Salt domes, also called salt diapirs, have been used for several decades for the storage of sensitive materials such as hydrocarbons and radioactive waste. These geological structures are particularly suitable for these uses due to the specific properties of salt. This article explores the characteristics that make salt domes ideal for the storage of sensitive substances, as well as the advantages and challenges of their use in this context.

Halite (more commonly known as "salt") has physical properties that make it an effective natural barrier for containing hydrocarbons and radioactive waste. Indeed, it is virtually impermeable to fluids and gases, preventing the migration of stored substances to the external environment. In addition, salt domes deform in a ductile manner, they do not fracture under pressure, cracks and fractures fill and "heal" naturally. This ensures the safety of long-term storage. It is also a chemically stable material and reacts little with other substances, which reduces the risk of unwanted reactions with stored materials.

Right photo : Halite crystal with fluid inclusions and natural moving bubble from Bochnia Mine, Lesser Poland Voivodeship, Poland

Storage in salt domes relies on the creation of underground cavities. In the case of hydrocarbon or hydrogen storage, the cavities are formed by dissolving the salt with water, a technique called "leaching". Once the cavity is created, hydrocarbons, such as natural gas or oil, can be injected and stored safely. These cavities are used as strategic reservoirs. Radioactive waste, including long-lived and high-level waste, is enclosed in specialized containers before being placed in deep galleries dug into the salt domes. The plasticity of salt ensures that these wastes remain isolated from the environment for extremely long periods of time, which is essential for their long-term management.

Diagram of the creation of a saline cavity during the different phases © Geostock

Storage of hydrocarbons in salt domes, particularly natural gas but also more recently hydrogen, is a common practice in many regions of the world, notably in Europe and the United States. Salt cavities are used as strategic or regulation reservoirs to meet fluctuations in energy demand. Indeed, the cavities created in salt domes offer large volumes for hydrocarbon storage and thanks to the impermeability of salt, the risk of leakage is considerably reduced. On the other hand, the gases stored in the cavities can be easily injected and recovered as needed, making them an ideal solution for seasonal storage or in response to peak demand. The Gulf of Mexico Basin (United States) is a good example of a region where many salt domes are used to store natural gas and refined petroleum products. The salt caverns of the Lower Saxony region (Germany) also contain large reserves of strategic natural gas for Europe.

Storage of radioactive waste in salt domes is considered one of the safest long-term solutions for the management of nuclear waste, especially high-level and long-lived waste. The plasticity of salt allows the waste containers to be sealed. They also have low water permeability, limiting any contact with the external environment for hundreds of thousands of years. The stable structure of salt domes also ensures that the cavities dug for storage do not collapse or fracture easily, even in the event of tectonic movements. The WIPP (Waste Isolation Pilot Plant) radioactive waste repository in New Mexico (USA) is a good example: it is used to store military radioactive waste in underground cavities dug into a salt diapir. Similarly, in Germany, the Asse II Project is a former salt mine used for the storage of low and intermediate level nuclear waste.