Building Confidence in Approaches for Monitoring the Weight of UF6 Cylinders1

The International Atomic Energy Agency’s (IAEA’s) primary safeguards objectives at declared facilities involve the timely verification of all declared nuclear materials (i.e., detect attempts to divert) and the detection of facility misuse (e.g., undeclared production or processing of nuclear materials). One of the more challenging operational activities for the IAEA to independently verify is the transfer of uranium hexafluoride (UF6) between the process and the storage/transport cylinders. These operations take place at numerous facilities in the front end of the nuclear fuel cycle (for light water reactors: UF6 conversion, uranium enrichment, and fuel fabrication). IAEA inspectors frequently inspect the feed and withdrawal (F/W) areas at these facilities, but the operations run continuously, providing realistic opportunities for the operator to process undeclared materials in-between inspections. Load cells are installed in most modern F/W stations to allow the operator to continuously monitor the weight of the UF6 in cylinders for process control. Typically, four load cells are installed in feed and withdrawal stations. The IAEA has reported that the sharing of these weight data from the F/W stations is an appealing solution, but sharing weight data presents several challenges: The operators are required to protect proliferation sensitive data (especially at uranium enrichment facilities), and the IAEA must have confidence that the process data are not being tampered with. Researchers from several institutions are investigating methods to overcome these challenges to facilitate the joint use of operator weighing systems in the F/W stations being used to increase the IAEA’s assurance that the plant is being operated as declared. This paper presents several approaches to improve an inspectorate’s confidence that the weight data from operator load cells are authentic. This paper describes four options (stacked load cells, dual bridge load cells, authenticated load cells, and hybrid approaches) that would provide an independent sensing element for collecting the safeguards-relevant weight data. The authors will also describe available cryptographic options to further improve an inspectorate’s confidence in the data.