Investigation of the Self-Interrogation Neutron Resonance Densitometry applied to spent fuel using Monte Carlo simulations

Abstract Self-Interrogation Neutron Resonance Densitometry (SINRD) is a Non-Destructive Assay technique that has the potential to directly quantify 239 Pu in spent fuel. SINRD is based on the detection of the neutron flux attenuation by 239 Pu around the 0.3 eV neutron resonance, here indicated as the SINRD signature. An effective deployment of the SINRD technique relies on demonstrating that the 239 Pu content in spent fuel can be correlated with the SINRD signature. Previous work from Los Alamos National Laboratory proposed to place a set of fission chambers on one side of the fuel assembly and to detect the passive neutron emission. Since it is expected that in water the signature degrades after a few millimeters from the fuel assembly, a different measurement approach is proposed in our work, namely to insert a set of fission chambers in the guide tubes of the fuel assembly. SCK•CEN conducted a research based on Monte Carlo simulations to investigate the phenomena affecting the SINRD signature. First, we studied the impact of measurement conditions on the energy distribution of the neutron flux at the measurement position. Then, we determined the sensitivity of the SINRD signature to 239 Pu content. In the final part of our research, we assessed the expected individual impact that other nuclides have on the SINRD signature. We concluded that a better sensitivity is expected when the fuel assembly is in air, surrounded by a slab of polyethylene. The obtained results also indicated that the ratio between the neutron flux in the fast region and the neutron flux around the 0.3 eV resonance is more strongly related to the 239 Pu content than other ratios. The impact of 235 U on the SINRD signature was found to be significant, while limited effects were due to other nuclides.