The degradation of organic substances and metal components is expected to generate gases in radioactive waste repositories. Therefore, gas-permeable plugs and seals concepts have been developed to manage gas pressure development while ensuring the containment of radionuclides and other non-radioactive contaminants.The Gas permeable Seal Test (GAST) is an international project aimed at testing the feasibility and functionality of a gas-permeable seal under realistic scale and boundary conditions. The seal is made of a mixture of 80/20% sand/bentonite mixture and emplaced in the Grimsel Test Site (Switzerland).After a decade of progressive seal saturation and pressurisation with water, gas flow tests were carried out between May 2022 and August 2023. Noble gases (i.e. He, Ar, Xe) were used as tracers of gas transport through the seal section of the experiment.Consolidated interpretation of the results indicate that the gas path developed quickly through the seal (end-to-end-flow). Furthermore, the presence of the injected gas tracers both at the extraction point and in gas samples taken from inside the emplaced sand/bentonite layers demonstrated the existence of a gas phase within the seal. Finally, the absence of measurable gas leaks within the tunnel further confirmed the overall functionality of the seal system. AcknowledgementsGrimsel Test Site Staff for support onsiteSolexperts for hardware components, support in field and with measurementsEntracers and Hydroisotop for onsite and offsite gas analyses

Fluid-rock reactions are of great interest in many engineered geological storage and disposal systems where the long term integrity of the system is key, and where fluid seepage through a permeable rock may lead to reaction and convective transport of material through the formation. If an unsaturated fluid displaces formation fluid in equilibrium with a reactive porous medium, a reaction front develops, across which the invading fluid becomes saturated with the soluble matrix material. Depending on the composition of the invading fluid, it may initially be less dense than the formation fluid, but following reaction it may become denser than the formation fluid. If the invading fluid displaces the formation fluid downwards through the porous layer, the reaction front may then be stabilized by buoyancy, but a Rayleigh-Taylor type instability can develop at the interface between the reacted fluid and the original formation fluid ahead of the reaction front. We present a series of new analog experiments of this process by injecting aqueous sugar solutions into a porous layer containing saturated salt solution, salt powder, and glass ballotini. As an analog of a reaction front, a dissolution front develops as the aqueous sugar solution dissolves the salt powder and becomes denser than the saturated salt solution. The buoyancy instability then leads to a growing finger interface. If the buoyancy speed of the fluid, uB, is smaller than uF−uR, where the speed of the advancing fluid-fluid front is uF and the speed of the dissolution front is uR then the instability grows as if the system were unconfined. However, if uB>uF−uR, then the supply of fluid at the dissolution front limits the growth of the instability. We present an idealised model for the speed of the non-linear buoyancy-driven fingers, and we consider the implications of our results for the long term integrity of a number of geological storage systems. Published by the American Physical Society 2025

Salt deposits are being considered in several countries as potential host rocks for deep geological disposal facilities (GDFs) for radioactive waste. Although the very low porosity and lack of water content are favorable properties of salt that make it well-suited for this purpose, brine is present in amounts that are not insignificant when considering potential corrosion of waste canisters and pathways for radionuclide migration. It is therefore important to understand the processes and mechanisms that can cause brine to flow into the GDF.In this paper a relatively simple model is described that represents damage and fracturing in rock salt caused by heating and subsequent cooling. The model is used to simulate brine inflow to a heated borehole, using data from the Brine Availability Test in Salt (BATS) which was carried out at the Waste Isolation Pilot Plant (WIPP) in New Mexico, USA. Key characteristics of BATS and other, similar, experiments are increased brine production during periods of elevated temperature and a very large but short-lived increase shortly after the heat source is removed. The latter is not directly relevant to the disposal of heat-generating radioactive waste, where the heat source will gradually diminish over time, but the ability to simulate this phenomenon demonstrates an understanding of the coupled thermo-hydro-mechanical (THM) processes that operate in salt-based systems.The modelling demonstrates that a certain level of complexity is needed (such as the inclusion of viscoplastic effects and damage) to capture the key behaviors, but complexity can be tailored to the purposes of the study. In this case, a 1D model and a simple representation of the damage directly linked to permeability change was sufficient to gain a detailed understanding of the system. This includes the importance of the excavation damage zone (EDZ) in controlling the availability of brine.

