The synthetic element technetium (99Tc) has a thermal fission yield of approximately 6 % and is present at appreciable concentrations in spent nuclear fuel. While the current UK disposal method of reprocessed spent nuclear fuel raffinate - vitrification within an alkali borosilicate glass matrix - is sufficient for immobilising a range of fission products, the volatility of 99Tc within this system is low and problematic from the perspective of waste loading. Moreover, the conditions utilised promote the formation of Tc2O7, which is highly soluble in the presence of water, and readily forms the mobile pertechnetate anion (TcO4 -). The resultant environmental mobility of TcO4 - therefore, poses significant post-disposal mobility risks.
Despite the cessation of reprocessing at the Thermal Oxide Reprocessing Plant (ThORP), the driver for new nuclear power and the resultant increase in global stockpiles of spent nuclear fuel means that the potential of a future UK reprocessing programme cannot be excluded. In the event of such an action, strong consideration will likely be taken for the implementation of a 99Tc rejection stream such as that operated at La-Hague (France). Due to the uncertainty surrounding robust methods of the long-term immobilisation of 99Tc worldwide, an in-depth understanding of the fundamental Tc chemistry is required, and due to its synthetic nature and lack of stable isotopes, this is lacking compared to the majority of elements present in spent fuel and associated radioactive waste streams.
The overarching aim of this thesis was to identify and explore three key areas relating to 99Tc, to further fundamental research and investigate potential wasteforms. The areas identified were:
1. Assessment of Re as a surrogate in mechanochemical synthesis of alkalipertechnetates (ATcO4)
The use of rhenium (Re) as a surrogate for 99Tc in the synthesis of volatile alkali pertechnetates (ATcO4) was evaluated with an emphasis on comparing Re-based outcomes for comparative AReO4 compounds (A = K, Rb, Cs, Tl, Ag). Mechanochemical synthesis was identified as an alternative approach that bypasses the need for wet chemistry, effectively reducing contamination risks and minimising the generation of radioactive waste streams requiring immobilisation. Characterisation of the synthesised compounds (KTcO4, RbTcO4, CsTcO4, TlTcO4, and AgTcO4) was conducted through X-ray Diffraction (XRD), X-ray Absorption Spectroscopy (XAS), and Scanning Electron Microscopy (SEM). The study demonstrated the effectiveness of Re as a Tc surrogate in mechanochemical synthesis, with synthesised compounds closely matching reported characterisations.
2. Immobilisation of 99Tc
The synthesis of a previously unreported Tc(VII) compound in octahedral coordination is reported. Solid-state synthesis of two Ba (A-site) double perovskites was achieved through B-site cation charge balancing using Na or Li. Subsequent Sr (A site) double perovskites were also attempted, and although successful primary phase formation was confirmed using XRD, SEM, and XAS, secondary phases (NaTcO4 or unidentified) were present, resulting in an inferior
wasteform.
3. Wasteform conditioning of 99Tc
Systematic doping of 99Tc into the SnO2 system was investigated via solid-state synthesis. Similarities in ionic radii of Sn and 99Tc resulted in the successful incorporation of 99Tc into the cassiterite (SnO2) structure, as confirmed by XRD. The co-location of 99Tc on the Sn site was verified through SEM-EDX with XAS confirming the oxidation state of Tc(IV) and Sn(IV). The single phase was achieved at doping of 5-25 mol %, with low-level ingrowth of Tc metal visible at 30 mol % 99Tc, indicating the system's solid solution limit.
