Formation of secondary phases after long-term corrosion of simulated HLW glass in brine solutions at 190 °C

Abstract

SummaryThe behavior of rare earth elements (REE) as chemical analogues for actinides during glass corrosion was studied with static long-term batch experiments (7.5 years) at 190 °C. Corrosion tests were carried out using a simulated inactive high level waste (HLW) glass powder. Two different highly concentrated salt solutions (NaCl-rich and MgCl2-rich) have been used to simulate the potential corrosion by aqueous solutions in geological salt formations. Samples with surface-to-volume ratios (S/V) of 1000 m−1and 10000 m−1were investigated. The chemical composition of the aqueous solution and the formation of secondary phases at the glass/solution interface as well as their elemental composition, especially with respect to the behavior of REE was studied. The glass corrosion behavior and secondary phase formation was significantly different in the two brines. In the MgCl2-rich salt solution, with a final pH < 3.5, hydrous Mg-silicates and (Ba,Sr)SO4solid solutions with considerable amounts of Sr (up to 15 wt.%) have been observed. Further, REE-sulfates and REE-molybdates with an indication of dissolution (etch pits) as secondary phases could be identified. In the NaCl-rich solution with a final high pH, REE-containing hydrous silicates (up to 35 wt.%) and (Ba,Sr)SO4solid solutions with small amounts of Ce (up to 5 wt.%) have been observed. Frequently observed Ca-molybdates (powellite) did not contain detectable amounts of REE.