The extent to which crystalline rocks formed at high temperatures become infiltrated by groundwater during prolonged residence near the earth’s surface is evaluated. The study was carried out because rock matrix diffusion, defined as diffusion through pore water in crystalline rocks, has been proposed to facilitate the dispersal of radionuclides from buried waste. This assumes that rocks are water-saturated, but ubiquitous granite minerals such as biotite are not stable at low temperatures in the presence of water, irrespective of water composition. Nevertheless, the filling of almost all pores in a 3-mm-diameter core of fine-grained granite over 7 days was demonstrated by in situ X-ray computed tomography. Water infiltration could be inhibited by the formation of reaction products if granite reacted with the water, which was investigated using polished granite cores to facilitate observation of reaction products and etch pits. After 4 weeks at a temperature of 50 °C, etch pits in biotite and plagioclase and small secondary platelets on biotite were observed. At higher temperatures of 100 and 150 °C, secondary growths became more pronounced. To provide a natural analogue of possible water penetration, the history of the Mountsorrel Granite in the UK was examined. Despite > 300 Ma of residing within a few kilometers of the surface, the rock has fresh primary minerals, except for areas very close to spaced joints. Infiltration of groundwater into granite takes place along fractures but penetration of the matrix porosity is strongly inhibited, probably by secondary minerals resulting from hydration reactions.

The long-term safety of deep geological repositories of radioactive waste can be affected by gas production, transport and consumption, i.e. via alteration of the chemical environment and/or gas pressure build up. The Full-scale Emplacement (FE) experiment recreates a 1:1 scale spent fuel (SF) emplacement drift constructed in the Underground Rock Laboratory at Mont Terri (Switzerland). FE is inside Opalinus Clay, with heaters simulating canisters and granular bentonite as buffer material. Since 2014, FE-G combines on- and off-site measurements of gases in the FE experiment, providing data to monitor and validate the prediction of gas safety-relevant processes. In this poster contribution we will present results from 10 years of gas monitoring, with particular focus on: The observed rapid loss of gaseous O2 after emplacement, with current understanding of (ir)reversible processes related to O2 from 3-D reactive transport model (COMSOL) investigations; The role of noble gases, in particular 4He, to describe the ongoing gas exchanges via diffusion (and advection) among the Opalinus Clay, the emplaced bentonite and the niche outside the emplaced tunnel.   Acknowledgements Swisstopo and Mont Terri Consortium Bill Lanyon (Fracture Systems Ltd) for modelling support Companies supporting field and lab work (Entracers, Solexperts, Hydroisotop)

The long-term safety of deep geological repositories of radioactive waste can be affected by gas production, transport and consumption, i.e. via alteration of the chemical environment and/or gas pressure build up. The Full-scale Emplacement (FE) experiment recreates a 1:1 scale spent fuel (SF) emplacement drift constructed in the Underground Rock Laboratory at Mont Terri (Switzerland). FE is inside Opalinus Clay, with heaters simulating canisters and granular bentonite as buffer material. Since 2014, FE-G combines on- and off-site measurements of gases in the FE experiment, providing data to monitor and validate the prediction of gas safety-relevant processes. In this poster contribution we will present results from 10 years of gas monitoring, with particular focus on: The observed rapid loss of gaseous O2 after emplacement, with current understanding of (ir)reversible processes related to O2 from 3-D reactive transport model (COMSOL) investigations; The role of noble gases, in particular 4He, to describe the ongoing gas exchanges via diffusion (and advection) among the Opalinus Clay, the emplaced bentonite and the niche outside the emplaced tunnel. Acknowledgements Swisstopo and Mont Terri Consortium Bill Lanyon (Fracture Systems Ltd) for modelling support Companies supporting field and lab work (Entracers, Solexperts, Hydroisotop)

