Nuclear power is a non-carbon emitting energy resource generating 18% of electricity to the UK. As with any type of industrial process the waste management strategy is an important step to define considering the environmental, economic and political factors. However, the nuclear industry faces ongoing challenges to underpin a well-defined waste treatment strategy due to the high heat load and the radioactive nature of the products produced. Reprocessing of spent nuclear fuel produces a highly active liquor (HAL) waste stream. HAL is currently stored in a number of highly active storage tanks (HASTs). Within the HASTs, solid materials are known to have precipitated from the HAL over time. Particle simulants provide a route for understanding the physical behaviour, it is the synthesis of the particle simulants and the characterisation of these solid-liquid systems that are the interest of this study.
An understanding of the HAL waste properties is required for its safe transport, storage and eventual disposal of the HASTs are to be safely emptied and decommissioned. Collaboration with the National Nuclear Laboratory (NNL), at Sellafield UK, provided the opportunity to manufacture the HAL simulants, caesium phosphomolybdate (CPM) and zirconium molybdate (ZM), on larger scale. Manipulation of the aspect ratio of ZM particles is also investigated. The initial step of the synthesis produces spherical CPM which leads to the production of cubic ZM, the final step requires the addition of an organic additive, citric acid, where cuboidal zirconium citratomolybdate (ZMCA) is formed. Molecular modelling analysis revealed growth inhibition of the {2 0 0}, {-2 0 0}, {0 2 0} and {0 -2 0} faces, due to greater number of Zr sites for citratomolybdate complex affiliation. Distinct particle properties are established for CPM, ZM and ZMCA and compared to a common oxide particle material titanium dioxide (TiO2).
The results of this study highlight the influence of key aspects of the HAL particulates, such as size and shape, on relevant solid-liquid properties such as sedimentation and rheology. The influence of bulk liquid properties such as electrolyte concentration and pH were also investigated. Sedimentation behaviour was characterised by fitting the experimental data to the Richardson-Zaki model, yielding a fitting parameter n (cognate to particle size and shape) and thus demonstrated a settling relationship with particle shape, sphere > cubic > cuboidal.
The rheological behaviour explored was categorised into four groups: (i) flow behaviour data was fitted to a simplified Cross model yielding two parameters K (related to viscosity) and n (extent of shear-thinning); (ii) dependency of viscosity on particle volume fraction was characterised using the Krieger-Dougherty model yielding fitting parameter [µ] (particle’s contribution to suspension viscosity) and maximum packing fraction m, this demonstrated the relationship, cuboidal > sphere > cube; (iii) yield stress was characterised using an empirical model derived by Heymann et al (2002) yielding a fitting parameter σ^* (cognate to particle shape and size) and demonstrating a relationship, sphere > cuboidal > cubic; (iv) characterisation of compressive yield stress demonstrated the relationship, cuboidal > cubic > sphere.
The results indicate various possible behaviours within the tanks which may impact the storage, remobilisation and pipeline transport of this class of nuclear waste. Ultimately, it is of importance to establish the effect of solid-liquid properties on the behaviour of HAL for current processing, post operational clean out (POCO) and life-time assessment.
