Dissolution behaviour of UO2 in anoxic conditions. Comparison of Ca-bentonite and Boom Clay

In order to determine in how far the clay properties influence the dissolution of spent fuel, experiments were carried out with depleted UO2 in contact with Boom Clay water and in the presence of 2 kinds of clays: Ca-bentonite or Boom Clay. The leach tests were performed at 25°C in anoxic atmosphere for 2 years. The U concentrations in the clay water were followed during these 2 years, and the amount of U in the clay was determined after 2 years in order to determine the UO2 dissolution rate. The uranium concentration after 0.45 µm filtration was 50 times higher in the Boom Clay system (2.0x10-7 mol.L-1) than in Ca-bentonite system (6.5x10-9 mol.L-1), probably due to colloid formation in the Boom Clay system. Most released uranium was found in the clay. The fraction of uranium, dissolved from the UO2 pellet and found on the clay represents about 42 % of total uranium release in the system Boom Clay system and more than 76 % in the system Ca-bentonite system and global dissolution rates were estimated at about 2.0x10-2 and 3.4x10-3 µg.cm-2.d-1, respectively. This is not much lower than for similar tests with spent fuel, reported in literature.