In situ object counting system (ISOCS™) technique: A cost-effective tool for NDA verification in IAEA Safeguards

Nuclear material measurements using the ISOCS technique are playing an increasing role in IAEA verification activities. The ISOCS capabilities include: a high sensitivity to the presence of U and Pu; the ability to detect very small amounts of material; and the ability to measure items of different shapes and sizes. In addition, the numerical absolute efficiency calibration of a germanium detector used in the technique does not require any calibration standards or reference materials. The ISOCS modelling software performs an absolute efficiency calibration for items with various container shapes, container wall materials, material compositions, material fill-heights, U/Pu weight fractions and even heterogeneously distributed emitting materials. In a number of cases, some key parameters, such as the matrix density and U/Pu weight fraction, can be determined in addition to the emitting material mass and isotopic composition. These capabilities provide a verification solution suitable for a majority of cases where quantitative and isotopic analysis should be performed. Taking into account these advantages, the technique becomes a cost-effective solution for nuclear material non-destructive assay (NDA) verification. At present, the IAEA uses the ISOCS for a wide range of applications including the quantitative analysis of U scrap materials, U/Pu contaminated solid wastes, U fuel elements, U hold-up materials. Additionally, the ISOCS is also applied to some specific verification cases such as the measurement of PuBe neutron sources and the quantification of fission products in solid wastes. In reprocessing facilities with U/Pu waste compaction or facilities with item re-batching, the continuity-of-knowledge can be assured by applying either video surveillance systems together with seals (requiring attaching/detaching and verification activities for each seal) or verification of operator declarations using quantitative measurements for items selected on a random basis. In some cases, the first option is too expensive and places a high demand on inspector and operator time. Quantitative NDA based on the ISOCS technique verifies these materials and significantly decreases the resources required for assuring the continuity-of-knowledge.