Under the Microscope: I'll bet you never suspected that nuclear reactors arise spontaneously in the natural world. Well, think again. Science has known since the 1970s of natural nuclear reactors that operated for hundreds of thousands of years about two billion years ago at Oklo in Gabon, west Africa, writes Prof William Reville
How this happened is a fascinating story in itself, but study of the phenomenon also offers valuable information on how buried nuclear waste migrates over time. Readers who wish to dig more deeply into the story should consult an article by Alex Meshik in Scientific American, November 2005.
In order to understand nuclear reactors we must first consider some elementary atomic physics. Matter is composed of elements and there are 92 natural elements. The smallest part of an element that can exist is an atom. An atom has two parts - a tiny central nucleus containing almost all the mass of the atom, and a surrounding cloud of electrons occupying almost all the volume of the atom. The nucleus contains two types of fundamental particles called protons and neutrons - each proton has a positive electrical charge, but neutrons have no charge. Each electron has a negative charge. The number of protons in an atom equals the number of electrons.
The identity of an element is determined by the number of protons in its nucleus. Every atom of an element has the same number of protons. Atoms of the same element can have different numbers of neutrons, thereby giving rise to varieties (isotopes) of the element that differ in mass but remain identical chemically. For example, there are three isotopes of the lightest element, hydrogen (H) - each has one proton, but one additionally has a neutron and one additionally has two neutrons. These isotopes are called ordinary hydrogen (H-1), deuterium (H-2) and tritium (H-3). The heaviest natural element is uranium (U) and there are several isotopes of U, each containing 92 protons. Uranium is present in rocks and the principal isotopes found there are U-238 and U-235.
Squeezing protons and neutrons together into the tiny nucleus isn't easy. If the ratio of protons to neutrons isn't right, the nucleus is unstable and spits out bits of itself until a stable ratio is achieved. This is the basis of radioactivity. The particles and rays emitted constitute ionising radiation. Some isotopes of the heaviest elements are so unstable that their nuclei can fracture into two roughly equal halves. This is called nuclear fission. In nuclear fission a small amount of mass disappears, converted into an enormous amount of energy according to the equation E=MC2 where E is energy, M is mass and C is the speed of light.
The fission fragments are themselves unstable and disintegrate in turn to form daughters that are also unstable. In this way a series of radioactive daughters is produced until, eventually, stable isotopes are reached. When an atom undergoes radioactive decay it changes its elemental identity because the number of protons in the nucleus changes. The rate at which an isotope undergoes radioactive decay is quantified as the half-life, ie the time taken for half of any given collection of radioactive atoms (radioisotopes) of an element to disappear.
The only naturally occurring isotope that is fissile to any significant extent is U-235. Natural uranium, as found in rocks, is about 99.3 per cent U-238 and 0.7 per cent U-235. After natural uranium is mined the U-235 component is enriched to around 3 per cent and the uranium can now be used in a nuclear fission reactor.
Power generation in a fission reactor depends on a fission chain reaction. An atom of U-235 undergoes fission when hit by a wandering neutron (there are always some around). It splits into two fission fragments and several neutrons are released, each free to tickle another U-235 atom into fission, releasing more neutrons, and so on. This is the start of a chain reaction which, if not controlled, will run amok into a nuclear explosion. The process is controlled in a civil nuclear power reactor by limiting the number of free neutrons. Also, the chain reaction fizzles out if the neutrons that initiate them move too fast. In a civil nuclear reactor care must be taken to slow the neutrons down by using a moderator. Water is a commonly used moderator.
So, we are now in a position to understand how the reactors at Oklo began. U-235 has a six-fold shorter half-life than U-238 and two billion years ago constituted 3 per cent of natural uranium. Water flowing through the rocks acted as moderator to sustain natural chain reactions. Heat generated by the chain reaction boiled off the water, which shut down the chain reaction temporarily until things cooled down and water accumulated again. One of the reactors studied studied was on power for 30 minutes and off for two and a half hours before coming on again. The average power output was about 100kw, enough to run a few dozen electric toasters.
The fission products and their daughters produced in nuclear reactors constitute the high-level nuclear waste that must be held for up to 200,000 years before it can be released into the environment. Analysis of the Oklo rocks has shown that fission products from two billion years ago have remained securely trapped in the rocks where they were born. This bodes well for current nuclear industry plans to store high-level waste in deep geological repositories.
William Reville is associate professor of biochemistry and public awareness of science officer at UCC (http://understandingscience.ucc.ie)