Dissolution Behaviour of Used Candu Fuel Under Disposal Conditions.

The dissolution behaviour of Zircaloy clad used CANDU fuel in aqueous solutions has been examined under mildly reducing conditions at 95°C for a period of 16 months. The effects of various container components and groundwaters on the dissolution of the fuel have been investigated. The components studied were titanium (container material), carbon steel (fuel bundle support basket) and soda-lime glass beads (container infill material). Leaching solutions included deionized water (DIW) and synthetic saline groundwaters. p]The presence of glass beads or carbon steel did not appear to significantly affect solution concentrations of 137Cs, 90Sr, 99Tc, or 238U. Radionuclide concentrations were one to two orders of magnitude higher in saline groundwaters than in DIW. After about 30 days leaching, continued radionuclide release from the fuel was strongly inhibited in the absence of oxygen, and radionuclide concentrations in solution remained virtually constant. X-Ray photoelectron spectroscopy (XPS) measurements of the oxidation state of fuel fragments leached in groundwaters, showed a surface composition of less than UO2.33, below the postulated threshold for oxidative dissolution.