Oxidation state and local environment of selenium in alkali borosilicate glasses for radioactive waste immobilisation

The oxidation state and local environment of selenium in alkali borosilicate glasses for high level radioactive waste (HLW) immobilisation were studied by Se K-edge XANES and EXAFS spectroscopy. An inactive surrogate of the UKʹs “MW” HLW-loaded glass and two waste-free glasses were investigated. Results confirm that the predominant Se oxidation state in all air-melted glasses is Se4+. Low levels (< 10% each of Se0 and Se6+) were also detected in the simulated HLW-loaded glass. The presence of Se6+ is consistent with moderately oxidising melting conditions arising from decomposition of nitrates in the waste. Results also suggest small but measurable Fe–Se redox interactions. Imposed atmospheres of air, N2, or H2 during melting resulted in increasingly reduced average Se valence as expected. Linear combination XANES fitting to quantify the relative abundances of these species was restricted by the limited availability of appropriate XANES standards for reduced Se species. EXAFS of all air-melted glasses provided robust fits indicating Se–O bond lengths of 1.71 ± 0.1 Å and CN = 3 ± 0.3, consistent with Se4+ being present in SeO32− selenite groups. Therefore, 79Se in UK alkali borosilicate HLW glasses is expected to occur predominantly as Se4+ in SeO32− selenite groups.