THE PROBLEM OF ATOMIC WASTE
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The spontaneous changes in nuclei that result in the
emission of radioactivity, you see, always
transform an atom into something else. If its chemical
properties are altered, the atom becomes another element.
On the other hand, if an atom's nucleus is changed but its
chemical properties remain the same . . . a different
isotope of the same element is formed.
RELATED CONTENT
Uranium 235, for example, decays in a long series of steps
that include the radioactive isotopes radium 226, radon
222, and polonium 218. The end result, finally, is the
chemically stable, non"hot", lead 206.
The process of this breakdown is statistically predictable,
even though the instant at which a single nucleus will be
spontaneously transformed isn't. For this reason, atomic
decay is measured in "half-lives", which indicate the time
needed for one-half of the billions of atoms in a small
quantity of material to undergo this transformation.
Let's take an example: One of the major short-lived
isotopes in used nuclear fuel elements—iodine
131—has a half-life of 8.1 days. This means that,
when 8.1 days have passed from any given time, half of the
iodine 131 will be gone. After 16.2 days, only a quarter of
the original quantity of isotope will be left . . . only
one-eighth after 24.3 days . . . and so on. A period of 20
half-lives (which is less than six months in the case of
iodine 131) will reduce the original radioactive isotope to
one millionth of its initial mass.
You can see, then, that if all fission products
had half-lives of about a week, the storage of these wastes
wouldn't present much of a problem. They could simply be
held at powerplant sites for a. year or so, and could then
be disposed of in any way suitable to their
chemical characteristics. Residual radioactivity
would be—by that time—practically nonexistent.
Unfortunately, however, many of the fission products
regularly produced in nuclear reactors have extremely long
half-lives. Those of strontium 90 and cesium 137 are 28 and
30 years respectively . . . which means that these isotopes
would have to be stored for 1,000 years before their
radioactivity could be safely ignored. And plutonium 239,
which is formed in reactors by the non-fission absorption
of neutrons into uranium 238, has a half-life of 24,400
years. This material, in short, should be kept out of the
environment for at least one-half million years . . . which
is something on the order of 100 times longer than
the human race has been recording its history!
