US nuclear rule for aircraft impact

Thursday, 19 February 2009

Reactor designers must address the effects of a large aircraft impact, but resistance to such an attack remains 'beyond-design-basis' under regulations.

 

The decision was announced yesterday by the US Nuclear Regulatory Commission (NRC). In future, when a utility applies to the NRC to build a new power reactor, it must assess the ability of the proposed design to "avoid or mitigate the effects of a large commercial aircraft impact."

 

The cage atop the Opal research reactor in Australia is designed to mitigate potential small aircraft impact.   In a US nuclear power plant, this kind of feature would not be mandated or require redundancy, but would nevertheless come in addition to a reinforced concrete containment structure and multiple redundant safety systems.

Nuclear power plants have always been constructed to remain safe during large fires, floods, earthquakes and hurricanes, all of which must be factored into the basic design. Designers must take precautions including the multiple redundancy of systems to maintain essential nuclear safety functions such as reactor core cooling, the integrity of the containment structure, used nuclear fuel cooling capability and used fuel pool integrity. These functions should be maintained after a large aircraft impact, the NRC said.

 

However, any design feature or functional capability of proposed new nuclear power plants solely meant to help in the event of a large aircraft impact should "meet high quality standards but is exempt from NRC design-basis regulations, such as regulations for redundancy."

 

NRC chairman Dale Klein said the ruling was "a common sense approach" to the issue. He added: "I'm quite confident that this rule will be an important element in the regulatory framework for new reactor applications that will result in a margin of safety far beyond that required to achieve a reasonably assurance of public health and safety."

 

The ruling comes after a February 2002 order from the NRC for nuclear plant operators to "develop strategies to mitigate the impact of large fires and explosions that could be caused by an aircraft impact." Other terrorism-related changes have related to the potential squads of assailants that nuclear plant staff must be prepared to repel.

 

Nuclear power plants are already recognised as being among the most robust structures, which also boast the most comprehensive security of any industrial facility. Impacting aircraft would not be expected to penetrate the containment of a reactor building, which is composed of reinforced concrete at least one metre thick. Some modern design plants such as Areva's EPR and the Russian VVER-1200 boast two containment walls.