(GRS 168) Comparison of safety assessments for spent CANDU or LWR fuels in repositories in rock salt

Dieter Buhmann, Alice Ionescu, Margarit Pavalescu, Richard Storck

A long-term safety assessment of a repository for either spent CANDU or LWR fuel elements in a salt formation has been performed. A hypothetical repository site has been considered, using data of the European Community project PAGIS for all parts of the system: near field, overburden, and biosphere. Three scenarios have been taken into account: subrosion as the normal evolution of the salt dome, a combined scenario with brine intrusion from the overburden and from undetected brine pockets, and a cavern scenario representing future human actions. Spent fuel elements have been assumed to be disposed of in big storage casks in drifts. For reasons of comparability, the same source term model has been applied for both waste types, but different inventories of radionuclides and different heat production rates have been taken into account. The key parameter
for assessing long-term safety was the radiation exposure in the biosphere.
The results of the calculations demonstrate that both types of waste can be disposed of safely. For the combined brine intrusion scenario and for the human intrusion scenario, the radiation exposures are below the limit of German radiation protection law. In the subrosion scenario, however, for conservative values of the input parameters, the limit is exceeded in some calculations. This reflects the situation of a flat emplacement area, which is quickly decovered by subrosion, leading to high nuclide concentrations. As the subrosion model is very simple and over-conservative, this result must not be over-interpreted. A comparison with the results of PAGIS, which have been derived from the disposal of high-level waste in deep boreholes, in general shows higher consequences in case of a direct disposal of fuel elements.