NRC SFST ISG-8: Burnup Credit in the Criticality Safety Analyses of PWR Spent Fuel in Transportation and Storage Casks

Title 10 of the Code of Federal Regulations (10 CFR) Part 71, Packaging and Transportation of
Radioactive Material, and 10 CFR Part 72, Licensing Requirements for the Independent
Storage of Spent Nuclear Fuel, High-Level Radioactive Waste, and Reactor-Related Greater
Than Class C Waste, require that spent nuclear fuel (SNF) remain subcritical in transportation
and storage, respectively. Unirradiated reactor fuel has a well-specified nuclide composition
that provides a straightforward and bounding approach to the criticality safety analysis of
transportation and storage systems. As the fuel is irradiated in the reactor, the nuclide
composition changes and, ignoring the presence of burnable poisons, this composition change
will cause the reactivity of the fuel to decrease. Allowance in the criticality safety analysis for
the decrease in fuel reactivity resulting from irradiation is termed burnup credit. Extensive
investigations have been performed both within the United States and by other countries in an
effort to understand and document the technical issues related to the use of burnup credit.

Revision 3