Washington (CNN) -- Mock commandos who staged attacks on 24 nuclear power plants in pre-announced drills last year were able to "damage" or "destroy" critical targets at two of the plants, according to the Nuclear Regulatory Commission.
The NRC did not identify the nuclear plants that failed the security tests, citing security concerns and other sensitivities. But it said inspectors remained at those plants until security shortcomings were addressed.
Each year, about a fourth of the nation's 104 commercial nuclear power plants undergo security tests, which involve several weeks of security inspections and table-top exercises that probe for holes in security, followed by three consecutive nights of "force-on-force" assaults. The assaults are pre-announced to avoid confusion with real-world events, and participants -- in a high-stakes game of laser tag -- attack plant defenses in an effort to "destroy" critical buildings or equipment.
Attack scenarios are changed nightly to give the attackers, called the "composite adversary force" (CAF), the upper hand.
Last year's results are roughly analogous to those in recent years. In the three previous years, between two and four plants were breached and targets "destroyed," NRC records show.
"We don't characterize (the results) as good or bad because the plants must adhere to our security regulations, period," said NRC spokeswoman Holly Harrington. "If there are these failures, which from time to time occur, they are fixed and the plants are told they have to meet these requirements."
According to an unclassified version of an NRC report released Wednesday, the NRC conducted force-on-force inspections at 24 nuclear power plants and one nuclear fuel facility during 2010. It identified 23 deficiencies, but none was in the upper range of severity.
At two unidentified sites, the utilities failed to "effectively protect" the target during mock attacks, the report says.
Four exercises were "inconclusive" because of "drill artificialities" and safety concerns for the participants, it says.
Harrington said the NRC increased the frequency and the strenuousness of the drills following the Sept. 11, 2001, terrorist attacks. "The realism went up," she said.
The assault teams include contractors who work as security guards at other plants. Participants use military-grade laser weapon systems known as MILES, for Multiple Integrated Laser Engagement System, to simulate gunfire. NRC inspectors witness the assaults and the response.
The attacks occur on three consecutive nights to ensure that each shift of power plant's security force, which rotate schedules, can participate in the drills.
The NRC does not publicly reveal the "targets" in each assault, but it includes "key safety equipment."
"We can't get more specific than that," Harrington said.
In 2004, the Government Accountability Office criticized the use of one security company to manage the adversary force, saying it had been accused of cheating on previous force-on-force exercises by the Department of Energy.
The NRC says it will conduct 25 inspections security drills this year, three of which are to assess corrective actions from previous drills.