I recently wrote in detail about the devastating effects of the detonation of a nuclear device in a major city. I have also written about the nuclear threat from North Korea. The big question is whether N.K. has or will have soon the capability to fire a nuclear-armed missile accurately enough to hit a major West Coast city. However, it is also possible that the detonation of a nuclear device many miles away can have a very damaging effect on the electronic devices and infrastructure of a city so perhaps accuracy is not so important. Such an attack is called an electromagnetic pulse or EMP.
The Report of the Commission to Assess the Threat to the US from EMP Attack states: “When a nuclear explosion occurs at high altitude, the EMP signal it produces will cover the wide geographic region within the line of sight of the detonation. This broad band, high amplitude EMP, when coupled into sensitive electronics, has the capability to produce widespread and long lasting disruption and damage to the critical infrastructures that underpin the fabric of U.S. society.” The pulse can also travel over power lines and destroy substations and power generators.
The EMP signal has three components in sequence. The first component is the most intense and damaging. It occurs withing ten billionths of a second. Intense gamma radiation from the blast rips the electrons off the atoms of the gases in the atmosphere and hurls them at nearly the speed of light over the area in line of sight from the blast. The pulse that is created can cause very high voltages in electronic devices that overwhelm regular surge protectors. Special surge protectors are able to withstand the effect and are being adopted more widely. The second part of the signal lasts from one-millionths of a second after the blast to about one second. It is generated by neutron collisions following the explosion and is similar to the signal generated by a lighting strike. Existing protection against lightning can protect against this part of the signal. The third part of the signal lasts from tens to hundreds of seconds after the blast. It is generated by the effect of the blast on the magnetic field of the Earth that is distorted and then snaps back to its regular configuration. This part of the signal is similar to geomagnetic storms and it can induce high voltages in very long conductors such as power lines.
As with other effects of nuclear detonations, the size of the EMP is related to the size of the nuclear explosion in terms of equivalences in tons of TNT. There are some assumptions that are made which simplify the calculation of the effects of a particular EMP. The detonation is a symmetrical sphere. The strength of the magnetic field of the Earth is ignored. Three percent of the strength of the explosion is expressed in the gamma rays that are generated. The gamma rays are produced within ten billionths of a second of the explosion. About six-tenths of a percent of the gamma rays produce relativistic electrons. The threshold of the electric field damage is fifteen thousand volts per meter or more for the first part of the signal.
When all these factors are taken into account, a simple equation emerges. The distance in kilometers of damaging effects from an EMP is equal to the strength of the explosion in kilotons. So a twenty kiloton blast would cause damage in a circle twenty kilometers in diameter around the point under the detonation. Biggest test detonation of a nuclear device in North Korea so far has been about twenty kilotons. So if N.K. could send a nuclear missile to the West Coast of the U.S., there would only be EMP damage out to about six miles directly under the blast. In conclusion, concerns about an EMP attack from N.K. are overblown.
EMP graph:
