Long-term performance of PO4-based backfill cements in repository environments for DNLEU disposal

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Through natural decay, Depleted Natural and Low Enriched Uranium (DNLEU) will become the most radioactive material in the UK geological disposal facility after 1 million years, which poses a significant and challenging-to-manage long-term risk. Despite this risk, backfill materials specifically designed to immobilise UK DNLEU have not been "studied in any detail" (RWM, 2016). This PhD project will fill this crucial knowledge gap.U solubility is minimal at near-neutral pH and increases under more oxidising conditions, although uranyl (U(VI)) phosphate phases (e.g. autunites) can have "extremely low" solubilities and favourable precipitation kinetics. Hence U-P minerals are excellent candidates for U immobilisation. Ca-P (based on apatite, Ca10(PO4)6(OH,F,Cl)2) and Mg-P cements (based on e.g. struvite-K, MgKPO4.6H2O) are thus identified as promising DNLEU backfill materials.However, while various promising phosphate cements based on these chemistries have been identified, their thermodynamic data are incompletely known. This lack in fundamental thermodynamic understanding of P chemistry (which is a prerequisite for understanding U-P chemistry) in cement systems is a key research gap in the UK DNLEU disposal context. This situation also exists in the scientific literature: thermodynamic data for Mg-P cements have only very recently been compiled. Many data gaps for phosphate cements remain.PO4DNLEU will develop the data and models needed to reliably predict the long-term performance of phosphate-based cements, which are promising backfill materials for DNLEU immobilisation. Specifically, these data/models will be validated using results from new experimental tests and applied to predict key phosphate cement material properties e.g. mineralogical composition and porosity under hydrothermal aging conditions and in the presence/absence of groundwater exposure. The project involves three work packages (WPs): (WP1) Develop(/complete) thermodynamic data/models for key solid phases in phosphate cements (WP2) Predict the properties of hydrothermally aged phosphate cement backfills designed to RWM specifications using thermodynamic modelling (WP3) Predict the durability of phosphate cement backfills exposed to groundwater