SAN JUAN CAPISTRANO, Calif. -- A flawed computer model and a poorly planned generator combined to trigger the malfunction that shuttered the San Onofre Nuclear Generating Station, federal regulators said yesterday.
Nuclear Regulatory Commission officials disclosed the information during a meeting here to provide residents with information about the Jan. 31 radiation leak.
NRC's investigation uncovered widespread problems in the generator where the radiation release occurred, officials told a standing-room crowd of more than 300 people.
Stress tests showed that multiple tubes were at risk of rupture, the first time in the history of the industry that more than one tube at the same facility has failed a pressure check.
"The strength of eight tubes was not adequate, and structural integrity might not be maintained during an accident," said Greg Werner, an NRC branch chief. "This is a serious safety issue that must be resolved to prevent further failures from occurring."
Many questions remain, NRC said, and there is no timetable for when the plant will be allowed to reopen. That means plant operator Southern California Edison Co. will be forced to rely on other energy generation in summer, when demand often peaks. The plant powers about 1.4 million homes.
San Onofre, located about 60 miles north of San Diego, has been closed since the leak was detected in Unit 3. The generator in Unit 2 had been turned off in early January for routine maintenance, and it was not reopened because of the federal investigation.
Tubes in both units were wearing much faster than they should have, NRC and Edison subsequently found. The generators were installed as replacements in 2009 and 2010, at a cost of $670 million. They were supposed to last 40 years.
The investigation found that the primary cause of the unexpected wear was "higher than expected" steam and water flow velocities in the generators, Werner said. That produced vibrations that caused tubes to rub together and degrade.
Both unit generators came from Mitsubishi Heavy Industries. When it made the equipment, the Tokyo-based company used a computer model to approximate water and steam levels, but the analysis was faulty.
"The computer simulation used by Mitsubishi during the design of the steam generators had underpredicted velocities of steam and water inside the steam generators by factors of three to four times," Werner said.
There also were flaws with the equipment, he said.
Unit 3, where the leak occurred, had far more degradation in its tubes than did Unit 2, Werner said. They were manufactured at different times, he said, and the more recent version put in Unit 2 has superior equipment for limiting vibrations.
"For Unit 3, the anti-vibration boards do not come in contact with the tubes as tightly as they do on Unit 2," Werner said. "Essentially, the tubes are not held in place fully enough, so it allows them to slide or vibrate."
In addition to the eight tubes that failed the stress tests, problematic wear was found on other tubes in both units.
Unit 3 had 326 tubes with "tube-to-tube wear" greater than 10 percent, said Peter Dietrich, Edison's senior vice president and chief nuclear officer. In Unit 2, there were two tubes with degradation exceeding that level. There are 19,454 tubes in each generator.
Edison should be required to apply for a license amendment as a condition of restarting the plant, Friends of the Earth said yesterday. It should have gone through that step, FOE said, when installing the new generators.
"The inconvenient truth is that Edison misled the NRC staff about the significance of the changes made in the new steam generators when they proposed them in 2006," FOE said in a statement. "They avoided the rigorous review required by the license amendment process for evaluating significant design changes in equipment."
The green group yesterday filed a petition asking NRC to compel San Onofre to apply for a license amendment.
People attending the meeting with NRC were at times hostile, interrupting with questions and jeering some answers.
Outside the meeting at the city's community center, people had put up banners that said "Fukushima Not Again," "Decommission San Onofre" and "Shut Down Now."
During a question-and-answer session, several people inferred NRC and the utility were hiding information and that they were more loyal to each other than to a full accounting of what caused the plant's problems.
Residents asked exactly how much radiation was released.
The radiation released in Unit 3 was 5.2 millirem, an amount 10,000 times less than what a person would receive from an X-ray of an arm, said Greg Warnick, senior resident inspector at San Onofre.
"It was negligible," he said, adding that he was at the facility wearing a radiation badge at the time and the radiation "wasn't picked up at all."
One audience member asked why there was any radiation release at all, given that there is a large dome over the generator.
Werner explained that the dome only keeps in radiation that stays within the equipment. Once there is a leak in a tube, he said, radiation escapes.
People also asked about the plant's ability to withstand earthquakes. Elmo Collins, NRC regional administrator, explained that licensing requires the plant to shrug off ground acceleration of six to seven times the gravitational force.
Collins' answer did not satisfy the audience.
"There's a big one expected sometime in the future," said a man who didn't identify himself. "It's going to affect those steam generators."
One man asked if NRC would commit to holding a full hearing with evidence and cross-examinations before making a decision to reopen the plant. Collins said he has been in discussions with his superiors about that possibility.
Donna Gilmore, with the group called San Onofre Safety, said there is a 40 percent surplus in generation capacity in the region, making nuclear unnecessary.
"Why are we taking this risk for energy we don't need?" Gilmore asked.