Combined microfocus XAS and XRD analysis of α-particle radiation damage haloes around thorium-containing monazite in Fe-rich biotite reveals changes in both short- and long-range order. The total α-particles flux derived from the Th and U in the monazite over 1.8 Ga was 0.022 α particles per atomic component of the monazite and this caused increasing amounts of structural damage as the monazite emitter is approached. Short-range order disruption revealed by Fe K-edge EXAFS is manifest by a high variability in Fe–Fe bond lengths and a marked decrease in coordination number. XANES examination of the Fe K-edge shows a decrease in energy of the main absorption by up to 1 eV, revealing reduction of the Fe3+ components of the biotite by interaction with the 24He2+, the result of low and thermal energy electrons produced by the cascade of electron collisions. Changes in d spacings in the XRD patterns reveal the development of polycrystallinity and new domains of damaged biotite structure with evidence of displaced atoms due to ionization interactions and nuclear collisions. The damage in biotite is considered to have been facilitated by destruction of OH groups by radiolysis and the development of Frenkel pairs causing an increase in the trioctahedral layer distances and contraction within the trioctahedral layers. The large amount of radiation damage close to the monazite can be explained by examining the electronic stopping flux.

An extensive set of porosity, ε, effective diffusion coefficient, D e, and hydraulic conductivity, K, data were obtained from 45 granitic samples from the Bohemian Massif, Czech Republic. The measured dataset can be used to define parameter ranges for data to be used in safety assessment calculations for a deep (>400 m) radioactive waste repository, even though the samples originated from shallower depths (<108 m). The dataset can also be used for other purposes, such as evaluating the migration of contaminants in granitic rock (e.g. from shallow intermediate-level radioactive waste repositories and chemical waste repositories).

Sample relaxation and ageing processes should be taken into account in research otherwise migration parameters might be overestimated in comparisons between lab results and those determined in situ.

The heat outputs of higher burn up spent fuels (SF) create problems for disposal in mined repositories, including needs for reduced container loadings and extended pre-disposal cooling. An alternative that is less temperature sensitive is deep borehole disposal (DBD) which offers safety, cost, security and other potential benefits and could be implemented relatively quickly using currently available deep-drilling technology. We have modified our previously proposed version of DBD to be more appropriate for higher burn-up fuels by using smaller (0.36 m diameter) stainless steel containers, a smaller (0.56 m diameter) borehole, and different support matrices. We present the results of new heat-flow modelling for DBD of UO2 and MOX SF with burn ups of 55 and 65 GWd/t showing how temperatures evolve, especially on the outer surface of the containers. Consequences for the performance of the support matrices and the disposal concept are discussed. The thermal modelling indicates DBD is a viable option for higher burn-up SF and could be a practical disposal route for many combinations of fuel types, burn ups, ages and container loadings. Further, the results suggest that DBD of complete fuel assemblies, a desirable option, would be feasible and require much shorter pre-disposal cooling than necessary for disposal in mined repositories.

The role of the microbial activity on the transport properties of host rocks for geological repositories, particularly in the far-field, is an area of active research. This paper compares results from experiments investigating changes in transport properties caused by microbial activity in sedimentary rocks in Japan (mudstones) and sandstone (UK).

These experiments show that both Pseudomonas denitrificans and Pseudomonas aeruginosa appear to survive and thrive in pressurized flow-through column experiments which utilized host rock materials of relevance to radioactive waste disposal. Indeed, despite there being a difference in the numbers of organisms introduced into both biotic experiments, numbers appear to stabilize at ∼105 ml−1 at their completion. Post experimental imaging has highlighted the distinct differences in biofilm morphology, for the chosen rock types and bacteria, with Pseudomonas aeruginosa derived biofilms completely covering the surface of the sandstone host and Pseudomonas denitrificans forming biofilament structures. Regardless of substrate host or choice of microbe, microbial activity results in measurable changes in permeability. Such activity appears to influence changes in fluid flow and suggests that the transport of radionuclides through the far-field will be complicated by the presence of microbes.

Two different approaches were tested in this study to suppress sulfide interference in the silicomolybdate method of silica determination, namely: oxidation of sulfide to elemental sulfur; and conversion of the sulfide species to H2S and stripping by air bubbling. Based on the results obtained the latter approach is deemed to represent the better and quicker option to eliminate sulfide interference in silica colorimetric analysis.

