Mobility Of Radionuclides Research Articles

Radionuclide transport and retardation in uplifting granitic rocks: Part 2 – Modelling coupled processes in uplift scenarios

Uplifting fractured granitic rocks occur in substantial areas of countries such as Japan. Some of these areas might be considered when siting a deep geological repository for radioactive wastes. A repository site would be selected in such an area only if it is possible to make a safety case, accounting for the changing conditions during uplift. The safety case must include robust arguments that chemical processes in the rocks around the repository will contribute sufficiently to minimise radiological doses to biosphere receptors. Numerical modelling is an important aspect of making these arguments. To provide confidence in the safety arguments, numerical models need to be sufficiently realistic, but also parameterised conservatively (pessimistically). However, model development is challenging because uplift involves many complex couplings between groundwater flow, chemical reactions between water and rock, and changing rock properties. The couplings would affect radionuclide mobilisation and retardation, by influencing diffusive radionuclide fluxes between groundwater flowing in fractures and effectively immobile porewater in the rock matrix (rock matrix diffusion, RMD) and radionuclide partitioning between water and solid phases, via: (i) mineral precipitation/dissolution; (ii) mineral alteration; and (iii) sorption/desorption. It is difficult to represent all this complexity in numerical models while showing that they are parameterised conservatively. Here we present a modelling approach, illustrated by simulation cases for some exemplar radioelements, to identify realistically conservative process conceptualisations and model parameterisations.

Evaluating radionuclide mobility in groundwater recharge areas of fractured natural barrier systems using multiple isotopes and microbial indicators

The distribution of uranium (U) concentrations, which reached up to 322 µg/L, was found to correlate with the pattern of fractures within the natural barrier system (NBS). Analysis of the vertical distribution of dissolved oxygen (DO), dissolved organic carbon (DOC), tritium (3H), microbial communities, and H2O and SO42– isotopes revealed insights into oxic water infiltration within the heterogeneous fractured system. Their distribution showed that the average infiltration depth at the KURT site is 200 m, while in external areas with a high frequency of fractures, oxic conditions extended down to 495 m. The SO42– isotopes suggested the potential for microbial sulfate reduction to play a role in regulating radionuclide mobility in the deep geological system. At approximately 500 m, genera capable of thriving under harsh conditions of low DO and high heavy metal concentrations, such as Novosphingobium, Comamonadaceae_uc, and Desulfuromonas_g2, were identified. These findings indicate hydrogeological variability and microbial adaptation within the deep NBS, highlighting the importance of understanding the deep geological environment for evaluating microbiome performance in regulating toxic radionuclides within repository systems. Overall, this study emphasizes the pivotal role of age tracers, stable isotopes, and microbiome in enhancing the assessment of the long-term stability of fractured granite barriers.

Influence of earthworms on the mobility and bioavailability of metals, metalloids and radionuclides in historically contaminated soil

While soil characteristics such as pH, cation exchange capacity and organic matter are known to influence the mobility of pollutants in soil and the transfer to vegetation, far less attention has been paid to the possible non-trophic influence of organisms on pollutant transfer to other species. When pollutants are present in a bound unavailable form in the mineral or organic fraction, they can be made available through bio-weathering and decomposition of organic material. Within this study, the impact of earthworms on the mobility of metal(loid)s and radionuclides in soil and their transfer to vegetation was evaluated and the pathways through which earthworms work were studied. Soil from a Belgian field, historically contaminated with metal(loid)s and radionuclides, was used and microcosm systems were applied to mimic the natural interactions between soil, earthworms (Lumbricus terrestris) and vegetation (Lolium perenne). Metal and radionuclide concentrations were determined in soil, pore water and grass. In addition, soil pH, (chromophoric) dissolved organic matter, the ionic composition of pore water and the soil microbial activity were analysed. In general, earthworm presence significantly increased the concentration of most pollutants in soil pore water. This could be a direct consequence of decreased soil pH and increased humification. The latter also lead to higher nutrient concentrations in pore water that were depleted in the root zone due to high accumulation by plants. Indications were found for earthworm-induced effects on the soil microbial activity. The results show that earthworm activity can increase leaching of metal(loid)s and radionuclides and enhance dispersion into the environment and transfer to other biota and the food chain. Biologically induced release or remobilization of pollutants in the soil should therefore be considered in the context of risk assessment and site management strategies.

