Integration of self-interrogation neutron resonance densitometry and differential die-away self-interrogation to quantify plutonium in a PWR 17x17 spent fuel assembly

The integration of Self-Interrogation Neutron Resonance Densitometry (SINRD) and Differential Die-away Self-Interrogation (DDSI) to more accurately quantify plutonium mass in a PWR 17x17 spent fuel assembly in water has been investigated via Monte Carlo N-Particle eXtended transport code simulations. Both of these instruments utilize the 244Cm spontaneous fission neutrons to self-interrogate the fuel pins. The sensitivity of SINRD is based on using the same fissile materials in the fission chambers as are present in the fuel because the effect of resonance absorption lines in the transmitted flux is amplified by the corresponding (n,f) reaction peaks in the fission chamber. Thus, the 235U and 239Pu content in the spent fuel can be measured using 235U and 239Pu fission chambers placed adjacent to the assembly. DDSI uses time correlation neutron counting in a 3He-based detector system, with triggering on the spontaneous fission events, to determine the spontaneous fission rate and the induced fission rate in the spent fuel. The time separation of counts from spontaneous fission (early gate) and induced fission (late gate) neutrons enables DDSI to independently measure the fertile mass and fissile mass in spent fuel. Based on the complementary characteristics of SINRD and DDSI, we believe that the integration of these instruments will provide a more robust verification method for spent fuel assemblies by improving the ability to measure plutonium. This research is part of the Next Generation Safeguards Initiative (NGSI) of the U.S. DOE and is being conducted in collaboration with several other national laboratories and universities throughout the U.S. Future work includes performing experimental measurements with both SINRD and DDSI on LWR fresh and spent fuel in water. (author)