Modeling the Failure of Nuclear Waste Containers

A model was developed to predict the failure of Grade-2 titanium (Ti-2) nuclear waste containers. Two major corrosion modes were included: failure by crevice corrosion (CC) and failure by hydrogen-induced cracking (HIC). A small number of containers were assumed to be defective and to fail within 50 years of emplacement. The model is probabilistic in nature, and each modeling parameter was assigned a range of values, resulting in a distribution of corrosion rates and failure times. The crevice corrosion rate (R_CC) was assumed to be dependent only upon properties of the material used and the temperature of the vault. CC was assumed to initiate rapidly on all containers and to propagate indefinitely without repassivation. Failure by HIC was assumed to be inevitable once container temperature (T) fell to ≤ 30°C. Depending upon the rate at which they were expected to cool, temperature-time profiles for individual containers were approximated by two-step or single-step temperature-time functions. These functions then were used with experimentally measured corrosion rates to compute the fractional failure rates and cumulative fractions of containers failed as a function of time. Approximately 97% of all containers were predicted to fail by CC. Only ~ 0.025% were predicted to fail before 500 years, the time considered a minimum for containment of nuclear waste. The majority of containers were predicted to fail between 1,200 years and 7,000 years.