Boltzmann equation
Neutronics basics - Neutron migration

The neutron population in a nuclear reactor can be described as both numerous and dilute. For example, in a pressurized water reactor operating under nominal conditions, there are permanently around one hundred million neutrons per cubic centimeter: however, these neutrons are approximately 10¹⁴ times less numerous than nuclei (there are on the order of 10²² per cm³ in ordinary matter), which means that neutron-neutron interactions are totally negligible compared with neutron-matter interactions; consequently, the equation describing neutron population density will be linear. This is the equation written by Boltzmann in 1879 as part of the statistical theory of gases, and which can be used in practically the same...