"Hunterston is unplanned and we are looking into it," a spokesman for the company said.
The trip occurred at around 23:00 yesterday, he said before adding that engineers had been sent off to try and establish the cause. There are no safety concerns, the spokesman claimed.
The reactor has been out of action in recent months for inspection and repair, as well as the preparation of safety cases - after boiling tube cracking issues. The company got permission to restart it last month.
Another reactor, unit seven at Heysham, has been taken out of action for refuelling, the spokesman added.
"Heysham is planned refuelling," he said.
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