We sometimes talk about living in the Nuclear Age, because it has only been in the last 50 years that we have managed to harness the power expressed by Einstein as E=mc2. But strictly speaking, uranium fission, which is what we think of when we think of nuclear power, isn’t new.
About 1.78 billion years ago (give or take a few million years) there was enough uranium trapped in the rock near Gabon, West Africa, for a chain reaction to begin on its own. For a million years, Gabon was home to the world’s first nuclear reactor.
Still, we egotistical humans don’t really pay much attention to things that happened before we were around to appreciate them. We generally consider the Nuclear Age to have been born on December 2, 1942, under the squash courts at the University of Chicago, when the “nuclear pile” built by Italian physicist Enrico Fermi (and the rest of the Manhattan Project scientists) started producing heat from the process of nuclear fission.
Fission was discovered in 1938 by the German physicists Otto Hahn and Fritz Strassmann. They were bombarding uranium with neutrons–uncharged particles that make up atomic nuclei along with the positively charged protons–when they identified barium nuclei among the fragments from the collisions. The barium nucleus has 56 protons, far fewer than uranium’s 92.
Two colleagues of Hahn and Strassmann, Lise Meitner and her nephew Otto Frisch, who had fled to Copenhagen to escape the Nazis, suggested that a neutron could break a uranium atom into two pieces, a process they named fission. They went on to prove their idea experimentally.
Danish theorist Niels Bohr, together with American John Wheeler, explained this with the “liquid drop” analogy. Think of the heavy uranium nucleus as a drop of liquid. If a drop of liquid is distorted enough from its original spherical shape, it will split into two drops, because two spherical drops require less energy to maintain than one distorted large drop. The nucleus behaves the same way, splitting into two pieces if it’s distorted enough.
This can happen spontaneously in some atomic nuclei. Uranium-238, for example, the most common form of uranium, exhibits this spontaneous fission (not to be confused with ordinary radioactivity, in which the nucleus sheds particles a few at a time). But this happens so slowly that the spontaneous fission half-life of uranium-238–the time in which it would take half of all uranium-238 nuclei to spontaneous split into smaller nuclei–is 8,000 trillion years. (In other words, don’t wait up.)
However, fission can also be induced by bombarding certain nuclei with neutrons, as Meitner and Frisch proved. A nucleus of uranium-238 will capture an energetic neutron and form an energized nucleus of uranium-239 (the numbers are the total number of protons and neutrons in the nucleus). However, the uranium-239 nucleus distorts much more easily, and within a tiny fraction of a second, it splits into two smaller fragments.
The two new nuclei aren’t the only thing produced, however. There are usually some smaller bits left over, including neutrons, because a large nucleus contains a larger proportion of neutrons than a small one. These neutrons may then bombard other large nuclei nearby, which will split and send out neutrons to bombard other nuclei, producing a self-sustaining “chain reaction.”
The total mass of two medium-sized nuclei is less than the mass of a large nucleus twice their size. Thus whenever a large nucleus is split, a certain amount of mass disappears–and reappears as energy. A lot of energy, as Einstein’s equation points out.
If the chain reaction is controlled, it produces energy in a nice steady flow that can be harnessed to heat water to make steam to turn turbines to create electricity to run your hair dryer. If the chain reaction is uncontrolled, and the uranium is tightly packed enough (“critical mass”) it can lead quickly to an extremely large and sudden release of energy and spoil your day in a hurry. (This did not go unnoticed during the Second World War, which is why on July 16, 1945, there was a rather impressive explosion in the desert near Alamogordo, New Mexico.)
Since an uncontrolled nuclear chain reaction is obviously not desirable under most circumstances, especially not if you’re standing nearby, some way has to be found to “moderate” the process. This is done by surrounding the fuel with some substance that slows down the neutrons set free by fission. In the CANDU (CANadian Deuterium Uranium) reactor, this moderator is “heavy” water, water in which the ordinary hydrogen atoms have been replaced with deuterium, a form of hydrogen containing a neutron in its nucleus in addition to the usual single proton. The CANDU reactor also uses heavy water as a coolant. Ordinary water can also be used as a moderator and coolant, while the infamous Chernobyl reactor used graphite as a moderator.
The uranium oxide that fuels reactors is contained in “fuel rods.” There are also “control rods”; these are made of boron or cadmium or some other material that absorbs neutrons and can therefore slow down or stop the reaction.
Nuclear fission has the advantage, as an energy source, of producing no noxious fumes or greenhouse-effect-enhancing gases and of making extremely efficient use of its fuel supply (one gram of uranium yields roughly the same amount of power as one tonne of coal).
Its disadvantages include problems of waste disposal and transport, high capital costs and public fears about safety. What’s really needed is a way to harness nuclear power that does not involve messy fissionable material like uranium and does not produce unpleasant side-effects like waste plutonium.
Which brings me to the topic of nuclear fusion–and next week’s column.