Switzerland's oldest nuclear power plant Beznau is seen in 2011 near Doettingen, northern Switzerland. A reactor at the Swiss nuclear plant shuts down automatically due to a defect, the operator said, stressing that the procedure had been completely safe.

A reactor at a Swiss nuclear plant shuts down automatically Wednesday due to a defect, the operator said, stressing that the procedure had been completely safe.

"Block 2 of the Beznau shut down automatically," operator Axpo said in a statement.

"This was triggered by a defect in the non-nuclear part of the power plant. All systems functioned perfectly during the rapid shut-down," it added.

Axpo spokeswoman Daniela Biedermann told AFP that the reactor had not yet returned to the grid and that it could only be recommissioned once the defect had been located and fixed.

"When the power plant was shut down, steam escaped from the non-nuclear part of the plant," the statement said, adding however that "the safety of the power plant was guaranteed at all times".

Switzerland reacted swiftly to the in Fukushima last year, with parliament deciding to phase out nuclear energy.