Dissolution rates of unirradiated UO2, UO2 doped with 233U, and spent f under normal atmospheric conditions and under reducing conditions using an isotope dilution method

This report contains the results of dissolution testing of unirradiated UO2 fuel pellet materials, specially prepared unirradiated UO2 fuel pellet materials that contained 5 or 10% 233U, and spent fuel. As the objective was to determine the effects of alpha, beta and gamma radiation on the rate of processing of solid through the solution phase. The alpha doping levels were chosen to simulate the alpha activity in spent fuel at times of 3000 and 10 000 years after disposal. Dissolution rates were measured using an isotope dilution technique. The purpose was to eliminate uncertainties due to precipitation effects. All the measurements were performed in a dilute, synthetic groundwater solution, buffered by sodium bicarbonate/carbonate (modified Allard groundwater). The work reported was performed as part of the IN CAN PROCESSES project of the European Commission 5th Framework Programme (2000-2003). Long-term isotope dilution tests with unirradiated UO2 fragments and pellets under normal atmospheric conditions gave higher concentrations of U in solution than expected based on experience with previous leaching experiments with spent fuel. The tests begun to show the effects of precipitation at the end of the experimental time of 800 days. Further testing is needed before the samples reach steady state. A conclusion was drawn that care must be taken in interpreting the results obtained in different laboratories because the results for the behaviour of unirradiated UO2 may be affected by manufacturing effects. The isotope dilution tests under actively reducing conditions were performed in the presence of a strip of pure Fe in solution. The data from the batch tests with undoped UO2 fragments indicated that the iron strip was very effective in producing reducing conditions for uranium. The [U] in solution was less than the detection limit of the analytical method (< 0.02 ppb) after a few days to a few weeks of testing. A 'puff test' procedure, that used several short exposures of the solid sample to solution, was developed to eliminate as much as possible any U that entered the solution through an initial rapid dissolution of slightly oxidized material. The results at the end of the tests showed that 90 % of the 235U added as spike had precipitated onto the solid sample or the iron strip. The results of the tests with 233U-doped UO2 fragments under reducing conditions showed only small effects that might be attributed to the presence of alpha activity in the solid samples. The doped samples seemed to dissolve faster in the first short contact periods (≤ 14 days). In the final tests there was no evidence of enhanced dissolution due to alpha radiolysis. Generally, strong sorption and/or precipitation phenomena in the experimental system with metallic iron complicated the interpretation of results. Further testing is needed using more sensitive analytical techniques to establish the dissolution rate of the 233U-doped materials. Testing with spent fuel under H2 atmosphere (10 bar) showed decreasing [U] together with increasing [Cs] indicating that the H2 was able to act as a reducing agent in the system. It was possible to calculate the dissolution rate for U from the spent fuel that was occurring in the presence of precipitation. Further testing of spent fuel is needed to obtain a dissolution rate in the presence of actively corroding iron. (orig.)