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Frontiers in Nuclear Engineering ·

Volume 5 - 2026 | https://doi.org/10.3389/fnuen.2026.1843726

The safety case for a deep geological repository in Switzerland: engineered barriers for low- and intermediate-level waste disposal

  • Lukas Martin1*corresponding author
  • Georg Kosakowski2
  • Typhaine Guillemot1
  • Alexandros Papafotiou1
  1. 1Nagra, Wettingen, Switzerland
  2. 2PSI Center for Nuclear Engineering and Sciences, Villigen, Switzerland
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Abstract

The safe geological disposal of low- and intermediate-level radioactive waste relies on a passive multi-barrier system. This paper presents the provisional Swiss design for the low- and intermediate-level waste (L/ILW) emplacement caverns and their expected evolution. In the current design, containers are stacked within the L/ILW disposal caverns, and the remaining void space is backfilled with a porous mortar. Because cementitious materials are used in large quantities, they largely determine the geochemical conditions in the L/ILW near field. A key property of these cementitious materials is their ability to maintain a high-pH porewater environment. Such conditions limit the degradation of organic materials and suppress microbial activity. They also promote the passivation of metal surfaces, resulting in slow corrosion rates and consequently reduced gas generation. The chemical evolution of the near field is strongly coupled to the repository’s saturation history, as water is required for chemical reactions such as for the corrosion of metals, degradation of organic matter or pozzolanic reactions. Given the low permeability of Opalinus Clay, water availability in the caverns is limited, and partially saturated conditions are expected to persist for several hundreds of thousands of years. These conditions help maintain elevated pH levels, particularly in the unsaturated regions of the emplacement caverns. The substantial presence of cementitious materials also enhances the retention and slow release of radionuclides from the L/ILW near field into the host rock, once the waste packages breach. As a result, the L/ILW near field contributes to key safety functions, including immobilization, retention, and controlled release of radionuclides, as well as ensuring compatibility among repository components. The properties and long-term behaviour of these components are sufficiently well understood to allow a robust description of their evolution over extended timescales, supporting the demonstration of their barrier function and long-term performance.

Keywords

1 Introduction

A wide range of disposal concepts have been developed around the world for the disposal of low- and intermediate-level radioactive waste (L/ILW), according to the needs and available infrastructure of the country (OECD/NEA, 2010; International Atomic Energy Agency, 2020). More specifically, five types of disposal can be listed: trench, aboveground mound, concrete vault, boreholes and rock cavern (cf. Garamszeghy, 2021; Jacques et al., 2025). Trenches are considered to be “near-surface” facilities and are still commonly used in many countries for high volume, low hazard wastes (e.g., France, Spain, United Kingdom, United States; International Atomic Energy Agency, 2017). They are generally considered appropriate only for wastes that will decay sufficiently within an anticipated period of institutional control (generally between 100 and 300 years).

An aboveground mound is in some respects very similar to a trench, except that it is built on the surface of the ground, then mounded over with a cap and cover system. This type of disposal for low- and very low-level waste, has been used in Sweden, France and the United States of America. Concrete vaults are one of the most common methods used for disposal of LLW (International Atomic Energy Agency, 2018). They consist of a thick-walled concrete structure of various dimensions. The concrete structure can be scaled to a range of small to very large capacities and offers the flexibility to allow emplacement of a wide variety of waste package sizes. They can also be constructed underground in different geological settings and at various depths: at the surface (e.g., France, Spain, United Kingdom), near-surface (e.g., Czech Republic), or up to several tens of metres (e.g., Scotland).

Boreholes are vertical shafts that can be bored to with various diameter and suitable depths. The diameter ranges from up to a few tens of centimetres and they can be filled with waste, then backfilled with a cement grout or other material. This type of disposal is mostly used in the United States of America.

Finally, the rock cavern repository consists of underground excavations at various depth in a suitable bedrock formation (e.g., salt, sedimentary and crystalline formations). It can be designed for the storage of ILW. Deep rock cavern repositories for L/ILW are currently operated in Germany, Hungary, South Korea and the United States of America. They are also planned for France and Switzerland.

In Switzerland, L/ILW is envisaged to be disposed in caverns in the L/ILW repository section as part of Nagra’s planned combined deep geological repository in Opalinus Clay (Smith, 2026). The disposal of L/ILW waste in a deep geological repository is required by Swiss federal law (KEG, 2003 Art. 31 Abs. 2 lit. a). This contrasts with the strategies adopted in other European countries–with the exception of Germany - which plan subsurface or surface disposal for low level waste and/or intermediate level waste. The Swiss approach goes back to the demand that radioactive waste must be disposed of in such a way that the protection of humans and the environment from its ionizing radiation is guaranteed in the long term (KEG, 2003 Art. 3), without imposing unreasonable burdens and obligations on future generations (ENSI, 2023). Furthermore, it is requested that the deep geological repository must be designed in such a way that no further measures are required to ensure long-term safety after its closure. This requirement ensures that future generations are not burdened with the monitoring, managing and maintaining the waste facilities. It should also be noted that L/ILW may contain long-lived radionuclides (e.g., 36Cl). Therefore, safe dispoal of L/ILW is to be demonstrated by Nagra up to one million years according to ENSI (2023). These demands can be fulfilled by disposing of L/ILW in a deep geological repository, as requested by the Swiss regulations.