Fundamental study on measurement of soil pH and electrical conductivity in batch tests for the determination of soil-soil solution distribution coefficient

Soil-soil solution distribution coefficient, Kd, is used to understand the mobility of radionuclides in the soil environment. Soil pH and electrical conductivity (EC) are the important factors affecting soil Kd, but existing methods for Kd employed different conditions, as well, standard soil pH and EC methods varied in the test conditions. It would be desirable to measure both pH and EC using the same soil solution for Kd with portable meters (non-glass electrode type) that need only a small liquid volume. In this study, at first, we utilized portable pH and EC meters to examine effects of different conditions on measurements, i.e., the water/soil ratios, contact time and standing time. The results showed that portable meter measurements of soil supernatant after 1 h shaking and 1 h standing could provide comparable pH and EC values with other standard soil tests. Then measurements of pH, and EC up to 168 h were carried out, considering the Kd methods for radiocesium (Cs); only slight differences in these values were observed compared to the standard test method. Considering these results, further analysis was carried out on the correlation between soil Kd-Cs and pH and found a clear decrease of Kd-Cs at pH < 5.

Coupled Modelling of Brine Availability in Salt-Based Disposal Facilities - Learning from DECOVALEX 2023

Salt is a potential host rock or caprock for deep geological disposal facilities for radioactive waste and has several favourable properties compared to other candidate rock types. In particular, intact salt has an extremely small porosity and permeability, which limits water availability and transmissivity. This has tended to lead to disposal concepts in salt being considered as ‘mostly dry’ and hence ideally suited to limiting groundwater interactions that could otherwise lead to corrosion of waste packages and mobilisation of radionuclides over isolation timescales (10 5 -10 6 years). However, there is plentiful experimental evidence to suggest that excavations in bedded salt can exhibit non-trivial inflows. These inflows often increase on heating and are thought to arise due to complicated interactions between thermal, hydrogeological and mechanical processes in the salt. Task E in the international collaborative DECOVALEX 2023 programme was established to attempt to better understand how these coupled processes affect brine availability in bedded salt. A summary of the UK team's learning from the task is given in this paper that may be of relevance when constructing models to inform future safety cases for radioactive waste disposal in salt, and other energy geoscience applications where brine interactions are a potentially complicating factor.

Potential applications of microbial genomics in nuclear non-proliferation.

As nuclear technology evolves in response to increased demand for diversification and decarbonization of the energy sector, new and innovative approaches are needed to effectively identify and deter the proliferation of nuclear arms, while ensuring safe development of global nuclear energy resources. Preventing the use of nuclear material and technology for unsanctioned development of nuclear weapons has been a long-standing challenge for the International Atomic Energy Agency and signatories of the Treaty on the Non-Proliferation of Nuclear Weapons. Environmental swipe sampling has proven to be an effective technique for characterizing clandestine proliferation activities within and around known locations of nuclear facilities and sites. However, limited tools and techniques exist for detecting nuclear proliferation in unknown locations beyond the boundaries of declared nuclear fuel cycle facilities, representing a critical gap in non-proliferation safeguards. Microbiomes, defined as "characteristic communities of microorganisms" found in specific habitats with distinct physical and chemical properties, can provide valuable information about the conditions and activities occurring in the surrounding environment. Microorganisms are known to inhabit radionuclide-contaminated sites, spent nuclear fuel storage pools, and cooling systems of water-cooled nuclear reactors, where they can cause radionuclide migration and corrosion of critical structures. Microbial transformation of radionuclides is a well-established process that has been documented in numerous field and laboratory studies. These studies helped to identify key bacterial taxa and microbially-mediated processes that directly and indirectly control the transformation, mobility, and fate of radionuclides in the environment. Expanding on this work, other studies have used microbial genomics integrated with machine learning models to successfully monitor and predict the occurrence of heavy metals, radionuclides, and other process wastes in the environment, indicating the potential role of nuclear activities in shaping microbial community structure and function. Results of this previous body of work suggest fundamental geochemical-microbial interactions occurring at nuclear fuel cycle facilities could give rise to microbiomes that are characteristic of nuclear activities. These microbiomes could provide valuable information for monitoring nuclear fuel cycle facilities, planning environmental sampling campaigns, and developing biosensor technology for the detection of undisclosed fuel cycle activities and proliferation concerns.

