On fire effects of nuclear weapons and military planning

by Wm. Robert Johnston
31 January 2004

Lynn Eden, a sociologist at Stanford Universityís Center for International Security and Cooperation, recently published a book titled Whole World on Fire: Organizations, Knowledge, and Nuclear Weapons Devastation. The first chapter was adapted into an article "City on Fire" appearing in the Jan./Feb. 2004 issue of The Bulletin of the Atomic Scientists. Her thesis is that the thermal effects of nuclear weapons have been "ignored" in a "systematic" fashion by U.S. military planners, with the result of "misleading" policy makers. The following discussion (which will focus on the "City on Fire" article) will illustrate that Edenís thesis is undermined by an insufficient treatment of the relevant physics and other problems--in particular, by her incorrect claim that thermal effects are relatively predictable.

* Eden notes that her one-warhead scenario for Washington, D.C. is unrealistic, quoting a "former commander in chief of the U.S. Strategic Command" as saying "We must have targeted Moscow with 400 weapons...I can think of 50 to a hundred targets right off....I would be comfortable saying that there would be several dozens of weapons aimed at D.C."

It is worth noting that there are substantial differences between targeting of Moscow and targeting of Washington, D.C. The 400 warheads for Moscow includes those targeted on the greater Moscow area, including hardened ABM silos and radar sites, hardened shelter facilities, etc. Additionally, the large number of warheads reflects the fact that the U.S. would use dozens of low-kiloton range bomber weapons. In contrast, the Soviets/Russians would use a smaller number of half-megaton-range warheads.

* "More than 95 percent of the energy initially released would be in the form of intense light." This is incorrect. In the first fraction of a second this large a fraction may be in the form of gamma/x-rays and thermal/kinetic energy of fission products and fireball matter. However, a majority of this energy is eventually converted to blast. After about one minute, about 50% of an airburst's energy will be blast, 35% thermal flash ("intense light"), and 15% ionizing radiations.

* "By the time the fireball approached its maximum size, it would be more than a mile in diameter. It would very briefly produce temperatures at its center of more than 200 million degrees Fahrenheit (about 100 million degrees Celsius)--about four to five times the temperature at the center of the sun." This is backwards. In the first few microseconds of detonation, parts of the vaporized warhead are at the stated temperature (60 to 100 million C). The resultant x-ray radiation disperses this heat to the surrounding air, creating a fireball which at maximum size has temperatures inside and out of order 3000-20000 degrees C. Also note that her figure of 100 million degrees C corresponds to only 180 million F, and is about 6 times higher than the temperature at the center of the Sun (16 million C).

* Eden dwells on the maximum luminosity of the fireball--for example "At this distance, for a split second the fireball would shine more than 5,000 times brighter than a desert sun at noon." However, she does not fully acknowledge the short duration of this maximum. Averaged over the 3 seconds it takes for 80% of the thermal energy to be emitted, the average intensity of radiant energy from the fireball at the distance referenced here is 1,900 times that of the Sun.

* "The marble on the Lincoln and Jefferson memorials would crack, pop, and possibly evaporate." Evaporation of marble at 1.3 miles from a 300 kt burst is unlikely. Note that some missile silos are designed to survive conditions at ground zero in her scenario.

In discussing issues more central to her thesis, Eden confuses the relevant fire effects, and uses this confusion to support her erroneous conclusions. She lumps several distinct phenomena together as "mass fire". In reality, much of what she describes is a firestorm. A firestorm (which happened at Hiroshima, Hamburg, and Dresden) is a massive fire with high inward winds, feeding oxygen to the fire. Temperatures in the fire zone get incredibly high, but the fire doesn't spread. In contrast, a conflagration (which happened at Nagasaki) is a mass fire which is spread by winds--this is a bit like the "line fire" Eden mentions, but covering a large area simultaneously. Temperatures don't get as high, but it will spread. The burned out area of Nagasaki spread beyond the flash zone as a result. By confusing the two types of fires, Eden claims that the intense effects of a firestorm would spread beyond the area of mass ignitions like a conflagration--a claim not consistent with relevant observations.

* Eden claims that fire effects are predictable, which is not true. Note that mass fires did not result from detonations on Pacific islands and in the Nevada desert. Mass fires require fuel. It is not clear that a nuclear detonation over a city will leave enough fuel widely exposed. Recall that the collapse of the WTC towers largely extinguished the mass fires of burning jet fuel and office contents. Some buried fires continued, but in a blast demolished city this won't support a firestorm. The mere examples of Hiroshima and Nagasaki (firestorm in one, conflagration in the other, both in comparable urban environments with comparable weapons) show unpredictability.

* Eden minimizes the effects of weather. Airbursts at altitudes chosen to maximize blast effects on soft (urban) targets could easily occur above a cloud cover. She talks about ways snow could maximize flash effects, but ignores the fact that it would mostly minimize the effects. In particular, snow covered roofs, lawns, etc., would be significantly protected.

* In minimizing the influence of snow, Eden cites the firestorm in Dresden. The analogy is flawed. The Dresden fire was set by incendiary bombs, with flammable liquids that burn (somewhat) regardless of the presence of snow. A nuclear explosion sets fires by light, which will be reflected by snow before it can reach fuel.

* Eden claims firestorms are well predicted from physical principles. The same physical principles govern formation of thunderstorm cells. Can the formation of individual thunderstorm cells be predicted? Not analytically, and not numerically. The chaotic effects of specific meteorological starting conditions are too significant.

* Claiming that military planners could take into account severe weather conditions is misleading. In a strategic war, thousands of targets will be hit: uncooperative weather is certain to be present somewhere in the U.S. Soviet/Russian targeters could adjust the burst height if Washington is under cloud cover, but you're unlikely to be able to select the time and date of your global thermonuclear war on the basis of weather at one target location.

* Eden discusses the fire effects on an aircraft carrier. While her point is partly valid, it should be noted that carriers have construction and procedures intended to prevent deck fires from necessarily sinking the ship. Cold War naval vessels actually have some design aspects motivated by resistance to effects like flash.

In short, flash effects really are unpredictable. They should be addressed more, because they are going to be significant--more so for civilian targets than military targets, for example. But Eden's claims that they are well quantifiable and thus an oversight in defensive planning are both false.

In a review of Edenís book on the National Security Archive web site, her book is described:

"Exploring why the military disregarded nuclear firestorms, Eden argues that the resulting underestimation of the destructiveness of nuclear weapons made top officials poorly informed about weapons effects and contributed to unnecessarily high levels of nuclear forces. The organizational failure to understand weapons effects, Eden argues, could have produced a nuclear catastrophe; similar organizational failures produced the Titanic disaster and the poor fireproofing of the World Trade Center."

This suggested even more questionable conclusions. The reference to "unnecessarily high levels of nuclear forces", a clear judgment call, presumably refers to the strategic nuclear arsenal from the 1950s onward. However, the levels of this arsenal was driven principally by efforts to address counterforce, not counter-value targets--in other words, to counter increasingly hardened military targets, not soft urban targets. Hence, the effect of weapons on cities was not the primary factor in decision-making on arsenal size.

Regarding the reference to "poor fireproofing of the World Trade Center" as an "organizational failure", note that the WTC as originally designed had superior fireproofing than the final product: during construction of the WTC, New York outlawed the use of asbestos fire insulation, forcing the use of a less satisfatory substitute. The "organizational failure" at issue here involves environmental regulatory agencies.

© 2004 by Wm. Robert Johnston.
Last modified 31 January 2004.
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