[6] A decision-in-principle for a fourth reactor to be built at the site was granted by the Finnish parliament in July 2010,[7][8][9] but in June 2015 TVO decided that it would not apply for a construction license for Olkiluoto 4.

[12] Major upgrades were carried out to the units in 2010 and 2011, including replacement of turbines and generators, isolation valves, electrical switchgear and seawater pumps.

[20] The environmental impact assessment program, published in January 2024, investigates options to extend the lifetime by 10 or 20 years at the current power or uprating to 970 MW (in 2028 at the earliest) in addition to the 10 or 20-year extension.

Japan Steel Works and Mitsubishi Heavy Industries manufactured the unit's 526-ton reactor pressure vessel.

[26] In February 2005, the Finnish government gave its permission to TVO to construct a new nuclear reactor, making Finland the first Western European country in 15 years to order one.

Areva later sold its majority stake in Framatome, its nuclear reactor and fuel business, to Électricité de France.

[25] In September 2022, the Radiation and Nuclear Safety Authority (STUK) granted a licence for the Olkiluoto 3 reactor to increase its power output to more than 60 percent to a full 4,300 MW of thermal capacity.

[2][3] In October 2022, damage was discovered in the feedwater pumps and unit 3 was shut down for an investigation, delaying the end of commissioning testing.

[40] In January 2023, it was announced that new more robust impellers will be installed in all four feedwater pumps, and test operation at full power should restart in February.

[42][6][43] Unusual for nuclear reactors in Scandinavia, OL3 reduced production in May 2023 due to negative market prices during the spring thaw when hydropower was unable to absorb surplus.

In a statement, the operator said it was "not pleased with the situation" although solutions to various problems were being found and work was "progressing", and that it was waiting for a new launch date from Areva and Siemens.

[47] In February 2014, TVO said that it could not give an estimate of the plant's startup date because it was still waiting for the Areva-Siemens consortium to provide it with an updated overall schedule for the project.

"[48] Later the same month it was reported that Areva was shutting down construction due to the dispute over compensations and unfinished automation planning.

[51] The delays have been due to various problems with planning, supervision, and workmanship,[8] and have been the subject of an inquiry by STUK, the Finnish nuclear safety regulator.

An apparent problem constructing the reactor's unique double-containment structure also caused delays, as the welders had not been given proper instructions.

[53] In 2010, Jukka Laaksonen, Director General of STUK at that time, analyzed reasons for the delay:[54] Construction of the turbine succeeded better under the responsibility of Siemens.

Even taking into account all OL3 construction delays the long-term target for operational production cost (not including investments and subsidies) for all three plants is 33 €/MWh.

[66] In 2009, Stephen Thomas wrote, "Olkiluoto has become an example of all that can go wrong in economic terms with new reactors," and that Areva and the TVO "are in bitter dispute over who will bear the cost overruns and there is a real risk now that the utility will default.

The outage was delayed from the original plan due to the shutdown process and inspections of the fuel elements taking longer than expected, and technical problems identified during the maintenance.

[101] On 21 April 2010, the government of Finland decided to grant a decision-in-principle to Teollisuuden Voima for the fourth reactor in Olkiluoto.

Economic Affairs Minister Jan Vapaavuori referred to the long delay of the 3rd reactor and to unsatisfactory assurances by TVO that the 4th unit would ever be built.

Nevertheless PM Stubb stated that the rejection didn't spell the end for the OL4 project, and that TVO would have the opportunity to apply for a construction license before the decision-in-principle expires in June 2015.

The power plant hosts the northernmost vineyard in the world, a 0.1 ha experimental plot that yields 850 kg Zilga grapes annually.

[105] In April 2014, a turbine steam condenser of unit 1 had a small seawater leak, at a rate of two litres per hour.

[107] The event caused a full-scale emergency response at the power plant and at the Radiation and Nuclear Safety Authority.

An immediate investigation found the cause to be a failure in the differential pressure measurement of the generator's sealing oil and hydrogen coolant.