NRC SFST ISG-6: Establishing minimum initial enrichment for the bounding design basis fuel assembly(s)

The Standard Review Plan, NUREG-1536, Chapter 5, Section V, 2 recommends that “the
applicant calculate the source term on the basis of the fuel that will actually provide the
bounding source term,” and states that the applicant should, “either specify the minimum initial
enrichment or establish the specific source terms as operating controls and limits for cask use.”
A specified source term is difficult for most cask users to determine and for inspectors to verify.
The specification of a minimum initial enrichment is a more straightforward basis for defining the
allowed contents. The specification should bound all assemblies proposed for the casks in the
application. Specific limits are needed for inclusion in the Certificate of Compliance.
Lower enriched fuel irradiated to the same burnup as higher enriched fuel produces a higher
neutron source. Sometimes fuel assemblies are driven to burnups beyond the value normally
expected for the given enrichment. According to the U.S. Department of Energy’s Characteristic
Data Base, the lower enrichment for fuel burned to 45,000 MWd/MTU is about 3.3%. The
neutron source for an initial enrichment of 3.3% is expected to be 70% higher than the neutron
source for 4.05% enriched fuel.