Faulty computer modeling caused the equipment problems that are expected to keep the San Onofre nuclear plant dark through the summer, federal regulators said Monday. The plant has been out of service since Jan. 31, when operators discovered a small leak in one of the thousands of steam generator tubes that carry hot, radioactive water used to create steam to turn turbines that generate electricity. That led to the discovery that other tubes were rubbing against support structures and adjacent tubes, and wearing out more quickly than expected. Eight tubes failed pressure testing, which NRC officials said ... is the first time in the nuclear industry that more than one tube at a plant has failed. The wear is a safety concern because tube ruptures can release radiation. The plant's operator, Southern California Edison; the NRC; and Mitsubishi Heavy Industries, the manufacturer of the steam generators, have been studying the cause and extent of the wear. The NRC has ordered Edison to keep the plant shuttered until it has determined the cause and how to fix it. 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 or four. The high rate of flow caused the tubes to vibrate and knock against each other, leading to the wear. It was not clear why the computer modeling was so far off.