Our nuclear power operations

Operations at a nuclear power plant are characterized by the high level of safety awareness among employees. There is a distinct safety culture at Uniper, whether in daily electricity generation or managing reactors that are no longer operational. Even when dismantling and demolishing nuclear power plants, we do so responsibly and safely.

Sweden has three active nuclear power plants, where Uniper has a unique position, since we are joint owners in all of them. We are the majority owner of OKG in Oskarshamn and a minority owner of Ringhals and Forsmark. We also own Barsebäcksverket, the first commercial nuclear power plant in Sweden, which is now decomissioned and is being demolished in line with the Swedish system.

Reactor facts

Reactor unit

Commercial date


Net capacity (MW)

Delivery 2018 (TWh)

Reactor type






Boiling water






Boiling water






Boiling water






Boiling water






Boiling water






Boiling water






Pressurized water






Pressurized water






Pressurized water






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Boiling water






Boiling water

The OKG reactors

Oskarshamn 3 is a boiling water reactor that was connected to the electricity grid on March 3, 1985, with an installed capacity of 1,050 megawatts (MW).

In 2009, a comprehensive modernization project was carried out that included a safety upgrade and increasing the capacity to 1,450 MW, making O3 one of the world’s largest boiling water reactors. The average annual electricity generation is enough to power half a million houses.

With the installation of independent core cooling completed in 2020, the reactor is now equipped for long-term operation until at least 2045. It’s also possible to further extend the lifetime of the facility to the 2060s, if it proves necessary for the Swedish electricity market.

Oskarshamn 1 and 2 are no longer operational. The extraordinary general meeting on October 14, 2015 resolved to close O1 and O2 ahead of schedule, given the low prices in the electricity market and the high level of taxation.

O1 went into operation in 1971, and is Sweden’s first commercial nuclear power plant with a light water reactor. The reactor was decommissioned on June 17, 2017 and by that point had delivered approximately 110,000,000 MWh of climate-friendly electricity.

O2 had already been taken out of operation in 2013 for a comprehensive modernization project that was intended to finish in 2015, but the reactor was never restarted.

Now both O1 and O2 are being dismantled and demolished along with the reactors at Barsebäck. Demolishing the four reactors in a joint portfolio and according to a joint strategy helps coordinate specialist expertise, safety, logistics and finances to our advantage.


Barsebäck’s nuclear power plant is in the municipality of Kävlinge in western Skåne, north of Malmö. The nuclear power plant has two reactors that were closed in 1999 and 2005, respectively, after a decision by the government at the time. Both reactors are undergoing a comprehensive decommissioning project, and the ambition is for the buildings to be declassified so that conventional dismantling can begin in the 2030s.


The Ringhals nuclear power plant is located on the Värö peninsula in Varberg municipality, in Halland. The powerplant is 70.4% owned by Vattenfall and 29.6% by Sydkraft/Uniper. The facility has four reactors, the two oldest of which were shut down in 2019 and 2020. The two newest reactors will remain operational for long-term use until the 2040s.


Forsmark’s nuclear power plant is in Östhammar municipality in Uppland. The power plant is 66% owned by Vattenfall, 24.1% by Mellansvenska Kraftgruppen and 9.9% by Sydkraft/Uniper. The power plant has three reactors, all of which will remain operational for long-term use until the 2040s.

How a nuclear power plant works

Nuclear power is a way of harnessing the energy available everywhere – in atoms, or rather, in their nuclei. Hence the name “nuclear power.”

It was Albert Einstein who first theorized it would be possible to use this power. He realized this thanks to his theory of relativity, E = mc2. This possibility then became a reality with the splitting of uranium-235. The energy released by splitting uranium-235 is used to boil water. This generates steam, which drives a turbine. In turn, the turbine drives a generator that produces some of the electricity you use in your home. Nuclear power supplies around 40% of Sweden’s energy needs.

Maintenance outages at a nuclear power plant

A nuclear power plant closes for maintenance every year. During this time, the reactors are refueled and maintenance is conducted at the facility. After a maintenance outage, which takes around one or two months, the reactor is ready for its next operating period.

Between 100 and 600 people are usually involved in a maintenance outage. Larger maintenance outages can require as many as 3,000 people for the task.

