Transportation cask contamination weeping: A program leading to prevention

摘要

This paper describes the problem of cask contamination weeping, and efforts to understand the phenomenon and to eliminate its occurrence during spent nuclear fuel transport. The paper analyses of field experience and scoping experiments, and concentrates on current modelling and experimental validation efforts. The ''weeping'' phenomenon associated with spent fuel transportation casks (also known as ''sweating'') is believed to be due to the conversion of fixed contamination on the external surface of the cask to a removable form. Spent fuel transportation casks are loaded under water at nuclear power plants in a spent fuel storage pool, exposing the cask surfaces to contamination by radionuclides present in the pool water including (sup 137)Cs, (sup 134)Cs, and (sup 60)Co. The external surfaces of loaded casks are routinely surveyed for removable contamination and decontaminated to 1/10 of the US and IAEA regulatory limits prior to being released for shipment (49CFR 1983, IAEA 1989). However, 3% to 8% of US spent fuel casks have arrived at final destinations with removable surface contamination in excess of that allowed by regulation, though many preshipment surveys have shown contaminant levels to be within allowable limits (Grella 1987). Attempts to reduce the incidence of weeping have met with limited success and resulted in time-consuming operational constraints and procedures that significantly increase cask processing times and occupational exposures at loading facilities. As the US Department of Energy (DOE) moves toward a high volume spent fuel transportation campaign beginning in 1998, the elimination of weeping occurrence and minimization of operational constraints has received increased attention.