This week, the federal Nuclear Regulatory Commission (NRC) released its determination that the excessive wear and decay in tubes which carry radioactive water in the San Onofre Nuclear Generating Station was caused by modifications to the plant's design.
At a community meeting Monday evening in San Juan Capistrano, NRC officials stunned the large audience with the news that generator manufacturer Mitubishi's computer codes to simulate how the generators would work were "off" by a factor of three or four.
NRC inspectors also considered other culprits -- such as flaws in fabrication or installation of the 400 additional tubes and their V-shaped supports. They concluded that "thermal velocities" were much higher than predicted, causing the tubes to vibrate against each other and against the support structure, causing premature wear.
San Onofre, which is owned by Southern California Edison, has been idle since January, when a broken tube in one of the four steam generators released a small amount of radiation.
NRC investigators soon discovered some tubes were so badly corroded that they could fail and possibly release more radiation. The findings were shocking, as the generators had been extensively overhauled in 2009 and 2010, and the equipment was virtually new.
Environmental groups such as Friends of the Earth contend that the NRC let Edison avoid public review of its large-scale changes to the design of the plant, instead allowing the changes to qualify as "in-kind," or essentially identical to the old equipment.
Friends of the Earth is now calling on the NRC not to allow the operator, Southern California Edison, to restart the plant without going through a licensing amendment and public review.
The overhaul of San Onofre cost $670 million.
About 7.4 million Southern Californians live within 50 miles of San Onofre.
NRC Regional administrator Elmo Collins was clear the plant operator, Southern California Edison, still has multiple questions to answer before it can even apply to restart.
"So far," he said, "these issue are not resolved to the NRC’s satisfaction."
The plant provided approximately 20 percent of the region's electricity.