Washington ? Making nuclear power plants crash-proof to an airliner attack by terrorists is impracticable and it’s up to the military to avert such an assault, the government said Monday.

The Nuclear Regulatory Commission, in a revised security policy, directed nuclear plant operators to focus on preventing radiation from escaping in case of such an attack and to improve evacuation plans to protect public health and safety.

“The active protection against airborne threats is addressed by other federal organizations, including the military,” the NRC said in a statement.

The agency rejected calls by some nuclear watchdog groups that the government establish firm no-fly zones near reactors or that plant operators build “lattice-like” barriers to protect reactors, or be required to have anti-aircraft weapons on site to shoot down an incoming plane.

The NRC, in a summary of the mostly secret security plan, said such proposals were examined, but that it was concluded the “active protection” against an airborne threat rests with organizations such as the military or the Federal Aviation Administration.

It said that various mitigation strategies required of plant operators – such as radiation protection measures and evacuation plans – “are sufficient to ensure adequate protection of the public health and safety” in case of an airborne attack.

The NRC unanimously approved the plan, which has been the subject of internal discussions for 15 months, in a 5-0 vote at a brief meeting without discussion.

“Nuclear power plants are inherently robust structures that our studies show provide adequate protection in a hypothetical attack by an airplane,” NRC Chairman Dale Klein said in a statement, adding that plant operators already must be able to manage large fires or explosions, no matter the cause.

Klein called the new rule “only one piece” of an effort to enhance reactor security and said the NRC would continue to examine and discuss the issue of airborne threats and take additional actions if found to be necessary.

The defense plan, formally known as the Design Basis Threat, spells out what type of attack force the government believes might target a commercial power reactor and what its operator must be capable of defending against.

While details are sketchy because of security concerns, the plan requires defense against a relatively small force, perhaps no more than a half-dozen attackers, but that they could come from multiple directions, including by water, and could include suicide teams.

The plan, which formally approves many of the procedures that have long been in place, reflects the increased concerns raised by the Sept. 11, 2001, terrorist attacks. It also includes measures to address cyber attacks, according to the NRC.