Advances in passive neutron instruments for safeguards use

Passive neutron and other non-destructive assay techniques have been used extensively by the IAEA to verify plutonium metal, powder, mixed oxide, pellets, rods, assemblies, scrap and liquids. Normally, the coincidence count rate is used to measure the 240Pu effective mass, and gamma ray spectrometry or mass spectrometry is used to verify the plutonium isotopic ratios. During the past few years, passive neutron detectors have been installed in plants and operated in the unattended/continuous mode. These radiation data with time continuity have made it possible to use the totals count rate to monitor the movement of nuclear material. Monte Carlo computer codes have been used to optimize the detector designs for specific applications. The new inventory sample counter (INVS-III) has been designed to have a higher efficiency (43%) and a larger uniform counting volume than the original inventory sample counter. Data analysis techniques have been developed, including the 'known alpha' and 'known multiplication' methods that depend on the sample. For scrap and other impure or poorly characterized samples, multiplicity counting has been developed and initially implemented in the plutonium scrap multiplicity counter. For large waste containers such as 200 L drums, the add-a-source technique has been developed, giving accurate corrections for the waste matrix materials. The paper summarizes recent developments in the design and application of passive neutron assay systems. (author). 16 refs, 3 figs