Mechanisms of contaminant migration from buried concrete structures

随着英国核许可场所的大规模退役，大量不活跃和具有潜在放射性的胶凝废物将需要处置。现场处置的选择可能包括在工程设施内埋葬；用于填补空隙；或故意留在原地。最终，这些处理这些废物的方法将根据具体场地进行评估和优化；然而，目前缺乏对这些近地表系统和量化数据的一般性技术理解，以使评估模型参数化。如果不发展这种理解，那么环境评估将需要采取过于保守的方法；这可能导致更多的废物被运输到不必要的地方，从而大大增加成本。胶凝废物通常产生高pH值的渗滤液，这是一种化学毒性危险，这些渗滤液还可能含有有毒金属和放射性核素。因此，有必要了解近地表处置内和周围碱性羽流的产生和衰减机制。事实上，英国核设施已经测量到水泥粉尘产生的高PH值羽流，如果它们排放到环境敏感地区，可能会对环境有害。该项目将调查在了解英国核电站碱性烟羽生成方面存在的一些关键不确定因素。首先，将测试不同退役废物类型的渗滤液产生潜力；并将使用最先进的高分辨率同步加速器和电子显微镜，结合传统的地球化学分析，表征相关废物中关键放射性核素(例如Cs、Sr、U)和污染金属(例如Cr、Pb、Ni)的固相组合。这将决定这些污染物在胶凝废物施加的pH条件下被淋洗的倾向。然后，将在曝气和不含空气的间歇试验中确定发生在邻近土壤中的关键碱度衰减机制(例如，表面滴定、矿物溶解、碳化)。最后，将通过批量和柱状实验确定控制垃圾掩埋场下坡土壤中污染物迁移的因素(例如解吸动力学)，这些数据将被用来对反应传输模型进行参数化，以评估长期的污染物行为和处置废物的稳定性。项目目标：1.确定英国核电厂水泥垃圾产生碱性渗滤液的可能性，以及与邻近土壤反应所发生的pH衰减和中和机制；2.通过实验和分子尺度分析表征放射性核素和污染物痕量金属在水泥垃圾和土壤中的去向和化学形态；3.通过实验量化污染物的解吸率，并为掩埋的混凝土结构内和周围的废物稳定化和污染物行为开发地球化学模型。

As large scale decommissioning of the UK's nuclear licensed sites proceeds, very large volumes of both inactive and potentially radioactive cementitious wastes will require disposal. Options for onsite disposal may include burial within engineered facilities; use for void filling; or being deliberately left in-situ. Eventually these approaches for disposal of these wastes will be assessed and optimised on a site specific basis; however currently the generic technical understanding of these near-surface systems and quantitative data to parameterise assessment models is lacking. If this understanding is not developed then environmental assessments will need to adopt an overly conservative approach; which may result in more waste being transported offsite than necessary, with substantially increased costs. Cementitious wastes commonly generate high pH leachate, which represent a chemotoxic hazard, and these leachates can also contain toxic metals and radionuclides. Therefore, there is a need to understand the mechanism of alkaline plume generation and attenuation in and around near-surface disposals. Indeed, high pH plumes from crushed cement burials are already being measured on UK nuclear sites and these could be detrimental to the environment if they discharge into environmentally sensitive areas. This project will investigate a number of key uncertainties exist in the understanding of alkaline plume generation on UK nuclear sites. Firstly, the leachate generation potential of different decommissioning waste types will be tested; and, the solid phase associations of key radionuclides (e.g. Cs, Sr, U) and contaminant metals (e.g. Cr, Pb, Ni) will be characterised in relevant wastes using state of the art high resolution synchrotron and electron microscopy in concert with traditional geochemical analysis. This will determine the propensity of these contaminants to be leached under the pH conditions imposed by the cementitious wastes. Then the key alkalinity attenuation mechanisms (e.g. surface titration, mineral dissolution, carbonation) that occur in adjacent soils will be determined in aerated and air-excluded batch experiments. Finally, factors that control contaminant mobility in soils downgradient of waste burial sites (e.g. desorption kinetics) will be determined in batch and column experiments, and this data will be used to parameterise reactive transport models to assess long-term contaminant behaviour and the stability of the disposed wastes.Project Objectives: 1. Determine the potential for cement wastes to produce alkali leachates on UK Nuclear sites, and the pH attenuation and neutralisation mechanisms that occur by reaction with adjacent soils; 2. Characterise the fate and chemical speciation of radionuclides and contaminant trace metals in cement wastes and soils using experiments and molecular scale analysis; 3. Experimentally quantify contaminant desorption rates and develop geochemical models for waste stabilisation and contaminant behaviour in and around buried concrete structures.