Synthesis and Characterisation of Highly Active Nuclear Waste Simulants

Caesium phosphomolybdate (CPM) and zirconium molybdate (ZM) are two known problematic fission products that precipitate out within the Highly Active Storage Tanks (HASTs) at Sellafield, UK. Currently these HASTs do not have an in situ monitoring system where the behaviour of these products can be directly observed. Therefore, non-radioactive simulants of CPM and ZM have been successfully synthesised and further characterised to aid behavioural understanding, specifically within a suspension, in the hope of assisting with the planning of future waste management strategy. These fission products are potentially contributing to both mobility problems and ‘hot-spots’ with the HASTs, as a result of their suspension behaviours within the tanks, causing difficulty in re-suspension for onward processing and feasibly blocking pipes. Consequently, an understanding of their dispersion stability and behaviour is of crucial importance in limiting the potential for unexpected issues during Post Operational Clean Out (POCO). Non-radioactive simulants of spheroidal CPM and two differing morphologies of zirconium molybdate, cuboidal (ZM-1) and wheatsheaf (ZMCA-1) were successfully synthesised; confirmed via Scanning Electron Microscopy (SEM), Powder X-ray Diffraction (PXRD) and Fourier Transform Infrared (FTIR) spectroscopy. The kinetics of the CPM synthesis was reported for the first time, where is was found to be a first order reaction in respect to phosphomolybdic acid. In addition, two supplied ZM simulants (ZM-2 and ZMCA-2) with an elongated bipyramid and wheatsheaf morphology, respectively were also analysed. For the onward processing and immobilisation of these waste dispersions, centrifugal analysis was utilised to understand their suspension behaviours; sedimentation, compressive yield stress and permeability in both aqueous and 2 M nitric acid environments (mimicking current storage conditions). The application of the LUMiSizer® for sedimentation, compressive yield stress and permeability experiments and the methodology applied for investigating the permeability of the simulants is, as far as the author is aware, reported for the first time in this research. Furthermore, mixed dispersions of CPM and ZMCA-2 were also investigated in depth for the first time where it was proposed that they have strong particle-particle interactions influencing their dispersion behaviour. Overall, in comparing individual simulants CPM is reported be the most challenging to re-suspend and keep within solution as a result of its high instability in both water and nitric acid. The suspension behaviour for different ZM simulants provided different results, highlighting the significant impact of ZM’s morphology, where it is suggested a range will be present within the HASTs. When considering the mixed system of CPM:ZMCA-2 within both Milli-Q water and 2 M nitric acid regardless of the weight ratio of the simulants, an open difficult to compress bed network is formed, suggesting strong particle-particle interactions and high aggregation. The key message from this research is that the systems are complex, unstable and likely to aggregate whether individually or mixed within the acidic conditions of the HASTs.