In June, a team of federal investigators announced that a botched computer analysis resulted in design flaws that are largely to blame for unprecedented wear in the tubes.
Overall, investigators found wear from friction and vibration in 15,000 places, in varying degrees, in 3,401 tubes inside the four generators. And in about 280 spots — virtually all in the Unit 3 reactor — more than 50 percent of the tube wall was worn away.
In Unit 2, investigators found that the wall thickness had been worn away by at least 20 percent in 147 tubes. When about a third of the wall thickness wears away, a tube is deemed too risky to keep in service. Edison has retired, or plugged, more than 500 tubes in Unit 2 because of damage or as a precaution, a number within the margin to continue operating the plant.
Gradual wear is common in such tubing, but the rate of decay at San Onofre stunned officials since the equipment was recently installed.
Dietrich said Unit 2 was susceptible to the same problems that ravaged Unit 3, but engineers believe that the extent of damage was different because of manufacturing and assembly differences that resulted in looser tubes in Unit 3. Running at lower power should correct the trouble, at least in Unit 2, he said.
The generators, which resemble massive steel fire hydrants, control heat in the reactors and operate something like a car radiator. At San Onofre, each one stands 65 feet high, weighs 1.3 million pounds, with 9,727 U-shaped tubes inside, each three-quarters of an inch in diameter. They were manufactured by Japan-based Mitsubishi Heavy Industries.
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 city of Riverside. The Unit 1 reactor operated from 1968 to 1992, when it was shut down and dismantled.
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