Radiation

The chief delayed effect of a nuclear explosion is the creation of huge amounts of radioactive material with long lifetimes (half-lives 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.

Measurement
A rad is a unit of measurement used to measure the level of radiation in an area. The Vaults are equipped with sensors that detect radiation levels. This measurement is reported to the residents over the public announcement system.

In Vault 101 on July 13, 2268, the public announcement system reported the level of radiation -- "Current radiation level - 0 rads, as always."

In our world, the rad is a real unit, meaning radiation absorbed dose, though it is largely obsolete, having been replaced by the "gray" (Gy) and "sievert" (Sv) for absorbed dose and equivalent dose, respectively.

Mutations
The various types of mutant creatures that inhabit the wastelands were mostly caused by radiation. This is where Mantises, Geckos, Spore Plants, 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. In Fallout 1, most of the ghoul population was 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, and indeed in the Fallout 3 era, circa 2277. Ghouls are generally as intelligent as normal humans, though some may lose their intelligence over time and turn feral. However, their extreme ugliness and physical frailty makes the life of a ghoul difficult at best. Ghouls are naturally immune to radiation, and, in fact, are also healed by it. 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 mostly true for us. Notable exceptions in the real world can for instance be found in the area around the detonated nuclear power plant of Chernobyl where, up to this day, children are born with disabilities due to the increased exposure to radiation.

Fallout, however, is different. In this world, in accordance with 1950's sci-fi physics, radiation just makes things bigger and meaner (if it doesn't kill them), though the player can still suffer radiation poisoning in the game and radiation in general is a bad thing. Nonetheless, this fact wasn't really popular in the pre-war society (as you get to know in Fallout 3): from car propulsion to the moon rockets, fission batteries to cola that glows in the dark, radioactive substances were used for almost anything, accepting the inevitable radiation as a by-product. Even after "the bombs fell", the inhabitants of the "Wasteland" have no repulsion towards radioactivity: For instance, the settlement named "Megaton" is built around an undetonated atomic bomb, which is even worshipped by some people.

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 of Harold's origin, he was transformed into a Ghoul-like creature not by radiation but by FEV exposure (see Barkskin perk), or Fallout 1 Harold dialogue files pertaining to his escape from Mariposa. It would seem that he is a special, ghoul-like person.

Fallout 3
Eating and drinking most food items or entering an irradiated zone causes the player to gain Rads. The player loses S.P.E.C.I.A.L. attributes at certain thresholds, and dies at 1000 rads. Radiation does not affect your hitpoints (until you die).

The Pipboy's dosimeter will appear in the upper right whenever you are exposed. There are five major ticks (200, 400, 600, 800, and 1000 rads), with minor ticks at multiples of 66.67, e.g., 67, 133, 200, 267, 333, 400, etc. You can also check your rad status in your pipboy to see your Rad Resistance and Rad level.

There is also a dial in the upper-left of your Pip-Boy that shows the approximate radiation level. This meter is difficult to read however, due to the needle's constant movement.

Rad exposure is usually limited; only very rarely will zones be so irradiated that venturing into them results in a quick death. You have to stand in +1 Rad water for a long time to die (16 minutes and 40 seconds), and more generally, you can splash briefly through radioactive water dozens of times before you reach the barely-annoying 200 Rad threshold.

The quickest way to die of massive exposure is near the surface entrance to Vault 87. Radiation can reach up to 3,443 rads per second. (It's possible to reach the entrance by frequently pausing to use Rad Away, but you need tons of it.) Trying to enter the G.E.C.K. chamber of Vault 87 is perhaps the second quickest way. Although it starts at 1 rad per second, it virtually doubles every couple of seconds, until you're receiving about 400 rads per second (and probably die). Jumping into the river off The Pitt Bridge is also a near instant death, reaching upwards of 600 rads a second. Also, if you use the save-before-you-fall glitch, the mouth of the blast furnace in the Steelyard is upward of 400 rads a second.

Rads can be flushed by:
 * Paying 100 caps to a doctor (removes all rads)
 * Using a Rad Away (removes a variable amount, depending on player's Medicine skill)
 * Using a personal infirmary (removes all rads)
 * The Rad Absorption perk slowly decreases your radiation level
 * The Nuclear Anomaly perk gets rid of all rads when activated
 * Consuming certain foods, such as Cave Fungus or Wild and Refined Punga Fruits. (Punga Fruits are found only in the Point Lookout add-on.)

Radiation can be resisted by equipping certain types of apparel, such as 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 (they don't stack). This resistance is applied to all sources of radiation, from the external environment to irradiated ingestibles. The Lead Belly perk halves the Rads you take from any irradiated water you drink while the Rad Resistance and Cyborg perks raise your overall radiation resistance.

In Fallout 3, NPCs are immune to Radiation, as can be seen in this video. This explains why NPCs like Confessor Cromwell (who stands in irradiated water at all times) don't die from Radiation Poisoning. Also if complete the 'Contract radiation sickness' part of The Wasteland Survival Guide with a reading of 600 Rads (the optional objective), you gain the Rad Regeneration perk.

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

Fallout

 * The Glow, with instant death awaiting any who venture without lots of Rad-X and Rad Away.

Fallout 2

 * Toxic Waste Dump encounter, which is the only significant source of radiation in the game.

Fallout 3

 * Vault 87 entrance (400-5000 rads a second), which is the highest level of radiation found in the game.
 * Near the G.E.C.K in Vault 87 (40-100 rads a second).
 * Megaton Ruins after you blow it up in The Power of the Atom (1-12 rads).
 * The Pitt's various rivers (around 600 rads).
 * Wheaton Armory, inside the main building (as high as 13 rads/second in the silo room).
 * The crashed Delta IX rocket near the Statesman Hotel (over 40 rads/second near the front of the rocket).
 * The chamber inside Project Purity during the quest Take it back! (around 17-30 rads/second).
 * Drinking the water around the Atom Bomb in Megaton nets 20 rads per second.
 * The Hole in The Pitt add-on (17+ rads without protection).

Note

 * If for any reason, going to your Pip-Boy to eat or drink anything that will make your rad level cross the 1001-Rad death threshhold, a notice will say that you are affected by "Fatal Rad Poisoning". At this instant you will not die; however exiting the Pip-Boy without taking any RadAway that would lower your radiation below 1001 will kill you instantly. If you do this it is interesting to note that Fatal Rad Poisoning has an effect of -10000 HP so if there is a way to increase max HP in the console, then it could be possible to live with this stage of radiation without making yourself invincible.
 * Another interesting thing to note about this stage of poisoning is that it will immediately drop your HP by 100, and each Stimpak will decrease it by the amount they would normally heal you.
 * For short-ranged players it may actually be worth playing with a 400 rads if you have the the Rad Regeneration perk, if you mind the loss in Endurance and Agility. The regeneration of limbs is very fast, as any crippled limb will heal within a few seconds.