Inducing the resumption of alteration in UK radioactive waste glasses

放射性废玻璃在与水接触的最初1 - 28天内，其溶解速度大约降低了千分之一。这种溶解速率的降低，即所谓的残余速率，与溶解的热力学驱动力的降低（当某些玻璃组分的溶解度极限接近时，不再有玻璃被溶解）以及对玻璃表面形成的改变层的保护作用有关，这种改变层抑制了与未改变的玻璃表面之间的运输。在国际（和成分不同的）玻璃配方中，在长时间尺度上偶尔观察到的一种现象是，玻璃溶解以接近初始溶解速率的速度恢复。这种现象被称为恢复改变，并且仍然是地质处置设施中放射性废弃玻璃长期性能的潜在不确定性。本研究项目将调查英国放射性废料玻璃恢复改造方案的适用性，并确定在任何恢复方面的主要控制措施，即其成分与国际放射性废料玻璃的差异。当溶液成分在玻璃表面形成二次相时，溶解的热力学驱动力恢复时，就会发生恢复。这种二次相的形成也可能引起表面保护性质的改变。该项目的两个主要目标是：区分被认为导致恢复改造的两种影响的相对重要性。确定恢复溶出的机制及其速率与初始速率和速率下降的关系。>确定该过程对ph值的敏感性。这是为了涵盖地下水通过与回填或ILW拱顶中的胶结材料接触而变得高度碱性的潜在情况。除了常规的溶液浓度测量外，还将使用扫描电子显微镜、EDX和微探针分析对表面材料进行详细分析。将部署固态核磁共振波谱技术，可以量化改变层的程度（通过1H NMR），并通过1H交叉极化技术选择性地探测和识别存在或再沉淀在玻璃表面的晶体和非晶成分的结构。同位素技术（柱分离和详细质谱）确定溶液中的同位素比率作为时间的函数，提供了溶液和表面在恢复过程中演变的补充观点。

The dissolution rate of radioactive waste glasses reduces by approximately a factor of one thousand over the first 1 - 28 days of contact with water. This reduction in dissolution rate, to what is known as the residual rate, is associated with a reduction in the thermodynamic driving force for dissolution (as solubility limits of certain glass components are approached and no more glass can be dissolved) and the protective effect on the glass surface of the formation of an altered layer that inhibits transport to and from the unaltered glass surface. A phenomenon that is occasionally observed at long time scales in international (and compositionally different) glass formulations is that glass dissolution resumes at rates approaching the initial dissolution rate. This phenomenon is known as resumption of alteration and remains as a potential uncertainty in the long-term performance of radioactive waste glasses in a geological disposal facility. This research project will investigate the applicability of a resumption of alteration scenario for UK radioactive waste glasses and determine the primary controls on any resumption with respect to their compositional differences from international radioactive waste glasses.The resumption is thought to occur when the thermodynamic driving force for the dissolution resumes as solution components form secondary phases on the surface of the glass. This secondary phase formation may also cause changes to the protective nature of the surface.The two main objectives of the project will be:> Distinguish between the relative importance of the two effects thought to cause resumption of alteration.> Determine the mechanism of the resumption of dissolution and its rate with respect to the initial rate and the rate-drop.> Determine the sensitivity of the process to pH. This is to cover potential scenarios where groundwaters have become highly alkaline through contact with cementitious material in back-fill or ILW vaults.In addition to conventional measurements of solution concentrations, detailed analysis of the surface material will be undertaken using scanning electron microscopy, EDX and microprobe analysis. Solid-state nuclear magnetic resonance spectroscopy techniques will be deployed that can quantify the extent of the altered layer (by 1H NMR) and selectively probe and identify the structure of both crystalline and amorphous components present or re-precipitated on the glass surface by 1H- cross polarisation techniques. Isotope techniques (column separation and detailed mass spectrometry) to determine the isotopic ratios in solution as a function of time provide the complementary views of the evolution of both solution and surface during the resumption process.