ONDRAF/NIRAS is developing and evaluating a surface disposal concept for low and intermediate level short-lived radioactive waste (LILW-SL) at Dessel, Belgium. In support of ONDRAF/NIRAS's assignment, SCK•CEN carried out long-term performance assessment calculations for the inorganic non-radioactive components that are present in LILW-SL. This paper summarizes the results obtained from calculations that were done for a heavily engineered surface disposal facility at the nuclear zone of Mol/Dessel. The calculations address the migration of chemotoxic elements from the disposed waste to groundwater.

Screening calculations were performed first to decide which non-radioactive components could potentially increase concentrations in groundwater to levels above the groundwater standards. On the basis of very conservative calculations, only 6 out of 41 chemical elements could not be classified as having a negligible impact on man and environment. For each of these six elements (B, Be, Cd, Pb, Sb, and Zn), the source term was characterized in terms of its chemical form (i.e., metal, oxide, or salt), and a macroscopic transport model built that would capture the small-scale dissolution processes relevant to element release from a cementitious waste container. Furthermore, reliable transport parameters in support of the convection-dispersion-retardation (CDR) transport calculations were determined. This included derivation of (1) solubility for a cementitious near field environment based on thermodynamic equilibrium calculations with The Geochemist's Workbench, and (2) distribution coefficients based on a compilation of literature values. Scoping calculations illustrated the effects of transport parameter uncertainty on the rates at which inorganic components in LILW-SL leach to groundwater.