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Radioactive waste (file A04)


by Ludwik Kowalski, Sylvie Leray and David Whittal


The major problem of the nuclear industry is the disposal of radioactive wastes produced in reactors. A typical large power plant generates about 30 tons of highly radioactive spent fuel each year. Most of the radioactivity is short-lived but some fission products and actinides have extremely long half-lives, for example, 213,000 years for 99Tc and 375,000 years for 242Pu. The byproducts of nuclear combustion (confined within the fuel rods) are usually stored in water pools near reactors; this storage is temporary. The ultimate destination of spent fuel is the subject of an ongoing debate. Up to quite recently the preferred U.S. option was the disposal of unprocessed radioactive wastes in stable geological formations (5). A permanent storage site, Yucca Mountain, has been selected for that purpose in the state of Nevada. It will be a long tunnel located 200 meters above the water table and 300 meters below the top of a stable rocky formation. The most dangerous wastes are expected to be reduced to glass-like blocks deposited in corrosion-resistant containers. Only a negligible amount of radioactivity is expected to escape with small amounts of rain water slowly percolating through the system.

But what should actually be deposited in deep geological formations? There is still some disagreement on that issue. The US has decided to bury all the spent fuel while the option currently advocated in France is to extract uranium and plutonium from the waste before burying. The extracted components are too valuable to be thrown away; they can be used to produce fuel elements. A related issue is what to do with the military plutonium, perhaps as much as 300 tons worldwide (6). One option is to bury it, another is to mix it with uranium and utilize it as fuel, together with plutonium extracted from wastes. The technical name of the plutonium enriched fuel is MOX (mixed oxides); it has already been fabricated and used to generate electricity in France (7) and Japan (8). The energy in each ton of the 239Pu is the same as in 14 million barrels of oil.

Spent fuel consists of two categories of elements: fission products (atomic numbers between 40 and 80) and actinides (atomic numbers above 89). All slowly decaying actinides are alpha emitters while all fission products are beta emitters. Unfortunately, we have no ways of significantly speeding up or slowing down the emission processes. The only way to get rid of the radioactive byproducts, both civilian and military, is to destroy them, that is, to turn them into non-radioactive substances or to substances whose half-lives are not excessive. But how can this be done on a large scale?

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