Beznau, located in the northern Swiss canton of Aargau near the German border, has two reactors and first opened in 1969.

Both reactors were shut down in March of 2015 for work that included enhanced protections against earthquakes and floods, as well as IT upgrades, site managers Axpo said in a statement.

The repairs featured 9,000 "tasks" and more than "20,000 routine inspections of plant systems and components.

"The plant fulfils all the safety-technical requirements for long-term operation of 60 years," the statement said.

The Beznau plant had become a touchstone of the heated debate about nuclear safety in Switzerland that intensified following the 2011 Fukushima nuclear disaster in Japan.

Greenpeace and two other environmental groups -- the Swiss Energy Foundation and the Tri-national Nuclear Protection Association -- went to court demanding the "definitive closure" of the plant, shortly after the repairs began.

In the aftermath of Fukushima, Switzerland announced plans to phase out nuclear energy and close its four plants, but no clear timeline has been set.

Roughly a third of the country's electricity is generated from nuclear power.