The Deadliest Mushroom

"Now I am become Death, the destroyer of worlds."

I'm going to test the nuclear device which will launch the rocket. If you see a Hiroshima Mushroom you do not come to the site; instead, turn around and leave immediately.

—Robert A. Heinlein's Rocket Ship Galileo

No, we don't mean Poison Mushroom.

A discussion of what nuclear weapons actually do.

For this discussion, we will detonate a 1 megaton (fairly small compared to some ICBMs with over 10 megaton payload) nuclear weapon in Los Angeles. 500 South Buena Vista Street, Burbank, California (Lat 34.155744, Lon -118.326766) will be ground zero and the bomb will detonate on the ground.^[1]

The Flash

The first thing you get with a nuclear explosion is the light flash. This is very, very bright and if you are close enough (21km (13 miles) on a clear day and 85km (53 miles) at night for a 1 megaton nuke) will cause at least temporary flash blindness. Unless you're Sarah Jane Smith and probably for her too, this is not good at all. But heaven help you if you look up at the sky while the bomb goes off.

That's not the worst of the problems, there's heat as well. If you're within 11km (7 miles) (Beverly Hills), you're going to get a bad sunburn on exposed skin. 9km (6 miles), permanent scars. 8km (5 miles), third degree burns. If you're sunbathing, you're toast in both senses of the word.

It's been estimated 50% of deaths at Hiroshima and Nagasaki were due to burns.

Overpressure

Then you'll get the blast wave. This is measured in terms of the sudden increase in pressure. 10 psi of overpressure equals your building getting hit by a 470km/h (294 mph) wind.

This is the big problem. The following things will probably happen:

• Within 2.45km (1.53 miles), most of Burbank, including our target, the Mt. Sinai Memorial Cemetery and a couple of parks, will just be gone. Nothing is going to survive on the surface.
• Out to 4.50km (2.81 miles), any ordinary house is gone and reinforced structures are going to be severely damaged. This would include Universal Studios and parts of North Hollywood.
• Beyond here, you'll get damage to everything out to Beverly Hills and Panorama City. Windows will shatter as far away as Santa Monica (~18 miles away).

For your average human, it's not the overpressure that will kill you. It's the building collapsing. Or being shredded by flying glass. Or bricks hitting your head. Or being thrown against a wall in a way that Goa'uld could only dream of doing with their hand device.

A lot of fires will be created. It's possible for a fire storm to ensue as fires merge together.

The Mushroom Cloud and Fallout

After the fireball disperses, you will see the mushroom cloud start to form from condensing vapor. This contains water, debris and general radioactive nastiness. It's not just a nuclear explosion thing- any large explosion will produce one, as well as volcanic eruptions or a meteorite impact: There's a 1937 description of an explosion in Shanghai that references a mushroom -- 8 years before the first nuclear explosions. Furthermore, one account of the eruption of Mount Vesuvius in Italy in 79 AD described it as having the shape of a pine tree; pine trees in Italy have a similar shape to mushrooms.

This mushroom cloud disperses in a matter of hours. During that time, fallout starts raining down on the ground. Most of it has half-lives short enough to disappear within days or weeks. However, stuff like strontium-90 (half-life of 29 years) or caesium-137 (30 years) have half-lives short enough to be really radioactive, but long enough to stick around and cause trouble for decades. It's worth noting us in the radiation biz generally use seven half lives as the rule of thumb when getting to 'zero' radiation, and that's not counting radioactive daughter products.

The initial radiation may well kill you, but at the distance it wouldn't, you're dead anyway from the other effects. The fallout stuff can cause hair loss, infertility, cataracts, tumors, heart failure and generally nasty death, much earlier than planned. To get something of an idea, watch Threads.

When it comes to radiation, it depends on the size of the nuclear weapon. For smaller nukes, especially rather small tactical devices, immediate radiation accounts for much more of the damage they inflict than the (still very substantial) explosive effects, although delayed radiation from fallout is insignificant. For larger nukes, of course, the radiation pulse is absorbed by the atmosphere before it can reach anyone who wasn't killed by the explosion, but delayed radiation is another matter entirely. For some types of small nuclear weapons, especially neutron bombs, the immediate radiation is the main kill mechanism, because the energy distribution from the blast is designed to favor that; that's another story, though.

