Evaluations of Accountability Verification Measurements

The majority of the nuclear material samples, collected during safeguards inspections and sent to an analytical laboratory for measurements, serve several purposes, (i) They are taken to verify the correctness of declarations for the amounts of nuclear material (element (U, Pu, Th) and isotope amounts U)) in inventories and in transfers into or out of a facility, 

The operator-declared data and the measurement results, obtained for the inspection samples, are stored in the inspectorate in a central operator-inspector database where they can be accessed for subsequent evaluations (IAEA 2004). This database also contains the results of verification measurements by nondestructive assay (NDA) methods. 

In the IAEA data analysis, various statistical techniques (IAEA 1998a) are used to derive separate estimates of the operator s and inspector s uncertainty parameters based on the collection of historical operator-inspector differences. The results of these evaluations are performance values, typical for a specific facility and for each stratum (material type) and measurement method combination. The actually observed verification measurement performance is then used for the planning (sample size calculations), the conduct (establishing reject limits), and the evaluation (material balance) of inspections in a given facility. 

The nuclear material contained and processed in a facility is stratified into items or batches that have similar physical and chemical characteristics. Grouping the material into strata simplifies verification and makes it possible to formulate the sampling plans needed to verify a material balance and to calculate its uncertainty. In calculating sampling plans, generally 

The sample size for a stratum defines the number of items to be verified in order to be able to draw conclusions about the total population. The total sample size can then be allocated among the accountancy verification methods for gross, partial and bias defects (IAEA 1998a Jaech and Russell 1991). The uncertainty values generally used are the verification performance estimates for the given facility and strata. The International Target Values could serve as a substitute (IAEA 2001). 

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