UF₆ mass flow measurement in gas centrifuge enrichment plants using passive process monitoring

Oak Ridge National Laboratory (ORNL) developed the fissile mass flow monitor (FMFM) for the Blend Down Monitoring System (BDMS), which is currently being used to monitor UF₆ flows in Russian facilities, where highly enriched uranium (HEU) is down-blended to commercial grade uranium [1]. The FMFM uses active neutron interrogation to determine the mass flow of ²³⁵U at these facilities and has worked successfully, with approximately 100 instrument-years of operation. Building on this experience, this paper proposes a different technical approach to measure ²³⁵U mass flow in gas centrifuge enrichment plants (GCEPs) using passive methods that do not require radioactive neutron sources. The proposed method takes advantage of the characteristics of compressors that are typically installed in product and tail piping at GCEPs. The compressor characteristics, specifically the power consumption and rotational frequency, are related to the volumetric flow rate of the compressor. The relationship between these characteristics and the flow may be complex and depends on the gas properties. Nevertheless, with a given set of compressor characteristics, the power consumption, and the rotational frequency, a correlation can be developed to estimate the volumetric flow rate (i.e., cubic meters per hour). To convert the volumetric flow rate to a mass flow rate, a passive 186 keV gamma ray measurement is proposed at the compressor inlet. Through calculations and benchmark analyses, a correlation can be developed that relates the 186 keV gamma ray counts per second to a concentration (i.e., kg per cubic meter) of ²³⁵U in the pipe. The product of this concentration times the volumetric flow rate gives the ²³⁵U mass flow rate (i.e., kg per hour) through the compressor. Continuous unattended monitoring of the ²³⁵U mass flow rate has significant safeguards value and can be used, along with other safeguard measures, to provide confidence that no fissile material is diverted. An additional advantage of monitoring the ²³⁵U mass flow, as opposed to the total UF₆ mass flow, is that the ²³⁵U product flow is the most significant component of the separative work unit (SWU) capacity declared by the plant (the other two components are the feed and tail enrichment); therefore, this measurement can be used to monitor the SWU usage of the plant, thus ensuring that SWUs are not diverted.