Application of Credibility Theory to Material Accountability Verification

Avenhaus, R. & Jewell, W.S. (1975). Application of Credibility Theory to Material Accountability Verification. IIASA Working Paper. IIASA, Laxenburg, Austria: WP-75-089

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Abstract

The nuclear materials safeguards system of the International Atomic Energy Agency (IAEA) in Vienna consists of two parts: the verification of the material flow and inventory data reported by the operator of a nuclear plant; the establishment of a material balance at the end of an inventory period with the help of the operator's reported data, which means that the book inventory (initial physical inventory plus receipts minus shipments) is compared with the ending physical inventory. By definition it is necessary that the plant operator maintains a complete measurement system for all nuclear materials processed in the plant.

In this paper, we consider an alternative inspection scheme which is based on material accountability too, but which does not make use of the data reported by the operator. Contrary to the IAEA safeguards system, the material balance in this system is closed only with the help of the data observed by the inspection team itself. Such a system could be important in situations where there is no reason for a plant operator to maintain a complicated measurement system, or where, for some reason, the records are not available.

It is clear that if the inspection team cannot measure the data of all material batches processed in the plant under consideration (e.g. if the inspection budget or time is limited), then some prior information about the average material contents of the different batches as well as the batch-to-batch variation have to be used. Therefore, a Bayesian approach seems to be natural for the treatment of problems of this kind. On the other hand, this prior information will not be very detailed, and so we will use the principles of credibility theory where only the first two moments of the prior distribution have to be known.

In this paper, we first consider only one class of material, and then R different classes (inputs, outputs, etc.) with the problem of material balance closure. Finally we discuss the problem of optimization of a given inspection effort.

As the batch-to-batch variation of the true material contents within one class normally is much larger than the measurement variance, we will neglect the measurement errors here; they could easily be taken into account, if necessary.

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