Geochemically derived non-gaseous radionuclide source term for the Asse salt mine -: assessment for the use of a Mg(OH)2-based backfill material

The Asse salt mine was used as a test site for radioactive waste disposal from 1967 to 1978. Low- and intermediate-level waste forms (LLW/ILW) were emplaced, containing a total radionuclide inventory of 3 x 10(15) Bq, estimated for reference date January 1, 2005. It is expected that MgCl2-rich brine will enter the emplacement rooms and react with the cemented waste products. Possible microbal degradation of organic waste components in MgCl2-rich brine could produce significant quantities of CO,. resulting in an acidification and consequently in an increase of element solubilities. Application of buffering backfill materials is discussed for closure of the mine. Selection of backfill materials is based on geochemical modeling taking into account the corrosion of the cemented LLW/ILW and degradation of organic waste components. In the present Study the evolving geochemical milieu and respective solubilities of Am, Np, Pu. U. Th., Tc, Sr, Cs and I were modeled for each emplacement room. Laboratory experiments were Undertaken to selectively verify the modeling predictions. Geochemical modeling leads to the conclusion that Portland cement. a M-(OH)(2)-based material and crushed salt can be used in different combinations as backfill materials. According to the predictions Mg(OH())2-based backfill material controls the pH and concentration of dissolved inorganic carbon within ranges that are favorable with respect to actinide solubilities. A source term for radioelements and fission products is derived from a comparison of the respective solubilities and the radionuclide inventories. Calculated solubilities of Th. Pu and U are lower than their inventories. These are controlled by the thermodynamic solubility therefore. Maximum solution concentrations of all other elements are controlled by their inventory.