The successful implementation of plans to develop a repository for radioactive waste in the United Kingdom will require an assessment of the ability of the facility to meet appropriate regulatory safety criteria at all stages during its construction, operation and after closure. As part of this assessment, an environmental safety case (ESC) will be presented and this will contain a collection of claims, arguments and evidence which collectively demonstrate that long-term safety can be achieved and maintained. Natural analogues (NA) can be helpful in demonstrating understanding of aspects of repository performance by, for example, providing evidence that certain materials can survive for long periods. Appropriate NAs can be critical to providing long-term practical demonstrations to support the theoretical and mathematical arguments of the ESC and they may have a significant role in the overall process understanding. As part of the ongoing repository development programme in the UK, an updated version of an existing catalogue of NA studies (focussed on the requirements of the ESC) has been produced and the background to NA studies in general, and to the catalogue in particular, are presented here. Thematic collection: This article is part of the Sustainable geological disposal and containment of radioactive waste collection available at: https://www.lyellcollection.org/topic/collections/radioactive

In the 2023 phase of the international collaborative DECOVALEX modeling project, Task E focused on understanding thermal, hydrological, and mechanical (THM) processes related to predicting brine migration in the excavation damaged zone around a heated excavation in salt. Salt is attractive as a disposal medium for radioactive waste because it is self-healing and is essentially impermeable and non-porous in the far field. Investigation of the short-term, near-field behavior is important for radioactive waste disposal because this early period strongly controls the amount of inflowing brine. Brine leads to corrosion of waste forms and waste packages, and possible dissolution of radionuclides with brine transport being a potential transport vector to the accessible environment.The Task was divided into steps. Step 0 included matching unheated brine inflow data from boreholes at the Waste Isolation Pilot Plant (WIPP) and matching temperature observations during a Brine Availability Test in Salt (BATS) heater test. Step 1 included validation of models against a thermo-poroelastic analytical solution, and two-phase flow around an excavation. Finally, Step 2 required all the individual components covered in steps 0 and 1 to come together to match observed brine inflow behavior during the same BATS heater test.There were a range of approaches from the teams, from mechanistic to prescriptive. Given the uncertainties in the problem, some teams used one- or two-dimensional models of the processes, while other teams included more geometrical complexity in three-dimensional models. Task E was a learning experience for the teams involved, and feedback from the modeling teams has led to changes in follow-on BATS experiments at WIPP. The primary Task E lessons learned were the impact of hydrologic initialization methods (wetting up vs. drying down), the difference between confined and unconfined thermal expansion, and the large changes in permeability associated with heating and cooling.

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 which could otherwise lead to corrosion of waste packages and mobilization 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, which may be of relevance when constructing models to inform future safety cases for radioactive waste disposal in salt and in other energy geoscience applications where brine interactions are a potentially complicating factor.

Abstract. In line with the European waste directive 2011/70/EURATOM, the European Joint Programme on Radioactive Waste Management (EURAD) was launched in June 2019. EURAD aims to make a step change in European collaboration between advanced and early-stage waste management programmes. Monitoring Equipment and Data Treatment for Safe Repository Operation and Staged Closure (MODATS) is Work Package (WP) 17 of the EURAD programme. This WP aims to address a range of research needs relating to repository monitoring data and technologies. This abstract summarises research relating to the interactions between repository monitoring systems and the multi-barrier systems in which they are emplaced. A detailed features, events and processes (FEP) analysis is being undertaken to identify and describe the manner in which monitoring systems may interact with multi-barrier systems. Monitoring system components are generically considered in this research; they include data acquisition technologies, power technologies, data transmission technologies and data loggers. No specific disposal concepts have been selected; however, a range of commonly engineered barrier system materials and typical host rocks are considered. This FEP analysis research will provide an understanding of the impacts of monitoring systems, which could be used to aid monitoring system design optimisation, for example by supporting the screening of monitoring technologies (White and Scourfield, 2019). It may also provide evidence to demonstrate that monitoring systems do not unacceptably impact the safety functions of the multi-barrier system. Potential interactions may include the creation of gas migration pathways along monitoring power or data transmission cables positioned in the multi-barrier system. This process has been observed in underground research laboratory experiments, such as the Large Scale Gas Injection Test (LASGIT) experiment in the Äspö Hard Rock Laboratory, which involved a series of gas injection tests in a full-scale KBS-3V deposition hole. Other possible interactions could include (non-exhaustive) corrosion of metallic components, degradation of non-metallic components, microbial activity, gas generation, void introduction and formation and volume changes. The final output of this research will be a catalogue of FEPs describing the interactions between generic monitoring systems and multi-barrier systems. Similar to the Nuclear Energy Agency FEP lists, this FEP catalogue is intended to be a starting reference for waste management organisations to use to understand the potential interactions between their specific monitoring systems and multi-barrier systems.

