Almost everything on earth is made of atoms, and radioactivity is a property that atoms can have. Below is a technical explanation of this property, including its role in nuclear power and the concept of half-life. Other pages get into the types, units of measurement, effects on the human body, and general hazards of radioactivity.
There is a size limit to how large atomic nuclei can get and remain stable. Every element heavier than lead (atomic number 82) is radioactive to some extent. But small atoms can be unstable also, if they have more neutrons than protons. For example, the most common form of carbon on earth is carbon-12. Carbon has an atomic number of 6, meaning it has six protons in its nucleus. When it also has six neutrons, this adds up to an atomic weight of 12, and this isotope of carbon is stable. However, if you add two more neutrons you get carbon-14, which is unstable – that is, radioactive.
Likewise, the most common isotope of hydrogen has a nucleus composed of just a single proton. It is a stable atom. If you add one neutron to the nucleus, you get an isotope called deuterium that is rarer but still stable. However, if you add two neutrons, you get a form of hydrogen called tritium that is unstable and undergoes radioactive decay.
The isotopes of hydrogen – deuterium and tritium – are the only ones to have their own names. All other isotopes are designated by giving the name of the element followed by atomic weight of the isotope, as with carbon above.
When these unstable nuclei decay, they transform themselves into a different element, releasing radiation and heat as part of the process. This new element, or radiation product, may itself be radioactive and decay further – releasing more radiation – or the product may be stable, in which case the process stops.
The heat from radioactive decay is what makes these elements a usable source of power; the radiation is what makes them dangerous. In nuclear reactors, that heat is used to heat a liquid that drives a turbine, which in turn generates electricity. Sometimes the radiation is used to transform materials into others that are useful for medicine or science; but mainly the radiation is a hazard, which continues to be a problem long after the material no longer generates enough heat to be useful in a reactor.
Half-life is the other relevant parameter for radiation. We never know when specific atoms in a substance are going to decay; but we do know the rate at which some random atoms in it will decay. This rate is expressed as the element’s half-life, which is the time it will take half the atoms of a radioactive isotope to decay. So if a substance has a half-life of four years, after four years half of it will have decayed into some other element; and after another four years, there will only be a quarter of the original isotope (half of that half) left.
Half-life can be a misleading characteristic: when you read that the half-life of plutonium-239 (the most common isotope of plutonium) is 24,100 years, that makes it sound like this horrible, horrible thing that’s going to be around forever. Which is true in some senses, but it also means that the plutonium is decaying so very slowly that it just doesn’t emit much radiation. Its chemical toxicity (see below) is a much more powerful effect on the human body.
On the other hand, a very short half-life means that a given isotope is stunningly radioactive, but this doesn’t last very long before it decays to a manageable level. So waiting for relatively short periods of time can also reduce the radioactivity of dangerous materials tremendously. Storing certain kinds of spent nuclear fuel for only three years lowers the heat generated by radioactive decay “by a factor of nearly 12,000” at which point it’s a lot safer to handle.
The sweet spot for a half-life that’s most troublesome to humans belongs to two isotopes that are common products of the radioactive decay of uranium and plutonium: cesium-137 and strontium-90. They both have a half-life of about 30 years, which is strong enough to make them dangerous emitters of beta and gamma radiation, but long enough so that once an area is contaminated with these radionuclides, it’s going to stay dangerous for the rest of your life.