If you're unlucky enough to be targeted with a neutron bomb, the neutrons might make other material radioactive through a phenomenon called neutron activation. While this might happen with any nuclear bomb, it's only with enhanced-radiation bombs that it's likely to be a more significant problem than fallout and blast damage (for normal thermonukes, anything close enough to be neutron activated is likely to be blown apart or incinerated instead, so the small amount of matter that's made radioactive just contributes slightly to fallout).

The Doomsday Device

At one point, a design was on the drawing board for a nuclear device designed to produce extra fallout. It was called the cobalt bomb. The design called for the explosive core of the bomb to be surrounded by a tamper made of (non-radioactive) cobalt-59. When the bomb went off, the neutrons zipping out of the reaction would turn the cobalt-59 into radioactive cobalt-60, and then the blast would hurl these tiny fragments of cobalt-60 far and wide. It would have been a "dirty bomb" on steroids, and enough of them could have contaminated the entire surface of the Earth with radioactivity. Even its designers referred to it as a Doomsday Device.

Mr. EMP

There is also electromagnetic pulse. The mechanism is rather complicated, needless to say; when it comes to the effects, they can be felt with relatively low-yield and low-altitude bursts, although over a smaller area than if one were to, say, detonate a 25-mt device 500km over Kansas. That might knock everything that depended on electronics in orbit out of commission and destroy every unhardened electronic device in North America, actually. Your car would probably refuse to start, for instance, because the electronics it depends on to function would be fried. They found about this in the 1950s and 1960s, when they were conducting high-altitude nuclear tests. Later the Soviets developed something called a Fractional Orbital Bombardment System, which was designed to detonate a large warhead high over the US in an attempt to destroy every unhardened electronic device in North America. Another issue with high-altitude bursts is particle radiation becoming trapped in the Earth's magnetic field; this leads to temporary, although nasty, artificial radiation belts.

The EMP occurs when the intense flux of gamma radiation from a nuclear explosion produces an ionized region in the surrounding medium via a mechanism known as Compton scattering. The gamma rays strip electrons off of things, producing Compton electrons and positively-charged cations. The electrons are much lighter than the cations and same sign charges repel; the electrons travel to the outer parts of the deposition region while the cations stay in the central part. The outer parts are negatively charged; the inner parts are positively charged. Because this deposition region is never symmetrical or spherical (it could only be so under ideal conditions) there is a net vertical electron current (that is, there's a net flow of electrons. This is in the opposite direction as the conventional current, which is positive, and obviously vertically-oriented). This produces an intense pulse of broadband electromagnetic radiation, which is the EMP, which radiates outwards at the speed of light. Electromagnetic waves have notationally infinite range, but in practice are limited by the inverse-square law and atmospheric attenuation. Anyway, this electromagnetic radiation can be picked up by conductive objects in the same manner that an antenna picks up a signal, and once transmitted to electronics, damaging them. In fact, the EMP is so intense that it can lead to very strong currents, although for only very short durations, in things that normally aren't very conductive. Close to the Earth, the ground, which conducts electricity well, allows the electrons an alternative return path to the central deposition region, which results in an intense magnetic field in the air and ground, but in those areas there's more to worry about from the actual explosion. Effects from the emitted EM radiation can be felt over a greater area.

The mechanism is a little different for high-altitude bursts. The deposition region ends up being in a large region of the upper atmosphere. Only the gamma rays that travel to what becomes this region have much of an effect; otherwise, there's not much to interact with. Anyway, across this very large area Compton electrons are produced. Phillip J. Dolan, in The Effects of Nuclear Weapons, wasn't very specific, but he just said that "[the] electrons are deflected by the earth's magnetic field and are forced to undergo a turning motion about the field lines...[this causes] the electrons to be subjected to a radial acceleration which results, by a complex mechanism, in the generation of an EMP that moves down toward the Earth." The electric field is rather less intense, but for obvious reasons the areas affected are much larger. This way, electronics across entire regions may be damaged. You can check the relevant publication out here.

As implied above, you can harden electronic devices against EMP, with things like a Faraday cage. You can recognize a Faraday cage as being similar to the shield with holes on it that covers the glass window in the door of your microwave oven; if it wasn't there, the microwaves would pass through the glass and cook you!

Ground Burst vs. Air Burst

We've had our bomb go off on the ground. If it was detonated in the air, you'd get more damage due to fewer buildings being in the way. More importantly, far less fallout is generated as a smaller fraction of the fireball will intersect the ground. If none of it intersects the ground, it is termed an air burst. For our 1 megaton bomb, this requires a detonation height of at least 3,000 feet.^[2]

Air bursts cause a phenomenon known as the Mach Effect. When you detonate a bomb in the air, it creates a spherical shock wave (the direct wave). When the wave hits the ground, it literally bounces off, creating a second shock wave that moves faster than the direct one. Chances are, this second wave will overtake the first and combine, producing a skirt around the base of the shock wave bubble where the two shock waves have combined. This skirt sweeps outward as an expanding circle along the ground with an amplified effect compared to the single shock wave produced by a ground burst.

