Spent Fuel Project Office, Interim Staff Guidance - 8, Revision 2

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.
The U.S. Nuclear Regulatory Commission (NRC) Standard Review Plan for Transportation
Packages for Spent Nuclear Fuel, NUREG-1617, provides the existing recommendations for
the staff to proceed with acceptance, on a cask-specific basis, of a burnup credit approach in
the criticality safety analysis of pressurized water reactor (PWR) spent fuel casks. Based on
the technical information provided in the literature, together with confirmatory analyses by NRC
research program, the following Recommendations expand the existing guidance in the
Standard Review Plan to: (1) extend the range of allowed burnup and cooling time; (2) allow
loading of assemblies exposed to burnable absorbers; (3) removing the loading offset for initial
235U enrichments between 4 and 5 percent; and (4) indicate an acceptable source for selecting
a bounding axial burnup profile(s). The Recommendations endorse only burnup credit based on
actinide compositions. A document providing more detailed and background information for
this Interim Staff Guidance is provided in an attachment to a separate memorandum from staff.1
The NRC staff will issue additional guidance and/or recommendations as more information is
obtained from research programs directed at 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.