The Interaction Between Nuclear Waste Glasses and Clay-II
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THE INTERACTION BETWEEN NUCLEAR WASTE GLASSES AND CLAY-II P. Van Iseghem, K. Berghman and W. Timmermans, SCK/CEN, Boeretang 200, B-2400 Mol, Belgium ABSTRACT The interaction between simulated HLW glasses and clay has been investigated, as part of a larger programme to evaluate the performance of vitrified HLW in repository conditions. Experimental conditions were carefully chosen to simulate repository conditions as close as possible. The glass corrosion seems to be controlled by a diffusion process, but there are indications of a (small) final corrosion rate. The leaching of the radio-isotopes 134 Cs, 90Sr, 2 39Pu and 24 1Am is characterized by extremely small mobile concentrations leached (for 239pu and 24 1Am, below detection limit). The presence of a gamma irradiation field does not increase the glass dissolution. The production of radiolythic hydrogen is strongly reduced by the presence of clay. INTRODUCTION The enhancing effect of the presence of clay (bentonite, smectite, ... ) on waste glass corrosion has been observed and recognized by various authors (ref. I - 5). The enhancement is a function of the type of clay considered. Most of the studies considering the effect of clay were directed toward the selection of candidate backfill materials. The Belgian situation, however, is different. A clay layer of 100 m thick, situated 190 m deep, has been selected as a possible geological repository site. The long-term performance of the various candidate waste glasses in clay has to be thoroughly quantified and understood, in order to evaluate the barrier function of the high-level waste glass in the multibarrier disposal concept, and to optimize the other engineered barriers. This paper reports on results of corrosion tests with some simulated waste glasses of interest to Belgium, in solutions containing various clay ("Boom" clay) concentrations. The glasses are : SON 68 (the Cogema-AVH reference glass), SM 513 and SM 527 (the DWK-PAMELA reference glasses for the low - and high - enriched waste concentrates resulting from the Eurochemic reprocessing activities). The reducing and low flow conditions expected in the clay repository were carefully respected. This paper focusses on the leaching mechanisms of the glass matrix constituents and the specific leaching of some radionuclides of interest (Cs, Sr, Pu, Am). The influence of an external gamma irradiation field is evaluated as well. These data will be the basis to model the corrosion of waste glass in clay. Future corrosion experiments will enlarge the data base to Np and Tc, the influence of the engineered barriers (canister, overpack, backfill), and the corrosion of fully active waste glass samples. This R & D work was performed and partially funded in the frame of the research programmes of the European Communities. EXPERIMENTAL The composition of the waste glasses can be found in literature (e.g. ref. 6). Glasses SON 68 and SM 513 are "common" HLW borosilicate glasses, but SM 527 has an unusually large A12 0 3 concentration (about 20 wt %). The glasses used i
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