"An error message from a valve led to a manual quick stop being carried out," Vattenfall writes on its website.
The problem has been fixed and the restart will begin tomorrow. Then it will take a while, about two days, to reach the desired effect, says Helene Andersen, acting communications manager at Forsmark.
The reactor, one of three at Forsmark, was recently restarted after annual maintenance. It has also been upgraded to produce about ten percent more power than before. However, it was not at full capacity when it was shut down.
The other two reactors, Forsmark 2 and 3, have normal production.
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