The problems involve the Ringhals plant in western Sweden, The Local reported.
The Radiation Safety Authority discovered the device was not functioning when the reactor was shut down during a March audit, the Swedish newspaper said. The device closes valves automatically in the event of an accident to keep radiation from escaping.
The failure was classified as a Category One safety breach, the most serious kind. Sixty safety breaches in Category One and Two or security problems have been discovered at Ringhals so far this year.
In some cases, Ringhals employees have tested positive for drugs or alcohol.
Special supervision means increased scrutiny of security at the plant but not an authority takeover of operations.
The Ringhals plant has four reactors. It is located about 25 miles south of Gothenburg on Sweden's west coast.
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