Mechanisms of contaminant migration from buried concrete structures

随着英国获得许可的核设施的大规模退役，将需要处理大量非活性和潜在放射性的胶凝废物。现场处置的选择可能包括在工程设施内埋葬；用于空隙填充；或者被故意留在原地。最终，这些处置这些废物的方法将根据具体地点进行评估和优化；然而，目前缺乏对这些近地表系统的一般技术理解和参数化评估模型的定量数据。如果没有这种理解，那么环境评估将需要采取一种过于保守的方法；这可能导致更多的废物被运出现场，而不是必要的，从而大大增加了成本。胶凝废物通常会产生高pH值的渗滤液，这是一种化学毒性危害，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及其周围碱性羽流产生和衰减的机制。事实上，在英国的核设施中，已经测量到了由碎水泥掩埋产生的高pH值羽状物，如果它们排放到环境敏感地区，可能会对环境有害。该项目将调查一些关键的不确定性存在的理解碱性羽流产生在英国的核场所。首先，测试不同类型的退役废物产生渗滤液的潜力；关键放射性核素（如铯、锶、铀）和污染金属（如铬、铅、镍）的固相结合将在有关废物中利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进行表征。这将决定这些污染物在胶凝废物施加的pH条件下被浸出的倾向。然后在曝气和不透气的批量实验中确定邻近土壤中发生的关键碱度衰减机制（如表面滴定、矿物溶解、碳酸化）。最后，控制废物掩埋地点土壤中污染物迁移的因素（如解吸动力学）将在批和柱实验中确定，这些数据将用于参数化反应迁移模型，以评估污染物的长期行为和处置废物的稳定性。项目目标：1；确定水泥废物在英国核设施产生碱渗滤液的可能性，以及与邻近土壤反应产生的pH值衰减和中和机制；2. 利用实验和分子尺度分析确定放射性核素和污染物微量金属在水泥废物和土壤中的命运和化学形态；3. 通过实验量化污染物解吸率，并开发埋地混凝土结构内及其周围的废物稳定和污染物行为的地球化学模型。

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.