An NRC Perspective on Physical Inventories and Inventory Differences at Facilities

The physical inventory program is important to nuclear safeguards because, if established and maintained adequately, it confirms the presence of nuclear material and the accuracy of the book inventory, and thus provides assurance that the facility’s material accounting aspect has been effective. Periodic physical inventories enable a facility to adjust material accounts to accurately reflect the status of nuclear material inventory within the facility. Comparisons of the book inventory to the physical inventory normally result in a difference between the book inventory quantity and the ending inventory quantity which is denoted as the inventory difference (ID). These inventory comparisons serve as a quality control check on the performance of the material control tests employed for material loss detection. Although physical inventories do not provide a very timely function for abrupt loss detection like process monitoring and item monitoring programs, the capability of the physical inventory program still serves the purpose of confirming that other timely accounting and control measures are functioning properly with respect to detecting non-abrupt material losses or revealing potentially unauthorized removal of nuclear material. Each NRC-licensed facility is required to have a well defined system for evaluating total plant IDs and taking actions when IDs exceed certain predetermined thresholds. This paper will provide an overview of the NRC’s oversight perspective in determining ID regulatory limits and providing guidance on response levels for excessive IDs for different facility categories. An excessive ID may be an indicator of unauthorized removal of nuclear material or of other insider’s activities that could lead to degradation of the performance of the facility’s material control and accounting (MC&A) program. This paper will also discuss the NRC’s responsibility and authority for compliance evaluation and effectiveness evaluation with respect to the facility’s physical inventory program and reported ID.