Feasibility of Fast Neutron Spectroscopy for Safeguards and Verification of Spent Fuel in Dry Cask Storage

Spent nuclear fuel is being stored in dry casks as an interim storage solution. This current research supports the development of a neutron spectroscopy system using helium-4 gas scintillation fast neutron detectors to convert the neutron spectra into quantifiable signatures. The goal is to use these signatures to identify diversion/removal of fuel and verify spent fuel parameters, such as cooling time and burnup. To validate this concept, Monte Carlo simulations were used to produce realistic neutron spectra from PWR assemblies inside a dry cask storage system. A detailed methodology is discussed for generating neutron spectra using the Next Generation Safeguards Initiative (NGSI) spent fuel libraries and a high-fidelity model of the HI-STORM 100S dry cask system. ORIGEN-S calculates the neutron emissions from the spent fuel rods and MCNP 6.1 performs the transport and shielding calculations. Various diversion scenarios were analyzed to determine the ability to detect missing assemblies around the periphery and center of the cask. Although a significant neutron flux is emitted from the cask, neutrons less than 1 MeV dominate the spectrum. There is limited feasibility of using the total and relative contributions of spontaneous fission and (alpha,n) to predict fuel parameters. However, simulations show the feasibility of using count rate statistics to identify diverted assemblies around the periphery of the cask. Fluxes outside the cask but nearby the missing assembly location are 6% to 8% lower when compared to fluxes from a fully loaded cask.