Field testing of a fast-neutron coincidence collar for fresh uranium fuel verification

摘要

The Fast-Neutron Coincidence Collar (FNCL) is an active non-destructive assay system for the measurement of uranium-235 in fresh nuclear fuel assemblies using the neutron coincidence counting (NCC) approach. Unlike traditional NCC instruments, such as the Uranium Neutron Collar (UNCL), the FNCL contains liquid scintillator to detect fast neutrons (＞0.1 MeV) with nanosecond-level time resolution. The subsequent reduction of the NCC coincidence window by a factor of one thousand (from microseconds to nanoseconds) allows the uncertainties associated with accidental neutron coincidences to be virtually eliminated resulting in precise and accurate assays of fresh fuel assemblies (FFAs) in significantly less time than alternatives. The measurement of fast neutrons also minimizes the systematic uncertainties due to burnable neutron poisons commonly present in the fuel matrix. The FNCL was tested at two nuclear fuel fabrication facilities in Brazil and the Republic of Korea with continued support from the two States. 39 measurements of FFAs were obtained with varying geometry, enrichment and burnable neutron poison content. These field test measurements and conclusions drawn pertaining to the FNCL performance are discussed.