[en] Following several decades of committed implementation of disposal strategies in Finland and Sweden, as well as cooperation in the development of a safe disposal solution based on a Swedish design, the first ever deep geological repository for spent fuel is being constructed in Olkiluoto, Finland. Sweden, along with other countries, is also working towards building such a facility. After spent fuel is removed from nuclear power reactors, it continues to generate significant heat for several decades. It is therefore placed in water pools or in dry storage facilities to cool down. Storage pools and containers ensure that spent fuel maintains its integrity and no radiation or radioactive materials are released, thereby protecting people and the environment from exposure. However, spent fuel remains highly radioactive for several thousands of years and needs to be isolated for several hundred thousand years. One way to dispose of spent fuel — when declared as waste — once the heat has decayed is to bury it in engineered facilities several hundred metres below ground level, in deep geological disposal facilities. The objective is to contain its radioactivity by encapsulating the spent fuel in robust and leak-tight containers and isolating it by burying it. Such facilities consist of a system of tunnels or chambers, built at a site geologically suitable for ensuring the longterm safety of the buried material (see the Science box). The facility being built in Finland is based on the ‘KBS-3’ disposal concept, which was developed by the Swedish Nuclear Fuel and Waste Management Company (SKB), in close cooperation with Posiva, the Finnish company responsible for the disposal of spent nuclear fuel. The KBS-3 method consists of encapsulating spent fuel in corrosion-resistant copper canisters and embedding the canisters in swelling clay inside the repository’s tunnels up to 500 metres below ground level.