Nuclear blasts and electrical disruptions

[Keeves]

The way I remember it from the movie “The Day After”, the electromagnetic radiation from the bomb disabled electrical devices even a great distance away from the blast. This included things like cars and transistor radios.

I think I understand how microwaves can melt computer memories (because the circuitry is to tiny, shaking the molecules even slightly will basically melt the whole thing). But would this really happen to other devices as well?

[Undead_Dude]

Nuclear blasts produce enormous amounts of gamma radiation in a very short burst. Gamma radiation is classified as “ionizing radiation”, because when it strikes an atom, it flings electrons off of that atom. In a nuclear burst, in a fraction of a second an enormous number of electrons are flung around. As a result, very strong electric and magnetic fields are created. These fields will induce electric currents in any conductive material they find. Electronic devices are simply vulnerable to the fact that a current far too strong for them to handle may be induced in their circuits.

[AuraSeer]

Microwaves are not a big problem for electronic devices. The waves that kill electronics are much lower in frequency.

Have you ever heard a nearby radio station being faintly received on stereo speakers, even though the stereo was turned off? The strong broadcasted signal is inducing a voltage in the speaker wire, as if it were an antenna. That voltage is not amplified by the stereo’s electronics, but it’s still enough to move the speaker cone and produce faint sound.

Immediately after a nuke strike, there’s so much energy flying around that it can induce a voltage in almost any wire, even the small ones inside a computer. When every circuit in a device receives a voltage spike at the same time, a lot of important stuff is liable to burn out. (Imagine what would happen to your speakers if the radio station suddenly broadcast a burst of static at 1,000,000 times its normal power.)

---

Of course I don’t fit in; I’m part of a better puzzle.

[Keeves]

Have you ever heard a nearby radio station being faintly received on stereo speakers, even though the stereo was turned off?

Actually, I did once hear a local FM station coming from my kitchen telephone! Until then, I did not believe people who claimed that their dental fillings picked up radio stations, but now I do.

Thanks for the explanation. It makes sense. I guess the reason why our electrical brains would continue to function is the comparative lack of metal?

[Undead_Dude]

I guess the reason why our electrical brains would continue to function is the comparative lack of metal? – Keeves

I would guess that yes, the fact that the brain is not as conductive as metal would help. Also, most electronic devices contain induction coils, which would dramatically increase their vulnerability. I would expect that there would be a point at which the EMP could effect the brain, probably causing a grand mal seizure. Since I’ve never heard of this, perhaps that threshold is close enough to the blast to instantly kill/vaporize.

[AuraSeer]

I’ll agree with Undead Dude once again. There must be a radius in which the EMP is strong enough to screw up nerve impulses, but I’d say that the “kill zone” of the blast and radiation is much larger.

Note that EMP in general does not come only from a nuclear blast; some amount is emmitted from every electrical device. The difference is one of degree, because nukes dump out so damn much energy at once.
There are (unfounded, AFAIK) rumors that the US military is attempting to develop EMP as a weapon. If you had a powerful EMP-gun of some kind, you could theoretically incapacitate enemy soldiers and wreck lots of their equipment, without having to actually kill anyone. (But note again that I’ve heard these as rumors, in the same breath as stories about UFOs at Area 51.)

---

Of course I don’t fit in; I’m part of a better puzzle.

[PovertyPoo]

to what Aura said:

I seem to recall this report on 60 Minutes or 20/20, or one of those news magazine shows… Anyway, it dealt with “New Terrorists”. Their main point was this guy who was ably to make these machines, abotu the size of a suitcase, which emitted I believe microwaves, which were powerful enough to screw up electronics from quite a distance. Not exactly an EMP gun, but the same application

---

---

Poverty P’uh

[Robb]

Cecil has a column related to EMP. It suggests some of the devices vulnerable to EMP.
http://www.straightdope.com/classics/a3_294.html

[mangeorge]

Now just a minute there fellers. Doesn’t the circuitry in a radio translate the RF broadcast into audio frequencies? And then amplify it, so we can hear Ole Donnovan singing about saffron?
Peace,
mangeorge

---

Work like you don’t need the money…
Love like you’ve never been hurt…
Dance like nobody’s watching! …(Paraphrased)

[Undead_Dude]

Now just a minute there fellers. Doesn’t the circuitry in a radio translate the RF broadcast into audio frequencies? – mangeorge

Well, there are two frequencies in a radio broadcast. There is the frequency of the carrier wave, and then there is the frequency of the sound encoded within it. So for example, with an AM broadcast, if someone struck a standard concert “A” tuning fork, the amplitude of the carrier wave would rise and fall 440 times per second.

