In the beginning ... (more or less)
Stars were formed by clouds of hydrogen collapsing under their own weight, with the energy released making them hot enough to start combining hydrogen atoms into helium. This process too released energy (whether it can be reproduced usefully on Earth remains to be seen) and still powers the emission of light and heat from the sun. In time it went further, forming higher element though releasing much less energy per step. In fact, to make elements beyond a certain point would require a great input of energy, which could however be supplied by explosions in dying stars. So were formed the heavier elements such as uranium, which were scattered into space and available to be gathered into later generations of stars and planets.
Atomic structure and radioactivity
On the simplest view, atoms comprise three types of particle:
Protons and neutrons are almost two thousand times heavier than electrons, and are confined to a nucleus of about a ten-thousandth of the atomic diameter; the rest of the space is occupied by a surrounding cloud of electrons equal in number to the protons when the atom is complete. Thus atoms can interact directly only through the electrons, and chemical identity is determined by the number of electrons or protons - the atomic number.
Protons repel each other electrically, so apart from hydrogen with only a single proton, all nuclei need a similar or larger number of neutrons to hold them together. In fact the heavier the atom, the greater the proportion of neutrons needed. Beyond lead, all elements are to some extent unstable, although atoms may last for billions of years before disintegrating.
Because chemical identity depends on the protons, the number of neutrons is not necessarily the same in all atoms of an element, giving rise to isotopes of the element. The number cannot vary very widely without causing instability: too many neutrons and one may convert to a proton, emitting an electron as a beta-particle to balance the charge; too few, and a proton may convert to a neutron by emitting a positron (anti-electron) or absorb an electron from the surrounding cloud. All such changes are to a state of lower energy within the atom, with the excess divided between the remaining nucleus, emitted material particles and electromagnetic radiation - gamma rays.
The heaviest elements have two other possible ways of decaying: they may emit a helium nucleus or alpha-particle, or split into two major parts - fission.
A susceptible atom may split spontaneously, but much more readily if struck by a neutron. For some is required a very energetic neutron; these are called fissionable; for others, which are fissile, any energy will serve, and apart from some especially favourable energy levels, the lower the better since the density of neutrons in any space is inversely related to the speed with which they pass through it.
Fission generates not only the two major fragments, but also on average two or three free neutrons that under favourable conditions may go on to cause fission in other nuclei, and so on in a chain reaction that may be self-sustaining (the condition of criticality). It releases much of the energy absorbed from the exploding parent star when the nucleus was originally assembled, eventually appearing as heat.
The only fissile isotope naturally present in any quantity on Earth is uranium-235. Over 99% of uranium is however U-238 which is only fissionable; it can however absorb a neutron to form the short-lived U-239, which by emitting two electrons changes (transmutes) into plutonium-239, another fissile isotope. U-238 is therefore described as fertile.
Because the proportion of neutrons to protons rises with increasing atomic number, the fission fragments have too many, and on average need to convert about five of them to protons on the way to stability. The earlier steps are generally rapid, but the last one or two may be delayed; hence the prolonged radioactivity of nuclear waste. The moment when a radioactive atom actually decays is purely a matter of chance (and until that happens it emits no radiation), but in any time interval there is a certain probability; thus half of any particular isotope decays in a characteristic time called the half-life. It follows that the longer the half life, the lower the radioactivity.
The purpose of a power reactor is to maintain a nuclear fission chain, control it at a steady rate, and deliver the energy released (in practice as steam) to electricity generators. Nearly all at present are of the thermal type in which the initially very energetic neutrons released by fission are slowed down so as to interact more readily with fissile nuclei.
The essential components are thus:
A practical reactor also requires shielding to protect operators from radiation, and containment for any leak of radioactive material. The natural reactors that ran nearly two billion years ago in West Africa didn’t have them, but the surviving remains are still there.
The fuel is generally based on uranium as metal or oxide, usually enriched with an increased proportion of U-235 (obtained by partial separation from the raw stock, leaving the rest somewhat depleted); plutonium may be used instead, and will in any case be generated from U-238 during operation, partially compensating for the U-235 consumed. The whole is encased in an inert metal to contain the fission products.
The moderator must be a poor absorber of neutrons and be of light elements for effective energy transfer. Water is most commonly used (despite absorbing some neutrons unless highly enriched in deuterium, the second isotope of hydrogen), but sometimes graphite.
Carbon dioxide may be used as coolant, but water is much more common, and then usually serves also as moderator.
The control system uses movable rods of strongly neutron-absorbing materials to keep the neutron flux at the required level. A feature crucial to stability is negative feedback: any momentary power increase raises the energy of the flux, automatically reducing its ability to continue the fission chain and so limiting the power change. In one tentative design concept it might provide automatic load-matching without action by operators.
The RBMK type of power reactor, used in early Soviet stations, is unique in combining water coolant with graphite moderator. In some circumstances, bubbles formed by boiling could reduce neutron absorption with less effect on moderation, so increasing net reactivity and creating the positive feedback that led to the explosion at Chernobyl. No other type is capable of such behaviour.
A drawback of thermal reactors is that in a single pass of the fuel they can deliver only a hundredth of the energy theoretically obtainable from the uranium that went into its manufacture, including the depleted tails from enrichment. By separating and recycling the uranium and plutonium from the discharged fuel, the proportion can be somewhat increased. For anything like complete utilisation, however, it would be necessary to use fast-neutron reactors in which not only U-235 and plutonium-239 but also U-238 and the other plutonium isotopes formed from it by neutron absorption can all undergo fission. Enough free neutrons are then produced for generation of plutonium to outweigh slightly the consumption of fissile material, and the system is then said to breed. It can thus utilise the stocks of largely low-grade plutonium and recovered or depleted uranium remaining from the earlier processes and now needing storage. The higher the neutron energy the better, so moderating materials must be avoided as far as possible and water cannot serve as coolant; instead may be used helium or a molten metal such as sodium or lead. The higher temperatures then possible allow a more efficient conversion of heat into mechanical and then electrical energy.