Terminal storage facility: a place for storage of things that we no longer wish to deal with, at least not within any finite period of time.

Terminal storage facilities usually serve for long-term, ideally ►eternal, disposal of radioactive elements — or, casually speaking, of nuclear waste. Radioactive elements characteristically dissolve into other elements, thereby emitting irradiation more or less hazardous to your health. A half-life is the time that is takes for half of the ►atoms in a radioactive substance to dissolve. The half-lives of some of the elements contained in radioactive waste are as follows:

Element   Half-life
uranium 238U 4.5 billion years
iodine 129I 17 million years
plutonium 239Pu 24,000 years
cesium 137Cs 30 years
strontium 90Sr 29 years
krypton 85Kr 11 years

Let us assume we place 1 kg cesium-137 into terminal storage. Thirty years later, half of the material will have dissolved; consequently, its radioactivity will have decreased by half. Of the material that remains, half again will have dissolved after the next thirty years. Thus, the degree of radioactivity decreases in accordance with the following infinite ►geometrical sequence:

1, 1/2, 1/4, 1/8, 1/16...

In practice, however, we do not have to wait an infinite period of time for complete dissolution of the radioactive material because matter is not infinitely divisible. Cesium, for instance, contains approximately 4.4 · 1024 atoms* per kg; hence, one kilogram of cesium requires about 2500 years to dissolve completely. A kilogram of plutonium, by contrast, requires one million years for its complete dissolution. And it would not be worthwhile to wait for the dissolution of a kilogram of uranium-238; the ►end of the Earth will happen first.

Storage for Eternity

To date, elements with low to medium radioactivity have been stored in facilities above and below the ground that are accessible to constant maintenance. For highly radioactive substances, by contrast, a maintenance-free storage period of one million years is preferred. This requires concepts reaching far into the future. Over the next million years, oceans may decline or rise, deserts develop or disappear, small mountain ranges unfold, and whole continents shift dramatically. In addition, the radioactive waste produces constant irradiation and heat. The terminal storage facility must be able to handle all of this without releasing any radioactivity.

To date, no such ideal terminal storage facility has been developed. The standard method is to store the material within stable geological formations many kilometers beneath the surface of the Earth. There are plans for terminal storage of highly radioactive material in the volcanic tuff of Yucca Mountain, Nevada, as well as in the granite underneath the island of Olkiluoto in Finland. Salt deposits would be suitable candidates as well, since they usually do not change over a million-year span. However, investigation of the possibility of a German terminal storage place in the salt deposits near Gorleben has been suspended since 2000 due to the current political climate.

* number of atoms in 1000 g cesium-137:   1000 / 137 · 6.0221415 ∙ 1023 = 4.4 · 1024
Complete dissolution period: 30 a * ln(4.4 · 1024) / ln(2) = 2456 a

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