Corrosion studies on the material combination used for the POLLUX cask for direct disposal of spent fuel elements in rock salt formations

A high rank is attributed to the packaging under the concept of direct disposal of spent fuel elements in a rock salt formation. The packaging has to ensure over a period of 500 years (working hypothesis) the safe containment of the wastes against the mechanical load of the rock pressure and the attack of corrosive salt brines in hypothetical accident scenarios in a repository. The POLLUX container system developed by DWK under these aspects provides a mechanically stable disposal container (steel 15 MnNi 6.3) with a corrosion protection layer of Hastelloy C4 applied by surface welding onto the body of the disposal container. Shielding against the gamma and neutron radiation is ensured by a surrounding nodular cast iron container and a polyethylene filler on a AlMgSi 0,5 carrier material. In the framework of activities reported here it was examined whether the corrosion behavior of Hastelloy C4 is influenced by surface welding and the simultaneous corrosion of the different POLLUX materials. For this purpose, specimens from two typical surface welds were examined in long-term immersion tests (18 months at the maximum) in three disposal relevant salt brines (two MgCl2-rich brines and one NaCl-rich brine) at 1500C. Besides Hastelloy C4, also the corrosion behavior of nodular cast iron in salt brines was studied. (orig./MM)