Very short introduction of the atomic nucleus...

All matter in the world is composed of atoms. These atoms consist of a nucleus, and a bunch of electrons flying around the nucleus. The nucleus itself is built up of protons (particles having an electric charge "+1"), and neutrons which have no charge. The number of protons in the nucleus must be balanced by the number of electrons (electric charge "-1") that fly around the nucleus. The electrons determine the chemical behavior of the atom, so the number of protons, being the same as the number of electrons, is specific for a certain chemical element. A nucleus of a chemical element has a fixed number of protons, but may have a variable number of neutrons. The nuclei with different numbers of neutrons are called 'isotopes' of the chemical element. If a nucleus looses or acquires a neutron, it will remain the same chemical element (but it will become another isotope). When the nucleus looses or acquires one proton it will become another chemical element. Isotopes are identified by specifying the numbers of protons and neutrons that make up the nucleus, or by stating the chemical element and the total number of particles in the nucleus. Because there is a one-to-one correspondence between the chemical element and the number of protons, the number of neutrons can always be uniquely calculated. Example: uranium-238 (92 protons, 146 neutrons), or 92238, or plutonium 239 (94 protons, 145 neutrons), or 94239. Not all configurations of protons and neutrons are stable:unstable isotopes will restore equilibrium by emitting charged particles or photons(gamma rays) until a stable configuration i s reached. This process is known as radioactive decay. Radioactive decay results in a new isotope of a different chemical element since charged particles are emitted.

Conversion of nuclear fuel

Not all nuclei are fissionable by introducing a neutron, in fact, the number of fissile nuclei is rather limited. However, neutrons interact with all nuclei. One type of interaction is absorption, where the incident neutron is absorbed into a nucleus and becomes part of a nucleus. Usually the resulting isotope is unstable and the newly formed nucleus shows radioactive decay. A special case of neutron absorption happens when a heavy, non-fissile nucleus,absorbs a neutron and decays to become a fissile nucleus. The most important 2 of these mechanisms are the U-238 chain and the Th-232 chain:

U-238 (non-fissile) + n -> U-239 -> Np-239 -> Pu-239 (fissile)
Th-232 (non-fissile) + n -> Th-233 -> Pa-233 -> U-233 (fissile)

In these reactions, new fissile material (nuclear fuel) is formed from material which was previously non-fissile. Most reactors in the world use uranium fuel with 95+% U-238, and in all nuclear reactors U-238 is converted to Pu-239. In a power reactor roughly 40% of all power is produced by fissions of Pu-239, so conversion is an important (and universal! ) effect in nuclear reactors.

Conversion ratio, breeding

It is important to know how many new fissile nuclei can be formed by conversion for every fissile nucleus consumed. The ratio of (# of new fissile nuclei / # of consumed fissile nuclei) is known as the conversion ratio. This ratio can be estimated as follows:

1. For every fissile nucleus consumed, X new neutrons are released
2. For a stable chain reaction, one neutron is needed to sustain the reaction: X must be larger than 1
3. To have 1 new converted nucleus for every fissioned nucleus, one neutron is needed: X must be larger than 2
4. Neutrons will leak from the reactor, so X must be appreciably larger than 2 to make a practical reactor with a conversion ratio > 1.

The value of X is highly dependent on the energy of the incident neutrons, as shown in the figure, and X grows rapidly for high-energy interactions. For thermal energies X = 2.4, which is not large enough to have a conversion ratio > 1. At high energy, X > 3 and this enables a conversion ratio > 1: at high energies more neutrons are available for conversion reactions, and it is possible to convert more nuclei than are consumed.This is known as breeding: the process of making nuclear fuel from material which was previously not fissile.


In this figure the Greek letter eta is the symbol for X. Pu-239 has the highest value of X in the high energy region and thus Pu-239 is the best breeding fuel available. At thermal energies X=2.44 for U-235, which is enough to get a conversion ratio of around 0.5: for every U-235 nucleus consumed, 0.5 U-238 nucleus is converted to Pu-239. At higher energies X=3 or higher, providing the possibility of getting a conversion ratio > 1.

As can be seen from the figure, breeding will be more effective at high energies, and this provides the impetus for the development of the Fast Breeder Reactor. The Fast Breeder Reactor makes more fuel than it consumes, providing excellent economics and a virtually endless power supply.