Switzerland’s approach stands out internationally. While most European countries plan to bury only their intermediate-level waste and keep low-level waste in surface or near-surface facilities — trenches, concrete vaults, or aboveground mounds — Swiss federal law demands that all radioactive waste go into a deep geological repository. The reasoning is rooted in a principle of intergenerational fairness: the protection of humans and the environment must be guaranteed in the long term without imposing unreasonable burdens on future generations. Once the repository is closed, no monitoring, maintenance, or repair may be required. That is a tall order when some of the waste contains long-lived radionuclides such as chlorine-36, which is why Swiss regulators require a safety demonstration stretching to one million years.

The provisional design reads like a blueprint for passive self-destruction of the barriers’ weaknesses — in the best possible sense. Waste from nuclear power plant operations and dismantling, along with material from research, medicine, and industry, is first conditioned in cementitious matrices inside 100 to 200 litre drums. These drums are packed into concrete containers, which are then stacked in dead-end emplacement caverns up to 250 metres long and roughly 11 metres in diameter. Every void between containers and cavern wall is backfilled with a deliberately porous mortar known as M1, made from uniformly sized aggregate bound by a strictly limited quantity of cement. The trick is that the cement paste is too sparse to fill the gaps between grains, leaving a coarse, gas-permeable pore network that can store and release gas while the grain-supported skeleton keeps the tunnels from converging once the rock support eventually degrades.

Sealing the system is an engineering feat in itself. Each cavern is closed with a gas-permeable but water-limiting seal built from an 80:20 sand-bentonite mixture, placed in direct contact with the Opalinus Clay after partial removal of the rock support. The sand grains form a rigid support matrix that carries external stresses, while the saturated bentonite swells to fill the pore space within that skeleton, creating a low gas entry pressure so gas can escape into the operational tunnels without building dangerous pressures. A transition layer of calcareous aggregates — chosen because they sit in chemical equilibrium with both cement and clay — prevents pore clogging at the interfaces, and a low-pH concrete abutment mechanically stabilizes the whole sealing section while guarding against accidental flooding and human intrusion.

But the real magic of the Swiss concept lies in its chemistry. Cementitious materials dominate the near field, and their defining property is the ability to maintain a high-pH porewater environment. Freshly hydrated cement produces porewater with a pH above 13, buffered by dissolved alkali hydroxides. As the cement ages through what the researchers describe as degradation stages, the mineral portlandite takes over as a chemical buffer, holding the pH above 12.5. Only when portlandite is fully exhausted does the pH drift down toward about 10, governed by the incongruent dissolution of calcium-silicate-hydrate phases, the C-S-H minerals that give cement its fabric and serve as the key phase for radionuclide sorption in Nagra’s safety assessment. This alkaline environment is a chemical fortress: it promotes the passivation of metal surfaces, keeping steel corrosion rates slow and gas generation correspondingly low, while simultaneously limiting the degradation of organic materials and suppressing microbial activity.

Here the story takes an unexpected turn — into the physics of water scarcity. Every chemical reaction that degrades the repository, from metal corrosion to organic breakdown to pozzolanic reactions, requires water. And Opalinus Clay is extraordinarily stingy with it. Because of the host rock’s low permeability, the emplacement caverns are expected to remain only partially saturated for several hundreds of thousands of years. The saturation history of the repository becomes the master variable controlling its chemical evolution. Water that does accumulate tends to pool at the bottom of the caverns, driven by gravity and the low capillary strength of the coarse backfill mortar, whose dominant pore sizes range from sub-millimetre to a few millimetres. The container concrete, by contrast, has much higher capillarity and will saturate first, particularly in the bottom row of containers, where reinforcement corrosion will eventually cause spalling and disintegration.

The capillary contrast between materials creates a subtle equilibrium that works in the repository’s favour. When two partially saturated porous materials with different water retention curves are in contact, suction equalizes at their interface, and the material with higher capillary strength draws in more water. This means very limited water transfer occurs from the saturated container walls toward the drier container infill mortar, at least for a long time. Even more striking, once the gas pressure generated inside the cavern reaches hydrostatic conditions, further inflow of water from the Opalinus Clay is physically prevented — a self-regulating mechanism that helps maintain partially saturated conditions for several hundred thousand years. In the unsaturated upper regions of the caverns, elevated pH levels persist longest, and Nagra is already exploiting this insight by strategically allocating critical waste packages to the upper part of the disposal area.

Modelling of a single barrel of heterogeneous operational waste with limited water availability, tracked over 160 years, delivered a finding with profound implications. The combined effect of high corrosion rates in the internal waste metal pipes, where pH is low and humidity transport is restricted, caused partial drying out inside the drum. Gas production rates were then controlled by the slow water and humidity fluxes from the outside toward the waste compartment, and they came out far smaller than rates calculated assuming unlimited water. The water initially present within waste packages is consumed relatively quickly, leading to desaturation, a drop in reactivity, and a final halting of chemical reactions until new water becomes available. In other words, the repository has a built-in chemical thermostat: it runs hot with reactivity only when water is present, and switches itself off when that water is exhausted.

The review also confronts the enemies within — the organic materials that make up a substantial mass of L/ILW. Polyvinylchloride, ion exchange resins, bitumen, plastics, polycarboxylates, cellulose, and Plexiglas are pressed into standardized 200 litre drums and embedded in cement matrices. Their degradation produces methane, contributing to the overall gas volume; carbon dioxide, which carbonates the cement and erodes the high-pH conditions; and complexing ligands that could enhance radionuclide mobility. Carbonation is the central concern, because dissolved carbon dioxide reacts with calcium from hydrated cement phases to form calcium carbonates, releasing water in the process and reducing both the sorption capacity of the hardened cement paste and its ability to passivate steel. Nagra’s safety assessment accounts for this through a mass balance approach that classifies waste into groups with corresponding sorption values and assigns waste types to slow- or fast-corroding conditions for gas calculations.

At the interfaces where cement meets clay, decades of experimental work at the Mont Terri underground rock laboratory and natural analogue studies at Maqarin in Jordan have shown that the interaction is remarkably tame. Fresh cement porewater, with its pH above 13 and high calcium and hydroxide concentrations, meets the neutral pH 7 porewater of Opalinus Clay, driving diffusion-dominated mineral reactions. Yet the alteration remains limited: clay minerals dissolve and transform into secondary minerals such as zeolites only within a few decimetres of the interface over 100,000 years, and beyond that a zone of elevated pH 8 to 9 develops without significant mineralogical change, extending just a few metres into the host rock. The reduction of free connected pore space at the interface actually slows the geochemical alteration processes, and the beneficial properties of the Opalinus Clay are not impaired. Taken together, the review concludes that the cementitious near field acts as an effective barrier to radionuclide transport over extended periods, delivering immobilization, retention, and controlled release of radionuclides — and that the system understanding gained through numerous studies over recent decades provides strong evidence for the robustness of its evolution and its ability to fulfil its safety functions for a million years to come.

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

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

Article References: Martin, L., Kosakowski, G., Guillemot, T., & Papafotiou, A. (2026). The safety case for a deep geological repository in Switzerland: engineered barriers for low- and intermediate-level waste disposal. Frontiers in Nuclear Engineering, 5, Article 1843726. https://doi.org/10.3389/fnuen.2026.1843726

Image Credits: AI Generated

DOI: 10.3389/fnuen.2026.1843726

Keywords: safety, case, deep, geological, repository, Switzerland, engineered, barriers, low-, intermediate-level, waste, disposal