NDA safeguards techniques for LMFBR assemblies

The significant safeguards concerns for liquid-metal fast breeder reactors (LMFBRs), and for the LMFBR fuel handling systems are the accountability, surveillance, and identification of fuel and blanket assemblies. The introduction of fuel assemblies with a high content of Pu into the receiving and shipping areas of the LMFBR fuel cycle does allow a more direct near-real-time assay profile of the disposition of Pu. Isotope correlations and neutron assay methods have been investigated and implemented for determining plutonium and burnup in fresh and spent LMFBR fuel assemblies. The methods are based on active and passive neutron coincidence counting (NCC) techniques. Preliminary studies on neutron yield rates from the spontaneous fission of plutonium and curium isotopes have indicated that the NCC system is a most effective measure in the verification of nuclear material flow in assembly form for the entire reactor fuel handling cycle, i.e., from the fresh- to the spent-fuel stage. A consequence of the high plutonium concentration level throughout the fuel irradiation period in an LMFBR, is that the spontaneous fission neutron yield from the 242-curium and 244-curium does not dominate the spontaneous fission neutron yield from the plutonium isotopes in the spent fuel stage