How does radiation work? (Chernobyl, Hiroshima - how do people get contaminated?)?

Radiation is all around you. Light is radiation. Heat is radiation. X-rays are radiation. Radiation can also exist as particles. An ALPHA particle is a helium nucleus that is traveling so fast it has left the electrons behind. It is a charged particle and because of its' size stops in an inch or two of air where it picks up 2 electrons to become a stray helium atom, and harmless. The problem with alpha particles is if material that emits them gets inside of you. The highly charged particle strips electrons from atoms in live cells, which can kill or otherwise damage the cell. When radioactive material such as Radium decays, sometimes it emits an alpha particle. The next particle is a BETA particle. A beta is simply a fast moving electron. IT too, is charged and causes other electrons to bounce around. It will also damage and kill live cells. It will travel several feet in air before coming to a stop. The last particle is a neutron, with no charge. A nuclear reactor relies on these to work. When an atom fissions, it splits into fragments and in the process gives off 2 or 3 of these. In a reactor, 1 or 2 are intentionally "lost" or absorbed so that only 1 goes around to fission another atom. THAT is what happens in a chain reaction, for every atom that fissions, one neutron is saved to cause another fission. Neutrons do damage simply by collision, knocking things about. As with the alpha and beta, it will ionize atoms in the body and cause damage or outright kill cells. Finally, gamma rays. Gamma rays are just like X-rays, only of higher energy. Gamma rays collide with matter and knock electrons out of orbit around a nucleus, making a beta. So, THAT is what happens with DECAY of radioactive materials. Particles and rays are given off. As long as they stay outside of the body, you can use time, distance and shielding to limit your exposure. However, if the material gets inside of you, time, distance and shielding no longer applies and you get it ALL in LIVE tissue with NO way to reduce your exposure. As for how long it takes to "disappear", that varies greatly. The term is "half-life". One half-life is how long it takes for half of the atoms to decay. Cobalt picks up a neutron to become cobalt-60. It has a half-life of 5.9 years at which time it decays to return to cobalt-59. 5 half-lives is used as a rule of thumb. After 5 half-lives have passed, only 3% of the original material remains, with 97% having decayed away. Loose radioactive material is called contamination. It is the contamination that is what is SO dangerous, because THAT is what can get inside of you. The major isotope involved is radioactive iodine, which can get inside and end up in your thyroid gland.There is also strontium which is similar to calcium and ends up in your bones. There is NO way to remove either from the body. What you CAN do is ingest iodine on purpose to "fill" the thyroid so it does not pick up any more iodine. Plain iodine and radioactive iodine are identical in the way they act as a CHEMICAL since they have the same number of electrons and protons. It is the number of neutrons in the nucleus which makes one radioactive and the other not. Chernobyl was bad news, and it will be so for a long time, many years. There was considerable contamination which was spread as a result of the fires which scattered contamination everywhere that smoke went. In Japan, it wasn't so much what was released, but what MIGHT be released as a result of the cores overheating. The heat comes from decay of fission fragments and after a core is shutdown, can be as high as 10% of the rated output. That is much like a electric stove, that instead of turning it off, you turn it down to 10% and then every once in a while, you turn it down a little more until after a month or so, it is at a point where it is putting out low enough heat that it can simply sit there and not melt itself. If the decay heat is not removed, the fuel will literally heat itself to the point that it melts. The earthquake did not damage things so much. The operating reactors did what they were supposed to do, shutdown. They had backup diesel generators to keep the cooling pumps running. Until the Tsunami flooded the generators, that is, and THAT is when all cooling was lost and decay heat became the biggest problem. There is MUCH more on this that I simply can NOT put here.