What is Olkiluoto-3? Reactor number three at the Olkiluoto nuclear power station in southwest Finland is a 1,600-MW EPR (European Pressurised Water Reactor) nuclear plant supplied by the Franco-German Areva-Siemens consortium. It is the largest nuclear power plant in commercial operation in Europe. On 1 May it officially began full commercial operation – almost 18 years after construction began on 12 August 2005. It has been undergoing testing since it was connected to the grid on 12 March 2022.