Chemical durability of vitrified high-level waste and spent fuel under simulated repository conditions of a Boom Clay formation

The Belgian programme is considering both the closed and open fuel cycle options. In the closed cycle, HLW glasses from the former Eurochemic reprocessing plant and from the French R7T7 plants are studied. R and D is focused on the interaction with the Boom Clay disposal host and potential near field environments. In surface laboratory testing we developed new set-ups to investigate various coupled processes: (1) combined glass leaching/Si diffusion in clay, (2) diffusion/sorption/precipitation of Si in clay, (3) effect of the presaturation of clay with Si, and (4) the mobile concentrations of e.g. Np, Tc, Se in clay slurries after leaching from glass. In the CORALUS in situ test, we installed four tubes in the underground laboratory (SCK-CEN site), and retrieved two of them after an interaction time of ∼1 year (90 deg. C) and ∼3 years (30 deg C). The R and D on the geological disposal of spent fuel in Boom Clay is focused on two major issues: (1) study the effect of α-activity on the dissolution of α-doped UO2 (simulating spent fuel ages between 150 and 90000 years), and (2) study the influence of Boom Clay and potential backfill materials (apatite, cement, sand) on the α-doped UO2 dissolution rate. Different experimental set-ups have been elaborated. Flowthrough tests in clay water and static tests in clay slurries were carried out in reducing conditions. We report the main results obtained on both HLW glass and doped UO2. Both HLW glass and UO2 show a similar evolution of the dissolution behaviour with time in the clay containing media. After an initial relatively fast dissolution characterized by sorption onto the clay, a much lower dissolution rate is observed. For both waste forms this results in expected lifetimes during geological disposal of 105 years or more. (author)