Corrosion and Alteration of Lead Borate Glass in Bentonite Equilibrated Water
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Corrosion and Alteration of Lead Borate Glass in Bentonite Equilibrated Water Atsushi MUKUNOKI1, Tamotsu CHIBA1, Takahiro KIKUCHI1, Tomofumi SAKURAGI2, Hitoshi OWADA2, Toshihiro KOGURE3 1 JGC Corporation, Minato-Mirai 2-3-1, Nishi-ku, Yokohama, Japan 2 Radioactive Waste Management Funding and Research Center, Tsukishima 1-15-7, Chuo-ku, Tokyo, Japan 3 Department of Earth and Planetary Science, Graduate School of Science, The University of Tokyo, 7-3-1, Hongo, Bunkyo-ku, Tokyo, 113-0033, Japan ABSTRACT The development of an iodine immobilization technique that can fix radioactive iodine in waste form for a long period and constrain its leaching into pore water is necessary in order to secure the long-term safety of geological disposal of transuranic (TRU) waste. Lead borate glass vitrified at a low temperature is regarded as a promising material for immobilizing the Iodine129 that is recovered from spent AgI filters generated by reprocessing plants in Japan and which may have a significant effect on the long-term safety of geological disposal. Batch leaching tests were conducted to understand glass dissolution behavior in various solutions that account for geological disposal conditions. Boron dissolved at the highest rate in all types of solutions to be used as an index element for measuring the glass dissolution rate. On the other hand, lead dissolved in these solutions at a much lower rate. These results are consistent with an electron micro-probe analysis (EPMA) of the altered glass surfaces that indicated the depletion of boron and enrichment of lead near the surfaces. The altered glass surfaces were further examined by scanning and transmission electron microscopy (SEM/TEM) and X-ray diffraction (XRD). SEM/TEM observation showed formation of a porous altered layer consisting of fine crystallites on the pristine glass and euhedral crystals on the altered layer. XRD analysis indicated that the fine crystallites and euhedral crystals are hydrocerussite, Pb3 (CO3)2(OH) 2, which was predicted by geochemical calculation as the precipitate for the experimental system. INTRODUCTION A recent review of TRU waste states that Iodine-129 is a key radionuclide in the safety assessment for the geological disposal of TRU waste.[1] It is anticipated that the development of an iodine immobilization technique that can fix iodine in waste form for a long period and constrain its leaching into pore water will offer an alternative that would reduce the release rates of Iodine-129 from primary containment under typical geological conditions. Low temperature vitrification with BiPbO2I (BPI) is one immobilization candidate.[2] Initially, H2 is used as a reductant to release iodine from a spent iodine filter. Then, the iodide ion is fixed into the BPI by reaction with an inorganic anion exchanger, BiPbO2NO3 (BPN). [3-4] The ion exchange reaction is as follows: (1) BiPbO2NO3 + I- ΔΊ%L3E22I + NO3In the vitrification process, a low melting temperature of 540qC is used to avoid iodine volatilization during the vitrification process.
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