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.