Nuclear Technology / Volume 181 / Number 2 / February 2013 / Pages 354-370
Technical Paper / Radiation Measurements and General Instrumentation
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 improve the detection of possible diversion scenarios for pressurized water reactor 17 × 17 spent low-enriched uranium (LEU) and mixed oxide (MOX) fuel assemblies was investigated via Monte Carlo N-Particle eXtended transport code (MCNPX) simulations. The following capabilities were assessed: (a) verification of the burnup of a spent fuel assembly, (b) ability to distinguish fresh and one-cycle spent MOX fuel from three- and four-cycle spent LEU fuel, and (c) sensitivity and penetrability to the removal of fuel pins. SINRD utilizes 244Cm spontaneous-fission neutrons to self-interrogate the spent fuel pins. The amount of resonance absorption of these neutrons in the fuel can be quantified using a set of fission chambers (FCs) placed adjacent to the assembly. The sensitivity of SINRD is based on using the same fissile materials in the FCs 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 FC. SINRD requires calibration with a reference assembly of similar geometry in a similar measurement configuration with the same surrounding moderator as the spent fuel assemblies. However, this densitometry method uses ratios of different detectors so that several systematic errors related to calibration and positioning cancel in the ratios.