Probability of a PWR Uncanistered Fuel Waste Package Postclosure Criticality

The purpose of this calculation is to estimate the probability of criticality in a pressurized water reactor (PWR) uncanistered fuel waste package during the postclosure phase of the repository as a function of various waste package material, loading, and environmental parameters. Parameterization on the upper subcritical limit that is used to define the threshold for criticality will also be performed. The possibility of waste package misload due to human or equipment error during preclosure is also considered in estimating the postclosure criticality probability. The internal degradation scenarious for the 21 PWR absorber plate waste package have been previously developed for the purpose of performing criticality evaulations of the possible degraded configurations (Ref. 7.4, Section 7.1). Figure 2-1 shows a schematic view of the expected sequence of degradation following breach of the package. Since the waste package (WP) interior was inserted with He prior to breach, the initial configuration will be the as-built basket (Fig. 2-1A). Within a few hundred years following the breach, the carbon steel and aluminum components will degrade to insoluble corrosion products as shown in Figure 2-1B (Ref. 7.4, Section 7.1, and Ref. 7.18). While structural calculations show that the absorber plates can support the load of the assemblies, localized corrosion in the crevice regions at the corners of each cell will likely cause collapse shortly after failure of the structural components. However, the majority of the borated stainless steel (BS-SS) absorber plates will remain largely undegraded and remain between the assemblies, along with corrosion products from the degraded carbon steel tubes (Fig. 2-1C).