UNATTENDED 235U FEED AND WITHDRAWAL MONITOR FOR GAS CENTRIFUGE ENRICHMENT PLANTS

This paper investigates the use of moderated boron-lined proportional counters for unattended measurement of 235U flows in feed and withdrawal (F/W) cells at gas centrifuge enrichment facilities. We are investigating the placement of portal monitor-type detectors near F/W cells to measure neutrons emitted from UF6 cylinders situated inside. This setup allows for the rapid detection of a misuse scenario involving the introduction of low-enriched uranium (LEU) into the enrichment process through existing feed cells, which would result in greatly enhanced flows of 235U. Neutron count rates recorded by the detector will depend on the 235U content of the cylinders, as the production of (a, n) neutrons on fluorine is largely driven by the abundance of 234U in the material, which increases along with enrichment. With reasonable assumptions about the configuration of material inside, 234U/235U ratios and UF6 flow rates, gross count rates together with changes in count rates over time can be used to measure the 235U transfer rate to sufficient accuracy to discriminate LEU from natural feed. Enhanced flow of 235U into the tails, associated with LEU feed, is similarly detectable. In this paper, we present the results of MCNP6 simulations featuring a 1’ x 5” x 7’ GE Reuter Stokes neutron detection module containing 7 boron-lined proportional counters. Simulations were conducted with the front face of the detector positioned 2m from the centerlines of 30B and 48Y cylinders filled to various levels and containing material enriched to different levels. We find that such a system provides a simple method for measuring the flow of 235U to the necessary accuracy, and a powerful tool for the timely detection and therefore deterrence of rapid misuse of commercial-scale enrichment plants for the production of HEU. We found that the ratio of signals in the front and back rows of proportional counters allowed detection of a modeled spoofing scenario involving neutron shielding applied to a cylinder containing LEU.