The Use of Burnup Credit for Spent Fuel Cask Design

The U.S. Department of Energy's (DOE's) Office of Civilian Radioactive Waste Management (DOE/OCRWM) is in the process of developing a new generation of high capacity casks to transport spent fuel from existing commercial nuclear reactor facilities to future federal waste facilities. The DOE's role in the Federal Waste Management System (FWMS) is defined by U.S. Federal law in the 1982 Nuclear Waste Policy Act (NWPA) and its 1987 amendment (NWPAA). The NWPAA requires DOE to use spent fuel and high level radioactive waste casks certified by the U.S. Nuclear Regulatory Commission (USNRC). Because of the high shipping rates anticipated, and since cooling times of spent fuel that wiU be shipped significantly exceed the design cooling times of existing casks, a decision was made to develop new higher capacity casks. The potential benefit of higher cask capacities, is fewer shipments. Fewer shipments result in health, safety, and cost benefits. In evaluating the needs of the cask development program a number of technical issues were identified that would further support improved cask capacities. Burnup credit is one of these technical issues (Sanders, et. al., 1987). The DOE/OCRWM program is currently designing casks for fuel having up to 4.5% initial enrichment of U-235. It is expected that this enrichment level will be exceeded by new fuel designs. Since burnup credit accounts for actual reactivity of fuel, and higher initial enrichments are used to allow greater fuel burnup, discharged fuel is expected to have similar reactivity regardless of initial enrichment (e.g., high initial enrichment with high burnup is similar to low initial enrichment with low burnup). Burnup credit casks permit accommodation of higher initial enrichments without redesign and physical modification of the criticality control system. The DOE/OCRWM program focuses its efforts on burnup credit on its use in spent fuel transport casks. However, the fundamental technical issues of burnup credit are addressed in a comprehensive manner, so that application of the information developed will be of use in other areas of the DOE/OCRWM program, such as spent fuel storage (Sanders, et. al., 1991). Burnup credit is the practice of accounting for the reduced reactivity of spent fuel in evaluating criticality safety. In the U.S., the USNRC transportation regulations (USNRC, 10 CFR Part 71) require subcriticality of transport systems. These regulations do not elaborate on how subcriticality should be assured, nor do they prohibit the use of burnup credit for criticality safety. The USNRC transportation regulations are based on those of the International Atomic Energy Agency (IAEA); however, the IAEA regulations and their accompanying guidance (IAEA, 1985 and 1987) do address actual irradiation experience for irradiated fissile material in determining subcriticality.