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.

With potential storage of several hundred years underground in a geological disposal facility (GDF) before closure, there is a requirement for radioactive waste packages to perform adequately. Over the past 25 years, cementitous grouts based on blast furnace slag (BFS) and ordinary Portland cement (OPC), have been used in the UK to immobilize intermediate-level waste (ILW), Magnox swarf, and currently such wasteforms are in surface storage awaiting geological disposal. Magnox fuel cladding will slowly corrode when encapsulated in alkaline cementitious grouts to produce hydrogen gas and an expansive corrosion product. Expansive corrosion products may lead to degradation of the wasteform and, if extensive, could affect the container. This study investigated the acute and chronic rates of corrosion of unirradiated Magnox swarf encapsulated in a BFS/OPC grout over a range of temperatures (25, 40, 60, 75 and 90°C) for curing times of up to 2½ years. Structural product degradation starts to develop as the tensile strength of the grout is approached by the expansive forces generated. Deformation of some experimental containers was also noted. An estimate of the time taken for a grouted product to fracture from this study due to the corrosion solely of Magnox is 350 years for storage at 25°C. Although not fully described in this paper, the main cement phases were calcium silicate hydrate, portlandite and gehlenite; the main product of Magnox corrosion was brucite. The curing temperature did not affect the compositions of the Magnox metal, corrosion product or grout.

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.

Mining is known to be a major source of metal contamination for fluvial systems worldwide. Monitoring and understanding the effects on downstream water and sediment quality is essential for its management and to mitigate against detrimental environmental impacts. This study aimed to examine the effects of the small-scale, abandoned, hydraulic Black Creek gold mine on the geochemical content of fine (<63 μm) stream bed and floodplain sediment in the Horsefly watershed (British Columbia, Canada), which is a part of the Quesnel River basin. Concentrations of As, Cd, Se and Zn were determined by aqua regia digestion and the modified BCR (European Community Bureau of Reference) sequential extraction procedure followed by inductively coupled plasma-optical emission spectroscopy or inductively coupled plasma-mass spectrometry. Age-depth models for the floodplain cores were constructed using excess Pb-210 and Cs-137 activity concentration profiles. The results show that the mine caused local As contamination of the Horsefly River floodplain sediment during the first years of operation, but at present the contamination is mainly contained in the Black Creek. Present-day and recent concentrations of Cd, Se and Zn in the Horsefly River are elevated and this is probably derived from other upstream mining activities. The spatial and temporal changes in heavymetal concentrations suggest a slight, but not particularly widespread, mining effect on the finesediment geochemistry in the Horsefly River system.

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.

Large volumes of depleted natural and low-enriched uranium exist in the UK waste inventory. This work reports on initial investigations of the leaching performance of candidate glass and cement encapsulation matrices containing UO3 powder as well as that of uranium oxide powders. The surface areas of UO3 powder and the monolith samples of UO3 conditioned in the glass and cement matrices were very different making leaching comparisons difficult. The results showed that for both types of monolith conditioned samples a steady increase of uranium concentration in solution with time was generally not observed. The wt.% of uranium leached from UO3 conditioned in the lead borosilicate glass wasteform was approximately five orders of magnitude less than that leached from UO3 powder. Similarly, the quantities of uranium leached from UO3 conditioned in composite cement made with ordinary Portland cement, and from magnesium phosphate cement, were approximately four and three orders of magnitude, respectively, less than that leached from UO3 powder. The performance of a mixed oxide borosilicate glass wasteform was only slightly better than that of UO3 powder. This work shows that wasteforms based on encapsulation in lead borosilicate glass and cement matrices have the greatest potential for further development.

