Spent Fuel Project Office, ISG-8 - Limited Burnup Credit in the Criticality Safety Analyses of PWR Spent Fuel in Transport and Storage Casks

When fuel is irrdiated in a reactor, the reactivity of the fuel decreases. This reduction of.
reactivity with bumup is caused by the change in fissile content of the fuel (i.e., bumup of U-235
and production of Pu-239 and other fissile actinides), the production of actinide neutron
absorbers, and the production of fission-product neutron absorbers. Until now, criticality safety
analyses for spent fuel casks, including storage, transport, and dual-purpose casks, were
performed under the assumption that the fuel was unirradiated. This '"fresh fuer assumption
was used as a bounding condition because of unresolved issues over the technical basis and
methods for including credit for fuel bumup in the criticality analysis of spent-fuel casks.
The U.S. Department of Energy (DOE) has been working on the development of a topical report
that proposes a method for taking bum up credit in casks for transporting and storing spent ·fuel
from pressurized water reactors (PWRs). DOE's proposal .has been submitted to the U.S.
Nuclear Regulatory Commission (NRC) and has gone through two cycles o.f revisions based on
NRC's review and comment. Based on the technical information provided in DOE's topical
report, with its supporting technical reports, and information available from other sources, both
foreign and domestic, the staff has·now found sufficient basis to approve a limited level and
scope of bumup credit while it pursues the development of a more complete basis for more
comprehensive bumup credit.
·As justified through the review of additional supporting data and analysis, the staff will issue
revised interim guidance to reflect the evolution of approved methods for greater levels of
bumup credit. Future revisions of the Standard Review Plans (NUREG-1617, NUREG-1536,
and NUREG-1537) will incorporate or reference the current guidance, as appropriate.