The publication of the British Geological Survey memoir on Rum and the Small Isles in 1997 was followed by a period of intense petrological and mineralogical research, leading to some 40 papers, books and other publications. The research progress since then is reviewed here and integrated with the information previously available to provide an overview of the current status of understanding of the centre. New data on the acidic and mixed acid/basic magmas of the early Rum caldera demonstrate that frequent mafic replenishments were the main driver for magmatic activity at Rum right from its initial stages. The caldera is bound by the Main Ring Fault, a structure which probably also exercised an influence on the emplacement of the subsequent basic and ultrabasic intrusions. The later emplacement of gabbros and ultrabasic rocks caused only limited thermal metamorphism of the surrounding Torridonian sandstones, contrasting markedly with the crustal isotope signatures of the early intracaldera ignimbrite magmas and the intense alteration of uplifted masses of Lewisian gneiss within the ring fault. Rare picritic dykes provide an indication of the possible parent magma for the mafic and ultrabasic rocks, but these, as with most other magmatic rocks on Rum, have undergone varying degrees of crustal contamination, involving both Lewisian granulite and amphibolite-type crust but, notably, no Moine metasedimentary compositions as is the case at the nearby Ardnamurchan centre. Detailed textural studies on the gabbroic and ultrabasic rocks allow a distinction between intrusive peridotites and peridotite that forms part of the classic layered cumulate units of Rum and, furthermore, this work and that on the chromite seams and veins in these rocks shows that movement of trapped magma and magma derived from later intrusions, may produce textures regarded previously as of primary cumulate origin. Sulfides in the chromitite seams and ultrabasic rocks, in turn, show possible influences from assimilated Mesozoic sediments. Igneous activity on Rum was short-lived, possibly only between 0.5 and 1 m.y. in duration and commenced at ∼60.5 Ma. The Rum Central Complex was extinct by the time the main activity at the nearby Skye Central Complex commenced (∼59 Ma). From recent apatite fission-track studies it seems probable that Rum, in common with other Palaeogene centres, underwent a brief, but significantly later heating event (∼45 Ma).

Separated stocks of UK civil plutonium are currently held as a zero value asset in storage, as there is no final decision about whether they should be treated as a resource for future use as nuclear fuel or as waste. Irrespective of future UK government strategies regarding plutonium, at least a portion of the UK civil plutonium inventory will be designated for geological disposal. In this context, we performed a high-level review of the performance of potential wasteforms for the disposal of separated civil plutonium. The key issues considered were the durability and chemical reactivity of the wasteforms in aqueous environments and the long-term radionuclide release under conditions relevant to geological disposal. The major findings of the review, relevant not only to the situation in the UK but to plutonium disposal in general, are summarized in this paper. The review showed that, in the event of a decision being taken to declare plutonium as a waste for disposal, more systematic studies would be required to constrain the wasteform performance under repository conditions in order to derive realistic source terms for a safety case.

The Environment Agency Guidance on Requirements for Authorisation (GRA) of a geological disposal facility (GDF) requires a demonstration that "the possibility of a local accumulation of fissile material such as to produce a neutron chain reaction is not a significant concern." A neutron chain reaction that is just self-sustaining is also known as critical.

Waste packages can be designed to ensure that criticality is impossible during the transport and operational phases of a GDF, and for a significant period post-closure. Over longer times, however, packages may degrade, and groundwater flows could lead to a localized accumulation of fissile material. Hence, even though the initial distribution of materials would need to change substantially, criticality cannot be ruled out completely.

This paper describes how an accumulation of fissile material could, hypothetically, lead to a critical configuration; how such a system could evolve; what the local consequences could be; and how the engineered and geological barriers could be affected. The conclusion from studies to date is that, even for large (and very unlikely) fissile accumulations, the consequences of a post-closure criticality event are not a significant concern.

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.

Cellulose degradation products (CDPs) can complex with radioelements causing solubility enhancement and sorption reduction, effects which are detrimental to the containment of radionuclides in the near field of a geological disposal facility and surrounding geosphere. Isosaccharinic acid (ISA) is the principal component of CDPs formed under the alkaline anaerobic conditions of a cement-based near field and appears to be largely responsible for the impact of CDPs on radionuclide solubility and sorption under near-field conditions. However, the situation appears to be more complicated under near-neutral pH geosphere conditions.

A combined experimental and modelling study was undertaken to compare the impact of a CDP leachate to ISA in a simple model ternary sorption system consisting of hematite as a single mineral substrate, thorium as the radioelement and ISA or a CDP leachate as the complexant. Thorium sorbs strongly to hematite. A triple layer model for thorium sorption to hematite was refined to fit to the experimental data in the absence of ISA or CDP leachate; the effect of ISA on thorium sorption was then predicted.

In the presence of CDP leachate, a significant reduction in thorium sorption was observed from pH 6 to 12 in good agreement with model predictions based on a high concentration of ISA. However, only a limited impact of ISA on thorium sorption was observed at pH 6 to 12, in contrast to predictions. The effects of ISA could be accounted for by assuming the formation of a ternary thorium–ISA–surface complex. The model has yet to be extended to the more complex CDP systems. Differences in the thorium speciation in solution due to the formation of a ternary calcium–thorium–ISA complex in the CDP leachate, which is absent from solutions with ISA only, provides the most likely explanation for the differences observed experimentally.