New insights into the behavior and biochemical mechanism of microbial Tc(VII) reduction via the investigation of electron transfer

Technetium-99 (99Tc), a long-lived and highly mobile radionuclide, is recognized as a problematic contaminant in groundwater. Although the microbial reduction and immobilization of Tc(VII) have been substantially documented, the reduction behavior of Tc(VII) and the associated biochemical mechanisms remain unclear. This study employed the facultative Klebsiella variicola strain X-21 (K. variicola X-21) to shed further light on these intricate processes, particularly on the electron donors for Tc(VII) reduction in the oligotrophic system and the mechanism of electron transfer. The results revealed that more than 68 % of the initial Tc(VII) (1 × 10-4 M) could be reduced and deposited by K. variicola X-21 under anaerobic conditions. The presence of Fe3+ could promote the Tc(VII) reduction induced by K. variicola X-21, while NO3– and Cu2+ inhibit the reduction. The self-secreted exopolysaccharides of K. variicola X-21 can be metabolized to supply electrons for the Tc(VII) reduction in the absence of effective electron donors such as pyruvate, glycerol, formate, and glucose. The electron transfer of the Tc(VII) reduction induced by K. variicola X-21 depends on succinate-CoQ reductase (complexes II), CoQH2-cytochrome c reductase (complexes III), and cytochrome c oxidase (complexes IV) of the respiratory chain. Combined with the location and valence state of the end products, the evidence becomes definitive that the Tc(VII) reduction mainly occurs within the cytoplasm of K. variicola X-21, resulting in the intracellular accumulation of technetium in the form of amorphous Tc(IV) species.

New Data on Potential Mobility of Anthropogenic Radionuclides in Bottom Sediments of the Yenisei River

New Data on Potential Mobility of Anthropogenic Radionuclides in Bottom Sediments of the Yenisei River

Machine learning surrogates for surface complexation model of uranium sorption to oxides

The safety assessments of the geological storage of spent nuclear fuel require understanding the underground radionuclide mobility in case of a leakage from multi-barrier canisters. Uranium, the most common radionuclide in non-reprocessed spent nuclear fuels, is immobile in reduced form (U(IV) and highly mobile in an oxidized state (U(VI)). The latter form is considered one of the most dangerous environmental threats in the safety assessments of spent nuclear fuel repositories. The sorption of uranium to mineral surfaces surrounding the repository limits their mobility. We quantify uranium sorption using surface complexation models (SCMs). Unfortunately, numerical SCM solvers often encounter convergence problems due to the complex nature of convoluted equations and correlations between model parameters. This study explored two machine learning surrogates for the 2-pK Triple Layer Model of uranium retention by oxide surfaces if released as U(IV) in the oxidizing conditions: random forest regressor and deep neural networks. Our surrogate models, particularly DNN, accurately reproduce SCM model predictions at a fraction of the computational cost without any convergence issues. The safety assessment of spent fuel repositories, specifically the migration of leaked radioactive waste, will benefit from having ultrafast AI/ML surrogates for the computationally expensive sorption models that can be easily incorporated into larger-scale contaminant migration models. One such model is presented here.