The transport and retardation of radioactive elements in hyper-alkaline conditions of radioactive waste repositories is a challenging field that is still poorly understood. In this study, the transport and attenuation of uranium in a column experiment was modelled by considering kinetic reactions, advection–dispersion and chemical/physical retardation processes. The modelling was first performed for three alluvium samples from Yucca Mountain in circumneutral pH to moderately alkaline conditions. Sorption of uranyl ( UO_2^2+ (U_VI) ) was found to strongly depend on the surface complexation model assumed, with no significant removal of U_VI by precipitation or ion exchange process. The surface/edge site reaction of Al-hydroxyl group in kaolinite was shown to have a high affinity for uranyl adsorption, while the hydrous ferric oxide edge on hematite adsorbed most of the uranyl ions. The model was then used to interpret uranium transport in a laboratory column filled with Hollington sandstone under hyper-alkaline (pH 13) conditions. The simulation results show that uranium adsorption on the Al-hydroxyl edge of kaolinite exceeds adsorption by the calcium silicate hydrate phase. This result may reflect the lack of surface complexation parameters for calcium silicate hydrate minerals. Hence, further studies are required in the field of surface complexation reactions for calcium silicate hydrate phases.

Safety of deep underground repositories for high-level radioactive waste is ensured by a multi-barrier system consisting of the geological barrier (host rock) and the engineered barrier system (EBS). Shaft seals are a major part of the EBS. The German regulator demands that the interplay of barriers has to be optimized in diverse redundancy to increase reliability and robustness of the barrier system. Shaft seals should also be constructed of diverse redundant components. The design of shaft seals for generic site models of Germany includes hydraulic sealing elements that are to be realized as Sandwich sealing systems. In contrast to conventional hydraulic seals of monolithic bentonite the Sandwich sealing system consists of sealing segments (DS) of bentonites and hydraulically conductive equipotential segments (ES) (Nüesch et al., 2002). Formation water, that is penetrating the hydraulic seal via preferential flow paths is contained in the ES and evenly distributed over the cross section of the seal. Thus, a more homogeneous hydration and swelling of the DS is obtained.In July 2019, after a two-years planning phase (Emmerich et al., 2019), a large-scale experiment was launched at the Mont Terri rock laboratory (MTRL) to demonstrate the feasibility of installation, to investigate the saturation process, to qualify monitoring techniques, and to assess the sealing effectiveness. The in-situ experiment is supported by laboratory experiments at different scales and by numerical simulation.The in-situ experiment consists of two experimental shafts of 1.18 m diameter and 10 ‑ 12.6 m depth located in the sandy facies of the Opalinus Clay. The DS in both shafts are constructed of German Ca-bentonites, while the ES consist of fine-grained sand. Both Sandwich sealing systems are hydrated with Pearson water type A3. Both shafts were drilled in 2020 and Shaft 1 was installed subsequently. Hydration of Shaft 1 started in May 2021. Shaft2 was installed about two years later so that experience from Shaft 1 operation could be included. Additionally, an excavation damaged zone had the chance to develop close to the shaft wall.The presentation will focus on the operation phase of the two shafts. Measurement systems and monitoring results, in terms of the evolution of water content, stress, pore pressure, and relative humidity in the sealing systems will be presented.AcknowledgmentThe Sandwich pre-project and the Sandwich in-situ experiment were/are funded by the German Federal Ministry for Economic Affairs and Energy under contracts 02E11587 and 02E11799.

