LOS ANGELES, Calif. -

Southern California Edison says it is seeking to hold Mitsubishi Heavy Industries Ltd. and Mitsubishi Nuclear Energy Systems liable for defective steam generators that forced the closure of the San Onofre nuclear power plant.

The utility says in a statement Thursday that Mitsubishi should be accountable for designing and manufacturing replacement steam generators that were supposed to operate for 20 years but leaked radioactive coolant after less than a year of operation.

The $670 million steam generators were supposed to extend the life of the twin-reactor plant on the coast between Los Angeles and San Diego, but the leak and discovery of excessive wear in the equipment force the utility to take it offline in January 2012. Southern California Edison subsequently announced the plant is being permanently closed.