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Nuclear Reactions (file A05)

by Ludwik Kowalski, Sylvie Leray and David Whittal

It is well known that a chemical reaction can not change one element into another. Nuclear reactions, however, can be used to perform such transformations. For example, aluminum bombarded with alpha particles can change to phosphorus according to

27Al + 4He ---> 30P + 1n

while iron bombarded with neutrons can change into manganese according to

56Fe + 1n ---> 56Mn + 1H

Such reactions have been used for decades to produce radioactive isotopes for medical and industrial applications. Bombardment of uranium by neutrons was initially undertaken to synthesize elements heavier than uranium; the discovery of fission was a byproduct of these investigations. Recent proposals for the destruction of radioactive wastes are based on nuclear reactions. The main idea is to create a very strong source of neutrons and to bombard the undesirable substances with them. One of such substance is 99Tc with half-life of 213000 years. By absorbing a neutron that isotope becomes 100Tc which decays into a non-radioactive product 101Ru; the half-life for the decay is only 17 seconds. Ideally one would like to extract Tc from the spent fuel before submitting it to the neutron bombardment but this may not be necessary; substances in a mixture will be destroyed by neutrons independently of each other. The word transmutation is often used to refer to nuclear reactions in which long-lived nuclei can be turned into short-lived or non-radioactive ones.

click to see Figure 1 (use the back button to return later).

Nuclear energy, as previously indicated, is now obtained through fission of 235U. That neutron-induced process is a reaction in which two fission products are formed from one nucleus, as illustrated in Figure 1 . Fission products are initially radioactive but most of them decay into non-radioactive isotopes in less then ten years. A small fraction of fission products, however, such as 99Tc and 129I, remains radioactive for much longer. In some cases a neutron is absorbed by an uranium nucleus but no fission takes place. Actinides are produced in that kind of reactions. According to (7) spent fuel generated by a commercial nuclear reactor in one year, after ten years of preliminary storage, still contains 440 kilograms of radioactive substances, 120 kg of fission products, 285 kg of plutonium and 35 of other actinides (mostly Np, Am and Cm). These substances can be destroyed in the same ways in which they were initially produced, by neutron bombardment.

Destruction by charged particles is also possible, at least in principle. For example, absorption of protons by radioactive 137Cs leads to formation of the non-radioactive 138Ba. Neutrons, however, are usually more effective transmuters than charged particles because they are not repelled from the atomic nuclei by electric forces.

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