Nuclear Fission and Fusion

Transmutation As we’ve learned earlier, the conversion of one element to another is called transmutation.   Transmutation happens naturally for radioisotopes; they decay to products and emit radiation spontaneously. Beginning around 1919, scientists were able to perform many transmutation reactions in the laboratory by bombardment of nuclei with alpha particles.

Artificial Transmutation As was often the case back then, Ernst Rutherford was the first: Rutherford effectively synthesized an isotope of fluorine, one that is unstable and quickly decays:

Nuclear Bombardment This reaction led to the discovery of the proton sometime later. The neutron was discovered in a similar fashion by James Chadwick in 1932: The first artificial radioisotope was synthesized two years later by Marie Curie’s daughter, Irene Joliot-Curie,

Magic Bullets: Alpha Particles and Neutrons Once scientists learned how to detect and produce neutrons, an avenue to produce new elements (transuranium elements) was available: This sequence of transmutation reactions was first described in 1940. Neptunium and plutonium were the first of the transuranium elements synthesized.

Nuclear Fission The importance of neutron bombardment of atoms was recognized quickly by Enrico Fermi, an Italian physicist who spent most of his illustrious career here in the US. In 1938, two German physicists, Otto Hahn and Fritz Strassman, bombarded uranium-235 with neutrons and got a very different result than the one shown earlier:

This came to be known as nuclear fission, the event that transformed geopolitical reality. Note that for every one reactant neutron, 3 product neutrons are produced, along with a large amount of energy. So one neutron produces three, three produces nine, nine makes 27, and so on.

Very quickly, an uncontrolled chain reaction takes place, releasing extraordinary amounts of energy nearly instantaneously. 1 kg of uranium-235 has the potential to release the equivalent of 2x104 tons of TNT.

Controlled Fission: Nuclear Reactors Russian-style graphite reactor, as in Chernobyl

American-Style Reactor

Nuclear Fusion Nuclear fusion is even more important than nuclear fission. The energy of the sun, which ultimately is responsible for most life on earth, is derived from nuclear fusion:

Fusion Reactors?? Fusion reactions release even more energy than fission reactions. But they require very high temperatures (4x107K) and pressures to take place. Fusion reactors would be far safer and efficient than fission reactors. The problem is re-creating the conditions inside the sun. Matter exists as plasma at these very high temperatures, and plasmas are very difficult to contain. Remember, plasma is a phase of matter where electrons are stripped from atoms. The repulsive forces between nuclei are formidable, and must be overcome if fusion is to take place.

Artificial Fusion There are four fusion reactions that are being studied:

Fusion: Pros and Cons If possible fusion would supply unlimited, clean energy. Fusion research has suffered a number of setbacks in recent years. Funding has been cut back as the reality of the technical hurdles and vast expenditures required has become clear. Currently, fusion reactors require a high ignition temperature packing nuclei into a small space using magnetic fields keeping the nuclei in that small space tight control of the fusion reaction materials that can withstand high temperatures and radiation levels