"Block 2 of the Beznau nuclear power plant shut down automatically," operator Axpo said in a statement.
"This was triggered by a defect in the non-nuclear part of the power plant," the company said.
"All systems functioned perfectly during the rapid shut-down."
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.
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"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 nuclear disaster in Fukushima last year, with parliament deciding to phase out nuclear energy.