Waterford - An error-ridden test of steam control valves for one of the main turbines at the Millstone Power Station ranks as one of the two "most threatening" safety lapses at U.S. nuclear power plants in 2011, according to the Union of Concerned Scientists.

In a report released Tuesday, the nonprofit safety and environmental advocacy organization examined what it termed "near misses" at nuclear plants around the country that triggered special investigations by the Nuclear Regulatory Commission. It concludes that the 15 incidents, including the Feb. 12, 2011, event at Millstone Unit 2, are evidence that plant owners and the NRC are not addressing known safety problems adequately. Millstone 2 is one of two operating nuclear power plants at the Waterford site.

None of the 15 events resulted in harm to plant employees or the public, "but their frequency - more than one a month - is high for a mature industry," the UCS said in a news release. The special post-incident investigations by the NRC are only done when the agency determines that an event had the potential to increase the risk of damage to the reactor core by a factor of 10 or more, noted David Lochbaum, author of the report and director of the UCS's Nuclear Safety Project.

In an email message, NRC spokesman Neil Sheehan took issue with the UCS's characterization of the Millstone 2 event as a "near miss."

"Our reactor inspection program is, by its very design, aimed at identifying low significant issues well before they would manifest themselves in 'near misses,'" Sheehan said. He emphasized that there were "no immediate safety concerns" as a result of the incident because it did not cause power levels in the reactor to exceed license limits or damage fuel.

"The plant is under increased NRC scrutiny until we are satisfied that root causes of the problems have been properly identified and corrective actions put in place to prevent recurrence," Sheehan said.

The Millstone event, which began when a member of a five-person crew running the test repeatedly turned a key dial the wrong way, caused a buildup of steam in the turbine and an unanticipated rise in the power level of the reactor. If the level had continued to rise, it would have triggered an automatic shutdown.

The incident was ranked with one at the Pilgrim Nuclear Power Station in Plymouth, Mass., as being the most troubling of the 15 incidents because they both involved multiple mistakes by several workers - "problems self-inflicted by plant operators" rather than the failure of equipment or procedures, the report said.

Lochbaum said the Millstone event stands out because the team of workers involved successfully completed training exercises in a simulator for two days before running the test on the actual equipment. The valves are tested quarterly, but this particular crew had not done it recently when the February 2011 test was scheduled to take place.

"It was a fairly routine test, and they did everything and more to prepare for it, and yet mistakes still were made," Lochbaum said. "It raises the question of how you'd perform under a greater challenge. In an accident, when there's more stress, you won't have rehearsed it and you won't have a peer checker."

Ken Holt, spokesman for Millstone owner Dominion, said the event was taken seriously by the company. He also noted that although errors were made during the test, reactor power never exceeded the margin of safety. After its investigation, the NRC issued a "white finding" of low to moderate safety significance - the second lowest level of four - for the violations that occurred. It required Dominion to take corrective actions.

"We've maintained all along that we were extremely disappointed with our performance," Holt said. "We conducted a root-cause investigation and brought in experts from the outside to investigate, and we've incorporated the lessons learned."

Lochbaum agreed that Dominion responded appropriately and constructively. "They didn't try to downplay it," he said.

Steve Shaffer, the NRC's senior resident inspector at Millstone, said the steam control valve tests have been run successfully at Unit 2 at least three times since the February 2011 incident. There is additional oversight, among other changes, he said.

"I was very confident in what I saw there," Shaffer said. He added that he disagrees with the UCS's characterization of the Millstone event as one of the two "most threatening" last year.

Shaffer also noted that Millstone is still in the process of completing corrective actions and that the NRC will be conducting a follow-up inspection in the coming months. "The workers involved in that were off shift for an extended period, and that crew was not reconstituted as it stood," he said.

j.benson@theday.com