BIogeochemical Gradients and RADionuclide transport. BIGRAD

UK Government will now dispose of high level and intermediate level radioactive wastes in a geological disposal facility (GDF). A key characteristic of the UK GDF concept for intermediate level waste is the role of cementitious backfill, intended to maintain strongly alkalin egroundwater conditionsin the GDF environment. The biogeochemical gradients that will develop across the poorly understood interface between an alkaline, deep GDF and the geosphere (in the Chemically Disturbed Zone; CDZ) will be critical controls on radionuclide behaviour and transport, and thus on the safety and environmental impact of a GDF. It is this CDZ that is the focus of this consortium.

1. to gain a holistic understanding of biogeochemical proces ses and their controls on radionucl ide behaviour in the CDZ (Chemically Disturbed Zone);
2. to develop predictive modelling capability, firmly rooted in scientific advances and experimental results to describe radionuclide mobility in the CDZ.

These objectives are then split into 3 complemen tary work packages:

WP1 Geosphere Evolution, where our specific objectives are:

• To identify the products and phases arising from reaction of high pH fluids with host rock;
• To assess the microbial ecology and scope of geomicrobiological processes occurring within the CDZ environment;
• To quantify the effects of reaction between the host rock, highpH fluid s and biogeochemical processes on permeability evolution.

WP2 Radionuclide Form Reaction and Transport, where our specific objectives are, for the CDZ, to define:

• the effects of solution components and colloid / mineral surfaces on speciation and mobility of radionuclides;
• the impact of geomicrobiological processes on radionuclide speciation and mobility;< /li>
• the thermodynamic and kinetic parameters that may be used to predict radionuclide be haviour.

WP3 Synthesis and Application, where our specific objectives are:

• To construct process-based conceptual models for radionuclide transport in the CDZ at different scales, which assist development of a computational model and the design of related lab studies
< li> To interpret and quantitatively describe radionuclide transport in the CDZ conceptu al model with the computational model, using experiment results from the process-based lab studies
• To test the CDZ concept at field-scale using the computational model and assess its performance in predicting radionuclide transport in the geosphere for a range of representative conditions.

Knowledge transfer permeates all of our objectives, and we have a clear statem ent of our KT related objectives in our impact plan.

Overall, this consortium will deliver world class strategic science that will significantly advance the UK capability for decision-making on repository design, site management and environmental risk posed by a geological disposal facility.

Over 50+ years of nuclear power generation and weapons development, the UK has created large quantities of radioactive wastes. In terms of total volume, the largest fraction (> 90 %) of the higher activity waste is Intermediate Level Waste (ILW). ILW does not produce heat but contains long-lived radioisotopes, and so cannot be disposed of near the Earth's surface. The Government has recently decided that the UK&rsquo;s ILW should be disposed of underground (200 - 1000 m) in a “Geological Disposal Facility” (GDF). The safety of a GDF depends on slowing the return of radioactivity from the GDF to Earth surface. It is therefore key to understand the processes which control the movement of radioactivity out of the GDF and through the surrounding rock.

The UK&rsquo;s ILW isvery diverse and includes discarded nuclear fuel, the metal containers used to hold fuel, as well as sludges and organic debris produced when processing these radioactive materials. The UK has treated many of these radioactive wastes by immobilising them in cement and a substantial fraction of ILW has now been cemented and awaits disposal. Once the wastes have been placedin the GDF, the intention is to backfill the remaining space with cement. No site has been identified for UK wastes as yet, but it is expected that the site will be under the water table and therefore be wet. This means that, after the waste is emplaced, the GDF will rewet as groundwater percolates through the wastes. Over a long time (from hundreds to millions of years) the ILW and its steelcontainers willdegrade, andthe cement will react with the groundwater to make it very alkaline. This is a design feature, as very alkaline, “rusty” conditions are expected to make most radioactive components of the ILW very insoluble. However, this alkaline water will react with the rock around the repository to form a “chemically disturbed zone” (CDZ). Up until now, no studies have examined the chemical, physicaland biological development of this CDZ and how this affects the mobility of radioactive contaminants from the GDF. We have chosen to study four long-lived radionuclides, the fission product technetium as well as uranium, neptunium and plutonium all of which will be present over the long timescales relevant to the CDZ.

In this project, we will try andunderstand how the CDZ will evolve over thousands to millions of years, so we can predict the movement of radioactivity through it, and help assess the safety of the GDF. To do this, we need to study the chemical, physical and biological changes which occur as the CDZ develops, and the way in which these different factors interact with each other. We will use experiments to understand theseprocesses and,based on these, we will develop computer models to predict what will happen in the future. We have divided our work programme into three parts: 1 Geosphere Evolution, where we will examine rock and mineral interactions, and how water flow within the rock is affected by chemical and microbiological changes caused by the water from the GDF; 2 Radionuclide Form, Reaction and Transport, wherewe will examine the chemical form and solubility of radionuclides, their interactions with microrganisms, and with rock surfaces, and the potential for microscopic particles to carry radioactivity; 3 Synthesis and Application, where we will bring all the experimental results together and design, develop and test our computer model to examine radionuclide transport in the CDZ. To ensure welink the different parts of the project effectively, we have identified two “cross cutting themes” (CCTs) - (i) biogeochemical processes in the CDZ; and (ii) predictive modelling of the CDZ, which will tie all the different pieces of work together. Our work will provide improved understanding of the controls on contaminant mobility across the CDZ, improve confidence in the safety ofgeological disposal and hence assist the UK in the crucial task of disposing of radioactive wastes.

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