The EMP commission’s executive report expresses the concern that “terrorists or state actors that possess relatively unsophisticated missiles armed with nuclear weapons may well calculate that… they may obtain the greatest political-military utility from one or a few such weapons by using them—or threatening their use—in an EMP attack.” Given that scenario, such a warhead would likely be launched by one of the Scud/No-dong/Shahab family of missiles. Since the payload of such missiles is limited to ~1000 kilograms, and only relatively crude technologies are available to such actors, we can safely assume that the yield would be on the order of ~1 kiloton [22]. By comparison, the gun-type U-based Little Boy (15 kilotons) weighed 4 metric tons (4,000 kilograms), and the Fat Man (21 kilotons) was an implosion Pu-based device and weighed 4.6 metric tons.
The EMP effects of a crude one-kiloton device , though still substantial, will be dramatically less than that of a one-megaton device. Firstly, a megaton-range EMP weapon is not very sensitive to the detonation altitude: any altitude between roughly 40 and 400 kilometers will yield a very strong E1 EMP pulse at ground level. On the other hand, the EMP effects of a smaller, one-kiloton warhead, is quite sensitive to the detonation altitude [16]. To boost the EMP lethality of a simple one-kiloton fission weapon, it must be detonated much lower than the hundreds of km that would expose the entire continental US to harmful electric fields. In fact, the “sweet spot” for maximizing the EMP lethality of such weapons would be a detonation altitude of about 40 kilometers—significantly higher, or lower, and the peak fields at ground level will decrease.
This lower altitude implies a smaller region on the ground will exposed to high E-fields, as the “horizon” (the farthest extent on the ground with direct view of the detonation) is closer to ground-zero. For 40 kilometers altitude, the maximum extent of the induced EMP E1-fields is within a 725-kilometer radius. In reality, this is an overestimate because the EMP far from the peak field region is inherently limited in strength by the lower initial gamma-ray yield for a small device, and the distant pulse also has a wider (and, thus, less threatening) pulse time-profile. Although in standard texts it is shown that the E-fields expected at the periphery of the exposed ground regions are roughly half the peak fields, this applies to large (>100 kilotons) devices [5]. For smaller devices the peripheral fields will be expected to be significantly below half the peak field. A reasonable estimate for the extent for the destructive EMP E1 fields from a one-kiloton burst at 40 kilometers is about 10 times the altitude, or ~400 kilometers radius [Fig. 1].
Thus, a standard “crude” one-kiloton device will not expose a very large area of the US to high E-fields, both because it will have to be detonated lower in the atmosphere to boost its EMP, and also because its EMP is inherently limited in strength.
Secondly, although a one-kiloton weapon could have a substantial peak E1 component in a limited region of the country, this component does not couple well to long-lines, and would not induce large currents in long cable runs. At the same time, a small weapon would have a significantly smaller E3 component (which is driven by the size of electrically charged fireball) than a megaton-range weapon, which, again, means that long-lasting country-wide power outages would not be expected.
Serious long-lasting consequences of a one-kiloton EMP strike would likely be limited to a state-sized region of the country. Although grid outages in this region may have cascading knock-on effects in more distant parts of the country, the electronic devices in those further regions would not have suffered direct damage, and the associated power systems far from the EMP exposed region could be re-started.
