3rd WP Probabilistic Criticality Analysis: Methodology for Basket Degradation with Application to Commercial SNF

This analysis is prepared by the Mined Geologic Disposal System (MGDS) Waste Package Development (WPD) department to describe the latest version of the probabilistic criticality analysis methodology and its application to the entire commercial waste stream of commercial pressurized water reactor (PWR) spent nuclear fuel (SNF) expected to be emplaced in the repository. The purpose of this particular application is to evaluate the 21 assembly PWR absorber plate waste package (WP) with respect to degraded mode criticality performance. The degradation of principal concern is the borated stainless steel absorber plates which are part of the waste package basket and which constitute a major part of the waste package criticality control. The degradation (corrosion, dissolution) of this material will result in the release of most of the boron from the waste package and increase the possibility of criticality. The results of this evaluation will be expressed in terms of the fraction of the PWR SNF which can exceed a given keff as a function of time and the peak value of that fraction over a time period up to several hundred thousand years.
The ultimate purpose of this analysis is to support the waste package design which defines waste packages to cover a range of SNF characteristics. In particular, with respect to PWR criticality the current categories are: (I) no specific criticality control material, (2) borated stainless steel plates in the waste package basket, and (3) zirconium clad boron carbide control rods (Ref. 5.4). The results of this analysis will indicate the coverage provided by the first two categories. With these results, this study will provide the first quantitative estimate of the benefit expected from the control measure consisting of borated stainless steel plates.
This document is the third waste package probabilistic criticality analysis. The first two (Ref. 5.12 for the first and Ref. 5.15 for the second) analyses were based primarily on the waste package criticality of the design basis fuel with a limited extension to SNF with other characteristics, and that only in terms of an infinite array (k~ versus keff) for the other fuel types. The previous analyses were also limited in the coverage of the range of possible input parameter values (material corrosion rates and the rate of water into the waste package). This analysis will show the sensitivity of criticality performance with respect to variations in the distribution of these parameters.