Pending the availability of an operational long-term spent nuclear fuel (SNF) repository or other disposal methods, SNF will be increasingly stored in interim dry casks. Casks loaded with commercial SNF may contain several significant quantities of plutonium, so appropriate nuclear material safeguards monitoring is in order. An external remote monitoring system (RMS) developed by researchers at Texas A&M University is proposed to further the current dry cask safeguards regime, which is limited to containment and surveillance mechanisms. In this study, neutron measurements of SNF in dry cask storage were performed with the external RMS at a commercial interim spent fuel storage installation. Corresponding neutron transport simulations using MCNP were conducted with two types of detector responses (tallies) and the results were compared with measurements.

The objectives of the study were to add dry cask measurement data to the literature, to assess the performance of the external RMS in full-scale dry cask measurements, and to investigate the degree to which measurements could be estimated with high-fidelity radiation transport simulations. The study demonstrated that the external RMS can acquire neutron count rate measurements with a relative error of less than 0.5% in 5 min or less through the shielding of a dry cask lid. Additionally, the developed simulation model matched trends in the measurement data to a degree that exceeds results in current literature, and normalization factors were calculated to better estimate the magnitude of neutron count rates.