Federal regulators have turned away a request from environmental activists to idle the Diablo Canyon nuclear power plant because of potential earthquake risks.
However, the Nuclear Regulatory Commission decision Thursday left unresolved questions about whether the agency allowed owner Pacific Gas and Electric Co. to operate outside the requirements of its federal license.
That issue was referred to an administrative judge at the Atomic Safety and Licensing Board, an arm of the NRC.
The decision stemmed from a petition filed last year by Friends of the Earth, which argued the company was concealing earthquake risks.
The group said it welcomed the decision, calling it a "turning point" is the long fight over earthquake safety at the coastal plant between Los Angeles and San Francisco.
PG&E says the plant is safe.