Pebble Bed Reactor Safeguards Using the Fast Neutron Collar

This work investigates the usage of the Fast Neutron Collar (FNCL) Instrument to quantify Pebble Bed Reactor (PBR) fuel for international safeguard applications. PBRs have unique challenges to Nuclear Material Accounting and Control (NMAC) practices including a large quantity of fuel pebbles actively in operation, the dynamic nature of these pebbles, and online refueling schemes. The preliminary design of the Kairos fuel canister and publicly available information for the generic fluoride salt-cooled high-temperature reactor (gFHR) core model and Hermes design are utilized. Using MCNP simulations, a first approximation of the quantification for the number of pebbles within a fresh fuel canister is made with the FNCL model. This is done using the singles and doubles rate(s) measured by the FNCL. The level of sensitivity to individual pebbles is investigated by performing this simulation with a varied number of pebbles in each canister. Additionally, this study investigates the neutron time interval distribution, or the interevent timing of neutrons, to further characterize the mass of fissile material. Further, diversion scenarios are tested by simulating mock fuel pebbles that are exclusively comprised of the matrix material. Differing signal rates from these diversion scenarios are investigated. This study produces a better understanding of the use of the FNCL for PBR safeguards applications. Understanding the sensitivity of the FNCL to potential diversion scenarios is crucial when determining safeguards-by-design features of PBRs. This work will gain insight on the capabilities of existing tools for advanced reactor safeguards.