Dynamic experiments: Intermediate topical report

This report presents the results of dynamic experiments performed in the period 2015 – 2016 as part of the programme of the Belgian Agency for Radioactive Waste and Enriched Fissile Materials (NIRAS/ONDRAF) on the geological disposal of high-level/long-lived radioactive waste. The tests were performed with depleted UO2 and Pu doped UO2 in cement waters at pH 12.5 and 13.5. With depleted UO2 at pH 12.5, the dissolution rate and uranium concentrations were similar as measured previously, indicating little oxidative dissolution. At pH 13.5, the dissolution rate and uranium concentrations were higher, indicating oxidative dissolution. This may be due to oxidation by traces of O2 in the glove box, combined with enhanced dissolution at high pH, although other aspects may also intervene (e.g. low density of the UO2). Although in the experiments reported here great care was taken to keep very low O2 levels in the glove box, (i.e. anoxic conditions), for future experiments with depleted UO2, an even better redox control will be applied to reproduce better the reducing conditions expected in situ, by addition of reducing agents to the solution. The dissolution rate and uranium concentrations at pH 13.5 were also higher than in previous tests with depleted UO2. The reason for this will be investigated further. The dissolution rate was smaller for Pu-doped UO2 F6 (alpha activity of a standard Belgian UO2 fuel with an age of about 90 ka) than for depleted UO2, and the low uranium concentrations in the leachate for the Pu-doped UO2 suggest little oxidation, in spite of the higher alpha activity of the doped UO2. This suggests that the doping of fuel with a small amount of Pu induces a lower sensitivity to oxidation. This should also be further investigated. New dynamic tests are proposed to verify the effect of reducing agents on the dissolution rate of depleted UO2 and to confirm the low dissolution rate of Pu-doped UO2 F6.