Spent Fuel Measurements: Passive Neutron Albedo Reactivity (PNAR) and Photon Signatures

Abstract

The International Atomic Energy Agency’s (IAEA) safeguards technical objective is the timely detection of a diversion of a significant quantity of nuclear material from peaceful activities to the manufacture of nuclear weapons or of other nuclear explosive devices or for purposes unknown, and deterrence of such diversion by the risk of early detection. An important IAEA task towards meeting this objective is the ability to accurately and reliably measure spent nuclear fuel (SNF) to verify reactor operating parameters and verify that the fuel has not been removed from reactors or SNF storage facilities. This dissertation analyzes a method to improve the state-of-the-art of nuclear material safeguards measurements using two combined measurement techniques: passive neutron albedo reactivity (PNAR) and passive spectral photon measurements. PNAR was used for measurements of SNF in Japan as well as fresh fuel pins at Los Alamos National Laboratory (LANL). The measured PNAR signal was shown to trend well with neutron multiplication and fissile content of the SNF. The PNAR measurements showed a 73% change in signal for a fuel burnup range of 7.1 to 19.2 GWd/MTHM of spent mixed-oxide (MOX) fuel and a 40% change in signal over a range of initial ^235U enrichment from 0.2% to 3.2% in UO2 fuel. Photon measurements were performed on a wide range of SNF pins to determine which photon signatures are visible in different sets of fuels. These signatures were then investigated and tested using a sensitivity analysis to determine the spent fuel parameters to which each signal is most sensitive. These photon signatures can be used to determine SNF parameters that can support PNAR determination of SNF fissile content. Based on the results from these measurements, we have concluded that spectral photon measurements can determine operating parameters to improve the implementation of PNAR. We also concluded that PNAR can accurately measure multiplication and fissile content in SNF with standard deviations of 1% and 4%, respectively. The PNAR and photon measurements can be used together as a powerful tool to support the IAEA safeguards technical objective.