Sensitivity study of the Self-Interrogation Neutron Resonance Densitometry for the safeguards verification of spent fuel

The Self-Interrogation Neutron Resonance Densitometry (SINRD) aims at the direct quantification of 239Pu in spent fuel assemblies. The 239Pu mass in a fuel assembly is estimated with SINRD by inserting in the central guide tube of a PWR fuel assembly a neutron detector wrapped in a set of Gd and Cd foils with defined thicknesses. These foils, also called neutron filters, have a cutoff energy for the neutron absorption and therefore allow the estimation of the neutron flux within the 0.2 – 0.4 eV energy region. This energy region is chosen because 239Pu has a significant resonance at 0.3 eV, therefore the reduction of the neutron flux in this energy region is correlated to the residual 239Pu mass in the fuel assembly. As part of the R&D work on SINRD and in view of its reliable implementation, a study to assess the sensitivity of SINRD to different measurement conditions was conducted at the Belgian nuclear research centre SCK•CEN. The sensitivity study was based on Monte Carlo simulations and considered the impact of the positioning of the neutron detector and the characteristics of the neutron filters on the SINRD signature. With respect to the neutron detector, the effect of asymmetric positioning of the detector in the central guide tube was investigated, and a comparison between neutron detectors with different lengths was made. The sensitivity of the SINRD technique to the neutron filters was evaluated by taking into account a non-ideal cover of the neutron detector by the filters, and by considering a 10% variation from the nominal filter thickness. This study shows that the incomplete cover of the neutron detector with the filter and the variation from the nominal filter thickness are the studied characteristics that most affect the results of the SINRD technique.