Radioactivity
Half-life and
timescales
Because radioactive atoms loose energy when they decay, a mass of radioactive material cannot stay radioactive forever and their activity decreases with time. Radionuclides decay at different rates.
The half life tells you how long it takes until the
original amount has decayed by one-half (i.e., to 50% of
the original amount).
Of more interest is the time for the radioactivity to decay to nothing - when all the original atoms have decayed.
Ten half-lives is a good estimate of this (i.e., to 0.1% of the original amount).
For some important radionuclides, like caesium-137, ten half-lives is around 300 years (half-life = 30 years). For others, like thorium-230, with a half-life of 10 billion years, ten half-lives is longer than the age of the universe!
Unfortunately not all decays lead to stable nuclides. Even short lived nuclides can decay into longer lived ones. The aim of isolating radionuclides from the biosphere requires that very long timescales be considered. Figure 1 indicates that most radioactive nuclides have short or very short half-lives. These are generally of no concern in waste disposal because they decay very quickly.
Nuclides with half-lives of around one year and above are important, and those with long and very long half-lives must be treated with care. There are quite a few on the right hand side of the figure. For comparison, some relevant timescales are also shown.
Radioactive waste forces us to consider timescales which are very unfamiliar to us.
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Decisions made today may have implications far into the future. |
Source: Galson Sciences Limited