The information was disclosed in a public hearing.
Mitsubishi Heavy Industry's computer modeling underpredicted the velocity of steam and water by a factor of 3 to 4.
According to the NRC who conducted the pressure test, it was "the first time in the history of the nuclear industry that more than one tube at a plant has failed". 8 tubes failed in the Unit 3's steam generators.
From LA Times (6/18/2012; emphasis is mine):
San Onofre's issues appear to be result of faulty computer modeling
The unusual equipment issues that have shut down the San Onofre nuclear power plant appear to be the result of faulty computer modeling by manufacturer Mitsubishi Heavy Industries, officials from the U.S. Nuclear Regulatory said Monday.
The agency used a public hearing in San Clemente to present the preliminary results of its inspection of the plant, which has been out of commission for more than four months after a steam generator tube sprang a leak, releasing a small amount of radioactivity. That led to the discovery that tubes were vibrating excessively and rubbing against each other, leading them to wear out more quickly than normal for equipment that had been operating for less than two years.
The NRC ordered plant operator Southern California Edison to keep the plant shut down until operators fully understand the wear and how to fix it.
Eight tubes in the plant's Unit 3 failed pressure testing, which NRC officials said Monday was the first time in the history of the nuclear industry that more than one tube at a plant has failed.
"This is a significant, serious safety issue," said NRC Regional Administrator Elmo Collins. "This is a very difficult technical issue, and to be honest, it's not one we've seen before."
NRC officials said it appears that simulations by Mitsubishi underpredicted the velocity of steam and water flowing among the tubes by a factor of three to four. The rate of flow caused the tubes to vibrate and knock against each other.
There were also issues with support structures intended to prevent vibration in Unit 3, but apparently not in Unit 2, officials said.
Mitsubishi did not have any representatives at the meeting and couldn't immediately be reached for comment. Collins said, however, that even though Mitsubishi did the computer modeling, the ultimate responsibility lies with Edison.
NRC and Edison officials did not give a timeline for restarting the plant, saying there are still unanswered questions and more inspections that need to be done.