Experimental analysis and prediction of radionuclide solubility using machine learning models: Effects of organic complexing agents

Radioactive wastes contain organic complexing agents that can form complexes with radionuclides and enhance the solubility of these radionuclides, increasing the mobility of radionuclides over great distances from a radioactive waste repository. In this study, four radionuclides (cobalt, strontium, iodine, and uranium) and three organic complexing agents (ethylenediaminetetraacetic acid, nitrilotriacetic acid, and iso-saccharic acid) were selected, and the solubility of these radionuclides was assessed under realistic environmental conditions such as different pHs (7, 9, 11, and 13), temperatures (10 °C, 20 °C, and 40 °C), and organic complexing agent concentrations (10−5–10−2 M). A total of 720 datasets were generated from solubility batch experiments. Four supervised machine learning models such as the Gaussian process regression (GPR), ensemble-boosted trees, artificial neural networks, and support vector machine were developed for predicting the radionuclide solubility. Each ML model was optimized using Bayesian optimization algorithm. The GPR evolved as a robust model that provided accurate predictions within the underlying solubility patterns by capturing the intricate relationships of the independent parameters of the dataset. At an uncertainty level of 95%, both the experimental results and GPR simulated estimations were closely correlated, confirming the suitability of the GPR model for future explorations.

Activity measurements and calculations for gamma-emitting radionuclides in concrete drill cores of unit 2 of the Greifswald NPP

Due to Germany’s nuclear phase-out, decommissioning and final disposal of construction materials of Nuclear Power Plants (NPP) become increasingly important. The reliable determination of radionuclides produced by neutron activation, the activity as a function of time since shutdown and investigation of subsequent radionuclide mobility are subject of a research project. Drill cores of the concrete shielding of unit 2 of the Greifswald NPP were retrieved. Specific activities of gamma emitters and the elemental composition were measured. The radiation transport code MCNP 6 was used for the calculation of spectral neutron fluences. A neutron radiation field calculation reveals that the maximum neutron fluence at the concrete component is located in the floor just below the RPV. The concrete structures closest to the reactor core are shielded efficiently against neutron radiation by the annular water tank. Measured and calculated specific activities of 152Eu, 154Eu and 60Co for the cement screed at the position of the maximum neutron fluence are surprisingly low. A specific exemption (i.e. release from radiation protection surveillance but mandatory for final disposal) of the screed sample according to Germany’s Radiation Protection Ordnance is expected to be possible approximately 4 decades after the shutdown of the NPP.

Radionuclide transport in fractured chalk under abrupt changes in salinity

Internationally, it has been agreed that geologic repositories for spent fuel and radioactive waste are considered the internationally agreed upon solution for intermediate and long-term disposal. In countries where traditional nuclear waste repository host rocks (e.g., clay, salt, granite) are not available, other low permeability lithologies must be studied. Here, chalk is considered to determine its viability for disposal. Despite chalk's low bulk permeability, it may contain fracture networks that can facilitate radionuclide transport. In arid areas, groundwater salinity may change seasonally due to the mixing between brackish groundwater and fresh meteoric water. Such salinity changes may impact the radionuclides' mobility. In this study, radioactive U(VI) and radionuclide simulant tracers (Sr, Ce and Re) were injected into a naturally fractured chalk core. The mobility of tracers was investigated under abrupt salinity variations. Two solutions were used: a low ionic strength (IS) artificial rainwater (ARW; IS ∼0.002) and a high IS artificial groundwater (AGW; IS ∼0.2). During the experiments, the tracers were added to ARW, then the carrier was changed to AGW, and vice versa. Ce was mobile only in colloidal form, while Re was transported as a conservative tracer. Both Re and Ce demonstrated no change in mobility due to salinity changes. In contrast, U and Sr showed increased mobility when AGW was introduced and decreased mobility when ARW was introduced into the core. These experimental results, supported by reactive transport modeling, suggest that saline groundwater solutions promote U and Sr release via ion-exchange and enhance their migration in fractured chalk. The study emphasizes the impact of salinity variations near spent fuel repositories and their possible impact on radionuclide mobility.

