for low linear power rating (LLPR) fuel. For high linear power rating fuel, ratios are considerably larger and may approach 1. For LLPR fuel, the ratios become larger when the leaching time is increased. It is proposed that these differences are related to the microstructure of used fuel. The results indicate that the source term for the instantaneous release of isotopes of cesium and iodine should include all of the fuel-sheath gap inventory. Power history data and calculated gas release data can be used to accurately estimate the contribution of the fuel-sheath gap inventory to the source term for radionuclide release.