A panel set up by Tokyo Electric Power Co. to investigate the company’s coverup of nuclear reactor faults has discovered that unauthorized bolts were used to repair a reactor at Tepco’s Fukushima No. 1 plant, company sources said Tuesday.

The bolts were used to secure a manhole cover on the plant’s No. 5 reactor, the sources said. The manhole is used to access the pressurized container and is located just below the reactor shroud. The cover was sealed with bolts after construction of the reactor was completed.

During a regular check, it was found that some of the bolts used for the cover were loose. Engineers tried to screw them tight, but the threads were stripped so the bolts had to be replaced, according to the investigation.

The engineers finally used new bolts not certified by the government, but Tepco failed to report the repair work to the government, the sources said.

Utilities are required to report any changes to a reactor core or pressurized containers to the minister of economy, trade and industry.

Although officials at the Nuclear and Industry Safety Agency failed to specify whether use of the bolts in question requires ministry authorization, they noted that the change should have been reported to the government prior to the repair work.

The repair of the manhole cover is mentioned in a list of 29 possibly falsified inspection reports submitted by Tepco to the agency last month, the sources said.

The utility claims the use of the unauthorized bolts did not affect the safety of the reactor, which was subjected to a regular inspection in January, the sources said.

But the utility may be ordered to shut down the No. 5 reactor of the Fukushima No. 1 plant if a further investigation shows that the bolts pose a safety hazard.

Tepco has 17 reactors in Fukushima and Niigata prefectures. It has already shut down two of these in connection with the falsification of inspection reports, with three other reactors set to be temporarily idled toward late October.

The sources quoted a company official as saying that additional shutdowns would probably deal a serious blow to the utility’s ability to supply power.

Restart not OK: METI

Industry minister Takeo Hiranuma on Tuesday denied a news report that the ministry may allow a halted nuclear reactor operated by Tokyo Electric Power Co. to resume operations for the purpose of checks without mending cracks that have been detected.

“The report is utterly unfounded,” the economy, trade and industry minister told a news conference, referring to an Asahi Shimbun report that Tepco is engaged in talk to this end with the ministry’s Nuclear and Industrial Safety Agency. Tepco said last month that during regular inspections it identified cracks in the shroud of the No. 3 reactor at the Kashiwazaki-Kariwa nuclear plant in Niigata Prefecture.

“The utmost priority issue for the moment is an investigation of the cause, so we are refusing to answer any question about what measures we would take,” Hiranuma said.

The utility has been rocked by revelations of damage coverups concerning other reactors at the Kashiwazaki-Kariwa plant, as well as reactors at the Fukushima No. 1 and No. 2 plants.

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