There are also nuclear warheads designed for ground penetration, i.e. to destroy missile silos. It's been estimated that the USSR pointed a gigaton worth of nuclear weapons just at Cheyenne Mountain, deciding to destroy the entire mountain. Should the US and Russia engage in a nuclear war, Stargate Command is toast.

By comparison, the combined total yield of all nuclear testing to date (as of early June 2008) is 510 megatons, or 0.51 gigatons. The largest individual nuclear weapon ever, the RDS-220 or Tsar Bomba, was designed with a yield of ~100 megatons but had this reduced due to fallout concerns to a yield of 50-58 megatons (sources differ) when tested in 1961 -- that is, one quarter of the Krakatoa eruption. At this point, the total yield of all the warheads targeted on Cheyenne Mountain exceeds total US strategic megatonnage.

As regards to weapon yields, it is worth noting that

• The Tsar Bomba was a huge, heavy, and stationary emplacement. It wasn't a practical device to weaponize.
  • You're mixing it with Castle Bravo device. RDS-220 was huge and unwieldy (27 tons, 8 m long, and so thick that bomb bay doors of Tu-95 bomber had to be removed to fit it there), but it was still a bomb, not a stationary device. Footage of the Tsar Bomba detonation can be viewed here.
    • Castle Bravo was sort of unwieldy but basically was actually a bomb that could be dropped. An earlier American thermonuclear test, Ivy Mike, was the one that was a building-sized pile before it went off.
• Multiple smaller blasts are vastly more efficient at spreading the damage of a huge area, and it is likely that where blast waves met enormous forces would result. Furthermore, widespread destruction is a little more likely to result in a firestorm.

Casualty Figures

Precisely how many people would die would depend on a lot on circumstances, for example:

• Time of day: If it's a weekday and people are at work, the area would have more people in it.
• Weather- sunny day, more people outside, but the blast has it easier to dissipate. Cloudy day, fewer people outside, but the heat waves may get reflected by the clouds back towards the ground, resulting in greater damage. Windy day, fewer people outside, but wider fallout. Wind direction is important too.
• Warning- a sudden strike would kill far more people than one after several days of conflict leading to a nuclear exchange. Whether air-raid sirens went off first (if they exist) or emergency broadcasts occurred first would be important too.
  • A general note on warnings- for a strike launched from Russia, the US would get about 20 minutes warning before the first explosions. The UK would get about ten (or even less).
    • The official early warning given to the public by the British Government would have only been four minutes - just enough time to make a nice cup of tea. But not to drink it.
• The availability of medical personnel and assistance, although they would be overwhelmed even in a small exchange.

Estimates for a nuclear war involving the United States tend to be a) somewhat out of date and b) prone to political slanting. It's fair to say, though, that at least two times the weekly audience of CSI (which would be c.120 million) would probably go in a "counter-value" (cities) strike on the US, with the aggressor taking a lot of casualties too. There would be fewer in a "counter-force" (silos, airfields etc.) strike, but the fallout would still be very nasty...

It is to be noted that the distinction between counter-value and counter-force can be an academic one in some countries. For example, many American submarine and bomber bases have major cities either within the blast radius of the weapons that would be used on them, or within the CEP (Circular Error probability, the amount to which the weapon is likely to be off target) of the delivery system used. The Soviet Union, by contrast, had their major strategic weapons located well away from their cities due to both security and environmental considerations (many of their bases were extremely cold). A lot of this was because the USSR was much, much larger than the US, and its major population centers were further apart (even though it was more than twice as large as the US by land area, the Soviet Union's population was only slightly larger, and was mostly concentrated in European Russia, Ukraine, and Belarus).

Here's something called The Effects of a Global Thermonuclear War, so that one can have some idea about, well, the effects of a global thermonuclear war. Very cheery.

There is an absolutely fantastic three part essay on the topic of nuclear war and nuclear policy-making: Nuclear Warfare 101, Nuclear Warfare 102, and Nuclear Warfare 103.

(With thanks to HYDESim)

1. ↑ What, you thought we'd let you get away with Hannah Montana?
2. ↑ about 914.4 meters