With the UK nuclear sector transitioning towards advanced fuels and advanced fuel cycles, current glass compositions used for waste immobilisation will need revising to accommodate the new waste chemistries expected. The revised glass composition will probably require an increased glass transition temperature (Tg) due to the higher heat loading from the expected future wastes. Several glass matrices from established nuclear sectors across the globe were selected for suitability testing in this initial study. These included 2 UK glasses (MW and CaZn); a modified US glass (C-104-M), a US-Brazil glass (BaBAl), a Japanese glass (P0798), and a number of modified Indian glasses (NaBSi, SB-44, SB-44-M, WTR-62 and WTR-62-M). Preliminary analysis showed that 5 candidate glasses could feasibly incorporate 25 wt% non-active surrogate waste loading and still form a stable waste form using the current UK HLW vitrification technology in use at the Sellafield site. Wider glass properties, including Tg, chemical durability (PCT-B), and melter corrosivity, were all tested. All 5 candidate glasses, down-selected from those tested, showed promising initial results, with compositional adjustments recommended for each prior to further R&D.
The WTR-62-M composition, adapted from an Indian glass formulation, for immobilisation of AFCP wastes in the current melter technology, would require compositional modification to counter its few, but potentially important, limitations. Optimisation of the composition to increase Tg would likely counter some of its more desirable features such as fluidity and low processing temperature, and while it shows good chemical durability the precipitation of metals is a cause for concern when considering the species not included in this simplified waste stream. The modified Indian SB-44-M and WTR-62-M glass formulations highlighted a range of compositional options available in these families, and whilst some barriers must be overcome (such as melting behaviour), these families warrant further investigation.
Although not conclusive from this initial study, water-soluble phases present in the waste glass samples are detrimental to long term durability. Based on this criterion alone sample SB-44-M would be suitable for long term storage. However, 7-day PCT-B durability testing results have shown that the other glasses have superior chemical durabilities, and Mo release was higher from the SB-44-M glass than some other glasses. Given that the other glasses have higher chemical durability by 7-day PCT-B, perhaps including Ca or Ba in those base compositions to form insoluble crystal phases would provide value. The better-performing glasses overall contained divalent cations such as Ca, Zn, Ba and Pb, which appear to help prevent formation of water-soluble alkali molybdate phases. For this reason, further development of divalent cation-richer glass compositions, whilst also delivering an acceptable balance of melting behaviour, thermal properties, and chemical durability, is warranted.
Many of the desired properties interfere, either constructively or destructively, for example suppressing water soluble phases and improving chemical durability are constructive, whilst increasing Tg while keeping a low melting temperature is challenging. For this reason, iterations of selected glass compositions would be necessary to optimise glass properties for the AFCP wastes and processing technology. Overall, with further development, glass formulations that maximise waste loading whilst meeting the criteria for preparing acceptable AFCP glass wasteforms using the current WVP/VTR AVM process used at Sellafield and NNL, may indeed be achievable, with CaZn, P0798, C-104-M, WTR-62-M and SB-44-M glasses all showing initial promise.