Not long ago the North Koreans tested a nuclear bomb. What did not make the mainstream news was that the yield was only about the equivalent of 3 kilotons of trinitrotoluene (TNT) which has an energy release of about 4.184 MJ/kg. A metric tonne is 1 Mg, and a kilotonne is thus 1 Gg. Then the total bomb energy was about 12 TJ. This is not big as nuclear explosions go. Military and intelligence analysts have concluded that the bomb fizzled and that the yield was not nearly as high as should be expected.

However, one former CIA nuclear weapons analyst noted that the North Korean bomb fit the signature of a kind of electromagnetic pulse (EMP) bomb the Soviets had designed. These are nuclear fission bombs that are optimized to produce a maximum-strength electromagnetic pulse. Detonated 200 km over the center of the USA (Kansas), at the altitude of low-earth orbit, it would take only one such bomb to produce an EMP event consisting of three phases, E1 through E3. E1 lasts about 100 ns and is caused by the early release of an intense burst of gamma rays – high-frequency (and therefore high energy) EM radiation that upon encountering air in the mid-stratosphere generates by the Compton effect (photons freeing electrons from atoms) a flux of near-light-speed electrons. This electric current, flowing downward through the atmosphere and being deflected into a spiral trajectory by the geomagnetic field, has a current density of about 48 A/m2 . After a few hundred milliseconds, E2 follows, consisting of neutrons that cause an effect much like lightning. E3 follows E2 and lasts about 15 minutes as the geomagnetic field recovers. E3 is capable of destroying an electric distribution grid.

The U.S. government observed EMP effects in 1962 in the mid-Pacific Ocean when 1445 km (898 miles) west of Hawaii at an altitude of 400 km (250 miles) a 1.44 Mton bomb was exploded. It created an electric field of 5.6 kV/m that knocked out 300 street lights in Honolulu, Hawaii and damaged a telco microwave link there.

A nuclear EMP attack would leave buildings and infrastructure largely intact, but would destroy much of everything electrical. As the trend in electronics technology has generally been toward greater fragility, susceptibility to EMPs has increased. For a while, the Soviets were intentionally retaining electron tubes in their military equipment because they are less vulnerable to EMP malfunction.

Humans are apparently not adversely affected by strong electromagnetic fields, though increasingly, metal implants are finding their way into the human body. One such implant is for teeth. The structure has two parts: the tooth itself (the crown) and the metal-alloy base that is screwed into the jawbone. This base is often made of titanium or a titanium-aluminum alloy. A screw of titanium of 1 cm length positioned vertically in a nuclear EMP field of 50 kV/m strength will develop across it 500 V. If the titanium piece has a resistivity of about 1.7 μΩ·m, is 3 mm in diameter and 10 mm long, then its resistance is 2.4 mΩ. It is in series with surrounding bone and gum tissue with substantially more resistance. Human skin provides protection against external electric shock with typical body resistance from 20 kΩ to 200 kΩ. However, catheterized patients and internal tissue have much lower resistance.

It is difficult to produce a resistance value for the completion of an implant circuit through bone and gum tissue, but from measurements given on the Web, a value of 1 kΩ is assumed here. Then the current in the implant loop is, by Ohm’s Law, 500 mA and the power dissipated is 250 W for the duration of the E1 pulse, which can be anywhere from 10 ns to 1 μs. Using the worst-case value of 1 μs, the energy dissipated around the implant is 250 μJ, or 0.25 mJ. This is not a significant amount of energy and therefore, tooth implants should not pose a major hazard to those having them during a nuclear EMP event.

Similar calculations could be carried out for larger metallic implants. At the least, an implanted person would experience a brief jolt at the implant site from the high voltage. Heart pacemakers and hearing aids – devices with electronics – would pose a more severe problem, especially if they are life-critical.