To understand the fate and impact of gas produced within a repository for radioactive waste, a series of laboratory and field scale experiments have been performed on the Callovo-Oxfordian claystone (COx), the proposed host rock for the French repository. Results show the movement of gas is through a localized network of pathways, whose properties vary temporarily and spatially within the claystone. Significant evidence exists from detailed laboratory studies for the movement of gas along highly unstable pathways, whose aperture and geometry vary as a function of local stress, gas and porewater pressures. The coupling of these parameters results in the development of significant time-dependent effects, impacting on all aspects of COx behaviour, from gas breakthrough time, to the control of deformation processes. Variations in gas entry, breakthrough and steady-state pressures are indicative of microstructural heterogeneity which exerts an important control on the movement of gas. The localization of gas flow is also evident in preliminary results from the large scale gas injection test (PGZ) where gas flow is initially focussed within the excavation damaged zone (EDZ), which acts as a preferential pathway for gas. Numerical models based on conventional two-phase flow theory are unable to adequately describe the detailed observations from laboratory tests.

Higher activity radioactive wastes remain hazardous for extremely long timescales, of up to hundreds of thousands of years. Disposing of such wastes deep underground presents the internationally accepted best solution for isolating them from the surface environment on associated timescales. Geological disposal programmes need to assess potential releases from such facilities on long timescales to inform siting and design decisions and to help build confidence that they will provide an adequate degree of safety. Assessments of geological disposal include consideration of the wastes, the engineered facility, the host geology and the surface and near-surface environment including the biosphere. This paper presents an overview of recent post-closure biosphere assessment studies undertaken in support of the Nuclear Decommissioning Authority Radioactive Waste Management Directorate disposal system safety case for geological disposal of the United Kingdom's higher activity radioactive wastes. Recent biosphere studies have included: (1) ensuring that the United Kingdom's approach to consideration of the biosphere in safety case studies continues to be fit for purpose, irrespective of which site or sites are considered in the United Kingdom's geological disposal programme; (2) updating projections of global climate and sea level, together with consideration of the potential importance of transitions between climate states; (3) considering geosphere–biosphere interface issues and their representation, including redox modelling and catchment-scale hydrological modelling; and (4) identifying key radionuclides and developing a series of reports describing their behaviour in the biosphere together with an evaluation of associated implications for post-closure assessment calculations.

This paper gives an overview of the geosphere research studies being undertaken by the Radioactive Waste Management Directorate (RWMD) of the Nuclear Decommissioning Authority. The approach of the RWMD in the current generic phase of the UK managing radioactive waste safely (MRWS) programme is to maintain an understanding of key processes and to carry out research and development into techniques so capability can be built. Although RWMD can demonstrate a general understanding of geosphere processes at this stage in the UK project, it is recognized that this will need to be made site-specific as the MRWS programme progresses. An understanding of the geosphere at the selected site(s) will be an important part of the future programme. Where possible, the RWMD will participate in international studies so that relevant site-based information can be accessed. In this way, the RWMD will be prepared for site-specific work in stage 5 of the MRWS process.

The effects of the presence of ten organic ligands and common pH buffering agents – acetate, oxalate, tartrate, citrate, phthalate, EDTA, carbonate, TRIS, phosphate and borate – on aqueous Si concentration measurements made using the common molybdate blue method were quantified. The concentrations of these additives ranged from 0.01 to 0.1 mol/kg in the measured aqueous solutions. Whereas measured Si concentrations were not affected by the presence of up to 0.1 mol/kg acetate, hydrogen phthalate or TRIS, the presence of 0.1 mol/kg of all other selected additives altered substantially the measured Si concentrations using the molybdate blue method. For example the presence of 0.1 mol/kg of citrate, EDTA, and hydrogen phosphate decreased the ratio of measured to true Si concentrations to ~0.05. These variations are interpreted to stem from the formation of competing aqueous complexes in the aqueous phase limiting the formation of the characteristically blue SiMo12O404– complex.

The process of transforming hydromagnesite to magnesite is analysed in the context of the theory of solvent-mediated transformations. A series of experiments at 120, 150 and 180ºC with different heating times was designed to determine, by powder X-ray diffraction, the amount of magnesite generated as a function of time. The aqueous-phase composition was monitored by inductively coupled plasma-mass spectrometry and carbonate alkalimetry. From the analytical data, the evolution of saturation indexes with respect to both phases was determined using the geochemical code PHREEQC. Finally, two different methods were applied to obtain the activation energy of the process and a TTT (Temperature- Transformation-Time) graph was constructed to define suitable conditions in which to obtain magnesite.

