Neutron absorbers and detector types for spent fuel verification using the self-interrogation neutron resonance densitometry

摘要

The Self-Interrogation Neutron Resonance Densitometry (SINRD) is a passive non-destructive assay (NDA) technique that is proposed for the direct measurement of ~(239)Pu in a spent fuel assembly. The insertion of neutron detectors wrapped with different neutron absorbing materials, or neutron filters, in the central guide tube of a PWR fuel assembly is envisaged to measure the neutron flux in the energy region close to the 03 eV resonance of ~(239)Pu. In addition, the measurement of the fast neutron flux is foreseen. This paper is focused on the determination of the Gd and Cd neutron filters thickness to maximize the detection of neutrons within the resonance region. Moreover, several detector types are compared to identify the optimal condition and to assess the expected total neutron counts that can be obtained with the SINRD measurements. Results from Monte Carlo simulations showed that ranges between 0.1-03 mm and 0.5-1.0 mm ensure the optimal conditions for the Gd and Cd filters, respectively. Moreover, a ~(239)Pu fission chamber is better suited to measure neutrons close to the 03 eV resonance and it has the highest sensitivity to ~(239)Pu, in comparison with a ~(235)U fission chamber, with a ~3He proportional counter, and with a ~(10)B proportional counter. The use of a thin Gd filter and a thick Cd filter is suggested for the ~(239)Pu and ~(235)U fission chambers to increase the total counts achieved in a measurement, while a thick Gd filter and a thin Cd filter are envisaged for the ~3He and ~(10)B proportional counters to increase the sensitivity to ~(239)Pu. We concluded that an optimization process that takes into account measurement time, filters thickness, and detector size is needed to develop a SINRD detector that can meet the requirement for an efficient verification of spent fuel assemblies.