Spent Fuel Project Office, Interim Staff Guidance - 8, Revision 1, Burnup Credit in the Criticality Safety Analyses of PWR Spent Fuel in Transport and Storage Casks

Unirradiated reactor fuel has a well-specified nuclide composition that provides a
straightforward and bounding approach to the criticality safety analysis of transport and storage
casks. 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 typically 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 burnup credit. Much of this work has been considered in the
development of the U.S. Department of Energy’s Topical Report (TR) on Actinide-Only Burnup
Credit for Pressurized Water Reactor (PWR) Spent Nuclear Fuel Packages (DOE/RW-0472).
The technical information provided in the literature and in the various TR revisions, together
with the initial confirmatory analyses by the U.S. Nuclear Regulatory Commission (NRC)
research program, have provided a sufficient basis for the staff to proceed with acceptance of a
burnup credit approach in the criticality safety analysis of PWR spent fuel casks as discussed in
the Recommendations below. Although insights gained from reviewing the TR submittals form
a part of the basis for the staff’s position, this interim staff guidance does not approve the TR or
its supporting documentation. The following recommendations provide a cask-specific basis for
granting burnup credit, based on actinide composition. The NRC’s staff will issue additional
guidance and/or recommendations as information is obtained from its research program on
burnup credit and as experience is gained through future licensing activities. Except as
specified in the following recommendations, the application of burnup credit does not alter the
current guidance and recommendations provided by the NRC staff for criticality safety analysis
of transport and storage casks.