As a rule, maintenance outages are carried out during the summer, when the need for electricity is at a minimum, since the reactors need to be shut down for this purpose. However, since Sweden has gone from twelve nuclear reactors to six, the capacity in southern Sweden has become more strained. The lack of dispatchable electricity generation and subsequent lack in capacity and support services make themselves known, even during the summer. There is no longer an ideal time of the year to carry out annual maintenance outages without affecting availability in the electricity system.

Nuclear power around the world

Today there are around 440 nuclear power reactors in 32 countries plus Taiwan, with a total capacity of approximately 400 GWe. In 2019, these reactors delivered a total of 2,657 TWh, or over 10% of the world’s electricity. Around 50 reactors are currently under construction in 16 countries, primarily in China, India, Russia and the United Arab Emirates.

Kärnkraften globalt

Expanding nuclear power around the world

Around 100 reactors with a total gross capacity of approximately 110 GWe are in the planning stage or already ordered, and over 300 have been proposed and could be approved in the future. Most of the reactors currently being planned are in Asia, with quickly growing economies and a rapidly increasing need for electricity.

Even Japan has started to reopen its nuclear power plants. The country currently imports 90% of its energy after shutting down the nuclear power network following the Fukushima incident. Since 2015, nine reactors have been restarted and an additional 16 are in the process of applying for authorization to start again.

In Europe, ten new reactors near the Baltic Sea are under construction or in planning. Another 15 new reactors are underway in eastern Europe and Turkey. Moreover, France and the UK have also announced a dozen new reactors.

There is also comprehensive research and development going on within the global nuclear power industry into new forms of nuclear power that will rely on small modular reactors (SMRs) and Gen4, the next generation of nuclear power with lead-cooled reactors and where the nuclear fuel can be reused for example. Some 70 designs have already been developed around the world. Last year, two of the first SMR units were put into operation in Russia.

The idea with SMRs is to create prototypes for series production at a lower price than larger, conventional reactors but with the same performance in terms of quality and safety. In Sweden, Uniper intends to work together with Blykalla and KTH to build and commercialize a new SMR reactor. The first test prototype will be powered by electricity and is expected to enter operation at OKG’s nuclear power facility in 2024.

According to IPCC, the UN’s climate panel, to keep global warming under the target threshold of two degrees Celsius, 80% of the world’s electricity generation needs to be fossil-free before 2050. Analyses from the International Energy Agency (IEA) indicate that the global need for electricity will increase between 80 and 130% by 2050. More and more international organizations have come to the conclusion that fossil-free nuclear power is essential for meeting climate goals. According to estimates from the World Nuclear Association (WNA), approximately 1,000 GW of nuclear power will be needed by 2050 – more than twice the capacity that nuclear power has today.

What nuclear power means for Sweden

Electricity is more complicated than you might think. Because large amounts of electricity can’t just be stored, electricity needs to be produced at the precise moment we use it. It also needs to have the right voltage frequency to avoid disruptions in the electricity system.

A brief production stoppage at a factory can lead to losses of several million Swedish kronor, and possibly even someone’s job. If the hospital’s backup system isn’t working, a sudden power cut affecting an operating theater at a hospital can put someone’s life in danger. That’s why it’s important for us to maintain a high-quality electricity system. We do this through baseload power.

There are currently six operational reactors remaining, and in 2021 they produced over 51TWh. This is approximately 31% of all the electricity produced in Sweden in 2021. If we convert this figure to the electricity used in Swedish households during the same period, we can see the following public benefits of nuclear power:

Kärnkraftens betydelse i Sverige


The electrification of Sweden

Access to electricity is a contributing factor to Sweden’s development as one of the world’s leading welfare states. Large-scale electrification began in the first half of the 1900s. Access to Swedish rivers helped expand hydropower on a large scale, making it possible to meet society’s increasing need of electricity due to developing industry and society’s expanding infrastructure. Electric machines and devices quickly became the standard in every home. The second half of the 1900s required further electricity generation, and the first nuclear power plants in Sweden were built in the 1970s.

Today, Sweden has one of the world’s premiere electricity systems in terms of delivery capacity, power quality and climate impact. Fully 98% of all the electricity produced in Sweden is free from fossil emissions. As digitalization and automation spread in society, it becomes even more essential for the electricity supply to be free from power cuts. We simply expect there to be electricity when we need it. Hydropower and nuclear power can provide this, which is why these power sources are also called baseload power. Access to baseload power is one of the Swedish industry’s most important competitive advantages.

