AIR DEFENSE    BALLISTIC MISSILE DEFENSE    CIVIL DEFENSE

The Shape of the Threat

Walmer E. Strope

    What would a terrorist nuclear attack look like? Perhaps only some members of the terrorist group have a good estimate. What we do know is that the terrorist threat is wholly different from the threat that we faced during the Cold War. Back then, in the 1970s and 1980s, a nuclear exchange between the United States and the Soviet Union would have involved the detonation of thousands of multimegaton warheads throughout the industrial nations of the northern hemisphere. Many distinguished scientists lamented that such an event would mean the end of civilization. Other scientists predicted that the thousands of mushroom clouds and smoke from burning cities would so hide the sun that the planet would freeze up in a “Nuclear Winter.” Although these predictions may be exaggerations, the concept of civil defense—survival and recovery—seemed ludicrous.

    The terrorist nuclear threat is nothing like the above. For one thing, it most likely will consist of a single nuclear explosion. The only ways that a terrorist group such as Al Qaeda can obtain a nuclear device is to build one surreptitiously or obtain one from a willing supplier. Building a nuclear weapon requires not only an adequate supply of fissile material but also nonnuclear parts and facilities for precision machining and the like. North Korea had all that but when they tested their bomb it did not work. The mass media has saluted North Korea as a new member of the “nuclear club” of nations possessing nuclear weapons but all we know is that North Korean weapons don’t work. Al Qaeda probably wouldn’t buy one.

   Iran also is trying to get a nuclear weapon. “It seems hard to imagine that Iran does not already have them,” says historian Michael Ledeen. “Iranians are not stupid, and they have been at this for a minimum of twenty years in a world where almost all of the major components needed for a nuclear weapon—not to mention old nuclear weapons—are for sale.” But, so far, Iran has not found a willing supplier. We don’t know about Al Qaeda. We do know that Usama bin Laden has said that if he had several nukes he would put one in each of several US cities and attempt to detonate them simultaneously.

  The terrorist threat also is shaped by the size or yield of the nuclear explosion. Most analysts agree that the terrorist nuke will be about the size of those that ended World War II with the bombing of Hiroshima and Nagasaki—15 to 20 kilotons.  The reason for this judgment is that the least amount of fissile material (highly-enriched uranium or plutonium) needed to form a critical mass at the moment of detonation was used. The amount of fissile material needed is by far the most costly component and most difficult to obtain. Actually, the amount of fissile material used in those very-first nukes was slightly larger than the minimum needed. When the North Koreans tested their nuclear device, they expected a yield of 10 kilotons. They got virtually no fission yield as the critical mass blew apart before only a tiny portion could engage in a chain reaction.

  Another possible source of a terrorist nuclear weapon is said to be one lost when the Soviet Union dissolved. There are inadequate records for some weapons but these are not high-yield ICBMs but rather battlefield devices with yields around 10 kilotons. If there are such not under Russian control, their possible use by terrorists does not change the shape of the terrorist threat.

  An important aspect of the threat is whether the explosion would be high in the air as it was in the Hiroshima and Nagasaki attacks (an air burst) or on or close to the ground (a surface burst.) There appears to be no advantage to terrorists from attempting to deliver an air burst and several drawbacks. If the terrorist objective is to kill as many Americans as possible, the surface burst is superior, as it threatens to kill by blast, fire and fallout whereas the air burst has only blast and fire. A 10-kiloton surface burst produces a crater 280 feet wide and 65 feet deep. No survivors there, but there were several survivors at Hiroshima’s ground zero, directly under the air burst.

  A surface burst is the natural location for a concealed terrorist nuclear device and surprise is essential to the terrorist purpose, as it was at 9/11. Lack of warning is central to the terrorist threat and it means that civil defense cannot avoid a grievous loss of life. Even so, civil defense can save most of whom would otherwise die. Do both: try to prevent and be prepared to save.

  The unavoidable deaths and injuries from an attack without warning will occur not only in the crater and its lip but also from blast and fire out to the 10-psi blast overpressure level, which occurs about 0.4 miles from ground zero. People outside and unshielded may be lost out to one mile where the blast overpressure is 2 psi. But outside one mile will be thousands who can be at hazard from the radioactive fallout from a surface burst. Consider the following for a 10-kiloton fission explosion:

            Dose Rate                                Distance                                   Max. Width

            r/hr@1 hr                                  miles                                          miles

                 500                                          3                                              < 1

                  50                                          14                                                2

                   5                                           60                                                6 

                  0.5                                        150                                               16

  This table, which is adapted from Table 9.90 in The Effects of Nuclear Weapons, Revised Edition, requires some explanation. Some selected values of the unit-time reference dose rate are shown in the left column, their downwind distance and width in the center and right columns. The distance and width in miles is for a uniform wind direction at all altitudes and a uniform wind speed of 15 miles per hour. 

  When a 10-kiloton nuclear device explodes, about 20 ounces of radioactive fission products are produced as a very complex mixture of over 200 different isotopes of 36 elements. This radioactivity is swept up with large amounts of crater material into the mushroom cloud. Further mixing and cooling occurs as this material falls gradually to back to earth as fallout. At weapons tests, which is where all the available data have been obtained, the wind direction and speed are never uniform and measurements on the ground occur hours and days after the event. The 20 ounces of radioactive fission products have been being depleted since they were formed in the chain reaction. Their “decay curve” is well known and is used to adjust the measured values to a common reference time; namely, one hour after the explosion. That is the left-hand column above. The location measurements are adjusted in a similar fashion to form the dimensions of the ellipses commonly shown to represent fallout areas.

  The important point is that the dimensions in the table do not account for the dynamics of the event. To do that, we note that all the dimensions are for one hour after the explosion. Where is the mushroom cloud at that time? It is 15 miles downwind. In the table we see that the 50 r/hr dose rate extends 14 miles. So the fallout event is over at 15 miles with a peak of about 50 r/hr. Moreover, the fallout event is over at all distances less than 15 miles with peak dose rates greater than the one-hour reference dose rate. For example, we might assume that the maximum width occurs at half the full distance of 50 r/hr reference dose rate, or at 7 miles. But fallout occurred there at about 30 minutes after burst when the dose rate was 2.2 times higher than the reference dose rate or over 100 r/hr. Obviously, the maximum width of the 50 r/hr peak dose rate was substantially larger than shown in the table. Conversely, peak dose rates beyond 15 miles will be lower than the reference dose rate because the fallout arrives later than one hour after burst. As an example, the extent of 5 r/hr reference dose rate is shown to be 60 miles but fallout would not arrive until four hours after burst when the peak dose rate would be about 1 r/hr.

  What have we learned about the shape of the fallout threat from a terrorist nuclear explosion? First, the extent of lethal levels of fallout is confined to areas where fallout arrives within one hour. That area might be about 10 miles long and 3 miles wide or 30 square miles.  Since the area subject to blast and fire is about 3 square miles, there may be ten times as many people  subject to lethal fallout as are subject to lethal blast and fire. We can also conclude that people in the lethal fallout area must be given very good shelter or directed out of the area within two to four hours after the explosion to avoid fatalities.