ESTIMATING THE UNCERTAINTY OF THE CHANGE IN HOLDUP INVENTORY

摘要

One of the most challenging aspects of Nuclear Material Accountancy and Control (NMAC) is monitoring and evaluating in-process nuclear material inventories. Some fraction of process throughput inevitably remains in the process equipment as holdup where it is difficult to measure. The amounts and statistical uncertainty of holdup can be large enough to completely dominate NMAC inventory loss detection limits. This paper discusses possible statistical approaches and methods used to determine the uncertainty in the amounts and change in holdup inventory between two successive periods. The amount of holdup will vary based on process design and chemical form of the material. Ideally, it is minimized through cleanout procedures. However, there is a certain amount of fixed holdup (material that is very difficult or impossible to cleanout) that will remain. History shows that an ingrowth to an equilibrium "fixed" holdup amount can never truly be assumed in an operating line. It is more likely for there to be a variation in time including the possibility for monotonic growth. The fixed holdup is periodically determined by an established nondestructive assay (NDA) measurement procedure. For loss evaluations, holdup inventory appears in the inventory equation as the term highlighted in red in the expression below [see page 2.51 NUREG-CR-2935]: Loss Detection for a process unit = [Receipts (loss detection) -- Shipments (loss detection)] + [(Uncertainty beginning holdup - Uncertainty ending Holdup)] These methods are part of NMAC which is an integrated approach to nuclear safety, security, and safeguards intertwined with facility operations. NMAC monitors nuclear inventories to detect unauthorized removals of nuclear material and to support physical protection and other measures.