L/ILW results from the operation and dismantling of nuclear power plants and from applications in research, medicine and industry (Guillemot et al., 2026) and as a result of the different sources, it is highly heterogeneous. Prior to emplacement, L/ILW waste is planned to be conditioned in cementitious materials, with the waste drums packed into concrete containers (Guillemot et al., 2026). The current provisional design and emplacement plan envisions containers stacked in L/ILW disposal caverns, with the void space backfilled with a porous mortar. Due to the high abundance of cementitious materials, these materials control the geochemical conditions within the L/ILW near-field (Kosakowski et al., 2023).

The most important property of the cementitious materials is that they ensure high pH porewater composition in the near field. This leads to the passivation and consequent slow corrosion of metals (e.g., steel) (Diomidis et al., 2023) and limits the degradation of organic materials and microbial activity (Guillemot et al., 2023), thus serving to minimise gas generation rates.

The chemical evolution in the near-field is strongly coupled to the overall saturation evolution of the repository, as water is a pre-requisite for chemical reactions that lead to the various degradation processes (Wieland et al., 2018; Kosakowski et al., 2020; Huang et al., 2021; Kosakowski et al., 2023). Opalinus Clay is a low permeability rock, and thus only partially saturated conditions are expected to persist for several hundreds of thousands of years in the L/ILW emplacement caverns (Martin et al., 2023; Nagra, 2024b; Papafotiou et al., 2023). In particular, spatio-temporal cement material evolution in the L/ILW repository might be controlled by local conditions of water saturation. Water availability in the bottom section of the cavern thus has a local impact on cement degradation (porewater pH), which in turn control various (bio-)chemical processes (Ma et al., 2024; Kosakowski et al., 2023). This helps to maintain elevated pH conditions, particularly in the unsaturated parts of the emplacement caverns, depending on the nature of the waste and cement materials employed (Kosakowski et al., 2023). Furthermore, the high abundance of cementitious materials, together with the beneficial sorption capacity of C-S-H, an abundant calcium-silicate-hydrated cement mineral (Tits and Wieland, 2023), contributes to the retention and only slow release of radionuclides from the L/ILW near-field into the host rock. As such, the L/ILW near-field contributes to the high-level safety functions of immobilization, retention and slow release of radionuclides, as well as ensuring compatibility among the components of the repository system (Smith, 2026).

The present paper summarizes the key aspects of Nagra’s concept for L/ILW disposal, and the current understanding of the evolution and performance of the L/ILW near-field. The paper also explains the functionality of the L/ILW near-field and its contribution to the multi-barrier system, which serves to safely isolate radioactive waste.

2 Description and evolution of the L/ILW nearfield

2.1 Operational phase

Nagra’s repository concept and the related pillars of safety are presented in Smith (2026). This paper focuses on the L/ILW repository section. An illustration of the current design of an L/ILW emplacement cavern is given in Figure 1. According to the current provisional design, the L/ILW caverns are dead-end structures, with a maximum length of 250 m and an inner diameter of approximately 11 m. The caverns are connected to an operations tunnel by shorter branch tunnels. During the construction process of the caverns, an excavation damage zone (EDZ) will be generated. Shotcrete in combination with gliding arches (curved tunnel support structures that guide loads into the surrounding ground through compression, reducing bending and deformation) will be installed directly on the surface of the Opalinus Clay as the primary rock support, needed for operational safety during the construction process. Applying shotcrete onto the Opalinus Clay will lead to a short-term heat release of few hours due to the hydration of the cement, and, due to the chemical gradient between the cement (high pH) and the Opalinus Clay (neutral pH), very local interaction on the millimetre to centimetre scale between the cement and Opalinus clay will occur. After the emplacement of the primary rock support, during the first few months an initial convergence will occur. When this convergence has ceased, an inner shell concrete liner will be emplaced using formwork and in situ pouring of concrete. Additionally, the base of the cavern will be constructed with a reinforced concrete floor with poured concrete on top.

Visualisation of the provisional design of an emplacement cavern for L/ILW disposal.
Figure 1. Visualisation of the provisional design of an emplacement cavern for L/ILW disposal.

Most of the waste is conditioned with cementitious materials in typically 100–200 L drums (Guillemot et al., 2026). These waste drums are then packed into concrete containers, with the void space between the drums backfilled with mortar. Waste comprising activated large metal pieces or activated concrete rubble is directly placed in concrete containers and backfilled with flowable mortar (Figure 1). The conditioned waste containers will then be emplaced in the L/ILW cavern, which is dimensioned such that several waste containers can be placed next to each other and stacked in several layers. Void space between the containers and cavern wall will be backfilled with cementitious material. The currently planned porous, gas-permeable cavern-backfill mortar comprises monograined aggregate (i.e., aggregate of uniform size) bound by a deliberately limited quantity of cement, ensuring that the intergranular pore space remains predominantly unfilled (type M1 mortar, Jacobs et al., 1994). The high porosity and permeability of this mortar serves as a gas storage volume and also allows gas release through the cavern seal into the backfill material emplaced within the operational tunnels (see below and Papafotiou et al., 2026), while the grain-supported aggregates in the mortar serve to limit tunnel convergence after the tunnel lining has degraded and lost its support function (Nagra, 2024c). This mortar, together with the concrete containers, the cemented waste packages and rock support, together constitute the cementitious L/ILW nearfield.