AbstractThe diffusive exchange of dissolved material between fluid flowing in a fracture and the enclosing wallrocks (rock matrix diffusion) has been proposed as a mechanism by which radionuclides derived from a radioactive waste repository may be removed from groundwater and incorporated into the geosphere. To test the effectiveness of diffusive exchange in igneous and metamorphic rocks, we have carried out an investigation of veins formed at low temperatures (<100°C), comparing the oxygen isotopic composition of vein calcite with that of secondary calcite in the wallrocks. Two examples of veins from the Borrowdale Volcanic Group, Cumbria, and one from the Mountsorrel Granodiorite, Leicestershire, UK, have remarkably similar vein calcite compositions, ca. +20‰(SMOW) or greater, substantially heavier than the probable compositions of the host rocks, and these vein calcite compositions are inferred to reflect the infiltrating fluid and the temperature of vein formation. Calcites from the wallrocks are similar to those in veins, with little evidence for exchange with the wallrocks. The results support existing models for this type of vein which suggest low-temperature growth from formation brines originally linked to Permian or Triassic evaporites. The results are consistent with flow through fractures being attenuated through a damage zone adjacent to the fracture and provide no evidence of diffusional exchange with pore waters from wallrocks.

A proposed strategy for the use of natural analogue (NA) studies in Nuclear Waste Services1 (NWS) UK geological disposal facility programme and safety case has been developed based on an extensive review of past and current international practices. In the review, it was found that information on the strategic implementation of NA information is not readily available. This may be partly due to lack of strategic planning for the utilisation of NAs, or simply because waste management organisations do not generally publish internal strategic discussions. Nevertheless, it is seen of importance that the topic is discussed in public to increase the visibility of NA information and its implementation in geological disposal programmes. Another main conclusion concerns the cultural aspects of treatment of NAs in international fora, which seems to set NA information in the category of complementary/alternative evidence rather than equal part of the knowledge base. Often, the emphasis is set on the uncertainties and qualitative nature of NA. However, this ignores the fact that uncertainties are also present in laboratory, URL (underground rock laboratory) and modelling studies: for example, NA uncertainties are related generally to ill-defined boundary conditions, while short term experiments have uncertainties in both spatial and temporal scales. When NAs are used only as “alternative” lines of evidence, there is a risk that the information is devolved from the other evidence and this creates a risk of over or underestimating processes if extrapolated. The strategy presented highlights the importance of: * knowledge management and systematic approaches to the use of data from the broadest range of sources (i.e. laboratory, URL, modelling and NA) possible * the continuous review of existing NA information * the potential benefits of new NA projects in the future national programme * the significance and experience of communication with NA information (at various levels across a broad range of stakeholder groups) The strategic study also includes an update of RWM’s NA catalogue (Milodowski et al. 2015), a starting point of the NWS’s NA knowledge base. Development of a strategic approach to utilise NA information naturally leads to activities to be undertaken in the immediate future and one of the activities emphasised is the potential use of regional (or self) analogues in support of the site characterisation programme. The emphasis and requirements for NA research will evolve and change as the focus of the geological disposal programme develops naturally as the programme matures so, for example, preliminary discussion on the operational phase is also included. References Milodowski, A.E., Alexander, W.R., West, J.M., Shaw, R.P., McEvoy, F.M., Scheidegger, J.M. & Field, L.P., 2015. A Catalogue of Analogues for Radioactive Waste Management. BRITISH GEOLOGICAL SURVEY COMMISSIONED REPORT CR/15/106. Keyworth, Nottingham British Geological Survey 2015. 1849p. 1 Following a merger with Low Level Waste Repository Limited, Radioactive Waste Management (RWM) is now part of Nuclear Waste Services (NWS)