Experimental study of water-extractable sulphate in Opalinus Clay and implications for deriving porewater concentrations

In northern Switzerland, the Opalinus Clay, a Jurassic claystone formation, is foreseen as host rock for a deep geological repository for radioactive waste. Characterizing its porewater is of particular importance for assessing the mobility of radionuclides and the stability of the engineered barriers. Although the porewater composition of the Opalinus Clay is fairly well known, there is still controversy on the sources of sulphate obtained by different porewater characterization methods. A striking observation is that sulphate concentrations from aqueous extraction and recalculated to in-situ conditions are consistently much higher than sulphate concentrations measured in borehole waters, squeezed waters and advectively displaced waters (“excess sulphate”). Accordingly, the main objective of this study is to better investigate the processes affecting dissolved sulphate concentrations during aqueous extraction and, thus, to reduce uncertainties in predicting the concentrations of this compound in the Opalinus Clay porewater. To this end, a series of extraction experiments were conducted using variable solid/liquid ratios, extraction times and extract solutions. In order to suppress sulphide-mineral oxidation, all the experiments were performed in a glovebox under oxygen-free conditions (atmosphere and solutions). Measurements of the sulphur and oxygen isotope composition of the dissolved sulphate in aqueous extracts are aimed to further constrain the source of the “excess sulphate”. Finally, the plausibility of the SO4 data from extraction experiments in terms of their representativeness for in-situ conditions was evaluated by simple geochemical modelling. The modelling shows that SO4 concentrations from aqueous extracts recalculated to in-situ conditions imply dissolved and exchangeable cation concentrations which are not consistent with measured data, thus, attesting non-conservative behaviour for sulphate during aqueous extraction. However, the various extraction experiments showed that pyrite oxidation was successfully suppressed during the experiments and neither contributions from e.g. organic material, congruent calcite dissolution and/or sulphate mineral dissolution provide enough SO4 to explain the “excess sulphate”. Ultimately, the various extraction experiments failed to definitely identify the source of the “excess sulphate” in aqueous extracts. However, the good agreement found between the δ18O and δ34S values of dissolved SO4 in aqueous extracts and those of borehole waters suggest that the “excess sulphate” might be weakly bound to mineral surfaces.

Pb-bearing Cu-(Fe)-sulfides: Evidence for continuous hydrothermal activity in the northern Olympic Cu-Au Province, South Australia

Lead-isotope data were obtained for a population of Pb-chalcogenides (galena, clausthalite and altaite) and Cu-(Fe)-sulfides in Cu-(Fe)-sulfide mineral separates from the Prominent Hill iron-oxide copper gold deposit, Mt Woods Inlier, South Australia. Each mineral displays wide variability in 206Pb/204Pb, 207Pb/204Pb and 208Pb/204Pb ratios, ranging from moderately radiogenic Pb signatures to strongly radiogenic. Any contribution from non-radiogenic (common) lead at the time of initial deposit formation (1600–1585 Ma) is minor. Lead is overwhelmingly derived from the decay of U, with a minor contribution from the decay of Th also present within the ores. The heterogenous nature of the recorded Pb isotope values suggests a mixing of isotope signatures during repeated cycles of fluid-assisted dissolution, remobilisation and reprecipitation of U- and Pb-bearing phases. Hydrothermal experiments showed that the incorporation of Pb-chalcogenide (galena) occurs rapidly (days) at relatively mild conditions (≤150 °C), via precipitation in reaction-induced porosity formed during coupled dissolution reprecipitation reactions of the host Cu-(Fe)-sulfides. Hence, even minor hydrothermal activity may result in some trapping of radiogenic Pb by pre-existing Cu-(Fe)-sulfides. Construction of Pb-Pb pseudo-isochrons from the data supports these assessments, returning a range of possible (theoretical) ages (0–0.535 Ga) for Pb-chalcogenide formation. An additional (two-stage) Pb evolution model is preserved by the data with separation of each reservoir from the bulk silicate earth at ∼ 830 Ma and mixing of the two reservoirs at ∼ 500 Ma. Although the Pb-isotope data can be interpreted as resulting from a number of discrete events that generated regional-scale fluid flow, near-continuous Pb (re)-mobilisation at the nm- to deposit-scales is an equally valid interpretation, and is also consistent with observations of modern radionuclide mobility in the ores. The observations made within this study should be considered when discussing the mobility of elements within and between mineral phases in ore-forming systems. They show that many mineral phases, particularly the sulphide mineral phases discussed here, should not be considered as closed systems, but potentially, as systems in which chemistry and mineral stoichiometry evolve continuously over geological time.

