The physics of fast reactors differs considerably from that of thermal reactors. The most important difference is that the composition of the fuel is different. In a fast power reactor the fraction of fissile material in the fuel is about 20-30% compared with 0.7-3% in a thermal reactor. In a reactor designed to consume fissile or waste materials it may be higher. This and the lack of a moderator means that fast reactor cores are much smaller, with dimensions of the order of 1 m compared with 3 m for light-water reactors and 10 m for graphite or heavy-water reactors, and the power density is much higher.

In a fast reactor thermal neutrons are almost absent so the mater­ials with high thermal neutron absorption cross-sections, which are so important in thermal reactors, do not affect the performance of a fast reactor nearly as much. Fission products such as 135Xe and 147Sm and impurities such as boron are relatively unimportant. There is no xenon poisoning problem for a fast reactor and the decrease of reactivity with burnup of the fuel due to the accumulation of fission products is much slower than in a thermal reactor. Because most materials have similar cross-sections for fast neutrons nuclear considerations place much less severe limits on the choice of materials for a fast reactor core.

The mean free path of fast neutrons is longer than that of thermal neutrons so the core of a fast reactor is more closely coupled than that of a thermal reactor. There is no question of zonal instability and there is less depression of the neutron flux in the fuel elements.

The temperature coefficients of reactivity come from entirely dif­ferent sources — the Doppler effect and coolant expansion in fast react­ors rather than moderator expansion and change in thermal energy in thermal reactors — but the magnitudes are similar so the dynamics of fast and thermal reactors are very similar in normal operation. Only in very rapid transients is there any difference because the prompt neutron lifetime is of the order of 10-7 s in a fast reactor, compared with about 10-3 s in a thermal reactor.

In spite of the simplicity of a fast reactor neutron flux calculations are much more complex because the simplifying assumptions valid for a thermal reactor cannot be made. In a thermal reactor most of the neutrons have energies in a narrow range and one-group or few-group calculations are useful. In a fast reactor the neutrons have a wide range of energies and multigroup calculations are essential. There is no fast reactor equivalent to the “four-factor formula”.