Corrosion and holes have been found in ventilation ducts at 12 reactors at seven nuclear plants across the country, the Nuclear Regulation Authority said Wednesday, raising concerns that workers could be exposed to radiation in the event of an accident.

The governmental nuclear watchdog released the results of a nationwide survey it had ordered following a revelation in December 2016 that corrosion had left multiple holes in the air ducts of the No. 2 reactor at Chugoku Electric Power Co.'s Shimane plant in western Japan. That reactor was not included in the survey.

Serious corrosion was found at the No. 3 unit of the Kashiwazaki-Kariwa plant in Niigata Prefecture operated by Tokyo Electric Power Company Holdings Inc. and may have abnormally affected ventilation of the central control room, the watchdog said.