Surface reconstruction and cleavage of phyllosilicate clay edge by density functional theory

Phyllosilicate materials (e.g., smectite) have been proposed to act as a waste barrier within long-term deep geological repositories. Aside from longevity, these clay materials must also limit the mobility of radionuclides, which is achieved through intermolecular interaction involving clay basal planes and edge surfaces. We aim to contribute to the fundamental knowledge of clay edges by investigating surface reconstruction in a dehydrated context. Using density functional theory, surface reconstruction, charge redistribution, and formation energies have been explored for pyrophyllite edges resulting from thirteen terminations. Three primary reconstruction mechanisms were identified: (1) the displacement of Al into the tetrahedral layer, (2) proton shift, and (3) sub-surface reconstruction involving sub-surface -OH or apical O and surface Si. Reduction in overall relative bond deficiency and charge redistribution to becoming more bulk-like were found to be the main driving force for reconstruction. Formation energies were also determined with dispersion correction for dehydrated and hydrated surfaces. The presence of water terminating agents reduced energetic costs by approximately one order of magnitude and the inclusion of dispersion correction resulted in indistinguishable formation energies between different hydrated surfaces. Linear regression models were also explored to predict surface formation energies based on relative bond deficiency and charge. These models were able to identify the key contributions to the surface energy; however, due to the small dataset they were not found to be predictive.

Investigating the role of Na-bentonite colloids in facilitating Sr transport in granite minerals through column experiments and modeling

Colloids play a crucial role in influencing the mobility of radionuclides in high-level radioactive waste repositories. However, the co-transport behavior of radionuclides and colloids in geological media remains insufficiently understood. This study investigated the transport of Strontium (Sr) in four types of granite minerals (quartz, biotite, K-feldspar, and plagioclase) in the presence and absence of Na-bentonite colloids (Na-BC) using column experiments. Employing a stepwise modeling strategy, this study first determined the basic flow and transport parameters through breakthrough curve analysis of the conservative tracer (Br). Then, the experimental data of Sr in the colloid-free Sr transport experiments and Na-BC in the Sr-colloid co-transport experiments were quantitatively explained using a two-site sorption model and a two kinetic sites model, respectively. Finally, the co-transport behavior of Sr and Na-BC was fitted using the colloid-facilitated solute transport model. The stepwise modeling allowed quantification of the kinetics of Sr sorption onto mobile and immobile Na-BC and highlighted the role of straining in Na-BC retention. In the absence of Na-BC, Sr transport experienced the greatest retardation in biotite, followed by plagioclase, K-feldspar, and quartz, respectively, which positively correlated with the specific surface area of the minerals. Moreover, Na-BC enhanced Sr transport with the same retardation order in all tested minerals, with recovery rate increments of 68.61%, 19.21%, 6.67%, and 4.33%, respectively. The transport of Sr in K-feldspar was found to be less affected by Na-BC compared to the other tested minerals, likely due to the strong cation exchange capacity of K+ among the cation components of minerals, making hydrated K+ more likely to exchange with Sr2+ on K-feldspar surfaces. These findings hold significant implications for assessing the risks associated with the transport of radionuclides in deep geological repositories.

