It has to do with the physics of nuclear particles. Basically, radioactivity is the decay of subatomic particles. An atom can be in what physicists call an “excited state,” which means that it is unstable and tends toward entropy–greater disorder–in much the same way as a snowfield is unstable and can cascade into an avalanche. When a nucleus is in this state, it tends toward entropy by emitting subatomic particles such as protons and neutrons. Not all radioactive elements have long half-lives. (“Half-life” refers to the amount of time it takes for one-half of the substance to decay). However, the compounds that are most often used to produce energy, whether in the form of controlled nuclear reactions in nuclear power generation or uncontrolled nuclear reactions in nuclear weapons, tend to produce effects that decay over very long periods of time. How long until an area contaminated with these products is safe depends on the precise element used, how much of it and under what circumstances it has been released. For instance, in Hiroshima and Nagasaki which were atom-bombed in 1945, there is not much detectable background radiation left from the blasts, but in parts of Australia where the British tested much stronger weapons in the 1950s and 60s, the level of radioactivity is so high that the test zones are estimated to be uninhabitable by humans for over 200,000 years.
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