Nuclear Byproducts and Waste

Nuclear wastes are classified according to their activity and lifetime. Some radioactive elements have a half-life of only a few seconds, while others have half-lives of millions or even billions of years, the danger of nuclear waste being inversely proportional to its lifetime (Table 8.5). The longer the half-life of a nucleus, the lower the number of disintegration events per unit of time.

With regard to nuclear waste management, most attention has been focused on high-activity spent nuclear fuels. Nuclear fuel rods are generally present in a commercial reactor for three to four years, during which time they are progressively used up; subsequently they have to be replaced by new rods in order to keep the electricity output constant. Used nuclear fuel is not a waste, however, as it still contains very large amounts of compounds with high energy value: 95% uranium (of which about 1% is 235U) and 1% plutonium. It also contains about 0.1% of actinides (neptunium, americium, curium, etc.) which could eventually be used to produce energy, and 4% of fission products which have no energy potential and are thus considered as waste which must be disposed. Treatment of the used fuel in special facilities in Europe and Japan enables the recycling of unused 235U, and plutonium. It also reduces by the same time the amount of highly radioactive waste produced and allows a better use of natural resources. For political reasons based on fears of nuclear proliferation, the reprocessing of nuclear fuel was banned in the United States in 1977. Despite a lifting of the ban some years later, however, reprocessing was only reconsidered in recent years. Origin-

Table 8.5 Half-lives (years) of some radioactive elements.

Uranium 238

4470 000000

Uranium 235

704000000

Neptunium 237

210000

Plutonium 239

24000

Americum 243

7400

Carbon 14

5730

Radium 226

1600

Cesium 137

30

Strontium 90

29

Cobalt 60

5.3

Phosphorus 30

2.55 min

ally forbidden by fear of nuclear proliferation, reprocessing is now, ironically, the preferred option to destroy stocks of military-grade plutonium by transforming it to usable MOX fuel for nuclear reactors. This enables the generation of energy while simultaneously reducing the amount and toxicity of spent fuels. It was also found to be cheaper than considering plutonium as waste and trying to dispose of it in depositories, such as under the Yucca Mountain in Utah. This facility was designed and built to store nuclear waste safely for at least 10 000 years, but following pressure from groups opposing the disposal of nuclear waste, standards were recently increased by the Environmental Protection Agency (EPA) to a 1 000 000 years guarantee to protect the next 25 000 generations of residents living near the site [86]. However, the underground storage of used nuclear fuel without reprocessing is clearly a waste of energy potential and resources, by placing in the ground toxic materials that would be perfectly useful to generate energy. It is thus not the best approach for nuclear waste management. Having said that, the disposal of nuclear material in specifically chosen underground facilities is feasible, safe for ten thousands of years, and does not involve insurmountable technical difficulties. Furthermore, the small quantities of used nuclear fuel generated each year from a commercial power plant are in the order of only a few tonnes and thus should be easily manageable. Technical solutions also exist for the treatment of all other nuclear wastes. The disposal of nuclear waste has never been an "unsolvable problem", as some antinuclear activists claim and would like us to believe. Rather than technical, the problem is more an ethical and political one. If we were able to build the atomic bomb, we certainly should be able to solve the problems of radioactive byproducts and waste.

In order to reduce the radiotoxicity of used nuclear fuels and make much better use of the uranium resources, reprocessing and the construction of breeder reactors to convert not only 235U but also most of 238U to energy is a preferable solution. The fast neutrons in breeder reactors are also able to brake the highly active minor actinides such as neptunium, americium and curium produced during fission reactions, which contribute (besides plutonium) to most of the radioactivity of the spent nuclear fuel.

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