After backfilling is completed, each L/ILW cavern will be closed with a V1-L/ILW seal, which is constructed in the branch tunnel and connects the cavern to the operation tunnel (cf. Figure 2; Nagra, 2021; Martin et al., 2023). The L/ILW seals are designed to be gas permeable but to limit water flow. The gas permeability of the sealing element allows gas generated in the L/ILW caverns to migrate into the operational tunnel, thereby increasing the available gas storage volume and mitigating gas pressure build-up within the caverns (Papafotiou et al., 2026). According to the current design (Nagra, 2021; Figure 2), cement-based material is separated from the sealing element by a transition layer, which comprises of calcareous aggregates. The loose material is held in place by partitioning walls. This transition layers serves to physically separate the cement-based elements (mortar M1 or the concrete abutment) from the sealing element. By using calcareous aggregates, which are in chemical equilibrium with cement and clay materials, pore clogging at the interfaces as a result of chemical interactions can be prevented (cf. Martin et al., 2023; Kosakowski and Smith, 2014). Afterwards, a sealing element, comprising of a 80:20 sand/bentonite mixture according to the generic design, is emplaced. In the section of the sealing element, the rock support will be, at least, partially removed, such that the swelling sand/bentonite mixture is in direct contact with Opalinus Clay. The sand grains form a support matrix (Romero, 2013) that carries the external stresses, while the bentonite (once saturated) can swell to effectively fill the pore space within this skeleton. This results in a low gas entry pressure (Manca, 2016), such that the gas can be released from the L/ILW caverns in the VF1 backfill (see below). After the sealing element, again separated by partitioning walls and a transition layer, a gas-permeable low-pH concrete abutment is installed. The abutment serves to mechanically stabilise the sealing section, and with respect to operational safety, it serves to prevent accidental flooding of the L/ILW repository section and hinders human intrusion.

Longitudinal section of a provisional V1-L/ILW seal. Distances are given in metres. Within the sealing section, complete to partial removal of the rock support will be considered. From Nagra (2021).
Figure 2. Longitudinal section of a provisional V1-L/ILW seal. Distances are given in metres. Within the sealing section, complete to partial removal of the rock support will be considered. From Nagra (2021).

The operational tunnels after the V1-L/ILW sealing elements will be backfilled with a VF1-backfill. According to the current concept, this material comprised of clay materials mixed with aggregates. This backfill material serves to provide a storage volume for gas, in particular for the gas released via the V1-L/ILW seals from the L/ILW emplacement caverns. As such, the backfill is required to maintain a high gas-permeability as well as a high gas-accessible porosity (cf. Martin et al., 2023; Nagra, 2021). The backfill materials have the further role of ensuring mechanical stabilisation of the underground openings, preventing or limiting tunnel convergence that might potentially cause damage to the geological barrier.

2.2 Post-closure phase

In the post closure phase, short term transient heat initially produced by the hydration of cement will result in a very limited temperature increase over a period of a few days to weeks. Radiogenic heat from L/ILW is low (Guillemot et al., 2026) and has no impact on the overall thermal evolution of the repository.

After the emplacement of L/ILW and the backfilling and sealing of the L/ILW emplacement caverns, residual oxygen present within the L/ILW cavern will be consumed quickly by chemical reactions such as corrosion of metal. As a result, anaerobic conditions will be established. The main gases produced in the L/ILW emplacement caverns under the reducing conditions expected to be established after cavern closure are hydrogen (H2), methane (CH4) and carbon dioxide (CO2). The timing of gas generation depends on the local availability of water and the degradation kinetics of the various constituents within the waste packages. For the purpose of the safety assessment, it is conservatively assumed that these processes start at the time of cavern closure. CO2 is, however, dissolved and consumed by carbonation of cementitious materials. H2 and CH4 remain in the gas phase due to their relatively low solubilities in water (Kosakowski et al., 2023). As a result of the hydraulic gradient caused by the excavation process, the partially saturated EDZ and rock support around the L/ILW caverns will largely resaturate with water and the EDZ fractures will re-seal (Martin et al., 2023), leading to the very slow resaturation of the L/ILW section of the repository with porewater, including the emplacement caverns (Figure 3a). This process occurs gradually from the bottom to the top and its rate is determined on the one hand by gas pressure in the tunnels that counteracts the inflow of porewater, the capillarity of the rock support, and on the other hand by gravity in the high-porosity backfill that drives water to the tunnel floor and gas to the tunnel roof (Nagra, 2024b). Water accumulation in the bottom section of the cavern thus has a local impact on cement degradation (porewater pH). This in turn controls various (bio-)chemical processes that back-couple to the saturation and degradation state, either in terms of water and gas transport properties, (diffusivity, porosity), e. g., at the OPA-rock support interface, or in change in local chemical conditions (Ma et al., 2024; Kosakowski et al., 2023).

**(a)** Illustration of evolution of L/ILW emplacement cavern: Different gray shadings from bottom to top indicate increasing saturation. Increased saturation of the lower part of the cavern, leads to increased degradation of concrete material in the lower region, while material in the upper part is hardly affected. **(b)** The high porosity of mortar M1 acts as a capillary barrier.
Figure 3. (a) Illustration of evolution of L/ILW emplacement cavern: Different gray shadings from bottom to top indicate increasing saturation. Increased saturation of the lower part of the cavern, leads to increased degradation of concrete material in the lower region, while material in the upper part is hardly affected. (b) The high porosity of mortar M1 acts as a capillary barrier.

