Sweden’s efforts to further develop underground storage vaults at Forsmark, to hold more radioactive waste and also create a vast vault for spent nuclear fuel, respectively, has enjoyed significant progress over the last 10 years or more.
The granitic rock mass under the coastline, north of Stockholm, offers extensive zones to hold quite different types of waste for the long-term, from radioactive materials now to, soon, spent nuclear fuel at deeper depths.
Excavations have commenced to extend Forsmark’s existing vault for short-lived radioactive waste. In addition, work began recently to prepare for the initial tunnelling to start creating a deep geological repository for spent fuel.
The deep geological repository will take high-level waste in sealed canisters and bury each vessel in its own silo, far below the ground surface. The repository is planned to be a vast warren of tunnels, carefully and progressively opened up in the chosen bedrock. The tunnels linking to the silo storage locations will also be sealed off, eventually, as will the entire vault, by its completion, due later this century.
Opening up the rock mass to carve out the deep repository will be done slowly, in stages, taking time. Decades. But the pattern is established. Along each ‘deposition’ storage tunnel will be small silos carved into the rock. They will be numerous but spaced, at regular intervals, each to receive a single canister. The process then has the backfilled, and sealed. When all the silo slots along a deposition tunnel are full then that tunnel itself will have been filled, finally, and then plugged and sealed off.
Eventually, all of what has been excavated and opened up to create the storage slots will be sealed off. The deep geological repository has a perspective of millennia for safe storage of spent fuel, which makes the coming decades of excavations and storage activities seem relatively short. However, in terms of infrastructure construction at a normal human lifetime scale, the development of the high-level waste storage vault is a long project – to be carefully executed, by generations of engineers.
For Forsmark, the journey of construction is only starting in the life of its deep geological repository. But the site in Östhammar municipality – which already hosts a nuclear power plant – is not new to underground storage of waste. At shallower depths, Forsmark hosted a vault for short-lived waste. Operating since the late 1980s, the vault is the final repository for radioactive waste from power plants in Sweden as well as from research, industry and medical uses.
The Swedish Nuclear Fuel and Waste Management Co (SKB) is now extending this waste vault to triple its capacity. Rock blasting for the extensive excavations began in late 2024. The full construction project is expected to take about six years to complete. Its excavations will be underway as those for the neighbouring deeper vault, for spent fuel, get underway in earnest.
A third storage system class is needed by SKB, for long-lived radioactive waste. It is still in planning and the site is yet to be selected. It would store metallic materials, such as reactor core components, control rods from water reactors, and reactor pressure vessels, as well as long-lived waste from medicine, research and industry. It would be the last and smallest of the three vaults, says SKB.
The expanding short-lived waste vault and coming deep vault for spent fuel are what SKB calls its final repository system at Forsmark. Whether the vault needed for long-lived waste is also located at Forsmark has yet to be determined, for there is another possible location – one that has competed to be a host site before.
Deep Geological Repository – The Journey Begins
Forsmark was not, from the outset, the predetermined site to provide deep burial of high-level waste. There was a key competitior and it looked to be winning, even though Forsmark already a power plant and an underground storage vault for short-lived radioactive waste. Forsmark was competing against the bedrock characteristics of another keen candidate, the Laxemar site, in Oskarshamn municipality, in the south of Sweden.
Oskarshamn already hosts an interim storage facility for spent fuel (Clab). The storage is provided by waterfilled concrete basins with steel lining, in vaults 30m down in the bedrock. It has been operational since the mid-1980s. SKB is seeking its expansion as existing capacity would be reached before the Forsmark deep repository is operational, tunnellers were told by SKB at the most recent World Tunnel Congress (WTC 2025), held in Stockholm, in mid-2025.
Excellent bedrock is the key to securely and safely storing nuclear spent fuel at the deepest levels possible underground. Strong and highly competent rock mass is needed, along with favourable groundwater characteristics. Together, the rock mass and extremely limited groundwater movements will act as an effective barrier, shielding the ground surface environment from the stored high-level waste. Effective distance is the key.