One of the key requirements for the deep geological disposal of high-level nuclear waste is the assessment of its long-term performance and safety. As any other barrier of the disposal system, waste containers must fulfil their respective safety functions for the required duration, which can vary from a few hundreds of years to several hundreds of thousands of years, depending on disposal system requirements. Sufficient corrosion resistance under repository conditions is one key requirement for container material to provide complete waste containment. Copper is an important part of many waste packaging and disposal concepts, e.g. KBS-3 developed in Sweden and Finland and Mark II developed in Canada. Much of the data available regarding its behaviour under repository conditions comes from short-term investigations, such as laboratory experiments at different scales and under controlled conditions. Observations made from copper analogue studies provide additional information on copper behaviour during the assessment time scale and under real geological environments. By this, they can support the argumentation in the safety case. Keweenaw native copper occurrences (Lake Superior, US) reflects more than one billion years of deposit evolution covering various geological (from bedrock to sediments and even anthropogenic mine site remnants) and geochemical environments (e.g., brines to meteoric water, anoxic vs. oxic, sulphur-free vs. sulphur-bearing). These deposits have been mined for a long time and there is a great deal of knowledge related to them as well as samples collected. However, data to be used in process based safety assessments for geological disposal is lacking and no formal review has been made from the geological disposal point of view. The current MICA Project Phase I systematically collect and review the existing literature and data on the Michigan copper analogue sites and available sampling potential. Based on the outcome, MICA Project Phase II will then study and analyse prospective sites and samples to address relevant questions regarding long-term behaviour of copper under disposal conditions. The MICA Project thus will provide a unique complementary data source to estimate processes governing behaviour of metallic copper and to support safety cases.

Abstract. Shaft-sealing systems for nuclear waste repositories are constructed to limit fluid inflow from the adjacent rock during the early stage after closure of the repository and to delay the release of possibly contaminated fluids from the repository at later stages. Current German concepts of shaft seals contain the hydraulic sandwich sealing system as a component of the lower seal in host rock (Kudla and Herold, 2021). The KIT-developed sandwich sealing system consists of alternating sealing segments (DS) of bentonite and equipotential segments (ES) that are characterized by a high hydraulic conductivity. Within the ES, fluid is evenly distributed over the cross section of the seal. Water bypassing the seal via the excavation-damaged zone or penetrating the seal inhomogeneously is contained, and a more homogeneous hydration and swelling of the DS is obtained. The functionality of such a system was proven in laboratory and semi-technical-scale experiments (Schuhmann et al., 2009). After a joint international pre-project (Emmerich et al., 2019) dedicated to the planning of a large-scale in situ test that demonstrates the feasibility and effectiveness of the sandwich shaft-sealing system in interaction with the host rock, the large-scale experiment was launched at the Mont Terri rock laboratory in July 2019 with partners from Germany, Switzerland, Spain, UK, and Canada. It consists of two experimental shafts of 1.18 m diameter and 10–12.6 m depth, constructed using a core drilling technique with a custom-made drill rig in a new niche in the sandy facies of the Opalinus Clay. The seal in shaft 1 consists of four DS (calcigel) of 1 m thickness and five ES (fine-grained quartz sand), each 30 cm thick (Fig. 1). Shaft sinking began in August 2020 and was completed in November 2020. In the following months, the sealing system and instrumentation of shaft 1 were installed. The sealing system is saturated from a pressure chamber located at the shaft bottom via an inclined lateral feeding borehole. Hydration of the system started in May 2021. Shaft 2 will host a slightly modified system emplaced 1–1.5 years later, in order to integrate experience obtained during the early operation phase of shaft 1. In contrast to shaft 1, the excavation-damaged zone around shaft 2 will have had time to develop. The seals and the surrounding rock are intensely monitored. Measurements in the rock (geophysics, pore pressure, and total stress) were started between August 2019 and March 2020. Characterization of the excavation-damaged zone along the wall of shaft 1 was performed by geophysical and surface packer measurements prior to seal emplacement. Measurements inside the shaft comprise water content, relative humidity, and temperature, pore pressure, stress, and displacements. The in situ work is backed by laboratory testing and model simulation. Data and experience obtained to date will be presented. The sandwich experiment is funded by the German Federal Ministry for Economic Affairs and Energy under contract 02E11799.

