Until now, the Nuclear Regulatory Commission's (NRC) approval of criticality safety evaluations for spent fuel in transport and storage casks has been based on analyzing the fuel as though it were fresh and without burnable poisons. The well-known nuclide composition of fresh fuel has provided a straightforward and bounding approach for showing that spent fuel systems will remain subcritical under normal and accident conditions. Burnup credit refers to the approval of criticality safety evaluations that consider the decrease in fuel reactivity caused by. irradiation in the reactor.

This paper reviews validation issues associated with implementation of burnup credit in transport, dry storage,
and disposal. The issues discussed are ones that have been identified by one or more constituents of the
United States technical community (national laboratories, licensees, and regulators) that have been exploring the
use of burnup credit. There is not necessarily agreement on the importance of the various issues, which
sometimes is what creates the issue. The broad issues relate to the paucity of available experimental data