The reactor, built in 1969, has been shut down since March 2015 after irregularities were found in its pressure vessel.
But on Wednesday its owner, energy company Axpo, said it had completed tests and analyses required by the Federal Nuclear Safety Inspectorate (ENSI) which show that the ageing reactor is safe to be restarted.
The tests were carried out on a replica of the reactor pressure vessel, it said in a statement.
“Tests confirm the proof for long-term operation,” it said.
“As a result, Axpo has no safety-technical concerns regarding the continued operation of the plant up until about 2030.”
On November 14th the company submitted its safety case to ENSI, which will review the findings alongside a review panel comprising international experts, and make a final decision on the restarting of Beznau I.
“Only if Beznau I fulfills the requirements of the legislators, then it will be able to start up again,” said ENSI Deputy Director Georg Schwarz in a statement.
Reacting to Axpo’s statement, environmental group Greenpeace said it had “serious doubts” regarding the energy company’s analyses and called for more transparency.
It had particular concerns about Axpo’s use of a replica of the reactor pressure vessel for testing .
“This method would probably not hold up against critical analysis,” it said.
Greenpeace called on ENSI to take great care when examining Axpo’s safety case “in the interests of the population”.
“Greenpeace demands that Axpo make public the documents upon which it based its case before the reactor is potentially put back in service.”
Greenpeace has long railed against Beznau I. Last year it launched legal action to demand it be shut down, saying that a 2012 Swiss safety probe that declared all Swiss reactors fit to withstand an earthquake was based on erroneous technical norms.
The news comes as the Swiss people prepare to vote next week on the future of nuclear power in Switzerland.
On Tuesday a survey suggested that a majority of voters will back a popular initiative aimed at shutting down all of Switzerland’s five nuclear reactors – three of them, including Beznau I, next year.
Backers of the initiative, including the Green Party, feel that Switzerland’s ageing reactors pose a safety risk.
However another survey indicated that voters’ support for the plan was not based on safety fears.
Beznau I sits on an artificial island on the Aare river, 37 kilometres from central Zurich and ten kilometres from the German border.
The second reactor on the complex was built in 1971.