Water accumulation in the bottom section due to the “gravity effect” originates from the low capillary strength of the backfill mortar M1, which surrounds the waste packages. As the amount of cement paste, which adheres the aggregate together, is that low that it does not fill the pores between the aggregates (cf. Jacobs et al., 1994; Martin et al., 2024), a characteristic (“dominant”) pore size results, which is in the sub-mm up to few mm range (Figure 3b). This coarse porosity has a low Van Genuchten capillary strength of 5.0 · 10−2 Pa (corresponding to a capillary rise of 5 cm) in comparison to the 1-2 · 10-6 Pa for the container concrete (Nagra, 2024a). The high capillarity of the container concrete will lead to the saturation of the concrete container, in particular in the lower most container row. Degradation of the concrete, enhanced by the corrosion of the rebars, which will lead to a spallation of the concrete, will lead to their disintegration in the long-run.

The capillary strength of the container infill mortar within the waste package is, however, lower as this material is no high-density concrete. When two partially saturated porous materials with different capillary strength (i.e., different water retention curves) are in contact, an equilibrium in suction will be established at the contact zones of the material. This continuity of suction in the state of thermodynamical equilibrium leads to a change in saturation in the contact zone of the two materials, with a higher degree of saturation occurring in the material with the higher capillary strength compared to the material with the lower capillary strength. Further details of this phenomenon are given in Nagra (2024b). As such, there will be just very limited water transfer from the container walls towards the container infill mortar, at least for some time. As further indicated by Papafotiou et al. (2026), once the gas pressure generated within the L/ILW emplacement cavern reaches hydrostatic conditions, further inflow of water from the Opalinus Clay into the L/ILW cavern will be prevented, which contributes to partially saturated conditions several 100,000 years within the L/ILW caverns (Martin et al., 2023; Nagra, 2024b).

3 Processes controlling the L/ILW near-field evolution

3.1 Evolution of cementitious materials

The most abundant materials within the L/ILW emplacement caverns are metals, organic compounds, aggregates, and cement-based materials such as hardened cement paste, mortar, and concrete. This implies that chemical conditions within the L/ILW near-field and within most of the waste packages are controlled by the cement porewater composition in equilibrium with the cement mineralogy (Kosakowski et al., 2023).

In general, cementitious materials such as concrete and mortar are made by mixing cement and aggregates with water, which causes an exothermic reaction and the material initiates hardening. The hydration process causes the mixing water to become bound to mineral phases, generating hardened cement paste. This cement paste glues the aggregates together. If the aggregate is only sand (grain size 0.063–2 mm), the material is called mortar, if larger aggregates are present, the material is called concrete. The hardened cement paste is composed of many mineral phases, with the main mineral phases being portlandite (Ca(OH)2) in Ordinary Portland Cement (OPC) and mineral phases comprising Ca, Si, Al and OH− in various proportions (C-S-H). C-S-H are also generating the firm fabric of cementitious materials by binding the aggregates together and is the key phase for the sorption of radionuclides in Nagra’s safety assessment (Tits and Wieland, 2023).

Regarding the performance of the near-field, the most relevant property of the cementitious materials is their ability to maintain a high pH in the pore water. High pH (>10.5) helps to limit the rates of corrosion of metals by promoting passivation (Diomidis et al., 2023), reduces the degradation rates and contributes to limit microbial activity (Small, 2019), which together also reduce the rate of gas generation. In addition, the presence of cement minerals favors the sorption of many radionuclides (Tits and Wieland, 2023; Wieland, 2014; Wieland and Van Loon, 2002).

Cementitious materials are, however, thermodynamically metastable in the presence of CO2, reactive silica and water and tend to degrade over time by different processes. The evolution of cement is analyzed in terms of four consecutive cement degradation stages (Figure 4; see also Kosakowski et al., 2023; Tits and Wieland, 2023).

Degradation stages for a typical cement-based material in terms of leached Ca<sup>2+</sup> due to aggregate-cement reactions (solid line) and carbonation (dashed line) with respect to the equilibrium pH of porewater. Sequential dissolution of cement phases results in a stepwise evolution of calculated equilibrium pH. It is to be noted that the different reactions occur at different kinetic rates, depend on different factors, and may vary relative to each other. The extracted curves refer to a temperature of 25 °C. Adapted from Kosakowski et al. (2023).
Figure 4. Degradation stages for a typical cement-based material in terms of leached Ca2+ due to aggregate-cement reactions (solid line) and carbonation (dashed line) with respect to the equilibrium pH of porewater. Sequential dissolution of cement phases results in a stepwise evolution of calculated equilibrium pH. It is to be noted that the different reactions occur at different kinetic rates, depend on different factors, and may vary relative to each other. The extracted curves refer to a temperature of 25 °C. Adapted from Kosakowski et al. (2023).

In cement degradation stage I, freshly hydrated cement/concrete has a porewater composition characterized by a high pH value (>13) due to significant amounts of dissolved alkali hydroxides (Figure 4). In cement degradation stage II, the chemical composition of the porewater is determined by the solubility of portlandite (Ca(OH)2), which controls the Ca-concentration and OH− concentration within the porewater in equilibrium with the cement paste and thus acts as a chemical buffer by maintaining pH > 12.5 (at 25 °C and 1 bar). In equilibrium with portlandite, C-S-H has a very high Ca/Si-ratio (>1.6). At the end of cement degradation stage III, when portlandite has fully dissolved, the pH is no longer buffered at 12.5 but is determined by the incongruent dissolution of the C-S-H phases leading to a pH of about 10, dependent on the temperature (cf. Kosakowski et al., 2023; Tits and Wieland, 2023).

