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Japan Atomic Power Co. kept running a nuclear reactor in Fukui Prefecture without reporting to the government that it had detected cracks in the core’s shroud, company sources said Wednesday.

The company found signs of cracks in the shroud of the No. 1 reactor at its Tsuruga nuclear plant, along the Sea of Japan, during a voluntary inspection in 1994, the sources said. Started up in 1970, the reactor is the nation’s oldest for commercial use.

It later replaced the shroud, telling the national and local governments it did so as preventive maintenance, the sources said.

A shroud is a stainless-steel cylinder made of welded plates that surrounds the core and regulates the flow of cooling water.

The disclosure follows revelations last month that Tokyo Electric Power Co. concealed on several occasions structural faults in its nuclear reactors. The government believes Tepco may have broken the law by covering up the existence of cracks and may pursue charges. If so, Japan Atomic Power may also face charges.

Japan Atomic Power officials said the company determined that it was not required to report the cracks to the government.

In a related development, Akira Matsu, parliamentary secretary for economy, trade and industry, said the government should introduce regulations to exert control over nuclear operators’ “voluntary” facility checks.

“It is strange that the law does not regulate the voluntary inspections of nuclear plants,” Matsu said before the Diet on behalf of the industry ministry, which oversees the Nuclear and Industrial Safety Agency. “It is very important that they come under the law.”

Meanwhile, the revelation may affect the nuclear plant operator’s plans to build a third and fourth reactor at the Tsuruga plant.

The Fukui Prefectural Government has given the go-ahead to the plans, but the operator has yet to clear some administrative procedures.

According to the sources, the operator began voluntary examinations of shrouds in 1991, including visual checks and the use of ultrasound detectors, in addition to the government’s regular inspections, following the findings of cracks in shrouds in the United States and other countries.

In 1994 it found indications of dozens of cracks of up to 47 cm in length and 19 mm in depth near the welding area of the shroud. It repeatedly came across cracks thereafter.

The company decided that the cracks posed no safety problem and continued operating the reactor without reporting the findings to the government.

During a regular inspection that ran between August 1999 and March 2001, it replaced the shroud with one made of material believed resistant to stress corrosion cracks, calling it “preventive maintenance.”

The company also assured the central and local governments that it had confirmed the “soundness” of the reactor in past inspections, without touching on the cracks that were detected, the sources said.

Launched in March 1970, the Tsuruga No. 1 boiling-water reactor, with a 357 megawatt capacity, is Japan’s first light-water reactor for commercial use.

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