Identifying safety-relevant knowledge gaps concerning radionuclide mobility – bringing together fundamental research and application in repository safety analysis

Abstract. The mobility of radionuclides in a nuclear waste repository is strongly influenced by their chemical form (e.g. oxidation state and speciation), which determines their solubility and chemical interactions with mineral surfaces. To date, significant efforts have been made to investigate the mechanisms (e.g. sorption and diffusion) underlying radionuclide retention. However, knowledge gaps persist, especially under complex environmental conditions, as may be expected in a deep geological repository (e.g. the complexation of radionuclides with organic compounds, the influence of elevated temperature and the effects of high ionic strength). In order to make reliable predictions about radionuclide mobility and, consequently, about post-closure repository safety, it is necessary to identify, evaluate and, where necessary, close these knowledge gaps. This workshop aims to bring together experts in both fundamental research and its applications to repository safety. It intends to facilitate knowledge exchange between the two fields, promoting more coordinated and goal-oriented safety research. Participants are invited to share their knowledge about the need for further experimental and computational investigations as well as their insight into the data requirements for reliable transport models and implications for safety assessment.

Deep borehole disposal as a potential solution for high-level waste (HLW) and spent nuclear fuel (SNF) in Norway – current status and a first generic safety assessment

Abstract. Deep borehole disposal (DBD) is currently being investigated as a potential solution for the disposal of the existing spent nuclear fuel (SNF) in Norway. Over the past years, BGE TECHNOLOGY GmbH (BGE TEC), together with the Finnish companies A-Insinöörit Oy (AINS Group), Mitta Oy and VTT Technical Research, has supported the Norwegian agency Norwegian Nuclear Decommissioning (NND) in the development of a deep borehole concept, including work on different technical details. At the end of 2022, the consortium was able to win a follow-up contract to further deepen and advance the work. For this second framework agreement, additional companies were added to further strengthen the expertise. The Geological Survey of Finland (GTK), Posiva Solutions Oy (PSOY) and Rambøll are now part of the new consortium group, which works under the acronym GeoReN (Geological Repositories for Norway). This paper provides an overview of the work carried out in the past for the deep borehole concept in Norway. In detail, this includes a basic description of the concept developed for the waste that originates from different institutions. Initial calculations indicate that a 69 BSK-R canister may be sufficient for the waste inventory to be considered. As the current conceptual borehole design provides the basis for most of the other work carried out, it will be presented and discussed within the paper. Furthermore, as part of the concept development, a first generic post-closure safety assessment was also performed for the DBD concept. The main results of the long-term calculations are discussed and presented in this paper as well. The main objectives of the generic safety assessment were the identification of critical parameters and assumptions for the model of the DBD repository and the performance of a first assessment of possible consequences of the potential post-closure release of radionuclides from the borehole. To be able to give a prediction at the present stage of the project, the assessment used conservative assumptions, models and data. The preliminary results for the normal evolution of the deep borehole repository indicate that highly mobile radionuclides, such as I-129, Cl-36, Ni-59 and Se-79, are the major dose contributors and that the expected potential radiological impact on future generations associated with these releases is extremely small. In addition, alternative scenarios have been assessed, which assume a hydraulic gradient leading to advective flow either upward through the borehole and surrounding excavation damaged zone (EDZ) or between certain parts of the borehole and the surface via a transmissive fracture system. Despite the pessimistic assumptions chosen for these scenarios, the maximum dose rates are significantly lower than the expected regulatory limit. However, it needs to be pointed that the present generic safety assessment is still on a conceptual level. With more detailed planning and updated information regarding the inventory, the geological environment, and the integrity and performance of the engineered barriers, repeated safety assessments will need to be performed to derive more substantiated results.

