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

通过自然衰变，100万年后，贫化天然铀和低浓缩铀（DNLEU）将成为英国地质处置设施中放射性最强的物质，这对管理长期风险构成重大挑战。尽管存在这种风险，但专门设计用于固定英国DNLEU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。铀的溶解度在接近中性的pH值下是最小的，在氧化性更强的条件下会增加，尽管磷酸铀酰（U（VI））相（例如autunites）可能具有“极低”的溶解度和有利的沉淀动力学。因此，U-P矿物是U固定的极好候选者。因此，Ca-P（基于磷灰石，Ca 10（PO 4）6（OH，F，Cl）2）和Mg-P水泥（基于例如鸟粪石-K，MgKPO 4·6 H2O）被确定为有希望的DNLEU回填材料。缺乏对水泥系统中磷化学的基本热力学理解（这是理解U-P化学的先决条件）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：Mg-P水泥的热力学数据只是最近才汇编出来。磷酸盐水泥的许多数据缺口PO 4DNLEU将开发可靠预测磷酸盐水泥长期性能所需的数据和模型，磷酸盐水泥是DNLEU固定的有前途的回填材料。具体而言，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，例如在水热老化条件下以及存在/不存在地下水暴露的情况下的矿物组成和孔隙度。该项目包括三个工作包（WP）：（WP 1）开发（/完成）磷酸盐水泥中关键固相的热力学数据/模型（WP 2）使用热力学模型预测按照RWM规范设计的水热老化磷酸盐水泥回填的特性（WP 3）预测暴露于地下水的磷酸盐水泥回填的耐久性

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