Few studies have sought to investigate the potential for dissolved organic matter (DOM) to bind (and thus potentially transport) radionuclides under the high pH regimes that are expected in cementitious disposal. We have used equilibrium dialysis to investigate uranyl binding to humic acid over a pH range of ∼10 to 13. The experimental results provide evidence that DOM can bind uranyl ions over this pH range, including in the presence of competing ions. There is a general decrease in binding with increasing pH, from ∼80% of total uranyl bound at pH 9.8 to ∼10% at pH 12.9. Modelling of the system with WHAM/Model VII can yield representative results up to pH ∼10.5.

The sorption behaviour of I−, Cs+, Ni2+, Eu3+, Th4+ and UO2+2on NRVB (Nirex reference vault backfill) a possible vault backfill, at pH 12.8 was studied. Sorption isotherms generated were compared to results obtained in the presence of cellulose degradation products (CDP). Whereas Cs was not affected by the presence of the organic compounds, a notable reduction in the sorption of Th and Eu to cement was observed. The results also indicated limited removal of Ni from solution (with or without an organic ligand) by sorption, the concentration in solution seemingly being determined solely by solubility processes. In the case of uranium, the presence of CDP increased the sorption to cement by almost one order of magnitude. Further studies into the uptake of CDP by cement are being undertaken to identify the mechanism(s) responsible.

Samples of artificially aged Nirex reference vault backfill (NRVB) were prepared by progressive leaching with deionized water, after which some of the samples underwent hydrothermal treatment. Compared to unaltered NRVB, X-ray diffraction (XRD) and differential scanning calorimetry (DSC) provided evidence for the ageing resulting in alteration of the mineralogy, in particular the absence of portlandite was observed. The specific surface area of NRVB initially increased due to leaching, but then decreased after further leaching.

Sorption distribution ratios (RD values) of uranium(VI), neptunium(IV), tin and zirconium onto aged NRVB samples were measured using the batch sorption technique. For all four elements, there was little difference between RD values for aged NRVB and those for untreated material. The most probable explanation for these findings is that even though the ageing treatments altered the NRVB mineralogy, calcium silicate hydrate (C-S-H) phases are responsible for most of the radionuclide uptake and their transformation to more crystalline forms has little effect on the bulk sorption capacity of the aged material.

The effect of competition from other metal ions on the complexation of Ni with isosaccharinic acid, gluconic acid and picolinic acid, at high pH, is described. The competing metal ions used were divalent Co, trivalent Eu and tetravalent Th. In the majority of cases, competition from these metal ions followed the predicted pattern, with most anomalies seeming to be caused by sorption of Ni to the many different solid phases formed in presence of the competing ions. The Ni solid phase was shown to change with time during the course of the study. No major unexplained competition effects were found.

In the UK, technetium may be disposed of in a high pH, low Eh cementitious repository. In such a scenario, its chemistry would be dominated by TcO4−, in aerobic conditions and the sparingly soluble TcO2(s) in anaerobic conditions. Repository heterogeneity could mean that both Tc(VII) and Tc(IV) are present simultaneously. The anthropogenic ligands EDTA and picolinic acid are commonly used as decontamination agents and are likely to be present in intermediate-level waste (ILW). They could complex with Tc(IV) increasing its aqueous concentration and mobility in the cement porewaters and beyond. The conditional stability constants (measured in 0.3 mol dm−3 NaOH) for the Tc(IV)–EDTA and Tc(IV)–picolinic acid complexes have been determined to be: βTc(IV)−EDTA = 1.6 × 1026, which corresponds to log β = 25.2±0.6; and βTc(IV)−PA = 8.65 × 1026, which corresponds to log β = 26.9±0.1, respectively.

Several European waste management organizations have started work on creating a technology platform to accelerate the implementation of deep geological disposal of radioactive waste in Europe. There is an increasing consensus in the international community about geological disposal as the preferred option for solving the long-term management of spent fuel, high-level waste and other longlived radioactive wastes. At the same time, European citizens have a widespread desire for a permanent solution for high-level radioactive waste disposal. A majority of European countries with nuclear power have active waste-management programmes, but the current status and the main challenges of those programmes vary. The most advanced waste management programmes in Europe (i.e. those in Sweden, Finland and France) are prepared to start the licensing process of deep geological disposal facilities within the next decade. Despite the differences between the timing and the challenges of the different programmes, there is a joint awareness that cooperation on the scientific, technical and social challenges related to geological disposal is needed, and that it is beneficial for the timely and safe implementation of the first geological disposal facilities. Such a demonstration of a viable solution for the management of high-level radioactive waste will enhance stakeholder confidence in Europe. It is envisaged that a technology platform would enhance European cooperation in this area. The platform will provide a tool for reducing overlapping work, to produce savings in total costs of research and implementation, and to make better use of existing competence and research infrastructures. From 2008, SKB (Sweden) and Posiva (Finland) led the preparation work to set up the implementing geological disposal of radioactive waste technology platform (IGD-TP). Since then other implementers from France, Germany, Switzerland, United Kingdom, Spain and Belgium have joined the project. To date a strategic research agenda for the platform has been prepared and consulted upon, which is now the basis for taking the platform into a deployment phase.

The international long-term cement studies (LCS) project aims to increase the understanding of the behaviour of cement within a radioactive waste disposal system and how hyper-alkaline leachates may interact with host rock. Such an understanding enables confident, robust and safety-relevant statements to be made concerning future system behaviour, irrespective of host rock, engineered barrier system, or waste type. The LCS project involves laboratory experiments, in situ tests and numerical modelling to address these issues. The agencies participating are Nagra (Switzerland), JAEA (Japan), the Nuclear Decommissioning Authority, Radioactive Waste Mangement Directorate (UK), Posiva (Finland) and SKB (Sweden).

