The radionuclide characteristics of light-water-reactor (LWR) spent fuel play key roles in the design and licensing activities for radioactive waste transportation systems, interim storage facilities, and the final repository site.

The purpose of this calculation is to evaluate the transient behavior and consequences of a worstcase criticality event involving intact pressurized water reactor (PWR) mixed-oxide (MOX) spent nuclear fuel (SNF) in a degraded basket configuration inside a 21 PWR waste package (WP). This calculation will provide information necessary for demonstrating that the consequences of a worst-case criticality event involving intact PWR MOX SNF are insignificant in their effect on the overall radioisotopic inventory and on the integrity of the repository.

The purpose of this analysis is to evaluate the transient behavior and consequences of a worst case criticality event involving intact pressurized water reactor (PWR) spent nuclear fuel (SNF) in a degraded basket configuration inside a 21 PWR assembly waste package (WP). The objective of this analysis is to demonstrate that the consequences of a worst case criticality event involving intact PWR SNF are insignificant in their effect on the overall radioisotopic inventory in a WP.

The objective of this calculation is to perform intact and degraded mode criticality evaluations of the US Department of Energy's (DOE) Advanced Test Reactor (ATR) Spent Nuclear Fuel (SNF) placed in the DOE standardized SNF canister. This analysis evaluates the codisposal of the DOE SNF canister containing the ATR SNF in a 5-Defense High-Level Waste (%-DHLW) Short Waste Package (WP) (Bechtel SAIC Company, LLC [BSC] 2004a), which is to be placed in a monitored geologic repository (MGR).

The responsible use of calculational methods in nuclear criticality safety includes a determination of bias and bias uncertainty that may exist between the calculated results and reality. Such biases exist due to approximations used to model the real world, uncertainties in nuclear data, and approximations associated with the calculational method (e.g., Monte Carlo method). The bias and bias uncertainty are typically determined by using the modeling approximations, nuclear data, and calculational method to model well-known, usually critical, systems.

The OECD/NEA “Burn-up Credit Criticality Benchmark” working group has studied the effect of axial burn-up profile on the criticality of a realistic PWR spent fuel transport cask (Phase II-B). The final results of this benchmark are presented and analysed in this report. Nine basic cases and two additional accident configurations were considered with the following varying parameters: burn-up (0 GWd/t for fresh fuel, 30 and 50 GWd/t), fuel composition (actinides only and actinides with fifteen fission products), axial burn-up discretisation (1 or 9 zones).

The Nuclear Regulatory Commission (NRC) Office of Regulatory Research (RES) has initiated a program to support effective implementation of burnup credit in the criticality safety assessment of transport and dry storage casks. The goal is to develop technical bases that can be used to provide criteria and guidance for use in licensing activities. The program is being conducted in a phased approach, with the initial focus on unresolved issues related to the use of actinide-only bumupcredit in transport and dry storage casks designed for spent fuel from pressurized-water reaciors (PWRs).