“THAAD: What It Can and Can’t Do”
There are many variables in calculating projected population casualties due to the effects of nuclear weapons. The causes of population fatalities and injury from nuclear weapons fall into two categories: prompt effects and fallout. Prompt effects include exposure to ionizing radiation (photons and neutrons), blast overpressure and thermal (skin burns, eye damage). Prompt effects occur immediately after a nuclear weapon is detonated. Fallout effects include long-term exposure to radioactive particles falling from the atmosphere downwind from a nuclear detonation.
The yield of nuclear weapons is based on estimates of current capabilities or projections of future capabilities. Cities are considered “soft” targets. Nuclear weapons allocated to “soft” targets are calibrated to detonate at optimal height (air burst) to produce the most damage. Air burst weapons produce negligible fallout which was the case for both Hiroshima and Nagasaki. If a nuclear weapon was ground burst or burst low enough where the fireball touches the ground then fallout would occur requiring more extensive calculations including variables for weather, wind direction and speed, fallout exposure time (hours, days, weeks, months), radioactive decay, and long term population sheltering.
Population and population density are known variables. The population density is the average density over the city’s entire area. The actual population density varies throughout a city. In Hiroshima, the population density near ground zero was far higher than the rest of the city. Related factors are the day of week and time of day when the detonation would occur. Population densities in downtown business areas are far higher during normal weekday work hours then they are at night or on the weekends. If the population was given sufficient warning of an impending attack, evacuations could occur, thus reducing the size of the population exposed. Various shelter protection factors that can reduce the effects of a nuclear detonation are based on the type of shelter such as is underground, frame house, multi-story upper or lower floors, concrete structures of various thickness, and vehicles (autos, trucks, buses). Sheltering can mitigate both prompt and fallouts effects.
Mathematical models are used to estimate the casualties from nuclear explosions. These models attempt to predict the probability of deaths and injuries from the effects of nuclear explosions. Mathematical models are created from extrapolations of information in the Hiroshima data archive, available in publications such as Glasstone & Dolan’s, “The Effect of Nuclear Weapons” . Scaling the yield of the Hiroshima weapon for peak blast overpressure and distance allows simulations with weapon yields larger than that of Hiroshima. Many computer programs have been created that use the mathematical models to provide rapid simulation for a large number of scenarios with a full range of weapon yields and dozens of input variables for both prompt and fallout effects. Some of these programs are available in the public domain . The results of these simulations can vary significantly depending on the scope and range of input variables and the sophistication of the computer models.
Traditionally the standard method for determining casualties is based on extrapolation of peak blast overpressure distances from Hiroshima data for airburst detonations—the “overpressure” model. There are other more elaborate models, one of which is the conflagration model (firestorm effects) whereby the casualties are increased because the population surviving the initial blast does not escape the firestorms found near the outer damage rings/zones . The following are the results of simulating a single 20kt nuclear warhead airburst at optimal height over Seoul, South Korea using the traditional “overpressure” model based on data from both Glasstone & Dolan  and Von Hippel, et al. . Seoul proper:
|Land Area Sq. Km.
|Density People/Sq. Km.
|Radius of City km
As noted previously, alternate input variables and alternate simulation models will result in a wide range of results. However, it is clear from the results of the simulations above that even a single Hiroshima/Nagasaki-like nuclear detonation (20kt) will cause significant casualties.
 US Department of Defense and the Energy Research and Development Administration, The Effects of Nuclear Weapons, by Samuel Glasstone and Philip J. Dolan, (Washington, DC, 1977), http://nnsa.energy.gov/sites/default/files/nnsa/inlinefiles/glasstone%20and%20dolan%201977.pdf. Includes a “Nuclear Bomb Effects Computer” circular slide rule.
 “HotSpot – Health Physics Codes for the PC,” Lawrence Livermore National Laboratory, https://narac.llnl.gov/hotspot; and Alex Wellerstine, “NUKEMAP,” The College of Arts and Letters, Stevens Institute of Technology, 2012-2014, http://nuclearsecrecy.com/nukemap/.
 William Daugherty, Barbara Levi and Frank von Hippel, “Casualties Due to the Blast, Heat, and Radioactive Fallout from Various Hypothetical Attacks on the United States,” Center for Energy and Environmental Studies, Princeton University (Report #PU/CEES 198, 1986).
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