Half-life

The time it takes for half of a given amount of a radioactive isotope to decay. It is not a measure of danger on its own: a short half-life means the atoms decay quickly, so a given amount is intensely radioactive for a short time, while a long half-life means weaker radioactivity that lasts longer. Tritium's half-life of 12.32 years means a stock loses about 5.5% a year to decay, which, together with a small world supply, is why a deuterium-tritium fusion plant must breed its own. The six standard groups of delayed neutron emitters in fission have half-lives from about 0.23 to 55.7 seconds, which is what slows a fission reactor's response enough for control rods and operators to keep up.

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