We have just discussed the energy changes which lead to the generation of electricity in a coal-burning electric power station. In a nuclear electricity generating installation the heat required for raising steam is provided by a nuclear reactor instead of a coal furnace.
Fig. 7.5 is a simplified diagram to show how a reactor works. It consists of a strong steel pressure vessel enclosing a core made of graphite bricks (Fig. 7.6). This graphite core has a number of vertical channels which are filled with rods of a very heavy
metal called uranium. Interspersed among the uranium rods are a set of boron steel rods which may be raised or lowered in similar channels in the graphite. These are the control rods, and their function will be explained in due course. The uranium used in the reactor consists of a mixture of two different kinds of atoms, of which the most important are called uranium-235. Quite spontaneously, some of these uranium-235 atoms explode or disintegrate to form other atoms of smaller mass. When this happens, energy is radiated from the central core or nucleus of the atom together with small high-speed particles called neutrons. If one of the
neutrons happens to strike the nucleus of a neighboring atom this may also disintegrate, with a further evolution of energy and the production of more neutrons. This splitting up of the nucleus is called fission.
The graphite of which the core is composed is called a moderator (Fig. 7.6). Its function is to slow down the speed of the neutrons, as it is found that fission of uranium-235 is more likely to occur with slow neutrons than with fast ones. In a small piece of uranium mixed with moderator most of the neutrons escape through the surface. If, however, the amount of material is increased the chances that a neutron will collide with an atomic nucleus will also increase, since there are more atoms present. Each nuclear fission which occurs produces two or three fresh neutrons which are, in turn, capable of promoting the fission of further nuclei. When the lump of uranium and moderator is above a certain critical size the fission process proceeds cumulatively in what is called a chain reaction (Fig. 7.7). This is where the above-mentioned boron steel rods play their part. Before the uranium rods are laded into the graphite core the boron rods are already in position, and these have <he property of being able to absorb neutrons which are shot out from the uranium, and so prevent the chain reaction from starting. When sufficient uranium rods have en added to effect critical conditions the pressure vessel is sealed and the boron rods raised out of the core. The uranium rods are now freely bombarded by one another’s neutrons and the chain reaction begins. The rate at which fission occurs can, of course, be controlled by raising or lowering the boron rods. If these are fully ill erred into the graphite core the reaction shuts down completely, and only the normal spontaneous nuclear fission takes place.
The heat energy released by the fission process is carried away as internal energy
in a stream of high-pressure carbon dioxide gas which is continuously pumped
through the pressure vessel. This hot gas circulates through a special steam boiler,
and the steam so raised is used to drive an electric turbo-generator in the usual way. Nuclear power installations working on the principle just described are called
Magnox reactors, and the first of these was built at Calder Hall, Cumberland in
Britain in 1956. Magnox is the name of the aluminium alloy in which the fuel rods are encased. In recent years, reactors have been designed which use other types of nuclear fuel with different moderators. Owing to the higher temperatures and pressures involved compared with Magnox reactors, some of these have presented safety design problems.