December 2001

Nuclear Plants Unfit to Withstand Impact of Aircraft Crashes

Nuke Info Tokyo, Jan./Feb 2002, n°87
Citizens' Nuclear Information Center (CNIC: "")
By Yukio Yamaguchi

[Posted 19/12/2001]

Since the events of September 11, we can no longer ignore the big question: Would nuclear facilities be able to withstand the impact of a large, fully fueled aircraft?

There are two matters to be addressed here: kinetic energy and chemical energy. First, by assessing the kinetic energy involved in a collision, the extent of damage caused by a large object crashing into a nuclear plant can be appraised. The other matter to be considered is to evaluate the extent of damage that can be done to facilities depending on the various forms that the chemical energy produced by aircraft fuel can take. It is relatively easy to make assumptions on the first issue on the basis of assessments or studies that have already been made. The second issue is not as straightforward because one key factor is extremely uncertain, complicating all hypothetical calculations: We cannot know beforehand which type of reaction -- combustion, deflagration, explosion, or detonation -- will be the main cause of damage. Reality can be very far removed from the results of laboratory experiments, with all their uncertain assumptions. The actual conditions in a real-life incident can produce diverse and unpredictable complications.

In what follows, I will discuss the minimum assumptions which should be taken into account in considering the likely impact of kinetic and chemical energy from an aircraft crash.

(1) Estimating kinetic energy

The kinetic energy Ek (in joules) of a mass (m) kilograms travelling with speed (v) meters per second is given by the formula:

Ek (J) = 1/2 mv2

In the court case fought over the legitimacy of the government's grounds for licensing the Uranium Enrichment Facility located at Rokkasho Village, Aomori Prefecture, one of the matters at issue was the safety of the facility in the event of an aircraft crash or a hit by missiles from the nearby U.S. Misawa army base and firing range. The ruling in this case will be handed down in March 2002. The two parties argued over whether or not damage would be caused if the engine of a phantom jet fighter (m = 20t, v = 150m/s) crashed into the facility's reinforced concrete walls.

The defendant's argument only evaluates the impact of a military fighter crash, but if a Boeing 747-400 (m = 375t) traveling at a speed of 916 km/h (= 254m/s) crashed into the facility, the kinetic energy produced would be about 54 times that of a phantom jet fighter:

(375/20) X (254/150)2 = 54 (approximately)

In court cases involving nuclear plants, the government has referred to an experiment conducted by the U.S. Sandia National Laboratories in which detailed data was collected on the impact of a F-4 Phantom (m = 12.7 t) traveling at a speed of 215m/s crashing into a 3.66m thick, 7m2 concrete block (1993). The government cites this experiment to support its argument that the walls of the nuclear reactor vessels can withstand aircraft crashes. The government also cites an experiment conducted in the former West Germany, in which a steel column (m = 1t) traveling at a speed of 222m/s was crashed into a reinforced concrete wall. The conclusion of this experiment was that if a wall is thicker than 70cm, it would withstand such a collision. Though accurate comparison cannot be drawn from these experiments, which were conducted under certain assumed conditions, if a Boeing 747-400 (m = 375t, v = 254m/s) crashed into, for example, the reactor building of Oi 3 (PWR, 1180MW), it can readily be assumed that the 110cm thick ceiling would be damaged.

(2) Evaluation of chemical energy

The chemical energy of aircraft fuel is:

4.2 X 104kJ per 1kg

This is 10 times the chemical energy of high-quality TNT.

If it is assumed that the Boeing 747-400 is loaded with a full tank of 145t (= 145 X 103kg) fuel, the chemical energy of the fuel equals:

145 X 103 X 4.2 X 104 = 6 X 109kJ (approximately)

This is about five hundred times that of the aircraft's kinetic energy:

1/2 X 375 X 103 X (2.54 X 102)2 X 10-3kJ (J = 10-3kJ)
= 1.2 X 10-7kJ (approximately)

Indeed, compared with the kinetic energy of the Phantom cited by the government in the Aomori court case, the chemical energy of the above-mentioned fuel is about 25,000 times greater:

54 (comparison of Ek) X 500 = 25,000 (approximately)

The assertion that the uranium facility could withstand a large-scale aircraft crash is shown by these calculations to be completely without support.

How much potential does chemical energy have to cause damage? Fuel can partially gasify and mix with liquid fuel to cause detonation. Ordinary combustion takes place with a combustion speed of between a few mm/s and a few cm/s; but detonation can far surpass the speed of sound (= 340m/s - approximately), traveling at an extremely high speed of a few km/s, and transmitting in an instant with a high-pressure and high-temperature shock wave. Most objects would be destroyed and melt from the high-temperature.

Steel embrittlement must also be taken into account. There are various types of special steel which can withstand high temperature and are used in the manufacture of boilers. When elements like Chromium (Cr), Molybdenum (Mo), Titanium (Ti), and Niobium (Nb) are added to steel, the permissible stress is improved by 100~150°C. However, it is not possible to prevent rapid decreases in permissible stress at temperatures of around 500~600°C (see Fig. 1).

When all these factors are taken into account, it is clear that reinforced concrete cannot be assumed to be heatproof against detonation. There is an immeasurable difference between the destructive force of chemical energy produced by the most common process, combustion, and that released by a detonation process. Considering that, although about 6,000 people are assumed to be missing, fewer than 300 bodies have been recovered from the wreckage of the World Trade Center towers in New York, I cannot help suspecting that detonation took place. The buildings were made additionally vulnerable by the fact that they were constructed with a double tube structure similar to a bird cage, in which units of three stories were joined together with bolts.

In the Aomori court case, the defendant assessed the damage that could be caused to the nuclear facility if a jet fighter lost its thrust and crashed to the ground at a speed of 150m/s, and if the 4m3 of fuel in the plane's reserve tank leaked into the uranium storage building, flowing down a floor that has a 1/100 slope, and catching fire. The defendant's conclusion was that because the fuel would burn up in about 2~3 minutes, 6 minutes at the most, the effects on uranium, stored in the facility in the form of uranium hexafluoride (UF6), could be safely ignored. It is clear from the arguments presented in this paper that this is a wholly unconvincing conclusion.

Nuclear energy, which can only be obtained by the use of massive amounts of steel and uranium, is, to borrow A. Lovins' term, one of the "hard energy paths." September 11 incident demonstrated all too vividly that this form of energy production has a defenseless Achilles' heel. Surely it is time to develop our reliance on the "soft energy paths."**

*Iron and Steel Material, Japan Metal Society edit., Japan Metal Society, 1985, p.200
**Soft Energy Paths - Towards a Durable Peace, A.B. Lovins, Friends of the Earth, Inc. 1977

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