The history of nuclear power

Research into splitting the atom made huge gains in the 1930s. Due to World War II, initial research and development was directed primarily towards military uses. It wasn’t until after the war that research into civilian applications started in earnest. The world’s first commercial nuclear power plant was built in England in 1954, and the first in Sweden in 1972.

OKG Historic image

But research started in Sweden as early as the 1950s. Sweden’s first nuclear power plant was Ågestaverket, a pressurized heavy water reactor that also functioned as a combined heat and power (CHP) plant. The plant went into operation in 1963 and had a capacity of 80 MW when it was closed in 1974. By that point it had delivered a total of 800,000 MWh of heating to Farsta and 415,000 MWh of electricity. The reactor was demolished in the early 1980s. The previous reactor hall still remains, but after losing its nuclear facility categorization the space has been used as premises for events, exhibitions and so on.

Sweden’s second nuclear power plant was Marvikenverket, built at Marviken near Norrköping. Large portions were completed by 1968, with an intended capacity of 200 MW. The goal was for the plant to produce both electricity and plutonium for Sweden’s atomic bomb program. But when Sweden signed the Non-Proliferation Treaty in 1968, Marviken was closed in 1970. It had never received any fuel or provided any electricity.

Sweden’s first commercial reactor, Oskarshamn 1, went into commercial operation in 1972 at Oskarshamn nuclear power plant (OKG). The equipment was supplied by Allmänna Svenska Elektriska AB (ASEA) and was a boiling water reactor developed in Sweden, with a capacity of 440 MW.

Nuclear power in the electricity system

Electricity is consumed the moment it is generated, which places high demands on the synchronization between electricity generation and distribution. The electricity system must be able to handle changes in the electricity supply and demand in real time. To ensure a balance between generation and consumption, the voltage frequency in the electricity grid should be 50 Hz – not less than 49.9 Hz, and not more than 50.1 Hz.

The ability of the system to also maintain the voltage frequency during rapid deviations is dependent on the rotational energy being offered by the various power sources. If there is a lot of rotational energy, the system will have the time it needs to adjust the voltage frequency and thus be more stable, and the power quality will be higher. If there is low rotational energy, the voltage frequency will be difficult to maintain, the system will become less stable, and the power quality will deteriorate.

Nuclear power and hydropower are driven by large generators, which provide most of the rotational energy in the system. Solar and wind energy do not currently contribute to rotational energy, and the higher the proportion of weather-dependent electricity generation the more unstable the system becomes.

A voltage frequency of less than 49.0 Hz causes disruptions in supply. The problems can start with a small local disruption, whereby the frequency drop can rapidly escalate and spread to other areas, followed by a total system collapse with power outages of several hours.

System inertia easily explained

Without nuclear power, the electricity grid would not be able to maintain frequencies of more than 49 Hz in the event of a sudden disruption to the system for 3% of the time, entailing a higher risk for about 11 days over the course of a year.

Nuclear power and safety

Each nuclear power plant has a comprehensive safety system that prevents errors, prevents errors from leading to breakdowns and mitigates the consequences of a breakdown. Safety is always first in operations at our nuclear power plants. At the least sign of danger, the reactor is taken offline. Each reactor also has a built-in system that allows it to quickly shut itself down in case of a technical error.

In Sweden, the Swedish Radiation Safety Authority (SSM) exercises control over operations at nuclear power plants. SSM has full insight into operations and can conduct controls and inspections at any time. This includes a mandate to shut down a reactor.

There is also an international network in the nuclear power industry that puts safety at the top of the agenda and where operators can share their new experiences, methods and procedures. Through this exchange of knowledge, all facilities around the world have the necessary conditions to improve their safety work and maintain the highest level of safety.

Find out more

Although renewable power sources are increasing rapidly, they will not be capable of replacing current production in its entirety. Therefore, it is time we begin looking into how to complement and eventually replace current nuclear power with new nuclear power systems. 

A lot has happened within nuclear power in the past decade, and many innovative projects aimed at making nuclear power an important part of the future’s fossil-free energy system are becoming a reality. One such innovation is the Small Modular Reactor, or “SMR.”

Nuclear power will be crucial part of the future electricity system. The nuclear power plants in Sweden today were built to last for a long time. 

Our global presence

We continue to strengthen our presence in key markets, develop innovative solutions and acquire companies in prioritized areas. Uniper solutions are sold to over 100 countries and we currently operate in over 40 countries.