THE DISSOLUTION OF UNIRRADIATED UO2 FUEL PELLETS UNDER SIMULATED DISPOSAL CONDITIONS

The rate of dissolution of unirradiated UO//2 fuel pellets under mildly oxidizing conditions has been investigated. A static leaching procedure was used, and the extent of dissolution was monitored by radiochemical analysis for uranium in the leachate. At 70 degree C, the average dissolution rate is approx. 5 multiplied by 10** minus **6 kg multiplied by (times) m** minus **2 multiplied by (times) day** minus **1, being smaller in granite groundwater than in distilled deionized water. The rate exhibits a first order dependence on the oxygen content (Po//2 equals 25-75 kPa) and on the carbonate and bicarbonate concentrations ( less than 10** minus **3 mol/L) of the leachant. The rate of dissolution increases with increasing temperature (30-90 degree C) in distilled deionized water, but shows the opposite behavior in granite groundwater. The rate increases with decreasing pH or pOH, and is minimal in the pH range 5-10. These data will aid in the characterization of used UO//2 fuel as a waste form, and in establishing reliable source terms at the vault-geosphere interface of a geologic nuclear fuel waste disposal vault.