Down blending high enriched uranium in Kazakhstan

At the end of the 1990s the BN-350 reactor in Kazakhstan was permanently shut down. Unused unirradiated fuel assemblies were transported to the Ulba fuel fabrication facility in Ust-Kamenogorsk, Kazakhstan. The fuel contained high enriched uranium (HEU) with enrichment values of up to 26% 235U and some depleted uranium (DU). To achieve the goal of reducing the inventories of HEU within the State, the Government of Kazakhstan arranged to have this fuel down blended at the Ulba facility. A separate material balance area (MBA) was designated, which covered the rooms where the down blending activities took place. Inside a glove-box line HEU dioxide pellets were removed from the fuel rods and converted to U3O8 powder by oxidizing them in air at high temperature. HEU powder was mixed with low enriched uranium (LEU) powder and finally dissolved so that the resulting LEU solution could be introduced to the parts of the Ulba plant that were certified for processing LEU with enrichment of less than 15%. The safeguards approach was based on utilizing standard containment and surveillance (C/S) measures, on-site non-destructive assay (NDA), sampling for destructive analysis (DA) and continuous inspector presence during active processing. Finally, dissolution of the mixed powders was observed to provide in-depth assurance that LEU could not be substituted to conceal diversion of HEU. This work demonstrated that the IAEA with the cooperation of the facility operator could provide assurance with a high confidence regarding the non-diversion of direct-use nuclear materials, as well as of LEU and DU during processing while simultaneously applying measures of material accounting, DA, NDA and C/S, and using safeguards inspection resources most efficiently. The IAEA has gained a great deal of experience to extend this model to future opportunities for downgrading HEU materials that are not covered under ongoing materials disposition programmes. (author)