The key processes that may lead to a change in the chemical conditions in the cement-based near-field over time are (i) the pozzolanic reaction and the aggregate-cement interaction; (ii) carbonation and (iii) cement-clay interaction (Kosakowski et al., 2023). The origin and impact of these processes on the near-field evolution and their relevance to the evolution of waste packages will be discussed in the following sections.

3.2 Aggregate-cement interaction and the pozzolanic reaction

Concrete materials are comprised of up to 70 vol% of aggregates, which typically have a high silica-content (Kosakowski et al., 2023). The silica and alkali ions from the dissolution of aluminum-silica aggregates and accessory minerals can promote pozzolanic reactions and alkali-silica reactions (ASR). ASR can originate from the interaction of alkalis (and hydroxyl ions) in alkaline pore fluid with reactive forms of silica in the aggregate. This is an expansive reaction which may damage concrete structures. In addition, there is also the pozzolanic reaction, where portlandite is consumed through interaction with siliceous compounds (aggregates, fillers such as microsilica/silicafume, fly ash, etc.), forming C-S-H phases with a decreased Ca/Si ratio. Due to the degradation of the portlandite, the buffering capacity of the solid phase is lost, and pH begins to decrease as silica is dissolved by the high-pH pore solution. This process continues until the cement-pore solution reaches saturation with silica, which occurs at a pH of about 10, dependent on the temperature (see the red continuous line in Figure 4).

Kosakowski et al. (2023) modelled the temporal progression of the out-of-equilibrium geochemical processes for various cement materials (Figure 5), accounting for the reaction of siliceous aggregates (quartz) with cement pore solution. Details of the models are given in Kosakowski et al. (2020), Huang et al. (2021).

Estimates of aggregate dissolution with time (upper figure) and pH evolution with time due to aggregate dissolution only (lower figure). The shaded areas show slowdown of aggregate dissolution rates of up to two orders of magnitude for the material at 42 °C due to lower reactive aggregate surface area and desaturation. In principle, for a certain slowdown, curves are shifted to the right, i.e., for 100 times slower dissolution rates the curves are shifted from the left boundary of the shaded area to the right boundary. In highly desaturated (“dry”) materials, aggregate dissolution rates are probably close to zero. Note that the pH evolution is temperature dependent. From Kosakowski et al. (2023).
Figure 5. Estimates of aggregate dissolution with time (upper figure) and pH evolution with time due to aggregate dissolution only (lower figure). The shaded areas show slowdown of aggregate dissolution rates of up to two orders of magnitude for the material at 42 °C due to lower reactive aggregate surface area and desaturation. In principle, for a certain slowdown, curves are shifted to the right, i.e., for 100 times slower dissolution rates the curves are shifted from the left boundary of the shaded area to the right boundary. In highly desaturated (“dry”) materials, aggregate dissolution rates are probably close to zero. Note that the pH evolution is temperature dependent. From Kosakowski et al. (2023).

The different cement materials show different temporal evolutions, depending on the ratio of cement stone to aggregates, the porewater pH in equilibrium with the cement mineralogy, and the different aggregate sizes, in which the reactive surface area of aggregates linearly influences the dissolution rates. This implies that the finer the material, the faster it will tend to dissolve (Kosakowski et al., 2023). The evolution of three different types of cement material is illustrated in Figure 5: the generic Stage II cement (which has been used as a reference for the cement sorption database in Nagra’s previous safety case) as well as mortar M1 were modelled as 2.8 mm monosized siliceous aggregates (for composition details, see Kosakowski et al., 2020) and have a low cement to aggregate content (cf. Section 2.1 in this work on the concept of mortar M1). For mixtures with monograins, the reactive surface area does not change strongly until equilibrium between the cement phases and the quartz aggregates is reached. As these mixtures were assumed to have a low cement content, only a small fraction of aggregates needs to be dissolved to reach equilibrium with silica. According to the conservative models used to generate the results in Figure 5, under fully saturated conditions it would take only several hundred years for a pH of ∼10 to be reached for these two cements. In contrast, for a normal structural concrete such as the “container concrete”, this process would take several thousands to tens of thousands of years (Figure 5). The reason for this is that the normal concrete has a wide grain size distribution (Kosakowski et al., 2020). The fine aggregate size classes (clay-silt-sand) dissolve quickly, due to their high surface to mass ratio, while coarse aggregates have a low surface to mass ratio and therefore dissolve slowly. In addition, the cement stone to aggregate ratio is also significantly higher than in the other two cements. As a result, the pH decrease over time for the container concrete is much slower.

These models give a good indication that low cement levels make materials more susceptible to this process, as for instance seen in the case of mortar M1. If high pH conditions for mortar M1 are deemed necessary, calcareous aggregates can be employed in place of siliceous aggregates, as CaCO3 is in chemical equilibrium with cementitious materials (Kosakowski and Smith, 2014; Wieland et al., 2018; Kosakowski et al., 2020). By this measure, aggregate-cement interaction processes can be excluded, and high pH conditions within mortar M1 can be maintained.

