The shutdown of Reactor 2 at the Beznau nuclear plant in northern Switzerland happened at 1:05 pm (1205 GMT) on Monday, energy company Axpo said in a statement, pointing to "an incorrect operation" during a routine inspection.

"The plant reacted according to design and was in a safe state at all times," it said, adding that supervisory authorities had been informed of the incident.

The reactor will be brought back online following consultations with the Swiss Federal Nuclear Safety Inspectorate, Axpo said.

"During the restart, water vapour will be visible" above the non-nuclear part of the plant, it said, stressing that "there is no danger to the population nor the environment".

Beznau's Reactor 2 first began operating in 1971, while Reactor 1 was first brought online two years earlier, making it one of the oldest nuclear plants in the world.

Axpo said late last year that it plans to run the plant until 2033.

Switzerland has placed no maximum life span on its nuclear power plants. The four currently in service, which provide around a third of the country's total electricity production, can run for as long as they are considered safe.