Nuclear safeguards is primarily concerned with the detection of significant quantities of special
nuclear material. Spent nuclear fuel offers a diversion pathway of significant quantities. The
gross neutron signal of spent nuclear fuel could be used to detect signal aberrations caused by
diversions, provided the temporal, enrichment, and environmental components of the signal are
understood and characterized such that these aberrations can be detected and noted. This study
characterizes the neutron source term of a spent fuel assembly for a range of burnups, initial
enrichments, cooling times, and counting environments. The GATORS code package was
written to automate the data transitions between the MCNPX computer code and the nuclear
SOURCES4C code. The relative contribution by source nuclide to the (alpha, n) and
spontaneous fission components of the gross neutron signal for a few selected state points is
presented. The net (subcritical) multiplication of the assembly at a few selected state points is
also presented. Analysis of the variation in the gross neutron signal showed that the fluctuations
are, in several instances, most likely too small to be differentiated, making detection of these
differences difficult using gross neutron techniques alone.
Analysis of the Neutron Source Term of a Spent Fuel Assembly