Abstract
AbstractStatic leach tests were conducted for simulated low-level radioactive waste (LLW) glass in deionized water at 90 °C for up to one year to investigate the dissolution mechanism of LLW glass. Widely studied leaching behavior of high-level radioactive waste (HLW) glass is referred in discussing the dissolution mechanism. LLW glass is characterized by higher sodium (Na) and aluminum (Al) contents than HLW glass, about twice as high as R7T7, with its SiO2 content close to HLW glass. Powdered simulated LLW glass of three different chemical compositions was tested with the glass-surface-to-water-volume ratio of 2,000 m−1. The release rates of boron (B), widely used as an indicator of dissolution for HLW glass, decreased with time during leaching, as commonly observed in similar tests for HLW glass. The pH of the leachate was stable around 11.3 - 11.6, which is higher than those in similar tests for HLW glass by one pH unit or more. The concentrations of Al in the leachates were higher compared to data for HLW glass by two orders of magnitude. The high concentration seems to be caused by higher pH. In the leachate condition of the present tests, a zeolitic mineral (analcime) is thermodynamically more stable than amorphous silica (SiO2(am)) which is known to control the concentration of dissolved silica (Si) with respect to HLW glass. The present results imply that dissolution of the LLW glass is accompanied with formation of analcime under virtually closed systems such as geological repository where the groundwater flow rate is quite low.
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