Natural Analogue Studies in Support of Post-Closure Safety Assessment of Deep Geological Disposal

Natural analogues are systems that have evolved over geological timescales with features similar to one or several components of a deep geological repository (DGR). Natural analogues complement short-duration laboratory studies since they are existing reflections of many long-term processes that might affect the performance of a repository. Mathematical models are often used for the post-closure safety assessment of a DGR. Confidence in the models’ predictions is enhanced when the models successfully simulate the past evolution of a natural analogue. This paper summarizes the Canadian Nuclear Safety Commission’s (CNSC’s) recent research on natural analogues to inform on (1) glacial erosion, (2) engineered barrier system, and (3) uranium reactive transport in the context of DGRs for radioactive wastes. Glaciation and its erosion are prominent factors impacting the performance of future DGRs at high latitudes in the northern hemisphere. The authors have reviewed the field data from the Greenland Analogue Project, developed a conceptual and mathematical model for the simulation of the thermal conditions within the Greenland ice sheet, as well as the thermal-hydraulic conditions at its base and the ice sheet velocity, and eventually estimated the erosion rate at the site. The Cigar Lake Analogue demonstrates the long-term radionuclide containment capability of the illite clay zone enveloping the ore body, serving as an analogy to the engineered clay barriers. The CNSC and University of Ottawa analyzed 129I in the Cigar Lake core samples, and modeled and correlated the diffusion-dominated transport of radionuclides over the geological evolution of the Cigar Lake deposit. The results provide information on the mobility of fission products and significant radionuclides in conditions analogous to the source, engineered barriers, and near-field host rock of a DGR. The reactive transport and geochemistry of the Kiggavik-Andrew Lake uranium deposit mineralization and remobilization was another natural uranium deposit analogue studied by the CNSC. A reactive transport model was established according to the conceptualized geochemical processes and run under specified boundary and initial conditions to validate the geochemical processes. The geometry, timing, geochemistry, and fluid composition were used as model constraints.

Assessment of salinity and heat impacts on natural organic matter composition in bentonite for used nuclear fuel storage

Bentonite clay (MX-80, Wyoming, USA) is a proposed sealing material for the safe and long-term management of Canada's used nuclear fuel in a deep geological repository (DGR). Due to the physiochemical properties of bentonite, this buffer layer can limit radionuclide mobilization, and minimize air and water circulation, and hence suppress microbial activity near the used fuel container. However, natural organic matter (NOM) in mined Wyoming-type bentonite (MX-80) may serve as microbial substrates and cause further corrosion to the used fuel container. In the proposed DGR, bentonite will be exposed to high salinity due to saline groundwater and heat released from radioactive decay of radionuclide inventory of the used nuclear fuel. There is limited knowledge about how NOM quantity and quality in MX-80 may change after being exposed to the anticipated levels of salinity and heat in a DGR, especially at the molecular-level. To explore this, organic carbon contents and NOM composition (both soluble- and solid-phase) in MX-80 after exposure to different saline (NaCl) solutions (1 M, 2 M, 3 M, 4 M and 5 M) with and without heat exposure (90 °C) were analyzed. Dissolved organic carbon (DOC) solubility decreased with increasing salinity but these differences were not statistically significant when compared to the control (P < 0.05). DOC solubilization was not significantly altered with heat (P < 0.05). Dissolved organic matter (DOM) composition was analyzed with solution-state 1H nuclear magnetic resonance (NMR) and ultraviolet–visible (UV–Vis) spectroscopy. The NMR results indicated that DOM samples with varied salinities and heat exposure had similar chemical signatures. The results from UV–Vis analysis revealed a small decrease in less unsaturated components in DOM with NaCl solution compared to deionized water. The chemistry of solid-phase NOM that remained in MX-80 after DOM isolation was similar and suggests an insensitivity to either heat and/or salinity. The solubilized DOC contributes less than 1% to the total organic carbon in MX-80, thus, the anticipated level of salinity and heat in a DGR is unlikely to enhance NOM solubilization and change NOM chemistry in the bentonite markedly. As the bentonite in this study was only exposed to the heat and salinity in a short-term experiment, further studies examining NOM composition and reactivity under long-term saline and heat conditions are recommended.