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:
o The large aircraft is assumed to be traveling at cruising speed,
which for commercial aircraft is approximately 500 miles per hour.
o 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.
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Assessment of Aircraft Impacts on
Nuclear Plant Containment Structures
o 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:
o 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.
o 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.
o 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
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Assessment of Aircraft Impacts on
Nuclear Plant Containment Structures
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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