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Inaccurate Prediction of Nuclear Weapons Effects and Possible Adverse Influences on Nuclear Terrorism Preparedness
Robert C. Harney The unthinkable is probably inevitable. At some time in the future a terrorist group will detonate a nuclear explosive in a major metropolitan area. Nuclear non-proliferation regimes are not working. The earliest U.S. policies failed to prevent the U.S.S.R., United Kingdom, France, and China from developing nuclear weapons. Later policies failed to deter Israel, South Africa, Pakistan, and India. They have not proven successful with North Korea or Iran and did not work in Iraq (unless you count invasion as an element of our non-proliferation policy). The few apparent successes (South Africa, Libya, etc.) can be attributed to internal factors as much as to the effects of non-proliferation activities. Once nuclear weapons are in the hands of unstable states or states that support terrorism, there is little doubt that one or more will ultimately wind up in the hands of non-state or state-supported terrorist organizations. Terrorist possession of a nuclear weapon will result in its use against a “highest-value” target – most likely a large city with major economic value, cultural and/or religious significance, and a dense population in which high casualties will result. The likelihood of an attack has prompted considerable public debate about what are the best steps to prevent such an attack. In many of these discussions estimates of the number of casualties or the size of the area that would be damaged by an attack are used to reinforce the importance of action.
1
Ironically, as discussed later, these estimates may evoke inaction in some critical areas. Paraphrasing many examples, they typically state: a Hiroshima-sized weapon detonated in a major metropolitan area will kill a million people or will vaporize everything within a half-mile of ground zero or some other equally dramatic claim (although some scenarios are less cataclysmic). To this author, the estimates do not ring true – they sound excessive. The estimates are often quoted or repeated by individuals who clearly lack technical expertise in nuclear weapons effects and original sources for the estimates are seldom cited. Although it is possible that some are the product of hyperbole used in political oratory to reinforce a point, the frequency is too high for this to always be the case. It is more likely that valid estimates made for a military attack scenario have been improperly extrapolated to the terrorist scenario. However, if the policymakers making such statements actually believe these estimates, then inaccurate information is being used to set policy, and something should be done to rectify the situation. Such “excessive” estimates have been used to establish emergency response planning guidance.
2
It remains to be seen whether this will result in over-preparation or under-preparation. Neither is desirable. The primary purpose of this paper is to discuss the accuracy of common effects estimates and describe how more realistic estimates might affect nuclear terrorism preparedness.
H
ARNEY
, N
UCLEAR
W
EAPONS
E
FFECTS
HOMELAND
S
ECURITY
A
FFAIRS
,
V
OLUME
V,
NO
. 3 (S
EPTEMBER
2009)
WWW
.
HSAJ
.
ORG
2
STANDARD EFFECTS ANALYSIS
The standard weapons effects prediction process occurs as follows. The desired type of nuclear explosive, its yield, and its height of burst are selected. The distances at which specific effects levels are expected to be achieved are estimated using relations derived from comparison of theory to measurements obtained during nuclear testing. Using these distances, areas are calculated that are associated with each effects level. The effects levels are then correlated with percentages of casualties. This correlation is somewhat subjective, but in the best cases is based on modeling that has been validated by the results from Hiroshima and Nagasaki. Once a target has been selected, population density data, the calculated effects areas, and the casualty correlations are multiplied to estimate the total numbers of casualties expected.
For purposes of example, we will assume that a Hiroshima-sized fission weapon (nominal 10 kT) is the most probable terrorist weapon. Slightly smaller or larger yields will not dramatically alter the results. Doubling the yield results in 22% larger blast damage distances and less than 49% larger areas (or casualties). Manhattan (New York City) is assumed to be the hypothetical target as it is arguably the highest probability target in the United States. It has the highest workday population density, it is the economic capital of the country, and it is a symbol of freedom and American might and prosperity. The “standard” analysis is an outgrowth of military effects analysis. Most experienced weapons-effects predictors learned their skills while addressing either global thermonuclear war or the tactical employment of nuclear weapons. Thus, virtually all examples used to guide novice or inexperienced effects predictors will be based on military analyses. With the exception of nuclear attacks on missile silos, deeply buried command centers, naval targets, and similar targets, an optimum altitude airburst is assumed in military nuclear-effects analyses. The optimum altitude airburst is far and away the most common analytical assumption in nuclear effects analysis. As we shall see, this may be the source of the putative overestimates. The range at which each effect level occurs can be estimated from simple relations that scale with the nuclear explosive yield
W
(in kilotons, abbreviated kT). Scaling relations allow the experimentally verified ranges at which specific effects are produced for a reference explosion of known yield (typically 1 kT) to be extrapolated to the ranges at which those same effects would be produced by an explosion with a different yield. Hundreds of atmospheric nuclear tests at Nevada Test Site, Enewetak Atoll, and elsewhere have contributed to the verification of these scaling relations. The scaling relation for the distance (in meters) at which a specific overpressure (i.e., the pressure in excess of atmospheric pressure) is produced by air blast from the explosion is given by
3
R
Xpsi
(
W
) =
R
Xpsi
(1kT)
W
1/3
where the scaling distance
R
Xpsi
(
1 kT
) for a 1 kT optimum altitude airburst can be shown to be
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H
ARNEY
, N
UCLEAR
W
EAPONS
E
FFECTS
HOMELAND
S
ECURITY
A
FFAIRS
,
V
OLUME
V,
NO
. 3 (S
EPTEMBER
2009)
WWW
.
