Assessment of the risks associated with continued storage of high-level waste in single-shell tanks at Hanford

The risks associated with continued future storage of high-level defense waste in single-shell tanks have been analyzed. The study considered the potential risks over the next few decades. The detailed analysis covered the first decade (1980 to 1990) to provide a specific time frame for computation. Decay of the nuclides reduces the risk after the first decade, so the results represent an upper limit to the risk during any one of the next several decades. The risk methodology developed and described here includes identifying the initiating events that may cause radioactive material to leave the tanks, tracing the possible transport of radioactive material to leave the tanks, tracing the possible transport of radioactive material offsite with a quantitative systems model, and calculating the population exposure and resulting health effects. A sensitivity analysis identifies the key initiating events and exposure pathways, and shows the implications of uncertainties in the input and model parameters. The results of the analysis show the possibility of one chance in 80,000 of a fatal cancer among the regional population from a decade of storage. The consequence distribution of the accidents is also presented. Individual risk is about one chance in 20 billion of a fatal cancer from a decade of storage, compared to one chance in four of fatal cancer from other causes. Genetic risk is about one chance in 50,000 of one genetic defect in the regional population over several generations. Congenital abnormalities from other causes occur in about 6% of live births. The higher consequence range of the distribution shows that there is about one chance in 40 million of an accident resulting in one or more fatal cancers. The sensitivity of the results to various assumptions is shown. Of the accident sequences examined here, the event with highest risk is structural failure leading to dome collapse