Leaching of Cesium and Uranium from Spent PWR fuel in the Gel-state Clays
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Leaching of Cesium and Uranium from Spent PWR fuel in the Gel-state Clays S. S. Kim, K. S. Chun, J. W. Choi, W. J. Cho and P. S. Hahn Radwaste Disposal Research Division, Korea Atomic Energy Research Institute, P.O. Box 105, Yuseong, Deajeon, Republic of Korea ABSTRACT The amounts of cesium and uranium released from crushed spent PWR fuel in the gel-state clays with a few ml of supernatant at hot cell temperature under Ar-atmosphere have been measured. The fractions of cesium dissolved from the fuel for 873 days were 0.29 and 0.25% in Boom clay/Boom-clay water and Ca-bentonite/synthetic granitic groundwater, respectively. These cesium fractions were very close to the gap inventory of cesium, which was determined to be around 0.30% in the previous experiment. The fraction of uranium released up to 193 days in the Boom clay media was 0.011% and this fraction has been retained until 873 days. Such this phenomenon was also obtained in the Ca-bentonite media even though the released fraction was higher than that in Boom clay. The increase of less than 0.001% in the dissolved uranium fraction between 193 and 873 days suggests that the long-term leach rate of uranium from spent fuel would be much less than 24 µg·m-2·day-1. INTRODUCTION For the safe isolation of spent nuclear fuel from biosphere, a repository for its disposal is based on multibarriers such as host rock, backfill, buffer and canister. The leaching behavior of spent nuclear fuel in the near field of geological repository after corrosion of metal canister should be understood, since the fuel itself can be considered as an innermost barrier to the release of radionuclides as well as the source of radionuclides. Many experimental results have been reported for the dissolution behavior of spent nuclear fuel under various conditions [1-5]. The reported studies on the dissolution of spent fuel to be contacted with bentonite after corrosion of metal canister were related with the dissolution of UO2 matrix [6-9], leaching behavior of radionuclides from fuel [10-12], the transport of these nuclides in the bentonite layer [13-15] and the identification of the formation of uranium alteration phases which causes a solubility-limit solid phases of uranium in bentonite water [16,17]. In our laboratory, we have performed the dissolution experiment of a spent PWR fuel in Cabentonite and Boom Clay with a few ml of supernatant under Ar atmosphere up to 873 days. The fractions of cesium released from the spent fuel in two kinds of clay media with leaching time
were compared with its gap inventory, and the leach rates of uranium were determined. EXPERIMENT The spent PWR fuel used in this study had a burn-up of 35 GWD/MTU, initial U-235 enrichment of 3.2 wt.%, cooling time for about 20 years and fission gas release of 0.22% from the fuel rod. Pellets were crushed in a brace cell and clad was removed out. The fuel powder was sifted with screens of -14/+40 mesh (calculated specific surface area of the powder was 6.33 cm2/g [18]), weighed and put in the under-part of a spent fuel
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