HSAJ
.
ORG
3
= 2125 meters for 1 psi overpressure = 1290 meters for 2 psi overpressure = 700 meters for 5 psi overpressure = 405 meters for 12 psi overpressure. These four overpressure levels are those used in the Office of Technology Assessment casualty correlation described later.
4
The relation for distance (in meters) at which different levels of thermal radiation is produced by the explosion is given by
R
2cal/cm2
(
W
) = 1180
W
1/2
R
8cal/cm2
(
W
) = 590
W
1/2
R
20cal/cm2
(
W
) = 375
W
1/2
The thermal radiation ranges are strongly dependent on atmospheric transmission. The values shown assume a perfectly clear day (no atmospheric attenuation). Ranges for hazy days will be shorter; the hazier the day, the shorter the thermal range. The relation for the distance (in meters) at which specific doses of direct nuclear radiation can occur is given by
R
70rad
(
W
) = 1200 + 500 log
W
R
300rad
(
W
) = 950 + 500 log
W
R
800rad
(
W
) = 800 + 500 log
W
Given a 10 kT airburst at the optimum altitude, the blast effects distances and their associated levels of damage are seen to be:
12 psi = 870 m for severe damage (steel-reinforced structures damaged) 5 psi = 1510m for moderate damage (wood/masonry structures destroyed) 2 psi = 2780 m for minor damage (wood/masonry structures damaged) 1 psi = 4580 m for light damage (windows shattered).
The thermal effects distance from ground zero is:
2 cal/cm
2
= 3730 m for first-degree skin burns (equivalent to a sunburn). 8 cal/cm
2
= 1865 m for severe skin burns & ignition of easily flammable materials. 20 cal/cm
2
= 1180 m for ignition of most flammable materials.
The distance associated with direct nuclear radiation effects (assuming no shielding) is:
70 rads = 1700 m for the threshold of radiation sickness (mild symptoms). 300 rads = 1450 m for the radiation sickness lethal threshold (approx. 5% fatalities). 800 rads = 1300 m for 100% fatal radiation sickness.
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H
ARNEY
, N
UCLEAR
W
EAPONS
E
FFECTS
HOMELAND
S
ECURITY
A
FFAIRS
,
V
OLUME
V,
NO
. 3 (S
EPTEMBER
2009)
WWW
.
HSAJ
.
ORG
4
The next step in the analysis is to correlate casualties with weapons-effects levels. Although nuclear radiation and thermal radiation produce casualties, overpressure appears to be the best single predictor of casualty levels.
5
The correlation used by the Office of Technology Assessment (OTA) and summarized in Table I is often used.
Table I. Correlation of casualty levels with overpressure.
Peak Overpressure Fraction of Population Density (psi) Dead Injured Uninjured >12 0.98 0.02 --- 5-12 0.50 0.40 0.10 2-5 0.05 0.45 0.50 1-2 --- 0.25 0.75 <1 --- --- 1.00
Consider now a 10 kT airburst in Manhattan. The average daytime population density in the Central Business District (Manhattan south of 60
th
Street) is 83,000 per square kilometer.
6
The maximum local daytime population density occurs in the half-mile (0.8km) area around Grand Central Terminal
7
and is approximately 330,000 per km
2
. Circular damage areas are calculated using the overpressure distances above. The areas are multiplied by the appropriate population densities and by the OTA correlation fractions to determine casualties. Details are summarized in Table II. Roughly 66 km
2
are damaged, over six million people are directly affected, and total casualties are estimated to be in excess of 2,700,000. The areas and the casualty estimates determined in this fashion are consistent with those mentioned in the public debates. The injury estimates may be too high as the 1-2 psi area includes large portions of the surrounding rivers.
Table II. Casualty analysis for a 10-kiloton airburst over Manhattan.
DAMAGE POPULATION ASSOCIATED ZONE RADII AREA DENS. TOTAL CASUALTIES (psi) (km) (km
2
) (km
-2
) --- DEATHS INJURIES >12 < 0.87 2.38 330,000 785,400 769,692 15,708 5-12 0.87 - 1.51 4.78 83,000 396,740 198,370 158,696 2-5 1.51 - 2.78 17.12 83,000 1,420,960 71,048 639,432 1-2 2.78 - 4.58 41.62 83,000 3,454,460 0 863,615 TOTALS 4.58 65.90 --- 6,057,560 1,039,110 1,677,451
This traditional casualty analysis coupled with observations of Hiroshima and Nagasaki presents a nearly “hopeless” picture. That is, one would expect that the southernmost one-quarter of Manhattan would be devastated. Roads through damaged areas would be impassable. Evacuation to mitigate fallout effects would probably be impractical in some areas. Power, water, communications,
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