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

经过自然衰变，贫化天然低浓缩铀(DNLEU)将在100万年后成为英国地质处置设施中最具放射性的物质，这构成了重大的和具有挑战性的长期风险管理。尽管存在这种风险，但专门为固定英国DNLEU而设计的回填材料尚未进行任何详细研究(RWM，2016)。这个博士项目将填补这一关键的知识空白。铀的溶解度在近中性的pH下最小，在更多的氧化条件下增加，尽管铀酰(U(VI))磷酸盐相(如奥陶石)的溶解度可能“极低”，并具有有利的沉淀动力学。因此，铀-磷矿物是铀固定化的极佳候选者。Ca-P(基于磷灰石，Ca10(PO4)6(OH，F，Cl)2)和Mg-P水泥(基于鸟粪石-K，MgKPO4.6H2O)被认为是有前途的DNLEU回填材料。然而，尽管基于这些化学成分的各种有前景的磷酸盐水泥已经被鉴定出来，但它们的热力学数据并不完全清楚。缺乏对水泥体系中磷化学(这是了解U-P化学的先决条件)的基本热力学理解是英国DNLEU处置背景下的一个关键研究空白。这种情况也存在于科学文献中：镁磷水泥的热力学数据直到最近才被汇编。PO4DNLEU将开发所需的数据和模型，以可靠地预测磷酸盐水泥的长期性能，这种水泥是DNLEU固定化的有前景的回填材料。具体地说，这些数据/模型将使用新的实验测试结果进行验证，并应用于预测关键的磷酸盐水泥材料特性，例如在热液老化条件下和在有/没有地下水暴露的情况下的矿物成分和孔隙率。该项目涉及三个工作包(WP)：(WP1)为磷酸盐水泥中的关键固相开发(/完整)热力学数据/模型(WP2)使用热力学模型(WP3)预测按RWM规范设计的水热老化磷酸盐水泥回填材料的性能(WP3)预测暴露在地下水中的磷酸盐水泥回填材料的耐久性

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