An integrated waste management approach for irradiated graphite was developed during the European Commission project ‘Treatment and Disposal of Irradiated Graphite and other Carbonaceous Waste’. This included the identification of potential options for the management of irradiated graphite, taking account of storage, retrieval, treatment and disposal methods. This paper describes how these options can be assessed using multi-criteria decision analysis (MCDA) for a case study relating to a generic power reactor. Criteria have been defined to account for safety, environmental, economic and socio-political factors, including radiological impact, resource usage, economic costs and risks. The impact of each option against each criterion has been assessed using data from the project and the wider literature. A linear additive approach has been used to convert the calculated impacts to scores. To account for the relative importance of the criteria, example weightings were allocated. This application has shown that MCDA approaches can be used to support complex decisions regarding irradiated graphite management, accounting for a wide range of criteria. Use of this approach by individual countries or organisations will need to account for the specific options, scores, weightings and constraints that apply, based on their national strategies, regulatory requirements and public acceptability.

Summary Coupled thermo‐hydro‐mechanical‐chemical modelling has attracted attention in past decades due to many contemporary geotechnical engineering applications (e.g., waste disposal, carbon capture and storage). However, molecular‐scale interactions within geomaterials (e.g., swelling and dissolution/precipitation) have a significant influence on the mechanical behaviour, yet are rarely incorporated into existing Thermal‐Hydro‐Mechanical‐Chemical (THMC) frameworks. This paper presents a new coupled hydro‐mechanical‐chemical constitutive model to bridge molecular‐scale interactions with macro‐physical deformation by combining the swelling and dissolution/precipitation through an extension of the new mixture‐coupling theory. Entropy analysis of the geomaterial system provides dissipation energy, and Helmholtz free energy gives the relationship between solids and fluids. Numerical simulation is used to compare with the selected recognized models, which demonstrates that the swelling and dissolution/precipitation processes may have a significant influence on the mechanical deformation of the geomaterials.

Radioactive waste disposal facilities for heat generating waste are designed to maintain a compressive stress regime in the host rock. Field-scale tests have been undertaken to study how heating of the host rock can reduce the effective stress on the rock. However, it is not possible to carry out experiments at the scale of a whole repository (km-scale), so variation of host rock properties that might be encountered at this scale is not captured by the experiments. Previous numerical modelling of field scale experiments has demonstrated that the physical processes leading to changes in the stress regime are well understood. Here those models are applied at the km-scale, considering approaches to representing a whole repository and the effect of variability in the properties of the host rock. A dataset from the Callovo-Oxfordian Claystone at the Meuse/Heute-Marne Underground Rock Laboratory is used to characterise the natural variability of a potential host rock at the scale of a disposal facility. The modelling demonstrates that for understanding the generation of thermal stresses, considerable learning can be gained from models that employ symmetry conditions and represent a small part of the repository, hence reducing the size of the computational problem. The host rock at the mid-point between waste cells remained in vertical compression for all the parameter combinations used, which builds confidence that horizontal fracturing of the host rock between waste cells due to thermal pressurisation is unlikely for the disposal concept discussed here. Ground surface uplift was also considered and is in the range of 9-13 cm for disposal system studied. Spatial variability in the properties of the host rock has the potential to lead to significant variability in temperature, pressure, effective stress and displacement around the facility. In particular, thermal conductivity, permeability, Young's modulus and coefficient of thermal expansion need to be well characterised.