SAN JUAN CAPISTRANO, California (Reuters) - Tubes that leaked radioactive steam at a California nuclear power plant, leading to an indefinite shutdown, were not properly tested by the manufacturer prior to installation, nuclear regulators told an overflowing public hearing on Monday.

The San Onofre Nuclear Power plant, located in Orange County, has been shut down since January 31, when plant operators discovered a small radiation leak in one of the plants' two units. The 2,150-megawatt plant is operated by Edison International's Southern California Edison utility.

The nuclear station is located halfway between Los Angeles and San Diego and is critical to the grid to import electricity into southern California. Its extended shutdown raises the possibility of rolling power outages as warmer temperatures boost demand for power over the summer.

The U.S. Nuclear Regulatory Commission on Monday pinned the blame for the leak on Mitsubishi Heavy Industries, which it said underestimated the velocity of water and steam surging through the generator by a factor of three or four times in its computerized test of the equipment.

The tubes were also not held together tightly enough inside the troubled Unit 3 reactor, allowing them to rub against each other and causing premature wear, regulatory officials said.

Eight of the 129 tubes tested by the Nuclear Regulatory Commission since the shutdown at the plant's troubled Unit 3 generator failed pressure testing, an unprecedented number, said Elmo Collins, regional administrator for the Region IV office of the NRC.

"We've never seen that before," he said of the test results. "This is a significant, serious safety issue."

Officials from the Nuclear Regulatory Commission and Southern California Edison said they would not allow the plant to reopen until it was safe to do so, and declined to give a specific timeline for restarting the plant.

"Both San Onofre units will be shut down until repairs are made and we and the Nuclear Regulatory Commission are satisfied it is safe to operate," said Pete Dietrich, senior vice president and chief nuclear officer for the power plant.

While the regulatory commission has some authority over contractors such as Mitsubishi, Collins made it clear that it's Southern California Edison that will ultimately be held accountable if penalties are eventually levied by the government.

A crowd of over 400 people showed up for the hearing, many asking pointed questions about the competence of the Southern California Edison and the regulatory commission, as well as raising questions about the safety of nuclear power.

Dozens of environmentalists held a rally prior to the meeting with anti-nuclear signs, including one banner that read "Fukushima not again!" - a reference to the Fukushima Daiichi nuclear disaster last year following the earthquake and tsunami that hit Japan.

Damon Moglen, climate and energy director for green group Friends of the Earth, said that Southern California Edison made significant design changes to the plant without seeking an amendment to its existing license, as is required by the regulatory commission.

His group submitted petition to nuclear regulators on Monday to require the company to obtain a new license, complaining that in his view the commission was "asleep at the regulatory wheel."

The Nuclear Regulatory Commission said its investigation into what went wrong at the plant was ongoing and promised to keep the public apprised of any new developments. A written report on the findings will be released next month, regulators said.