Neutron Monitoring of Vacuum System Cold Traps to Detect Undeclared HEU production at Gas Centrifuge Enrichment Plants

We describe a new radiation monitoring approach to detect undeclared uranium enrichment activities at Gas Centrifuge Enrichment Plants (GCEPs). Currently, the IAEA method for detecting undeclared highly enriched uranium (HEU) production is to use a gamma header pipe monitor to deliver a “go-no-go” indication, where a “no-go” alarm occurs if the 235U enrichment in the header pipe exceeds 20%. This approach is based on gamma measurements of (1) 235U, (2) 238U and (3) wall deposits (hold-up). The enrichment is the ratio of 235U/238U corrected for the wall-deposit term. However, at some GCEPs (e.g., Urenco) the header pipe UF6 pressure is =1.5 Torr, which leads to low gamma activities and difficulties in obtaining accurate and precise enrichment values. Our new approach targets the vacuum system cold traps (VSCTs), located in the UF6 handling area of GCEPs, which purify feed or enriched product UF6 of light gases. These VSCTs are readily accessible to inspectors under current international agreements. Unlike desublimer cold traps, the VSCTs are integral to the vacuum system, and prevent frictional heating and loss of separative work capacity that would otherwise be caused by elevated gas pressure in the centrifuges. During enrichment, light gases are preferentially concentrated with the product UF6 relative to the tails. Product UF6 accumulates in the VSCT to a mass of 20 kg (measured with an in situ load cell) before the trap is emptied. The large mass accumulation, and ability to verify the mass, makes the VSCT a good target for enrichment monitoring. Long termmonitoring of the VSCTs using a neutron coincidence counting system is proposed to achieve relatively small uncertainties on measured changes in enrichment. Calculations suggest that this method could not only meet the IAEA requirement for a go-no-go monitor, but also has the added advantage over gamma header pipe monitors in that undeclared enrichment activities could be detected even if the system is operated at very low gas pressures. Future work to improve the model calculation will include the effects of a, (a,n) neutron-induced fission of 235U contributions to the coincident neutrons at relatively high enrichments, and Monte Carlo modeling with the MCNP code.