Photo/IllutrationTokyo Electric Power Co.’s Kashiwazaki-Kariwa nuclear plant in Niigata Prefecture (Asahi Shimbun file photo)

Corrosion and holes were found in ventilation ducts in the central control rooms for 12 reactors at seven nuclear power plants, which could have exposed workers to radioactive materials in the event of an accident, Japan's nuclear watchdog reported.

The plants included Tokyo Electric Power Co.’s Kashiwazaki-Kariwa plant in Niigata Prefecture, according to an inspection report released by the Nuclear Regulation Authority on May 23.

Since the deterioration could have allowed an influx of radioactive materials that would have put the health of workers at risk, electric power companies will take measures including replacing problem ducts or repairing them.

The nuclear watchdog found holes in ducts in five reactors at four nuclear power plants and corroded sections in ducts at seven reactors, according to the NRA.

No abnormalities were found in the ventilation systems at facilities operated by all the electric power companies.

The NRA will check the potential danger in the No. 3 reactor at the Kashiwazaki-Kariwa plant, where larger holes were found in its ventilation ducts.

After holes were found in its ducts in the central control room of Chugoku Electric Power Co.’s Shimane nuclear power plant in Shimane Prefecture in 2016, the nuclear watchdog instructed electric power companies to inspect all ventilation ducts at their plants.

The NRA concluded that corrosion and holes in the ducts occurred mainly because they were designed without filters equipped with a fresh-air intake to let air into the room.

Holes were also found in the ventilation ducts at the No. 7 reactor of the Kashiwazaki-Kariwa plant; Chubu Electric Power Co.’s No. 3 reactor of the Hamaoka nuclear power plant in Shizuoka Prefecture; Japan Atomic Power Co.’s Tokai No. 2 nuclear power plant in Ibaraki Prefecture; and the No. 1 reactor of the Shimane plant.

Corrosion was discovered in ventilation ducts at Tohoku Electric Power Co.’s No. 3 reactor at the Onagawa plant in Miyagi Prefecture; Tokyo Electric Power's No. 6 reactor at the Fukushima No. 1 nuclear power plant in Fukushima Prefecture and its Kashiwazaki-Kariwa nuclear plant’s No. 4 and No. 6 reactors in Niigata Prefecture; Hokuriku Electric Power Co.’s No. 1 reactor at the Shika nuclear plant; and Chubu Electric Power’s No. 4 and No. 5 reactors at the Hamaoka nuclear plant.