Initial Characterization Of A DD Neutron Generator Driven Fast Neutron Coincidence Collar

For more than 30 years, the quantitative assay of the 235U content of light water reactor fresh fuel assemblies relied on measuring coincidence neutrons from fissions induced by an Am(Li) neutron source using 3He based detectors. The Fast Neutron Collar (FNCL), developed by the International Atomic Energy Agency, replaces traditional 3He proportional counters with an array of liquid scintillator detectors arranged about the fuel assembly to provide improved measurement precision and reduced sensitivity to gadolinium poison rods. The FNCL still relies on Am(Li) neutron sources that are no longer commercially available. This work examines the replacement of Am(Li) sources with a commercial off-the-shelf DD neutron generator. In addition to mitigating supply concerns, the neutron generator offers advantages in measurement precision and potential automation of sequential passive/active neutron measurements. This paper presents the initial performance results for both the integrated DD/FNCL and Am(Li)/FNCL assays of compact depleted uranium, low enriched uranium, and highly enriched uranium standards along with an estimate of the expected performance for fresh fuel assemblies.