Estimating the Uncertainty of the Change in Holdup Inventory

One of the 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 as hold-up where it is difficult to measure. The amounts and statistical uncertainty of hold-up can be large enough to completely dominate NMAC inventory loss detection limits. This paper discusses the 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 clean out) that will remain. History has shown that ingrowth to an equilibrium “fixed” hold-up 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 Hold-up)]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 provide detection against unauthorized removals for illicit purposes.