Radiation

The chief delayed effect of a nuclear explosion is the creation of huge amounts of radioactive material with long lifetimes (half-lifes ranging from days to millennia). The primary source of these products is the debris left from fission reactions. A potentially significant secondary source is neutron capture by non-radioactive isotopes both within the bomb and in the outside environment.

Nature of radiation
When atoms fission they can split in some 40 different ways, producing a mix of about 80 different isotopes. These isotopes vary widely in stability; some are completely stable while others undergo radioactive decay with half-lifes of fractions of a second. The decaying isotopes may themselves form stable or unstable daughter isotopes. The mixture thus quickly becomes even more complex, some 300 different isotopes of 36 elements have been identified in fission products.

Short-lived isotopes release their decay energy rapidly, creating intense radiation fields that also decline quickly. Long-lived isotopes release energy over long periods of time, creating radiation that is much less intense but more persistent. Fission products thus initially have a very high level of radiation that declines quickly, but as the intensity of radiation drops, so does the rate of decline.

A useful rule-of-thumb is the rule of sevens. This rule states that for every seven-fold increase in time following a fission detonation (starting at or after 1 hour), the radiation intensity decreases by a factor of 10. Thus after 7 hours, the residual fission radioactivity declines 90%, to one-tenth its level of 1 hour. After 7*7 hours (49 hours, approx. 2 days), the level drops again by 90%. After 7*2 days (2 weeks) it drops a further 90%; and so on for 14 weeks. The rule is accurate to 25% for the first two weeks, and is accurate to a factor of two for the first six months. After 6 months, the rate of decline becomes much more rapid. The rule of sevens corresponds to an approximate t^-1.2 scaling relationship.

These radioactive products are most hazardous when they settle to the ground as fallout. The rate at which fallout settles depends very strongly on the altitude at which the explosion occurs, and to a lesser extent on the size of the explosion. If the explosion is a true air-burst (the fireball does not touch the ground), when the vaporized radioactive products cool enough to condense and solidify, they will do so to form microscopic particles. These particles are mostly lifted high into the atmosphere by the rising fireball, although significant amounts are deposited in the lower atmosphere by mixing that occurs due to convective circulation within the fireball. The larger the explosion, the higher and faster the fallout is lofted, and the smaller the proportion that is deposited in the lower atmosphere. For explosions with yields of 100kT or less, the fireball does not rise above the troposphere where precipitation occurs. All of this fallout will thus be brought to the ground by weather processes within months at most (usually much faster). In the megaton range, the fireball rises so high that it enters the stratosphere. The stratosphere is dry, and no weather processes exist there to bring fallout down quickly. Small fallout particles will descend over a period of months or years. Such long-delayed fallout has lost most of its hazard by the time it comes down, and will be distributed on a global scale. As yields increase above 100kT, progressively more and more of the total fallout is injected into the stratosphere.

An explosion closer to the ground (close enough for the fireball to touch) sucks large amounts of dirt into the fireball. The dirt usually does not vaporize, and if it does, there is so much of it that it forms large particles. The radioactive isotopes are deposited on soil particles, which can fall quickly to earth. Fallout is deposited over a time span of minutes to days, creating downwind contamination both nearby and thousands of kilometers away. The most intense radiation is created by nearby fallout, because it is more densely deposited, and because short-lived isotopes haven't decayed yet. Weather conditions can affect this considerably of course. In particular, rainfall can "rain out" fallout to create very intense localized concentrations. Both external exposure to penetrating radiation, and internal exposure (ingestion of radioactive material) pose serious health risks. Explosions close to the ground that do not touch it can still generate substantial hazards immediately below the burst point by neutron-activation. Neutrons absorbed by the soil can generate considerable radiation for several hours.

The megaton class weapons have been largely retired, being replaced with much smaller yield warheads. The yield of a modern strategic warhead is, with few exceptions, now typically in the range of 200-750 kT. Recent work with sophisticated climate models has shown that this reduction in yield results in a much larger proportion of the fallout being deposited in the lower atmosphere, and a much faster and more intense deposition of fallout than had been assumed in studies made during the sixties and seventies. The reduction in aggregate strategic arsenal yield that occurred when high yield weapons were retired in favor of more numerous lower yield weapons has actually increased the fallout risk.

Mutations
The various types of mutant creatures that inhabit the wastelands were mostly caused by radiation. This is where Mantises, Geckos, Spore Plants, Deathclaws (Correction: in Fallout 2, it is revealed that Deathclaws were genetically engineered by the US government before the war), Radscorpions, Brahmin and the various mutant rodent species come from. Also, this is how Ghouls, decrepit, ragged, almost rotting, zombie-like victims of massive radiation poisoning, are made. Most ghouls were created from vault dwellers living in Vault 12 under the city of Bakersfield (better known as the Necropolis after the War). As part of the vast Vault Experiment Program, the Vault 12 vault door was designed not to close properly. Thus, massive amounts of radiation leaked in affecting those within the vault, most of whom were turned into the current ghoul population. Generally, in the Fallout universe, massive exposure to radiation causes humans to die, however prolonged exposure seems to be capable of transforming people into Ghouls. Also, all ghouls are completely sterile. There is only one generation of ghouls in the wasteland and it is the last. Furthermore, the Ghoul transformation grants its subjects extremely long lives. Ghouls created in the Great War of 2077 were still alive in 2241. Ghouls are generally as intelligent as normal humans. However, their extreme ugliness and physical frailty makes the life of a ghoul difficult at best. Ghouls are, naturally, immune to radiation. Radiation poisoning can't really get worse for them.

Many would question this and say that radiation wouldn't create giant scorpions and zombies, it would just kill you. That's entirely true for us. Fallout, however, is different. In this world, in accordance with 1950's sci-fi physics, radiation just makes things bigger and meaner (when it doesn't kill them), though the player can still suffer radiation poisoning in the game and radiation in general is a bad thing.

Another as-of-yet unconfirmed theory suggests that the majority of mutations caused in the Fallout universe are due to FEV exposure caused by creatures getting in or near the vats after the war, OR by it surviving the heat of the nuclear blast and having it scattered across the wastes. This is backed up by the revelation in Fallout 3 that Harold was not turned into a Ghoul by radiation but by FEV exposure (see Barkskin perk).

Fallout 3
Eating and drinking most food items like Nuka-Cola or Dirty Water, or entering an irradiated zone cause the player to gain Rads. The player can withstand up to 1000 rads before dying, as well as losing S.P.E.C.I.A.L. attributes at certain thresholds. Rad exposure is usually limited; only very rarely will zones be so irradiated that venturing into them results in a quick death. Rads can be flushed out by paying 100 caps to a doctor (to remove all rads) or by using a Rad Away (to remove a variable amount of rads, depending on the player's Medicine skill. The player can also purchase a personal infirmary for his or her house to remove all rads for free.

Radiation can be resisted by equipping certain types of apparel; typically Radiation Suits or Power Armor. A dose of the chem Rad-X also grants the player a radiation resistance based on his or her Medicine skill. This resistance is applied to every source of radiation, from external environment to even drinking irradiated water and eating irradiated foods.

Certain enemies, such as Glowing Ones, have radiation-based attacks that can increase the player's rads.