Project activities have included: the development of conceptual and theoretical models of cement–rock interaction; testing of numerical models against data from laboratory experiments and industrial and natural analogues of cement–rock reaction; and the synthesis and incorporation of performance assessment (PA) relevant data from analogue studies. Key threads running through these studies include an analysis of issues relating to upscaling of processes and data to the greater temporal and spatial scales relevant to PA, and investigations of modelling the changes in physical properties that accompany geochemical reaction. Here we present examples of the results from model test cases, highlighting the important issues that have arisen.

The implementation of a geological disposal facility requires the demonstration of confidence that such a facility would be safe during both the operational period and in the long-term after the closure of such a facility. The generic environmental safety case described in this paper is the vehicle used to demonstrate an understanding of environmental safety. It will be used to prepare a site-specific environmental safety case in due course. The approach taken will be consistent with a staged development and approval process, as advocated by the environmental regulators.

This paper provides a summary of the research programme undertaken in support of the Low Level Waste Repository's 2011 environmental safety case (ESC). The programme has been developed, based on an understanding of safety issues and the requirements of the ESC. The research requirements to underpin the safety case have been identified by means of an auditable process, and subjected to scrutiny by both the regulators and a peer review group. Key research priorities for the future are identified.

The 2008 UK government White Paper, published as part of the Managing Radioactive Waste Safety programme, identified benefits to disposing of all of the UK's higher activity wastes at the same site. That is, a single geological disposal facility (GDF) could be constructed that consists of a module for low- and intermediate-level waste, and a module for high-level waste and spent fuel.

A safety case for a co-located GDF will have to consider the extent to which evolving thermo-hydro-mechanical-chemical and gas (THMCG) conditions in and around one module may affect conditions in the other module, including the extent to which barrier performance and radionuclide migration behaviour could be altered. Several research projects have been undertaken on behalf of Radioactive Waste Management Directorate aimed at understanding and evaluating the THMCG interactions that might occur during the disposal facility operational and post-closure phases.

This paper describes research on THMCG interactions between disposal modules based on illustrative GDF designs for different host rock environments. Interactions were evaluated using simple analytical solutions and detailed three-dimensional models. The analyses demonstrated that interactions can be controlled by design constraints.

A three-year European Commission project entitled performance assessment methodologies in application to guide the development of the safety case (PAMINA) was conducted in the period 2006–2009 and brought together 27 organizations from ten European countries, including the Nuclear Decommissioning Authority and Galson Sciences Ltd from the UK. The PAMINA project had the aim of improving and developing a common understanding of performance assessment (PA) methodologies for disposal concepts for spent fuel and other long-lived radioactive wastes in a range of geological environments.

Work undertaken within PAMINA focussed on four areas: (1) review of PA methodologies in participating organizations; (2) treatment of uncertainty in PA and the safety case; (3) other methodological advancements in PA; and (4) relevance of advanced PA approaches to practical cases.

The state of development of a radioactive waste disposal programme has a strong influence on the type of safety case and supporting PA that is produced. A range of PA methodologies has been developed by different waste management organizations. This paper presents a selection of conclusions from the PAMINA project, in the context of general understanding developed on what would constitute an acceptable safety case for a geological disposal facility, and outlines areas for further development.

Disposal in deep geological formations aims to provide isolation of long-lived radioactive waste for hundreds of thousands of years. This raises the question of the long-term governance of the repository throughout its lifetime. In the operational phase the repository is under active regulatory control. Once closed, there will be a phase of passive management control or indirect oversight. This will be followed, at some time in the future, by a period in which there is no oversight. This may be a result of a decision to cease management control or it may occur through loss of records or a change in priorities. The importance of the main exposure scenarios (exposure as a result of the gradual transport of radionuclides in groundwater, transport of radionuclides in gas, and exposure arising from inadvertent intrusion into the repository) are discussed with reference to these different phases. An interesting question is 'How do we minimize the risk of inadvertent intrusion in the far future?' Perhaps it is better to ensure that the repository is forgotten and should we try to plan for this? The different approaches are discussed and the importance of deciding on a strategy at an early stage is emphasized.

Programmes for the geological disposal of radioactive wastes are by nature extremely complex. A structured approach for making and documenting varied kinds of decisions is required to support programme design and implementation. At each programme stage, the decision-making process must be able to identify and justify key priorities for work, to reduce uncertainties.

To support structured decision-making evidence support logic (ESL) has been developed and applied to varied complex projects, nationally and internationally, in several industries. Evidence support logic involves breaking down a hypothesis that informs a decision into a hierarchical 'decision tree'. Examples of hypotheses are 'the geology associated with site x will provide sufficient disposal capacity', 'container x will contain waste form y for z years' and 'the engineered barrier system will provide the required safety functions'. Independent evaluations of confidence 'for' and 'against' bottom-level hypotheses allow the level of remaining uncertainty (or conflict) to be recognized explicitly, and the overall confidence (and uncertainty) relevant to the overall decision, and key sensitivities, to be represented clearly and succinctly.

Thus ESL can help (1) break down decisions into a manageable and logical structure, assisting clear presentation; (2) identify key uncertainties and sensitivities to inform prioritization; and (3) test whether the outcomes of specific studies have improved confidence.