Mechanisms of contaminant migration from buried concrete structures

Project Outline: 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.