Physico-chemical characterization of bituminized Eurochemic medium-level waste Task 3 Characterization of radioactive waste forms A series of final reports (1985-89) No. 5

The questions addressed concern swelling phenomena during leaching of large samples, the leaching of radio-isotopes under clay repository pressure condition, and the ageing behaviour of real Eurobitum samples. The swelling behaviour upon leaching was investigated using inactive Eurobitum simulates; the leaching and ageing behaviour were evaluated using real, small Eurobitum samples. The swelling behaviour due to water uptake proceeds almost linear with the leaching duration. The swelling is not controlled by the water diffusion into the bitumen matrix, but rather by the hardness of the bitumen. An increase of the external pressure from 0.1 to 4.0 MPa, corresponding with the lithostatic pressure of the clay repository, yields a decrease in swelling with a factor of two. The presence of clay has only a marginal influence on swelling. The inactive simulates are not relevant to conclude about the swelling behaviour of active Eurobitum, but they seem so for the leaching behaviour of the inactive waste salts. The leaching of NaNo3 and CaSO4, which account for 80% of the waste, is diffusion controlled. A full size drum would be 90% NaNO3 depleted after about 5000 years (ambient temperature). The leaching of real Eurobitum was investigated using small samples, in claywater, in oxidizing conditions. The leached fractions for the radio-isotopes are very divergent: after one year, values are 15% (Sr90), 1.3% (Co60), 0.01% (Cs137) and below 0.1% (actinides). This means that the immobilization of the actinides is relatively efficient. Data of the physical properties until 4 and 7 years ageing were obtained. The most important observation seems to be the hardening of the bituminizates with ageing; the softening point, e.g. increases from about 1010C upon production to about 1090C after seven years