Unfortunately for Forsmark, its competitor looked able to hold the required high-level waste storage vault at slightly deeper depths. That is, until the possibilities for Forsmark received a boost from researchers finding that new measurements and analysis rock stresses showed that “these stresses have less impact on the repository depth” than had been assumed, according to SKB’s report ‘Site Investigation: Forsmark (2002-2007)’. The data came from almost six years of site investigations, including with trial drillings. Therefore, for a given stress the vault could be located deeper than first thought – and at a depth more comparable to that offered by Oskarshamn, at around 470m below the surface.
With extra distance from the surface, the shielding offered at Forsmark would be thickened, effectively.
Coupled with favourable groundwater characteristics, and less need for excavation it was argued to open up the storage tunnels, it was added, the balance tilted towards Forsmark. Aspects related to post-closure safety were decisive in choosing Forsmark, WTC 2025 heard.
After many years of studies and field research at sites across the country, starting in the late 1970s, SKB made its choice in 2009, in favour of Forsmark to host the deep geological repository. Hundreds of scientific reports had been produced by then, 25 cored boreholes drilled (19 to more than 500m depth; nine more to more than 1km depth, says SKB in its project briefing), and soil boreholes were sunk, too. There were also percussion drilled boreholes, and geo-scientific and ecological surface mapping.
In 2011, SKB filed an application to build and operate the high-level storage facility at Forsmark.
Äspö Hard Rock Laboratory
Whichever of the competing host sites for spent fuel would come to be chosen, the Äspö Hard Rock Laboratory was developed underground to provide important research data to the entire selection process. It just happened to be located in Oskarshamn municipality, slightly north of the city of Oskarshamn. Äspö is where much of Sweden’s research to develop a final repository for spent fuel has taken place. The research has involved international collaboration.
Survey work at Äspö began in the mid-1980s. The tunnels were excavated over 1990-95 and are nearly 500m below the surface. The research, development and demonstration programme envisaged for the network of tunnels was planned to last until 2010.
Part of the early works involved cross-checking and verification of geological data from the extensive excavations with that predicted from the site investigations from the surface, and sampling with boreholes. These methods were then employed in site investigations carried out for the competing host sites, nearby in Oskarshamn area and, much farther north, at Forsmark.
Underground, the Äspö complex has been used to test different technological solutions at full scale in a realistic setting, says SKB. These studies included examining the interaction of the sealed canisters and bentonite clay that would pack around each vessel in its silo, and also perform experiments on the use of the rock as a barrier.
Equipment systems for deposition of canisters were also tested.
The rock laboratory was extended over 2011-12, adding another 350m of tunnels. SKB says that in recent years the activities at the Äspö laboratory have focused more on adapting technology and methods to the industrial process to be used in operating the real repository, to be built at Forsmark.
Tunnelling
The government granted approval for the deep repository to be located at Forsmark in early 2022, SKB told WTC 2025. A year ago, SKB was given the greenlight with an environment permit to start construction. By mid-2025, it had signed a collaboration agreement with contractor Implenia for the major initial tunnelling works required, to begin major excavations of what will be, eventually, a huge underground network.
Under the agreement with SKB, services being provided by Implenia include planning, design and construction of the tunnels and shafts needed to reach down to the repository level, the a ‘central’ base area at that depth, and also the first main excavations to launch the underground storage grid of tunnels.
More particularly, Implenia is to perform detailed design and then excavations for the 5km-long spiralling access tunnel to reach some 470m below the surface (it is expected, WTC 2025 was told); the first of the local grid of large tunnels for various operational activities at the base of the shafts and access tunnel; and, also the initial transport tunnels that will open up the first repository storage zones. Branching off at regular intervals from the transport tunnels will be arrays of long ‘deposition’ tunnels. Along each of these slightly narrower tunnels will be storage silos, carved into the rock and each one awaiting a single sealed canister that will then be backfilled and buried in its solitary void.