Atomic force microscopy has been used to determine more precisely the mechanism of the initial stages of dolomite dissolution. Analysis of outflow solutions initially shows fluctuations of both Ca and Mg release with concentrations of Ca >> Mg. The dolomite surface dissolves congruently in the presence of slightly acidified water as confirmed by the regular spreading of characteristic rhombohedral etch pits. Direct in situ observations show that a new phase precipitates on the dissolving surface simultaneously. As the Ca and Mg release decreases with time, the precipitated phase can be seen to spread across the dolomite surface. These observations indicate that the apparent incongruent dissolution of dolomite is a two-step process involving stoichiometric dissolution with the release of Ca, Mg and CO3 ions to solution at the mineral–fluid interface coupled with precipitation of a new Mg-carbonate phase. The coupled element release confirms the interface-coupled dissolutionprecipitation mechanism.

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 (R D 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 R D 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 thermodynamic solubility product for baryte, determined at standard conditions, from data in commonly used compilations, was compared with published experimental solubility products for baryte and with the solubility products from databases used by the PHREEQC geochemical speciation code and MultiScale, which is often used by oil companies to predict or describe baryte scaling. The values in the various databases agree well with experimental data (10–10.05–10–9.96; Melcher, 1910; Neuman, 1933; Templeton, 1960; Davis and Collins, 1971; Blount, 1977; Felmy et al., 1990), which agree within uncertainty with the values presented in the compilations of Robie et al. (1979), Wagman et al. (1982), Lide (2005), Raju and Atkinson (1988), as well as Nordstrom and Munoz (1994), whose values have the least uncertainty. In solutions of 50% seawater mixed with 50% reservoir formation waters, the data predict baryte supersaturation, both at standard temperature and at the temperatures expected in the reservoir, completely consistent with field observations. This provides confidence that the Pitzer approach for activity correction and the database is valid for investigations of baryte precipitation rates in high ionic strength solutions.

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.

The Cyprus Natural Analogue Project was carried out due to the requirement to support ongoing laboratory and modelling efforts on the potential reaction of the bentonite buffer with cementitious leachates in the repository engineered barrier system. Although it is known that the higher pH (12.5–13) leachates from ordinary Portland cement will degrade bentonite, it is unclear if this will also be the case for the lower pH (10–11) leachates typical of low alkali cements. Ongoing laboratory and underground rock laboratory programmes, which are currently investigating this, face the obstacle of slow kinetics and the production of short-lived metastable phases, meaning obtaining unambiguous results may take decades. It was therefore decided to implement a focussed natural analogue study on bentonite/low alkali cement leachate reactions to provide indications of the probable long-term reaction products and reaction pathways to provide feedback on the existing short-term investigations noted above and to ascertain if any critical path research and development needs to be instigated now. The results of the analyses presented here, in this short overview of the project, suggest that there has been very limited alkaline groundwater reaction with the bentonite. This is generally supported by both the geomorphological evidence and the natural decay series data which imply groundwater/rock interaction in the last 105 a.

Gases will be generated in waste packages during their transport to a geological disposal facility (GDF), this generation will continue during GDF operations and after GDF closure. The range of gases produced will include flammable, radioactive and chemotoxic species. These must be managed to ensure safety during transport and operations, and the post-closure consequences need to be understood. The two primary post-closure gas issues for a GDF are the need for the system pressure to remain below a value at which irreversible damage to the engineered barrier system and host geology could occur, and the need to ensure that any flux of gas (in particular gaseous radionuclides) to the biosphere does not result in unacceptable risk. This paper provides an overview of the research of the Nuclear Decommissioning Authority, Radioactive Waste Management Directorate into gas generation and its migration from a GDF.