Experience with environmental sampling at gas centrifuge enrichment plants

Environmental sampling has been used routinely by the IAEA since 1996 after the IAEA Board of Governors approved it in March 1995 as a new technique to strengthen safeguards and improve efficiency. In enrichment plants it is used to confirm that there has been no production of highly enriched uranium (HEU), or production of uranium at above the declared enrichment. The use of environmental sampling is based on the assumption that every process, no matter how leak tight, will release small amounts of process material to the environment. Even though these releases of nuclear material are extremely small in gas centrifuge enrichment plants, and well below levels of concern from a health physics and safety standpoint, they are detectable and their analysis provides an indication of the enrichment of the material that has been processed in the plant. The environmental samples at enrichment plants are collected by swiping selected areas of the plant with squares of cotton cloth (10x10 cm) from sampling kits prepared in ultra clean condition. The squares of cotton cloth sealed in plastic bags are sent for analysis to the Network Analytical Laboratories. The analysis includes the measurement of the uranium isotopic composition in uranium-containing particles by Thermal lonization Mass Spectroscopy (TIMS) or Secondary ION Mass Spectroscopy (SIMS). Since the implementation of environmental sampling, swipes have been collected from 240 sampling points at three gas centrifuge plants of URENCO, which have a total throughput of more than 8,000 tonnes of uranium per year. The particle analysis results generally reflected the known operational history of the plants and confirmed that they had only been operated to produce uranium with enrichment less than 5% 235U. The information about the content of the minor isotopes 234U and 236U also indicates that depleted and recycled uranium were sometimes used as feed materials in some plants. An example is given of the TIMS particle results obtained for two samples taken in May 1997 from the cascade hall at one of the enrichment plants. The evaluation of the results, using model enrichment calculations, showed that at least two enrichment processes had taken place - feeding with natural uranium and with recycled uranium. The effectiveness of environmental sampling was also demonstrated by analyzing a series of swipe samples taken at regular intervals in the process area of a newly commissioned centrifuge enrichment cascade in which the product enrichment was stepwise increased up to 5% 235U. Experience is still being gained with environmental sampling at centrifuge enrichment plants but it is already considered to be a very useful technique, which allows determining past and current enrichment activities, in particular providing complementary assurance that no clandestine HEU production has occurred. Sampling both inside cascade areas and in cylinder filling and blending areas has proven to be useful and appropriate. A safeguards approach based on the traditional safeguards measures combined with environmental sampling for routine use is under development. This approach will enhance the efficiency of inspector's work in the field and the detection probability for clandestine activities