Implenia is also to design and build three deep shafts down to the repository level. The shafts are for ventilation and a lift, also to serve and support the activities of the underground complex over the coming decades of its operational life.
In Q3, the contractor began mobilising on site. Upon final safety approvals, from the national radiation safety authority, and after further preparatory works on the surface, the first steps in main excavations are to begin.
Excavation of the shaft, access tunnel and the initial repository-level tunnels in the package are anticipated to take about eight years to complete.
The full scale of the underground repository, though, is far more extensive. The network of tunnels and storage zones will only be extended in stages, matched to requirements. Some areas will have been filled with canisters and themselves sealed off, and new operational areas opened up.
Current expectations are than the repository will have been fully extended in some 60 years’ time, by the 2080s. By then, it is anticipated that 66km of tunnels will have been excavated, with in a grid network covering an area of 4km2, SKB told WTC 2025. Expectations are that 2.7 million m3 of rock will have been excavated, slowly and carefully, to form the tunnels at the repository level, and also the access tunnel and shafts from the surface. The scale of excavation is greater than that discussed before (1.85 million m3) when Forsmark was picked over Oskarshamn as favoured candidate to be the host site.
Disposal
SKB says that when the repository has finished excavating its first access, base, transport and storage tunnels then disposal of spent fuel can begin. The present timeline anticipates the disposal activities in the deep geological vault should be able to commence in the 2030s.
The plan is for Forsmark to host approximately 12,000 tonnes of spent fuel. This quantity of fuel is to be held in 6000 canisters, specially designed and sealed, and each to be individually placed in its own tightly carved slot. Each copper canister will be packed into the space with a surrounding of bentonite clay, the silo then plugged, and sealed off. This is the ‘KBS-3’ storage concept.
The burial slots are to be spaced out at regular intervals along the deposition storage tunnels, which will terminate in dead-ends. A few of these branch tunnels are to be in use at any one time, the storage activities working between them, as each of these will also be sealed up to where the most recent silo has been buried.
Operational storage activities, therefore, will gradually back out of each deposition tunnel, eventually to be filled completely, back to the junction with the main transport tunnel. The pattern of working across a few branch tunnels and gradually filling each will be ongoing.
Eventually, and ultimately, with all the branch deposition tunnels leading off a particular transport tunnel having been filled, then that main tunnel itself be filled.
Meanwhile other sections and arrays of branch tunnels will have been opened up in the granitic rock for what is planned to be a vast repository. The process will continue, systematically, over decades.
Other Excavations at Forsmark
Excavations started at Forsmark recently for the extension of the shallower-depth vault for short-lived radioactive waste.
The existing vault is about 60m and deeper below the ground surface and has a storage capacity of about 63,000m3. Rock basting for the new section began almost a year ago and is to add twice as much storage capacity as the original, at depths of about 120m-140m below the surface. The tunnelling works involve creation of 6 x 275m-long caverns, tunnellers were told mid-year by SKB at the WTC 2025 conference. The new storage section will take the total capacity of this vault to about 180,000m3.
The expansion project received approval from environmental authorities in late 2022. Tunnelling work is being undertaken by contractor Skanska, which was appointed in mid-2023. With SKB obtaining approval for the works from the radiation safety authority, in November 2024, first blasting began soon after. It is anticipated the construction work and fit-out will be completed around 2030.
In addition to holding radioactive waste from the healthcare and industry sectors, and research, the expanded vault is also to take waste from the decommissioning and dismantling of Swedish nuclear power plants. The power plant waste is to include reactor components and building materials.
Recently, the maintenance teams on the existing vault have borrowed lessons and experience from the neighbouring power plant teams to use the latest drone technology for remote and effective surveys, to monitor key areas of infrastructure assets. SKB says this helps with the accessibility, speed, frequency and cost of inspections of the existing underground storage facility.
In future, a vault for long-lived radioactive waste is also required and SKB has not yet selected the site.
The vault is foreseen as requiring a storage capacity of approximately 16,000m3 and be located at a depth of about 500m.
