Fissile assay of spent fuel using a slowing down time device

Effective techniques for assaying separately the fissile isotopes in spent reactor fuel are vitally important for adequate safeguards. Such techniques also are important in assigning credits for spent-fuel fissile contents, in confirming spent-fuel contents when taking credit for depletion in design of spent-fuel facilities, and in testing reactor analysis methods. Specifically, it is essential for safeguards to distinguish plutonium fissiles 239Pu and 241Pu from the uranium fissile 235U. This is important because plutonium can be chemically separated from the slightly enriched uranium used in most power reactors, and this plutonium then can be used in a nuclear explosive. The slowing down time (SDT) assay device is the only method that accomplishes this for spent fuel. The SDT assay method has been under development at Rensselaer Polytechnic Institute (RPI) for more than 10 yr. Results have been reported on SDT measurements and calculations of the sensitivity of assay of the various fissiles in small samples, in single fuel pins, and in a depleted U3O8 parallelepiped simulating a fuel element. In this paper the authors report measurements and analyses of SDT assay for a boiling water reactor (BWR) assembly mock-up fresh fuel