So the real sound frequency is in there. It doesn’t need to be translated so to speak. It just needs to be extracted.

[falcon2]

The high temperatures and energetic radiation produced by nuclear explosions also produce large amounts of ionized (electrically charged) matter which is present immediately after the explosion. Under the right conditions, intense currents and electromagnetic fields can be produced, generically called EMP (Electromagnetic Pulse), that are felt at long distances. Living organisms are impervious to these effects, but electrical and electronic equipment can be temporarily or permanently disabled by them. Ionized gases can also block short wavelength radio and radar signals (fireball blackout) for extended periods.

The occurrence of EMP is strongly dependent on the altitude of burst. It can be significant for surface or low altitude bursts (below 4,000 m); it is very significant for high altitude bursts (above 30,000 m); but it is not significant for altitudes between these extremes. This is because EMP is generated by the asymmetric absorption of instantaneous gamma rays produced by the explosion. At intermediate altitudes the air absorbs these rays fairly uniformly and does not generate long range electromagnetic disturbances.

The formation EMP begins with the very intense, but very short burst of gamma rays caused by the nuclear reactions in the bomb. About 0.3% of the bomb’s energy is in this pulse, but it lasts for only 10 nanoseconds or so. These gamma rays collide with electrons in air molecules, and eject the electrons at high energies through a process called Compton scattering. These energetic electrons in turn knock other electrons loose, and create a cascade effect that produces some 30,000 electrons for every original gamma ray.

In low altitude explosions the electrons, being very light, move much more quickly than the ionized atoms they are removed from and diffuse away from the region where they are formed. This creates a very strong electric field which peaks in intensity at 10 nanoseconds. The gamma rays emitted downward however are absorbed by the ground which prevents charge separation from occurring. This creates a very strong vertical electric current which generates intense electromagnetic emissions over a wide frequency range (up to 100 MHZ) that emanate mostly horizontally. At the same time, the earth acts as a conductor allowing the electrons to flow back toward the burst point where the positive ions are concentrated. This produces a strong magnetic field along the ground. Although only about 3x10^-10 of the total explosion energy is radiated as EMP in a ground burst (10^6 joules for 1 Mt bomb), it is concentrated in a very short pulse. The charge separation persists for only a few tens of microseconds, making the emission power some 100 gigawatts. The field strengths for ground bursts are high only in the immediate vicinity of the explosion. For smaller bombs they aren’t very important because they are strong only where the destruction is intense anyway. With increasing yields, they reach farther from the zone of intense destruction. With a 1 Mt bomb, they remain significant out to the 2 psi overpressure zone (5 miles).

High altitude explosions produce EMPs that are dramatically more destructive. About 3x10^-5 of the bomb’s total energy goes into EMP in this case, 10^11 joules for a 1 Mt bomb. EMP is formed in high altitude explosions when the downwardly directed gamma rays encounter denser layers of air below. A pancake shaped ionization region is formed below the bomb. The zone can extend all the way to the horizon, to 2500 km for an explosion at an altitude of 500 km. The ionization zone is up to 80 km thick at the center. The Earth’s magnetic field causes the electrons in this layer to spiral as they travel, creating a powerful downward directed electromagnetic pulse lasting a few microseconds. A strong vertical electrical field (20-50 KV/m) is also generated between the Earth’s surface and the ionized layer, this field lasts for several minutes until the electrons are recaptured by the air. Although the peak EMP field strengths from high altitude bursts are only 1-10% as intense as the peak ground burst fields, they are nearly constant over the entire Earth’s surface under the ionized region.

The effects of these field on electronics is difficult to predict, but can be profound. Enormous induced electric currents are generated in wires, antennas, and metal objects (like missiles, airplanes, and building frames). Commercial electrical grids are immense EMP antennas and would be subjected to voltage surges far exceeding those created by lightning, and over vastly greater areas. Modern VLSI chips are extremely sensitive to voltage surges, and would be burned out by even small leakage currents. Military equipment is generally designed to be resistant to EMP, but realistic tests are very difficult to perform and EMP protection rests on attention to detail. Minor changes in design, incorrect maintenance procedures, poorly fitting parts, loose debris, moisture, and ordinary dirt can all cause elaborate EMP protections to be totally circumvented. It can be expected that a single high yield, high altitude explosion over an industrialized area would cause massive disruption for an indeterminable period, and would cause huge economic damages (all those damaged chips add up).

A separate effect is the ability of the ionized fireball to block radio and radar signals. Like EMP, this effect becomes important with high altitude bursts. Fireball blackout can cause radar to be blocked for tens of seconds to minutes over an area tens of kilometers across. High frequency radio can be disrupted over hundreds to thousands of kilometers for minutes to hours depending on exact conditions.