After months of investigation, federal regulators have determined that design flaws appear to be the cause of excessive wear in tubing that carries radioactive water through California's troubled San Onofre nuclear power plant, a top federal regulator said.

The twin-reactor plant between Los Angeles and San Diego has been idle since January, after a tube break in one of four, massive steam generators released traces of radiation.

A team of federal investigators was dispatched to the plant in March after the discovery that some tubes were so badly corroded that they could fail and possibly release radiation, a stunning finding inside the virtually new equipment.