Mitigation of Non-Fixed Cesium Surface Contamination on Spent-Fuel Transport Packages – Confirmation of Ion Exchange Mechanism

A “cask weeping” incident occurs when a spent-fuel shipping cask that swiped “clean” for nonfixed contamination at the point of origin is later found to have removable contamination on its surface, or to have contaminated the adjacent environment. It was hypothesized that when a shipping cask is submerged in a spent-fuel storage pond during loading, 137Cs sorbs strongly onto clay particles affixed to the shipping cask surface. Then, during shipping, ion exchange processes release the 137Cs and contaminate the surrounding environment, i.e., “weeping” occurs. Implicit in this model are technologies for minimizing the possibility of weeping. This paper discusses our success in loading the Cs-sorbing sites prior to immersion into a pool so that the amount of Cs initially retained by a cask surface is much diminished. Technologies were also successfully developed for post-immersion Cs removal. Results are compared with 60Co, another problem radioisotope in weeping incidents, but one whose retention and release is not controlled by an ion exchange mechanism or greatly affected by the presence of clays.