The Solubility of Ba in a New Cs Waste Form, Cs2TiNb6O18
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The Solubility of Ba in a New Cs Waste Form, Cs2TiNb6O18 George Day 1, Geoffrey L. Cutts 1, Tzu-Yu Chen 1, Joseph A. Hriljac 1 and Yina Guo 2 1 School of Chemistry, University of Birmingham, Edgbaston, Birmingham, B15 2TT, UK 2 Materials and Surface Science Institute, University of Limerick, Sreelane, Limerick, Ireland ABSTRACT We report experimental and computational studies of Ba doping for Cs in Cs2TiNb6O18, a material with potential to be an exceptional ceramic waste form for Cs sequestration. Three co-doping (simultaneous metal reduction for charge balance of Ba2+ for Cs+) schemes have been experimentally tested: Ti4+ for Nb5+, Ti3+ for Ti4+ and Nb4+ for Nb5+. Unfortunately, none showed conclusively that the co-substitution was successful. Atomistic modelling was then performed on all three schemes using novel potentials to assess the energetic feasibility, from these the most favourable scenario is reduction of Nb5+ to Nb4+. INTRODUCTION 137
Cs is one of the most problematic radionuclides formed from the nuclear fission process. It is one of the primary heat and radioactivity contributors in spent fuel with a halflife of 30.2 years and a fission yield of around 6 %. Furthermore, many salts of Cs are highly soluble and therefore Cs becomes readily mobile if released into the biosphere. As a result of these characteristics, Cs is often separated from waste streams using a number of different processes and materials, typically ion exchange with inorganic exchangers. Inorganic exchangers are often employed on account of their high selectivity towards Cs, radioactivity and heat resistance and compatibility to be converted into a final waste form. Hot isostatically pressing (HIPing) Cs-loaded IONSIV produces a robust Cs waste form, Cs2TiNb6O18 [1]. This has been shown to have comparable leaching properties to hollandite, the Cs-containing phase in SYNROC [2]. When considering Cs waste forms, it is important to know if 137Ba2+, the transmutation product of 137Cs+, can be retained. By demonstrating the successful synthesis of mixed Cs/Ba hollandites, it has been claimed in previous studies that this process will occur naturally in hollandites [3]. Therefore synthesis of Ba doped Cs2TiNb6O18 could provide evidence that Ba could be retained and give reassurance of the long term integrity of the waste form. It was thought that Ba incorporation could be achieved via three stoichiometric mechanisms: ସା • Scheme 1: ଶି୶ ୶ ଵା୶ ହା ି୶ ଵ଼ ଷା ସା • Scheme 2: ଶି୶ ୶ ୶ ଵି୶ ଵ଼ ହା • Scheme 3: ଶି୶ ୶ ସା ୶ ି୶ ଵ଼
Scheme 1 served only as a ‘proof of concept’ as is not a realistic scenario in a waste form. Scheme 2 and 3 however represented ‘real life’ scenarios where Cs+ decays to Ba2+, releasing a β- particle which subsequently reduces either Ti4+ to Ti3+ (scheme 2) or Nb5+ to Nb4+ (scheme 3). A series of samples following schemes 1, 2 and 3 have been synthesized and analyzed.
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