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| Fission fragments, trans-uranics, and 238U |
After a reactor has been operating for about 18 months, it is shut down for a period of time and the fuel rods are replaced. What remains of the old fuel rods is called nuclear waste. This waste is the product of a very large number of different nuclear processes, and contains many different isotopes. Most of these isotopes are radioactive. Some are fission fragments, elements like Ba and Kr; some are trans-uranic elements, produced when neutrons are absorbed by the 238U in the fuel rods. It is important to realize that 238U itself, that part of the fuel rods that has not undergone any nuclear reactions (and is only very slightly radioactive), is also a part of nuclear waste -- in fact a large part. Unless a chemical separation of the various elements in the fuel rods takes place, something that is not ordinarily done, the 238U remains mixed with the other radioactive elements, and, according to U.S. law, has to be stored or disposed of along with those elements. |
| Strontium-90 The Test-ban Treaty |
As an example of what happens to fission fragments, consider the 9035Br nucleus. It undergoes three quick beta decays (half-lives less than a few minutes) and ends up as 9038Sr. This strontium-90 has a half-life of 29 years, so it stays around. Strontium is chemically very similar to calcium, so if it is in the environment it can appear in the same places that calcium appears, such as in milk. When nuclear weapons were being tested in the atmosphere in the 1950s, strontium-90 was one of the components of the radioactive "fallout". Its presence in milk and other foods meant that it would be incorporated in children's bones and teeth. Citizen pressure eventually led to a ban on testing in the atmosphere, signed in 1963. Testing underground continued until recently. |
| Relation between half-life and intensity |
In dealing with the question of nuclear waste, the half-lives are an important factor. In general there is an inverse relation between the half-life and the intensity of radioactivity of an isotope. Isotopes with a long half-life decay very slowly, and so produce fewer radioactive decays per second; their intensity is less. Istopes with shorter half-lives are more intense. In nuclear waste, isotopes with very short half-lives, say a few days or even a few weeks, are not the major concern. They will decay to negligible amounts within a year or two. Isotopes with very long half-lives, more than 1000 years, are likely to be less intense. But one has to plan storage and protection for the public on a time-scale of thousands of years. We cannot be very confidant about guaranteeing this protection reliably. Some trans-uranics are in this category. Isotopes with intermediate half-lives (say from 10 to 100 years), need only be secured on a time-scale of a few hundred years, although they are likely to be more intense. While this storage is also a serious problem, we can point to human institutions that have endured for centuries (the American Constitution, for example; medieval churches for another). Isotopes with intermediate half-lives typically are fission fragments. After 90Sr, the most important isotope is 13755Cs (cesium). Its half-life is 30 years. It is chemically similar to potassium, which is taken up by the body for use in various fluids and in the nervous system. |
| Long-term Storage |
The problem of radioactive waste is finding a way to keep it isolated, over a long period of time, from the biosphere -- particularly from underground water sources. It cannot simply be placed in ordinary containers. Radioactivity itself tends to damage materials like steel and other metals. Furthermore, a large quantity of radioactive matter tends to get very hot, and this also weakens containers. One important approach is to incorporate waste in certain kinds of glass and ceramic materials that are very resistant to being dissolved in water, or to any chemical reaction with the environment. Certain kinds of natural underground sites are effective in preventing the flow of chemicals, and thus can keep the waste isolated. |
| Yucca Mountain |
In 1987 Congress chose a site under Yucca Mountain, a barren volcanic mountain in Nevada, as the prime candidate for the storage of nuclear waste. After considerable scientific and political controversy, the Energy Department decided (in Jan. 2002) to recommend this as the site for storage of waste from both military and civilian reactors. The recommendation remained controversial, and in 2009 President Obama announced his opposition to the Yucca Mountain site, saying "we can do a better job." At present a Blue Ribbon Commission is re-evaluating the problem, and is to present a report in 2011. |
| Concentrated Storage Site? |
Whether it is a good idea to concentrate all nuclear waste at this one site, or to have multiple sites closer to the sources of the waste, is also a question with arguments on both sides -- even given concern about possible terrorist attacks. A single site would be easiest to defend, but the process of transporting waste thousands of miles along U.S. highways creates the risk of terrorist attacks along the way. Look for heated arguments on this. |
KEY CONCEPTS
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