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is the period of time it takes for a substance undergoing decay to decrease by half. (ie the radiation of the substance is decreased by 50%)
Plutonium-239's half-life is 24,110yrs for example, its radiation levels will decrease by 1/2 in 24,110 years. and still be unsafe... Plutonium-239 would have to go through 8 half-lives to eliminate all radiation from that particular source... that's 192,880years.
Fallout is the residual radiation hazard from a nuclear explosion, so called because it "falls out" of the atmosphere after the explosion.
After an air burst, fission products, un-fissioned nuclear material, and weapon residues vaporized by the heat of the fireball condense into a fine suspension of small particles 10 nm to 20 µm in diameter. These particles may be quickly drawn up into the stratosphere, particularly if the explosive yield exceeds 10 kt.
Atmospheric nuclear weapon tests almost doubled the concentration of radioactive 14C in the Northern Hemisphere, before levels slowly declined following the Partial Test Ban Treaty.
Initially little was known about the dispersion of nuclear fallout on a global scale. The AEC assumed that fallout would be dispersed evenly across the globe by atmospheric winds and gradually settle to the Earth's surface after weeks, months, and even years as worldwide fallout. Nuclear products were deposited in the Northern Hemisphere becoming "far more dangerous than they had originally been estimated."
The radio-biological hazard of worldwide fallout is essentially a long-term one because of the potential accumulation of long-lived radioisotopes (such as strontium-90 and cesium-137) in the body as a result of ingestion of foods containing the radioactive materials. This hazard is less pertinent than local fallout, which is of much greater immediate operational concern.
Corium, also called fuel containing material (FCM) or lava-like fuel containing material (LFCM), is a lava-like molten mixture of portions of nuclear reactor core, formed during a nuclear meltdown, the most severe class of a nuclear reactor accident.
It consists of nuclear fuel, control rods, structural materials from the affected parts of the reactor, products of their chemical reaction with air, water and steam, and, in case the reactor vessel is breached, molten concrete from the floor of the reactor room.