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Pipes were worn thinner than government standards in thermal reactors operated by four of the nation’s nine electric power companies, according to the results of their internal inspections released Tuesday.

The pipes were below regulation thinness in 23 places at nine reactors at seven thermal power plants, they said.

The utilities conducted the inspections in line with instructions from the Nuclear and Industrial Safety Agency following a fatal Aug. 9 accident at Kansai Electric Power Co.’s Mihama Nuclear Power Plant.

In the accident, five workers died and six others were injured after being blasted with superheated steam that burst from a ruptured pipe.

It was later learned that the pipe had not been inspected since the plant began commercial operations in 1976. It had worn down to less than 10 percent of its original thickness.

The inspection results show that most of the nine thermal reactors with sub-standard pipe thickness have been in operation for at least two decades.

They include the No. 1 reactor at Kepco’s Kainan power plant in Wakayama Prefecture, the No. 2 reactor at Hokkaido Electric Power Co.’s Date power plant, the No. 3 reactor at Chugoku Electric Power Co.’s Tamashima power plant in Okayama Prefecture and the No. 6 reactor at Okinawa Electric Power Co.’s Makiminato power plant in Urasoe, Okinawa Prefecture.

Chugoku Electric said it plans to replace the eroded pipes at the Tamashima plant in the near future.

Kepco said earlier this month that of its relatively new thermal reactors — those less than 20 years old — there were three spots where pipes were eroded beyond regulations. Two such spots were found at the No. 5 reactor of its Himeji No. 1 plant. The third was confirmed at the No. 6 reactor.

Inspections carried out on the newer reactors found that more than 90 percent of the roughly 67,000 spots considered prone to erosion had never been inspected.

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