Probabilistic analysis of uncertainties in the disposal of nuclear waste
Abstract
To determine a probabilistic analysis of the safety of a nuclear waste repository, a separated and sequential treatment of the waste migration problem is developed. The new approach consists of a description of the transmission of waste nuclei from interface to interface by a transfer kernel. The separation provides valuable information about the performance of each individual medium and how they contribute to the overall system performance. It also provides a framework for comparison of other safety assessment studies. The equations to describe the release rate of radionuclides for a simplified two-region model are obtained. The two-region model simulates the release of radionuclides from the repository and its subsequent transport through an overlying aquifer. Data that characterizes an actual proposed repository emplaced in a salt dome is used. A probabilistic analysis to investigate the parameter uncertainties at the level of the transfer kernel attempts to calculate the average release rates based on probability distributions assigned to the controlling variables. The widely considered scenario of accidental intrusion of water from the overlying rock into the filled repository during the post-operational period is generalized to have a probability distribution for possible intrusion times. Averaging over the brine intrusion time distribution gives the 'overall' average release rate. Numerical results for two important 'waste' nuclides, both with long half-life, are provided. The results include the average release rates from the aquifer and the repository for I-129 and Np-237 for five different brine intrusion times. The average release rates from the aquifer are quite different than the release calculated with a set of single conservative values. A reduction of about two orders of magnitude is observed, pointing out the significance of the probabilistic results over a single deterministic calculation. The release rates averaged over a brine intrusion distribution are also provided for three different mean times of brine intrusion, resulting in a further smearing out of the release rate. The averaging has a lesser effect in the repository region than in the aquifer domain. Thus, in general, the release rates from the aquifer are only slightly lower than the release rates from the repository. (Abstract shortened with permission of author.)
Degree
Ph.D.
Advisors
Ott, Purdue University.
Subject Area
Nuclear physics|Environmental science
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