Nuclear Plant Containment Structures

Assessment of Aircraft Impacts on

Nuclear Plant Containment Structures

Purpose:Provide an assessment of the effect of an impact of a large aircraft into a nuclear facility containment.

Summary:The NRC requirement to examine aircraft hazards is outlined in section 3.5.1.6 of the Standard Review Plan (Reference 1, attached). In summary, the Standard Review Plan calls for an assessment of the probability of an aircraft impact based on aircraft accident statistics and the proximity of the plant to expected flight paths. If the probability of aircraft accidents resulting in radiological consequences greater than the regulatory exposure guidelines is greater than 1 x 10-7 per year, a detailed review of aircraft hazards must be performed. Only three plants in the United States have performed this analysis: Three Mile Island, Seabrook, and Limerick.

Aircraft impact studies on reactor buildings typically evaluate two effects, building collapse and wall penetration. Building collapse is dependent on the weight and speed of the entire aircraft. The probability of wall penetration is dependent on the impact of the aircraft engines because the engines are the most rigid structures in the aircraft. The aircraft body and wings, being made of aluminum and, in the case of the body, largely empty space, crushes. It is important to note that an aircraft engine is a small fraction of the total weight of the airplane. Engines on large commercial aircraft weigh about 5,000 to 7,500 pounds.

The study that has been cited frequently in relation to aircraft hazards is the Probabilistic Assessment of Aircraft Risk for Nuclear Power Plants (Reference 2, attached). This study concludes that an impact of a large commercial aircraft on a flat reinforced concrete wall would have the following effects, expressed as probabilities of failure:

Penetration:

2-foot thick wall – 84%

6-foot wall – 32%

Collapse:

1 ½ foot thick wall - > 50%

The following assumptions are used in Reference 2:

• The large aircraft is assumed to be traveling at cruising speed, which for commercial aircraft is approximately 500 miles per hour.
• The analyzed walls are flat and reinforced with #9 reinforcing bars (1 1/8 inch diameter) spaced on 9-inch centers on both sides of the slab.
• In the penetration study the engine impacts the wall perpendicularly.

A summary of typical reactor containment design features is provided in Attachment 3.

When the flat reinforced concrete wall of Reference 2 is compared to the design information in Attachment 3 we can make the following observations:

• It is very difficult to impact a containment wall perpendicularly because:
• Primary containment surfaces are curved (cylindrical walls and a domed roof). In order to strike a curved surface perpendicularly it must be struck in a direction precisely on line with its center of curvature. In addition, in order to maximize the probability of penetration, most large commercial airplanes must be aimed so that the body strikes the containment obliquely in order for an engine mounted on the wing to be driven perpendicularly into the containment.
• The containment is a relatively small target that must be struck precisely while traveling at a high rate of speed and avoiding any surrounding structures and other obstacles.
• The flat concrete walls (reactor building wall in the collapse mode analysis of Reference 2) are not as strong as the walls of a primary containment:
• Primary containment walls are typically thicker than the walls analyzed (see Attachment 3).
• Primary containment is more strongly reinforced (larger reinforcing bars with closer spacing).
• The walls of primary containments are curved, which is an inherently stronger design than a flat wall.
• In a boiling water reactor the primary containment is completely contained within a reinforced concrete reactor building (see Attachment 3) similar to the building analyzed in the collapse mode analysis.
• The walls analyzed did not include a metal liner that would further inhibit penetration and equipment damage from fracture of the inner concrete surface.

Reference 2 includes an assessment of post accident fire associated with the aircraft accident. The study states: “In the event that the building wall is not breached, most fires would dissipate themselves to the environment and only cause local cracking of the concrete unless fuel was spilled into the ventilation ducts for the control room or other critical area.” The issue of ventilation ducts is not applicable to reactor containments as there are no ventilation ducts penetrating the containment structure.

Conclusions:An aircraft impact will not cause collapse or penetration of a containment building at a nuclear power plant.

References:

1. Standard Review Plan (NUREG 0800) Section 3.5.1.6, Aircraft Hazards
2. Ian B. Wall, Probabilistic Assessment of Aircraft Risk for Nuclear Power Plants, Nuclear Safety, Vol. 15, No. 3, May – June 1974
3. Reactor Containment Design Features

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