Demands placed on waste package performance testing and modeling by some general results of reliability analysis

Waste packages for a U.S. nuclear waste repository are required to provide reasonable assurance of maintaining substantially complete containment of radionuclides for 300 to 1000 yr after closure and of permitting only controlled release of radionuclides for 10,000 yr. The waiting time of failure for complex failure processes affecting engineered or manufactured systems is often found to be an exponentially distributed random variable. Assuming that this simple distribution can be used to describe the failures of hypothetical single-barrier waste packages, bounding calculations show that the mean time to failure would have to be [gt]10[sup 7] yr in order to provide reasonable assurance of meeting this requirement. With two independent barriers, each would need to have a mean time to failure of only 10[sup 5] yr to provide the same reliability, illustrating that the use of redundant independent barriers is the key to both achieving and demonstrating regulatory compliance. However, even this demonstration would require testing tens of thousands of two-barrier systems for several decades. As more barriers are added, the mean lifetime required of each individual barrier decreases, and the demonstration of performance becomes more feasible, although still requiring extensive testing and observation during the preclosure period for performance confirmation.more » In any case, the results illustrate that neither the engineered barrier system nor the geologic barrier system alone is likely to provide the required degree of assurance of repository safety.« less