Radionuclide Source Term for the ASSE Salt Mine: Geochemical Assessment for the Use of Magnesium(II) Based Backfill Material

Low- and intermediate-level radioactive waste was emplaced in the Asse salt mine. Application of backfill materials to provide mechanical stability, plugging of openings and chemical buffering is discussed for closure of the mine. If no remedial action is carried out in the emplacement rooms, degradation of the organic matter cannot be excluded which may result in acidification of the geochemical environment or actinides may be complexed by HCO₃⁻ / CO₃²⁻. 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, Tc, Sr, Cs and I were modeled. The solubilities and the radionuclide inventory define the geochemical source term for each emplacement room. Laboratory experiments were undertaken to verify the modeling predictions. The modeling leads to the conclusion that Portland cement, a Mg(OH)₂-based material and crushed salt should be used in different combinations as backfill materials. According to the modeling predictions, the Mg(OH)₂-based backfill material provides a long-term stable geochemical environment and scavenges CO₂ by formation of (Ca,Mg)-carbonates. The source term calculations were carried out for the presence of the backfill material in the emplacement rooms. The inventory of Pu and U in the waste is sufficient to allow these actinides to reach solubility limits. Maximum concentrations of the other radionuclides are limited by their inventories, which are too low to reach solubility limits. Regarding corrosion of cemented waste forms and the backfill material in MgCl₂ rich brines, experimental data and calculated results agree well. Experimental results demonstrate the ability of the Mg(OH)₂-based material to buffer the pH and to limit the HCO₃⁻ / CO₃²⁻ concentration in the brines. Based on both the modeling and the experimental results, it is highly recommended to apply the Mg(OH)₂-based backfill material in the Asse salt mine.