The Importance of Mobile Fission Products for Long-Term Safety in the Case of Disposal of Vitrified High-Level Waste and Spent Fuel in a Clay Formation

In Belgium, the possibility to dispose of high-level radioactive waste in clay formations is studied since 1976. In the PAGIS report (published in 1988) the most important contributors to the total dose via a water well pathway were 237Np, 135Cs and 99Tc. Since 1988, several elements that strongly influence the calculated doses have evolved: - the inventory of mobile fission and activation products in vitrified high-level waste has been improved; - the half-life of 79Se has been re-estimated; - progress was made in the determination of migration parameters. In recent performance assessments, the actinides and 135Cs do not significantly contribute to the total dose, as they remain confined in the host clay formation during several millions of years due to sorption on clay minerals. Consequently, the total dose resulting from the disposal of vitrified high-level waste or spent fuel is essentially due to releases of mobile fission and activation products. The most important contributors to the total dose via a water well are now in the case of disposal of high-level waste: 79Se, 129I, 126Sn, 36Cl, and 99Tc.