After lengthy delays, the Swiss nuclear safety watchdog Ensi has now declared the site safe.
Beznau 1, which was built in 1969, will be gradually brought back online after having been given the thumbs up from Ensi, said Axpo, the company that operates the plant, on Tuesday.
A battery of safety tests will be conducted during start-up.
The operator said detailed investigations had confirmed the presence of aluminium oxide inclusions in its reactor vessel. According to Axpo, the problem stemmed from the original forging of the pressure vessel in 1965 and were not related to the reactor's operations.
But Axpo said it had spent some 2.5 billion francs on updates to the facility meaning the Beznau 1 reactor was now expected to be operational until 2030, Axpo said on Tuesday.
Green groups were quick to react to the news, speaking of a “black day for safety”, Geneva daily the Tribune de Gèneve reported.
Beznau is currently the subject of a legal case before Switzerland's Federal Administrative Court. Environmental groups and local residents accuse Ensi of falsely certifying the reactor as earthquake-proof in a bid to keep it operational.
Socialist Party Vice President Beat Jans criticized Ensi for failing to wait for a judicial decision in the case while the Greens on Tuesday called for the “scrap metal reactor” to be taken offline for good, Swiss daily the Tages Anzeiger reported.
Switzerland currently has five nuclear reactors which provide a little over a third of all energy produced in the country.
In 2016, Swiss voters rejected an initiative which would have forced three of the country's five nuclear reactors to go offline in 2017.
However, in May 2017, voters in the country backed Switzerland's Energy Strategy 2050 which will see the nation's nuclear reactors slowly shut down in favour of renewable energy sources.