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Title: Ceramic wasteforms for wastes arising from potential future fuel cycles
Author: Bailey, Daniel J.
Awarding Body: University of Sheffield
Current Institution: University of Sheffield
Date of Award: 2017
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The UK produces considerable volumes of radioactive materials as a result of nuclear power generation and subsequent reprocessing of spent nuclear fuel. Spent fuel is currently reprocessed using the PUREX process and the subsequent High Level Waste is immobilised in a borosilicate glass matrix, however, some elements present in the waste complicate the vitrification process either through volatilisation or the formation of undesirable secondary products. Proposed future fuel cycles offer the opportunity for enhanced segregation of wastes and therefore open up the possibility of more tailored disposal routes such as immobilisation in ceramic matrices. This thesis presents a series of studies on the immobilisation of problematic elements by the use of ceramic matrices. The wastes selected were: caesium, iodine, technetium, plutonium and Mixed Oxide Fuel residues. A summary of the main results for each waste are provided below: Caesium- The titanate phase hollandite was selected as the host matrix of choice for this study. A range of Cs containing iron hollandites were synthesised via an alkoxide-nitrate route and the structural environment of Fe in the resultant material characterised by Mössbauer and X-ray Absorption Near Edge Spectroscopy. The results of spectroscopic analysis found that Fe was present as octahedrally co-ordinated Fe (III) in all cases and acts as an effective charge compensator over a wide solid solution range. Iodine- Iodine immobilisation in the apatite structured iodovanadinite phase was studied using hot isostatic pressing (HIPing) to minimise iodine volatilisation. Increasing the overpressure during HIPing was found to yield products of superior density. The use of AgI as an iodine source was found to complicate the formation of the apatite phase and when used as the sole source was found to not be incorporated into the target phase at all. The possibility of co-immobilisation of Tc was investigated by using the surrogate molybdenum. Limited substitution of Mo into the apatite structure was observed however, this observation is complicated by the aforementioned non-incorporation of AgI. Further investigation is necessary to investigate the possibility of Mo incorporation when not using AgI as the iodine source. Substitution of Ba into the structure resulted in the formation of Ba3(VO4)2 structured phase with solid-solution behaviour observed between Ba and Pb. Mixed Oxide Fuel residues- Brannerite was selected as the potential host matrix for the disposal of MOX residues due to its high potential waste loading. Ce was used as an inactive structural surrogate for plutonium. The resultant phase assemblage was found to be dependent on both processing atmosphere and waste loading. In air, it was found that decreasing waste loading by substituting the neutron absorber gadolinium improved the phase assemblage. Reacting brannerites in a reducing atmosphere was found to produce a highly unfavourable phase assemblage with large amounts of retained UO2. The most favourable phase assemblage was found to be achieved by sintering in argon however, increasing substitution of Gd was found to have a negative impact. XANES study found that Ti oxidation state remained unchanged whereas Ce was found to reduce from the +4 to the +3 oxidation state in all cases. The use of μ-focus XANES confirmed that charge compensation was achieved by the oxidation of U(IV) to higher oxidation states. Technetium and plutonium- Zirconolite was selected as a potential host phase for the co-disposal of technetium and plutonium and studied using the non-active surrogates Ce and Mo. The formation of the zirconolite phase was found to be improved by reaction at higher temperatures and the use of CaTiO3 as the Ca precursor instead of CaCO3. Cold pressed and sintered zirconolites were found to be highly porous and this was attributed to the volatilisation of Mo at elevated temperatures. Hot isostatic pressing was found to improve the density of synthesised zirconolites however, the temperature limitations imposed by the use of stainless steel cans resulted in an unfavourable phase assemblage. HIPing for a longer period of time may compensate for the reaction kinetics or HIPing in an alternative can material at a higher temperature.
Supervisor: Hyatt, Neil C. ; Stennett, Martin C. Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available