Leaching/Migration of UO 2 -Fuel in Compacted Bentonite
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LEACHING/MIGRATION OF U02-FUEL
IN COMPACTED BENTONITE
Y. Albinssona, R. Forsyth b, G. Skarnemarka', M. Sk&lberga, B. Torstenfeltc and L. Wermed. Department of Nuclear Chemistry, Chalmers University of Technology, S-412 96 Goteborg, Sweden S-611 82 Nyk6ping, Sweden
b Studsvik AB,
Sweden and Waste Management Co., Sweden
S-721 63 V~sterAs, d ABB-Atom, Swedish Nuclear Fuel S-102 48 Stockholm,
Box 5864,
ABSTRACT The release and migration of the fission products cesium, europium and technetium, the actinides plutonium, americium and curium, and the activation product cobalt from spent nuclear fuel pellets in highly compacted bentonite clay has been measured after contact times of 101 and 386 days. Experiments at longer contact times are in progress. In some cases small amounts (0.51%) of powdered copper or iron metal, or vivianite (Fe 3 (P04 ) 2 ) have been mixed with the bentonite clay. The results indicate as expected a high mobility of cesium. The actinides have a very low mobility. After 386 days, plutonium has diffused less than 0.5 mm away from the fuel, while americium and curium appear to be somewhat more mobile. The behaviour of europium is similar to that of trivalent actinides. Very little technetium has been leached from all samples. Cobalt shows a strong retention in pure bentonite as well as in the presence of vivianite, while the mobility is much larger when iron or copper is added. INTRODUCTION In the Swedish concept for final disposal of high-level radioactive waste, bentonite clay has been proposed as a suitable backfill material. Diffusion rates of fission products and actinides in bentonite have previously been measured using radioactive tracers, but this paper presents a study where the source for the diffusing nuclides is corrosion of spent U0 2-fuel. Hence this study adresses the release behaviour and diffusion of fission products and actinides in bentonite at a radiation level with significant radiolysis. In previous papers, a preliminary study of the effects of a dilute bentonite suspension on leach rates from spent fuel has been reported (1,2]. EXPERIMENTAL A spent fuel pin (burn-up 41 MWd/kg) was sawed into =4 mm thick pieces with a diameter of 12 mm. The spent fuel pieces, weighing about 4.5 g including 1.5 g of the cladding, were placed between two cylinders of bentonite, and put into diffusion cells. The clay had a density of about 2000 kg/m3 . The diffusion cells were submerged in synthetic groundwater [3] that had been preequilibrated with clay. In some cases a small amount of powdered metallic copper or iron (0.5% by weight), or the phosphate mineral vivianite (Fe 3 (PO 4 ) 2 , 1% by weight) was added to the clay. Mat. Res. Soc. Symp. Proc. Vol. 176. ©1990 Materials Research Society
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The average alpha radiation field closest to the fuel pin was in all cases about 3"103 alpha particles/mm2 *s. After a certain contact time, the diffusion cells were opened and the clay cut into 0.1 mm thick slices. A detailed description of the experimental setup together with a set of prelimina
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