Previous work1 has shown that γ-radiation induced enhancement of leach rates from high-level radioactive waste (HLW) solids can be attributed to radiolytic acidification of the aqueous solution and to radiolytic formation of organic complexing species, for example, (COOH)2, HCOOH. Barkatt et al.1 compared the leach resistance of three HLW glasses and SYNROC D (specifically designed for incorporation of US defence wastes2) and found that, at low pH and with complexing agents, higher alumina contents in the waste form gave large enhancement of leach rates. Specifically, in acid solution, the elemental leach rates for SYNROC D were increased relative to the glasses by factors between 20 and 1,200. The formulation of SYNROC C, designed by Ringwood2 for incorporation of reprocessed waste from commercial power reactors, was not tested in these conditions. We report here that SYNROC C is considerably more resistant than the standard HLW glass PNL 76–68 to acid attack and to possible complexation reactions between 90 and 150 °C, despite an alumina content in the formulation that is much higher than that for PNL 76–68. The Al content of SYNROC C is much lower than that of SYNROC D (Table 1) and it is largely present in the hollandite phase after hot pressing. No aluminosilicate phases are formed in SYNROC C and, for this reason, the pH dependence is much less marked. These results are significant in assessing the effects of changes induced by the ionizing radiation to which the HLW solid and solution will be exposed during geological isolation.
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Levins, D., Smart, R. Effects of acidification and complexation from radiolytic reactions on leach rates of SYNROC C and nuclear waste glass. Nature 309, 776–778 (1984). https://doi.org/10.1038/309776a0
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