It is important to note that the calculations used for Figure 5 assume that the aggregates are fully immersed in a high pH pore solution and dissolve uniformly from all sides until the cement pore solution reaches equilibrium with Si, which occurs at a pH of ∼10 (continuous red line in Figure 4 and vertical line in Figure 5). This chemical reaction, however, leads to local water consumption and, due to the precipitation of C-S-H, reduces the contact area between cement pore solution and aggregate surfaces. As a result of the reduction in reactive surface area, aggregate dissolution rates are slowed and shifted to the right, as indicated by the shaded area in Figure 5 (i.e., for 100 times slower dissolution rates, the curves are shifted from the left boundary of the shaded area to the right boundary). The effective kinetic rates for dissolution of aggregates minerals are, however, uncertain, in particular under partially water-saturated conditions and with regard to the effective reactive surface area (Ma et al., 2024). This is due to the fact that C-S-H is often precipitated in the interfacial transition zone around the aggregates (Taylor, 1990). When highly desaturated (“dry”) conditions are reached at the aggregate-cement stone interface, the aggregate dissolution rate is close to zero (Kosakowski et al., 2023).

As indicated above, water availability in Opalinus Clay is highly restricted and the mixing water in cement mixes is adjusted so the amount of water is just sufficient for the hydration of the clinker minerals present. As a result of the ongoing chemical processes, such as aggregate-cement interaction (and additional processes such as corrosion of metals), the cement-based material quickly tends to dry out. If this occurs, the reactions come to a halt and steady-state conditions within the waste packages/concrete structure prevail until the repository slowly begins to resaturate again in the long-term.

3.3 Degradation of organics and cement carbonation

Polyvinylchloride (PVC), ion exchange resins (IER), bitumen, plastics, polycarboxylates, cellulose and Plexiglas® (polymethyl-methacrylate) are the most abundant organic materials in L/ILW in term of mass (Nagra, 2023b; Guillemot et al., 2023; Guillemot et al., 2026). Most of the contaminated organic materials are pressed, placed in standardised 200 L drums and embedded in a cement/mortar matrix. Products from the degradation of organics have the following effects on the L/ILW near field:

  • production of methane gas (CH4), contributing to the overall gas volume (Guillemot et al., 2026),
  • production of CO2, leading to the carbonation of cement material and hence to a decrease in the high pH conditions in the near field, and
  • formation and release of complexing ligands, which may contribute to enhanced radionuclide mobility and reduced radionuclide sorption in the near field (Guillemot et al., 2026).

Of relevance for the evolution of the cement-based near-field is the carbonation process. Substantial amounts of CO2 may be released through the degradation of organic materials within the waste packages, leading to the carbonation of the cement stone. The extent of this process depends on the amount and type of organic matter, the amount of cement stone, and water availability in a waste package. The rate at which this process occurs is controlled by the degradation rate of the organic material and water availability required to promote the organic degradation process. In the carbonation process, dissolved CO2 reacts with the Ca from hydrated cement phases such as portlandite, C-S-H, ettringite and other cement minerals to form calcium carbonates and other minerals (Venhuis and Reardon, 2001). In the course of this process, H2O is also released through the decomposition of cement hydrates. During the operational phase and in the early post closure phase, CO2 present in the air also causes cement carbonation at the surface of the concrete structures. However, the CO2 in the air is consumed relatively quickly in a sealed repository and therefore, in the long-term, CO2 released from the degradation of organic compound will be the main source of carbonation (Kosakowski et al., 2023).

The extent of carbonation affects (i) the sorption capacity of the hardened cement paste and (ii) its ability to passivate steel (cf. Section 3.5). The impact of carbonation on these processes is accounted for in Nagra’s safety assessment by means of a mass balance approach, where the extent of degradation of the cement is calculated (Nagra, 2023a) for each waste type in the Model-based Inventory of Radioactive Materials (MIRAM; Guillemot et al., 2026). Within this model, criteria are established that classify the radioactive waste into two waste groups. For these two groups corresponding sorption values are assigned (Nagra, 2023a; Tits and Wieland, 2023). In addition, waste types are also assigned to slow- and fast-corroding conditions (cf. Nagra, 2023a) for the gas calculations, which are conducted as part of the performance assessment (Papafotiou et al., 2026).

3.4 Cement-clay interactions

Interfaces between cement- and clay-based materials can feature strong (geo-)chemical differences, because fresh cement porewater is characterised by high alkalinity, high Ca2+ and OH− concentrations and thus a high pH (>13) (Kosakowski et al., 2023), while Opalinus Clay porewater has a neutral pH of ∼7 and much lower alkali and Ca concentrations (Mäder and Wersin, 2023). The diffusion-dominated exchange of porewater between these materials gives rise to mineral reactions and changes in the porewater pH. To assess the compatibility of these materials, the extent of alteration driven by cement-clay interaction has been studied over a period of decades, for instance in the framework of the cement-clay interaction experiment (CI experiment) at the Mont Terri underground rock laboratory (Jenni et al., 2014; Jenni et al. 2017; Maeder et al., 2017; Bernard et al., 2020) and in natural analogue studies such as at Maqarin in Jordan (Pitty and Alexander, 2011; Martin et al., 2016). The current findings of these studies are that this interaction process leads to dissolution of cement and clay phases and precipitation of secondary mineral phases at or near the material interfaces. The reduction of the free connected pore space leads to a reduction in the mass and solute fluxes across the interface, slowing the geochemical alteration processes. Such mineral reactions at the interface were also investigated using numerical models. According to these models, the clay minerals of the host rock become dissolved and transformed into secondary minerals (e.g., zeolites) up to a distance of a few decimetres from the interface in 100,000 years. Beyond this transition zone and further into the host rock, a zone with elevated pH of 8 – 9 is expected to develop, but without significant mineralogical changes. Under the expected diffusive transport regime, this zone extends a few meters into the host rock (Kosakowski et al., 2023). In summary, as the alteration of Opalinus Clay resulting from interaction with cement is limited in its extent, as found in experimental studies, observations in natural analogues and supported by modelling studies, the beneficial properties of the Opalinus Clay are not impaired (Prasianakis et al., 2022).

