Cracks and damage were found in the Japan-made steam generator for San Onofre Unit 3 reactor in 2009 before it was delivered to the plant, U.S. Nuclear Regulatory Commission (NRC) documents showed Tuesday. The steam generator, produced by Mitsubishi Heavy Industries Ltd., was installed at the unit 3 reactor less than a year before it leaked radiation January 31st of this year.

It remains unclear whether the cracks and the radiation leak were related.

At the nuclear plant, operated by Southern California Edison, abnormal attrition was also found earlier this month in the tubes of another Mitsubishi-made steam generator installed at the unit 2 reactor.

Mitsubishi Heavy and Southern California Edison both declined to comment, saying they are under investigation.

According to the NRC documents, cracks and damage were found in the steam generator of the unit 3 reactor during checks by Mitsubishi Heavy in March 2009.

A follow-up probe found heat treatment prior to welding caused the problem and the NRC informed the utility and others concerned.