Safeguarding Centrifuge Enrichment
Uranium enrichment is an essential component of the nuclear fuel cycle. Most nuclear power reactors require fuel ‘enriched’ in the uranium-235 isotope, which constitutes about 0.7% of uranium found in nature. Uranium-235 is far more likely than uranium-238 (which makes up almost all of the remaining 99.3% of naturally occurring uranium) to fission upon interacting with a neutron in a nuclear reactor. However, because the natural ratio (enrichment) of uranium-235 to uranium-238 is insufficient for a self-sustaining fission reaction in most nuclear power reactors, the fraction of uranium-235 in nuclear fuel must be increased, typically to 3-5% – via uranium enrichment.
Today, uranium enrichment is typically carried out at gas centrifuge enrichment plants (GCEPs). This enrichment method passing a gaseous uranium compound (uranium hexafluoride, or UF6) through centrifuges, which rotate at very high speeds to separate lighter uranium-235 atoms from heavier uranium-238 atoms. To enrich uranium to levels suitable for use in nuclear fuel, it must be passed through multiple centrifuges arranged in ‘stages,’ within a ‘cascade.’ Each stage incrementally increases the enrichment of the material that passes through it. This process can be continued, however, to produce uranium enriched to higher levels, suitable for use in a nuclear weapon. The potential of GCEPs for producing weapons-usable material necessitates robust verification measures to ensure that these facilities are only being used for legitimate purposes. In this regard, the International Atomic Energy Agency (IAEA) carries out safeguards verification activities at GCEPs in non-nuclear weapon states recognized by the Treaty on the Nonproliferation of Nuclear Weapons (NPT), as well as at GCEPs in NPT nuclear weapon states that have offered these facilities for safeguards. The European Atomic Energy Community (EURATOM) also conducts safeguards activities at GCEPs in its Member States.
Safeguards at declared facilities
Although commercial GCEPs have been operating since the 1970s, global centrifuge enrichment capacity has grown considerably in recent years. Drivers of this growth include needs to replace the capacity of gaseous diffusion enrichment plants that have shut down, as well as pre-Fukushima expectations of growth in the nuclear power sector. During this period, existing facilities have grown in capacity, and new facilities have been constructed. This growth, along with the appearance of new GCEP technology holders, raises important issues relevant to safeguards implementation at GCEPs:
- Detecting HEU production in a timely manner at large-scale GCEPs. The IAEA aims to be capable of detecting the diversion of material from or misuse of GCEPs in a timely manner, so as to deter these activities through the possibility of early detection. However, the timescale along which consequential misuse of a large-scale GCEP can occur can very short. This gives rise to two questions:
At today’s larger enrichment facilities, how quickly does detection need to be achieved? To answer this question, we have developed a simulation tool for characterizing misuse scenarios at generic reference facilities. This code can simulate the equilibration of a cascade over time, and to estimate enrichment values throughout a cascade during off-normal operation. This information can then be used to generate non-idealized production estimates for misuse scenarios.
What means are available for improving detection timeliness, and how might they be implemented? Over recent years, the IAEA has focused on improving its detection capability for the unauthorized enrichment of uranium through the deployment of unattended technologies that constantly monitor conditions at GCEPs. We are investigating the potential of novel unattended systems that could provide additional detection capability and/or reduce the vulnerability of existing systems to spoofing, without impeding the operation of commercial GCEPs.
- Improving safeguards efficiency at GCEPs. Safeguards activities at GCEPs are costly, particularly in terms of person-hours, and as facilities have grown over time, some of these activities have become more onerous, while the IAEA’s budget has faced zero real growth. We are interested in the development of new technologies and concepts that could potentially reduce person-days of inspection (PDIs) at GCEPs, thereby reducing the cost of inspection activities to the IAEA, as well as disruption to the activities of GCEP operators, while maintaining or improving the effectiveness of IAEA verification at GCEPs.
Detecting undeclared facilities
In addition to safeguards activities at known facilities that have been ‘declared’ to the IAEA, we are also interested in measures and approaches that can verify the absence of undeclared, or clandestine GCEPs. This task is a vital component of comprehensive safeguards agreements that are currently implemented in Non-Nuclear Weapon States, and may also become important in all States in the future, if a Fissile Material Cutoff Treaty is negotiated and enters into force. We are interested in the development of technical tools (such as satellite imagery constellations) that could aid in the identification of an undeclared facility, as well as how legal and implementation measures can be refined to increase the odds that construction or operation of a clandestine GCEP will be discovered.