Development of Self-Interrogation Neutron Resonance Densitometry to Quantify the Fissile Content in PWR Spent LEU and MOX Assemblies

A new nondestructive assay technique called self-interrogation neutron resonance densitometry (SINRD) is currently being developed at Los Alamos National Laboratory to improve existing nuclear safeguards and material accountability measurements for light water reactor fuel assemblies. The viability of using SINRD to quantify the fissile content (²³⁵U and ²³⁹Pu) in pressurized water reactor 17 × 17 spent low-enriched uranium and mixed-oxide fuel assemblies in water was investigated via Monte Carlo N-particle extended transport code simulations. SINRD utilizes ²⁴⁴Cm spontaneous fission neutrons to self-interrogate the fuel pins. The amount of resonance absorption of these neutrons in the fuel can be quantified using ²³⁵U and ²³⁹Pu fission chambers placed adjacent to the assembly. The sensitivity of this technique 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. SINRD requires calibration with a reference assembly of similar geometry. However, this densitometry method uses ratios of different fission chambers so that most systematic errors related to calibration and positioning cancel in the ratios.