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The generators, which resemble massive steel fire hydrants, control heat in the reactors and operate like a car radiator. At San Onofre, each one stands 65 feet high, weighs 1.3 million pounds and has 9,727 U-shaped tubes inside, each three-quarters of an inch in diameter.

Cracked and corroded generator tubing has vexed the nation’s nuclear industry for years.

Decaying generator tubes helped push San Onofre’s Unit 1 reactor into retirement in 1992, even though it was designed to run until 2004. The following year, the Trojan nuclear plant, near Portland, Ore., was shuttered because of microscopic cracks in steam generator tubes, cutting years off its expected lifespan.

San Onofre is owned by SCE, San Diego Gas & Electric and the California city of Riverside. The Unit 1 reactor operated from 1968 to 1992, when it was shut down and dismantled.