CRITICAL DISCUSSION OF RELEVANT PHYSICAL ISSUES SURROUNDING THE WEEPING OF NUCLEAR-WASTE CASKS

Spent fuel casks have arrived at fuel disposal sites on numerous occas ions with removable external surface contamination levels in excess of regulatory limits, although preshipment surveys indicated much lower contamination levels. This phenomenon is called ''weeping'' and a conc eptual model has been proposed here to describe the same. A passive, v itreous oxide film having M-O and M-OH bonds is believed to form on th e SS 304 cask surface which incorporates the pool-borne radio-nuclides into its network structure during cask-loading in the spent fuel pool . Reordering and crystallization of the film takes place some time aft er the cask has been removed from the pool, decontaminated and shipped . The incorporated nuclides are thus able to migrate through the more defective, crystalline film to the film exterior, leading to a ''weepi ng'' phenomenon. The model successfully takes into account the effect of various factors which are known to influence weeping, e.g., humidit y and temperature of the environment, cask exposure to sunlight, and i ts surface roughness.