Regarding the geochemical evolution, the nearfield alteration is dependent on the influx of water, which is very limited in Opalinus Clay. In addition, the composition of the envisaged rock support is blended with pozzolans, such as, for example, silicafume/microsilica or some other fillers. The addition of pozzolans serves to generate high strength and to decompose portlandite and reduce the chemical gradients between shotcrete and the Opalinus Clay. As such, these materials are already in cement degradation stage III after a relatively short time following emplacement. Furthermore, the partially saturated EDZ and rock support around the L/ILW caverns gradually resaturate and the EDZ fractures re-seal, leading to the very slow saturation of the L/ILW emplacement caverns (Papafotiou et al., 2026). As such, cement-clay interaction is not considered to negatively affect the evolution of the near-field.

3.5 Cement-metal interactions

In L/ILW, many metals are present in the inventory, some in large quantities (Diomidis et al., 2023; Nagra, 2024b; Guillemot et al., 2026). For some waste types, the inventories of metal per mass of hardened cement paste in the tunnel are large enough that corrosion products may have a considerable influence on the volume and composition of the altered cement paste. The precipitation of secondary corrosion products is characterized by a net volume increase, which may affect the integrity of the cement/metal interface, leading to mechanical stress, cracking and spalling off of the concrete above the rebars (Kosakowski et al., 2023). This is, however, a long-term effect and requires that pH in the cement pore solution has decreased to a level where passivation is lost. It should be noted that, for Nagra’s safety assessment, no lifetime for the integrity of the disposal containers and waste packages is assumed (radionuclide release is assumed to take place immediately at the time of repository closure; cf. Li et al., 2026). It is, however, expected that the reinforcement within waste packages, in particular in the bottom row of the cavern, will become corroded and therefore disintegrate first, as water will initially accumulate at the bottom of the cavern (cf. Papafotiou et al., 2026). Nagra is making use of this finding in the current project development phase, where strategic allocation of critical waste packages in the upper part of the disposal area is being planned.

3.6 Impact of local saturation evolution on degradation reaction and gas generation rates

All the above discussed processes except for cement-clay interaction are of relevance for the evolution within waste packages, which may release gas into the near-field. The waste package evolution depends on several interlinked processes that are connected via the availability and consumption/release of water and the porewater pH. The recipe for cement-based materials (e.g., the ratio of cement paste to reactive siliceous compounds) can influences the cement porewater pH evolution over time as demonstrated in various studies (Wieland et al., 2018; Kosakowski et al., 2020; Kosakowski et al., 2023). At pH values below 10.5, it is assumed that steel/iron corrosion will be strongly accelerated (Diomidis et al., 2023). Whether reactions proceed to pH < 10 is dependent on water availability within a waste package and the amounts of degrading organic materials generating CO2, which may carbonate the cement stone.

In various studies, the importance of water availability for the evolution of waste packages has been demonstrated (Wieland et al., 2018; Kosakowski et al., 2023; Kosakowski et al., 2020; Huang et al., 2021). As outlined above, however, water ingress into waste packages within a L/ILW emplacement cavern is limited, as water availability in Opalinus Clay is highly restricted. To transport water accumulated in the bottom of the cavern, across the concrete containers into the container infill mortar and finally into the waste packages, multiple barriers must be overcome (cf. Section 2.2). As such, water availability within a waste package is the process governing the extent of the chemical evolution and gas generation.

The evolution of a single barrel of heterogeneous operational waste with limited water availability has been modelled for a period up to 160 years. Details of the model setup are given in Kosakowski et al. (2020), Huang et al. (2021) and are summarized in Kosakowski et al. (2023). In the model-based assessment, Huang et al. (2021) found that the combined effect of high corrosion rates in the internal waste metal pipes within the drum, where pH is low and the humidity transport limited, caused partial drying out in the drum. Gas production rates were then controlled by the water/humidity fluxes from the outside to the waste compartment inside the steel pipe. As a result, gas production rates within the modelled waste drums were much smaller than those calculated using an unlimited amount of water (Figure 6), as summarised in Kosakowski et al. (2023).

Comparison between gas generation (corrosion of steel and degradation of organic matter) in a simplified waste package comprising of mixed waste (fast and slow degrading organic materials, various types of metal with mostly steel, embedded in a cement-matrix) calculated with a mixing tank model (dashed lines) and a multi-component two-phase reactive transport model (solid lines). The stepwise behaviour in the transport-model curves results from partial drying of the waste drum caused by corrosion and organic degradation. This drying limits gas generation, leading to lower production rates than those assumed under unlimited water availability. Adapted from Huang et al. (2021).
Figure 6. Comparison between gas generation (corrosion of steel and degradation of organic matter) in a simplified waste package comprising of mixed waste (fast and slow degrading organic materials, various types of metal with mostly steel, embedded in a cement-matrix) calculated with a mixing tank model (dashed lines) and a multi-component two-phase reactive transport model (solid lines). The stepwise behaviour in the transport-model curves results from partial drying of the waste drum caused by corrosion and organic degradation. This drying limits gas generation, leading to lower production rates than those assumed under unlimited water availability. Adapted from Huang et al. (2021).

