Determining site-specific drum loading criteria for storing combustible (sup 238)Pu waste

摘要

Waste containing hydrogenous-combustible material contaminated with (sup 238)Pu can generate hydrogen gas at appreciable rates through alpha radiolysis. To ensure safe transportation of WIPP drums, the limit for (sup 238)Pu-combustible waste published in the WIPP TRUPACT-11 CONTENT (TRUCON) CODES is 21 milliwafts per 55 gallon drum. This corresponds to about 45 milligrams of (sup 238)PuO(sub 2) used for satellite heat source-electrical generators. The Los Alamos waste storage site adopted a (sup 238)Pu waste storage criteria based on these TRCUCON codes. However, reviews of the content in drums of combustible waste generated during heat source assembly at Los Alamos showed the amount of (sup 238)Pu is typically much greater than 45 milligrams. It is not feasible to appreciably reduce Los Alamos (sup 238)Pu waste drum loadings without significantly increasing waste volumes or introducing unsafe practices. To address this concern, a series of studies were implemented to evaluate the applicability of the TRUCON limits for storage of this specific waste. Addressed in these evaluations were determination of the hydrogen generation rate, hydrogen diffusion rates through confinement layers and vent filters, and packaging requirements specific to Los Alamos generated (sup 238)Pu contaminated combustible waste. These studies also showed that the multiple-layer packaging practices in use at Los Alamos could be relaxed without significantly increasing the risk of contamination. Based on a model developed to predict H(sub 2) concentrations in packages and drum headspace, the site specific effective hydrogen generation rate, and hydrogen-diffusion values, and revising the waste packaging practices, we were able to raise the safe loading limit for (sup 238)Pu waste drums for on site storage to the gram levels typical of currently generated (sup 238)Pu waste.