The government’s nuclear inspections agency admitted Tuesday it failed to identify cracks in the core shroud of the Hamaoka No. 4 reactor operated by Chubu Electric Power Co., even though it detected telltale signs in 2001.

The failure is detailed in a report by the Industrial and Nuclear Safety Agency, which investigated a series of defects following revelations in August that Tokyo Electric Power Co., Japan’s leading power utility, covered up similar reactor faults.

In the wake of the scandal, cracks were identified at a number of Tepco reactors, as well as the No. 4 reactor at Chubu Electric’s Hamaoka plant in Shizuoka Prefecture and the No. 1 reactor at the Onagawa plant operated by Tohoku Electric Power Co. in Miyagi Prefecture.

The agency said it detected streaky patterns during inspections in 2001. It failed, however, to identify these patterns as cracks.

Regarding the Onagawa No. 1 reactor, the agency said it identified no cracks during inspections conducted from 1994 to 1996. It therefore believes the cracks materialized recently.

It also reiterated that Tepco failed to confirm the appearance of telltale signs of defects and that the utility falsified inspection records.

The agency concluded that the Onagawa No. 1 reactor will not constitute a safety hazard over the next five years — even if the cracks continue to develop in its shroud.

A subpanel of experts operating under the Advisory Committee for Natural Resources and Energy, which advises the economy, trade and industry minister, endorsed the agency’s conclusions, paving the way for the reactor to start operating again without the cracks being repaired.

This marks the third time for a decision to be made on the basis of so-called maintenance standards that were introduced after the Tepco scandal. These standards allow power utilities to operate reactors without repairing defects, if the reactors are viewed as posing no current danger.

The green light for the Onagawa No. 1 reactor follows similar decisions for the No. 3 reactor at Tepco’s Kashiwazaki-Kariwa plant in Niigata Prefecture and the Hamaoka No. 4 reactor.

The agency issued its decision after testing sections of the shroud that are not cracked to see whether it could tolerate an earthquake.

The cracks must also be checked during the government’s regular inspections.

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