This finding has important implications for the evolution of waste packages in general: it has shown that the water initially present within the waste packages is consumed in a relatively short time. This process leads to a desaturation within the waste package, to a decrease of the reactivity and to a final halting of the chemical reactions. As such, it can be expected that a substantial amount of the L/ILW emplaced in the repository will relatively quickly run out of internal water and therefore, gas generation will come to a halt until new water again becomes available for chemical reactions. Overall, the availability of water is expected to determine the reactivity of waste over time (Kosakowski et al., 2023). Therefore, it can be assumed that high pH conditions in the disposal containers in the upper rows may be maintained for several hundreds of thousands of years.

4 Conclusion

In this manuscript, the design of the L/ILW near field and its evolution during the operational and post-closure phases have been described. The geochemical conditions in the L/ILW near field are governed by the large quantities of cementitious materials present in the rock support, backfill mortar, concrete containers, and waste packages. The most important safety contribution of these cementitious materials is the maintenance of a high-pH porewater environment as long as possible, which promotes steel passivation and slow corrosion. In addition, high pH limits the degradation of organic materials and suppresses microbial activity, collectively reducing gas-generation rates. The presence of cement minerals - particularly C-S-H - also favours the sorption of many radionuclides.

Cementitious materials are thermodynamically metastable in the presence of CO2, reactive silica and free water and degrade through various processes over time. Their evolution is therefore described in terms of distinct degradation stages. The key processes that may alter the chemical conditions in the cement-based near field include: (i) the pozzolanic reaction and aggregate–cement interactions; (ii) carbonation by CO2 released during the degradation of organic materials; and (iii) cement–clay interactions. These processes are strongly coupled to the overall saturation evolution of the repository, as water is a prerequisite for the chemical reactions that drive degradation. However, due to the low permeability of Opalinus Clay, water inflow into the L/ILW caverns is extremely limited, and water availability remains restricted. This results in only partially saturated conditions that are expected to persist for several hundreds of thousands of years. Such conditions help maintain elevated porewater pH levels, particularly in the unsaturated upper parts of the emplacement caverns.

Water availability within individual waste packages is of central importance, as local water consumption leads to decreasing reaction and gas-generation rates. Once the available water is consumed, these reactions come to a halt until more water becomes available. The chemical conditions within the cement, combined with the limited water availability in the L/ILW near field, contribute to maintaining elevated pH conditions over very long timescales.

A refined analysis of the processes and conditions in the L/ILW near field confirms that the cementitious near field acts as an effective barrier to radionuclide transport over extended periods. Furthermore, the system understanding gained through numerous studies over recent decades provides strong evidence for the robustness of the L/ILW near-field evolution and its ability to fulfil its safety functions.

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Footnotes

Edited by: Paul Smith, Safety Assessment Management GmbH, Switzerland

Reviewed by: Axel Liebscher, Federal Company for Radioactive Waste Disposal, Germany

Fiona Neall, Nuclear Waste Services, United Kingdom

Statements

Conflict of interest

Authors LM, TG, and AP were employed by Nagra. GK declares his contribution to this work was partially funded by Nagra.

Funding

The author(s) declared that financial support was received for this work and/or its publication. Funding of this project was provided by Nagra.

Author contributions

LM: Conceptualization, Project administration, Resources, Supervision, Validation, Visualization, Writing – original draft, Writing – review and editing. GK: Conceptualization, Data curation, Investigation, Methodology, Project administration, Writing – original draft, Writing – review and editing. TG: Conceptualization, Investigation, Project administration, Validation, Writing – original draft, Writing – review and editing. AP: Conceptualization, Data curation, Formal Analysis, Investigation, Writing – original draft.

Acknowledgements

The authors gratefully acknowledge the contributions of T. Steils, P. Marschall, A. Brown (Nagra), Y. Huang, S.V. Churakov, M. Glaus, W. Hummel, D. Kulik, B. Ma, D. Miron, N. Prasianakis (PSI, Switzerland), Paul Smith (SAM, Switzerland); E. Wieland (Chen Van Loon Consulting, Switzerland).

The content of this manuscript has been presented in part in Diomidis et al. (2023), Guillemot et al. (2023), Huang et al. (2021), Kosakowski et al. (2023), Kosakowski et al. (2020), Martin et al. (2023), Nagra (2023a), Nagra (2024c), Nagra (2021).

The authors thank A. Liebscher and F. Neall for the very helpful comments, which helped to improve the manuscript.

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Citation

Martin, L., et al. (2026). The safety case for a deep geological repository in Switzerland: engineered barriers for low- and intermediate-level waste disposal. Frontiers in Nuclear Engineering, Volume 5. https://doi.org/
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Copyright © 2026 Martin, Kosakowski, Guillemot and Papafotiou.

License: https://creativecommons.org/licenses/by/4.0/

*Correspondence: Lukas Martin

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