The world's oldest nuclear plant, built in 1969, Reactor block 1 at Beznau nuclear power plant, can restart after a three-year shutdown for repairs, the Swiss Federal Nuclear Safety Inspectorate said on Tuesday.
The Beznau facility in the most northerly Swiss canton of Aargau has been out of service since March 2015, when anomalies were found in its steam generator.
The safety authorities now say the reactor presents no security risk.
Following the announcement, utility company Axpo said it hoped to have the reactor fully up and running again by the end of March.
But environmental groups that called this "absolutely irresponsible", protested the announcement, the Swiss news agency SDA reported.
Beznau is one of two reactors that were shut down to check the quality of steel components used in their steam generators. The Beznau II reactor was built in 1971.
Switzerland has five nuclear power reactors which generate about 34.5 percent of the nation's energy needs. It uses nuclear energy to produce electricity, in research and medicine.
In May 2017, Swiss voters endorsed a new energy law that aims to promote renewable energy by banning new nuclear power plants and reducing energy consumption.
The Muhleberg nuclear plant near Bern, built in 1972, will shut down in 2019, according to the power station operators.
