The Leaching of Pu, Am, Np and Tc from High-Level Waste Glasses in Clay Media
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THE LEACHING OF Pu, Am, Np AND Tc FROM HIGH-LEVEL WASTE GLASSES IN CLAY MEDIA
K. LEMMENS, P. VAN ISEGHEM AND L. WANG SCK/CEN, Boeretang 200, 2400, Mol, Belgium
ABSTRACT The leaching behaviour of Pu, Am, Np and Tc from the high-level waste glasses of interest to Belgium has been studied. Corrosion tests were carried out on inactive glasses (the Cog~ma R7T7 glass SON68 and the DWK Pamela glasses SM513 and SM527), doped with the radionuclides. Nuclide leaching was studied in contact with a slurry of Boom clay, diluted with claywater, at 900 C and 400 C, for durations of up to 4 years. The retention of Pu and Np in the glass surface layers was considerable for SON68 and SM513, but small for SM527. The retention of these elements decreased at lower temperature. Tc was not retained. The largest part of the leached activity is adsorbed onto the clay in an immobile form. Nuclide concentration in mobile form (smaller than 105 MWU)was found to be independent on duration for Pu and Am. Increasing concentrations were found for Np and Tc. The mobile concentrations were smaller in the presence of corrosion products. INTRODUCTION As many other countries, Belgium investigates the feasibility of high level waste disposal in geological layers. A deep clay layer has been selected as a possible repository site. Conditioning of the high-level waste has resulted in the production of a number of waste glass types. The most important glass types for Belgium are the Cog~ma AVH reference glass SON68 and the DWK PAMELA glasses SM513 and SM527 (reference glasses for the lowand high-enriched waste concentrates resulting from the Eurochemic reprocessing activities). The waste form is one of the barriers in the multibarrier concept proposed by the Belgian authorities. In order to evaluate this barrier and to understand more fully the role of the other engineered and geological barriers, the leaching behaviour of the radionuclides from the glass must be examined. Since the long-term potential hazard is ascribed primarily to Am, Pu, Np and Tc [1], the experimental program upon which this paper was based focused on these elements. The following aspects will be treated: the retention of the radionuclides in the surface layers of the glasses, the mobility of the leached radionuclides and the influence of temperature and metal corrosion products on the radionuclide leaching behaviour. The characterization studies of the leached nuclides are discussed in another paper [2]. EXPERIMENTAL Three borosilicate glasses were investigated in the program : SON68, SM513 and SM527 [their composition can be found in 3]. We only mention that glass 5M527 has a particularly high A1203 content (>20 weightz). The doped SON68, SM513 and SM527 glasses were prepared by adding 110 to 150 MBq of Pu (70 Z 239 pu, 20 Z 240pu, 10 Z 238pu, 24 1Pu and 242pu) to about 40 g of the inactive glass pellets. Tracer amounts of 134Cs and 90Sr were Mat. Res. Soc. Symp. Proc. Vol. 294. i1993 Materials Research Society
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added to the same meltings, but these elements will not be disc
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