The Nuclear Decommissioning Authority Radioactive Waste Management Directorate have been participating in the current DECOVALEX-2011 project (development of coupled models and their validation against experiments) one task of which has been examining the Mont Terri Ventilation Experiment (VE). This long-term (>9 years), field-scale experiment in the Opalinus Clay near the Swiss–French border, was designed to examine the coupled hydraulic–mechanical–chemical changes caused in the tunnel and in the surrounding geology, by the controlled ventilation of a 1.65 m diameter micro-tunnel.

In contrast to many conventional benchmarking and validation exercises, a key aspect of the VE as examined in DECOVALEX was that some data were held back and participants were required to make predictions of key metrics for the future evolution of the system. This paper presents an overview of the work conducted by the Quintessa and University of Edinburgh team including selected results. The coupled models developed include multiphase flow, elastic deformation and chemical processes in both detailed and upscaled geometries.

The models have been able to replicate the observed desaturation around the tunnel, tunnel deformation and localized failure, vapour migration in the tunnel, and the transition in redox conditions into the host rock.

This paper gives an overview of the Nuclear Decommissioning Authority, Radioactive Waste Management Directorate approach to representing the biosphere in post-closure safety assessment studies. The assessments consider potential releases of radionuclides and chemically toxic substances from a geological disposal facility that may reach the biosphere by transport in groundwater and gas. It gives an outline of the key processes and understanding underpinning the representation of the biosphere and how the biosphere may evolve over the long timescales of relevance to post-closure safety. The current biosphere assessment approach and associated models are supported by research studies and collaboration that ensure they are consistent with international recommendations. Research studies are also commissioned to reduce uncertainty surrounding key contaminants and potential exposure pathways and to help ensure that the approach and models are adaptable and flexible enough to incorporate future developments, as the geological disposal programme moves towards site selection.

Consideration of the biosphere is important in the post-closure safety assessment of a geological disposal facility (GDF) as the biosphere acts as the receptor for any contaminants that may be released from the geosphere. Considerable uncertainty exists in the characteristics of the biosphere at times in the far future when any contaminant releases from a GDF would reach the accessible environment. These uncertainties include human behaviour, affecting environmental change as well as exposure modes.

A number of critical scientific issues have been identified through the practical application of the International Atomic Energy Agency reference biosphere approach within both site generic and site specific repository assessment projects. These issues are being addressed through an international collaboration programme, BIOPROTA. The purpose of this paper is to describe the BIOPROTA programme, its objectives and typical working method. The approach is illustrated with examples from the recent work programme including model intercomparison studies for the radionuclides 36Cl and 79Se.

At the post-closure stage of a geological disposal facility for higher activity radioactive waste several species of gas are likely to be generated in the near-field environment. These could alter the sealing and chemical properties of the bentonite buffer and the local geochemical environment significantly. The authors' attempt to simulate multicomponent gas flow through variably saturated porous media is presented. Governing equations have been developed for a reactive gas-flow model to simulate the thermo-hydro-gas-chemical-mechanical behaviour, with specific reference to the performance of highly compacted bentonite buffer subjected to repository gas generation and migration. The developed equations have been included in the bespoke numerical model COMPASS and some generic simulations are also presented. The model presented extends current capability to assess buffer performance.

Fluid flow through a two-dimensional fracture network has been simulated using a discrete fracture model. The computed field-scale permeabilities were then compared to those obtained using an equivalent continuum approach in which the permeability of each grid block is first obtained by performing fine-scale simulations of flow through the fracture network within that region. In the equivalent continuum simulations, different grid-sizes were used, corresponding to N by N grids with N = 10, 40, 100 and 400. The field-scale permeabilities found from the equivalent continuum simulations were generally within 10% of the values found from the discrete fracture simulations. The discrepancies between the two approaches seemed to be randomly related to the grid size, as no convergence was observed as N increased. An interesting finding was that the equivalent continuum approach gave accurate results in cases where the grid block size was clearly smaller than the 'representative elementary volume'.

The formation of crystalline rare earth element (REE) (e.g. La, Ce, Pr, Nd) carbonates from aqueous solutions was examined at ambient temperature using UV-Vis spectrophotometry, combined with X-ray diffraction, high-resolution microscopy and infrared spectroscopy. In all experiments REE-lanthanites (REE2(CO3)3·8H2O) formed via a highly hydrated, nanoparticulate and poorlyordered REE-carbonate precursor. The lifetime of this precursor as well as the kinetics of crystallization of the various REE-lanthanites were dependent on the specific REE3+ ion involved in the reaction. The induction time and the time needed to fully form the crystalline REE-lanthanite end products increase linearly with the ionic potential. The authors show here that the differences in ion size and ionic potential as well as differences in dehydration energy of the REE3+ ions control the lifetime of the poorly ordered precursor and thus also the crystallization kinetics of the REE-lanthanites; furthermore, they also affect the structural characteristics (e.g. unit-cell dimensions and idiomorphism) of the final crystalline lanthanites.