It is possible to make use of the neutrons that are not needed to maintain the chain reaction in various ways. The most important is for

breeding. When a neutron is captured in 238U the 239U that is formed decays in the following way:

The times shown are the half-lives for the decay processes. As far as reactor operation is concerned the long-lived plutonium isotope 239Pu is the end-product of the chain.

239Pu has nuclear properties quite similar to those of 235U and it can be fissioned by neutrons of all energies. Neutron capture thus provides a route for converting 238U into fissile material, so 238U is called a “fertile” isotope. 232Th, which is the only naturally occurring isotope of thorium, is also fertile. It behaves very similarly to 238U: the 233Th formed on capture of a neutron decays in a chain to 233U which is long-lived and fissile.

Thus there are two naturally occurring fertile isotopes, 232Th and 238U, and three related fissile isotopes: 233U, 235U and 239Pu. There are other fissile and fertile isotopes but these five are the most important.

This ability to convert fertile isotopes to fissile raises the possibility of “breeding” new fissile material, but this can be done only if enough neutrons are available. The average number of neutrons liberated in a fission is denoted by v. Its value depends on which isotope is being fissioned and on the energy of the neutron causing the fission, but in most cases it is about 2.5. We have seen that the fact that v is greater than 1 makes a chain reaction possible: the fact that it is greater than 2 is almost equally important. If we have a reactor in which on average one neutron from each fission causes another fission to maintain the chain reaction, and if in addition more than one of the other neutrons is captured in fertile material, then the total number of fissile nuclei will increase as the reactor operates. Such a reactor is called a “breeder”.

It is sometimes said that a breeder reactor generates more fuel than it consumes. This is rather misleading. The reactor produces more fissile material than it consumes, but to do this it depends on a supply of fuel in the form of fertile material.

Although v > 2 suggests the possibility of a breeder reactor the requirement for breeding to take place is more complicated. When a neutron interacts with a fissile nucleus it does not necessarily cause fis­sion. It may be captured, and if it is, it is effectively lost. The important quantity in determining whether breeding is possible is the average number of neutrons generated per neutron absorbed. This is denoted by n, where

n = vaf Kaf + °c )■

n, which is sometimes called the “reproduction factor”, is a function of the neutron energy E, and its variation with E for the three fissile isotopes is shown in Figure 2.

Of these n neutrons one is needed to maintain the chain reaction. Some of the remainder are lost either because they diffuse out of the reactor or because they are captured by some of the other materials present, such as the coolant or the reactor structure. The others are available to be captured by fertile nuclei to create fissile nuclei. If we denote the number of neutrons lost per neutron absorbed in fissile material by L and the number captured in fertile material by C, then C is the number of fissile nuclei produced per fissile nucleus destroyed and is given by

C ~ n — 1 — L

(This is only a rough value because there are other things that may happen to neutrons).

If C is greater than one, as it must be if the reactor is to breed, it is known as the “breeding ratio”. If it is less than one it is called the “conversion ratio”. There is no logical reason for the existence of two names for C. The usage grew up because different words were used in the contexts of different reactor systems.

In practice L cannot be reduced below about 0.2, so that breeding is possible only if n is greater than about 2.2. Figure 2 shows how this can be brought about. A fast reactor using any of the three fissile materials can be made to breed, although 239Pu gives the widest margin and 235U will allow breeding only if the energy of the neutrons causing fission is not allowed to fall much below 1 MeV. In all cases the higher the neutron energy the better the breeding ratio. A 233U-fuelled thermal reactor is just able to breed but the margin is very slender. The most widely favoured breeder system is based on the use of 238U and 239Pu in fast reactors, but there is also a certain amount of interest in the 232Th — 233U system, also in fast reactors.

A fast reactor does not necessarily have to be a breeder. The excess neutrons can be used in other ways. One such is to use them to con­sume radioactive waste materials by transmutation. This process can be applied to two classes of radioactive waste: fission products and “higher actinides” (i. e. nuclides with atomic numbers greater than 94). In both cases it is the most long-lived nuclides that are of interest because of a perceived difficulty in ensuring the integrity and safety of waste storage facilities over the very long periods, up to a million years, for which the waste remains dangerous. Fission products such as 93Zr, 99Tc, 129I and 135Cs have half-lives of the order of 106 years, as does the actinide 237Np.

In most cases it is not possible to transmute any of these nuclides into stable isotopes. However, the fission products can be made less hazardous by transmuting them into other radioactive materials with shorter half-lives. Higher actinides can be eliminated by fissioning them. Some of the fissile higher actinides can be used in effect as nuclear fuel, and in all cases benefit can be taken of the energy released when they are fissioned.

It is also possible to envisage a fast reactor that, instead of breeding, acts to consume fissile material. In this way it may be possible to use a fast reactor to eliminate unwanted stocks of weapons-grade plutonium.