Backfill-Waste Interactions in Repository Simulating Tests
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BACKFILL-WASTE INTERACTIONS IN REPOSITORY SIMULATING TESTS
NORIAKI SASAKI Tokai Works, Power Reactor and Nuclear Fuel Development Corporation, Ibaraki-ken, Japan; and SRIDHAR KOMARNENI, BARRY E. SCHEETZ AND RUSTUM ROY Materials Research Laboratory, The Pennsylvania State University, Park, PA 16802
Tokai-Mura,
University
ABSTRACT Candidate backfill materials such as montmorillonite and clinoptilolite with and without the presence of simulated nuclear waste solids such as borosilicate glass, glass ceramic, sintered ceramic and supercalcine ceramic were investigated under repository simulating conditions (=300'C and 30 MPa). Experiments were conducted under semi-wet as well as wet repository conditions. Montmorillonite and clinoptilolite did not seem to alter under both semi-wet and wet repository conditions as determined by XRD. However, the above backfill materials reacted extensively with wastes under both conditions. They altered to feldspar (oligoclase) in the presence of borosilicate glass and to analcime in the presence of particular ceramics under both semi-wet and wet repository conditions. Alteration of montmorillonite could not be detected either in the presence of the glass ceramic or supercalcine ceramic under both repository conditions. However, clinoptilolite altered to analcime in the presence of glass ceramic or supercalcine ceramic under wet repository conditions. Reactions of backfill materials with simulated wastes immobilized waste elements such as Cs, Mo, etc., by forming new phases such as analcime, oligoclase and powellite. In fact, analyses of product solutions from interaction runs indicated that the presence of backfill materials during the alteration of waste solids served to drastically reduce the concentration of some radionuclides in solutions. These results suggest that properly selected backfills can provide the simplest and most effective chemical (or thermodynamic) engineered barrier in an intelligently designed multibarrier system. Moreover, they indicate that the p, P, and T in the usual leach tests (e.g. Paige, IAEA, etc.) are such that results of such tests cannot have any value in evaluating waste form behavior under repository conditions. INTRODUCTION The backfill or overpack material surrounding nuclear waste containers is one of the components of a multiple barrier nuclear waste isolation system. The potential available in engineering of the backfill as an additional system's barrier for radionuclide-migration has been recognized only recently 11-31. However, there is very little published as yet on the concepts behind the design of backfill. In the Swedish (KBS) conceptual repository design, bentonite-sand mixture was proposed to be a suitable "buffer substance" and as a water adsorber which could be emplaced around the radioactive waste containers 12]. The
398 Pennsylvania State University has been a leader in the design of materials for Central to our conceptual framework is the understandthe backfill or overpack. ing that of the three intensive thermodyna
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