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 10 ¹⁴ times less numerous than nuclei (there are on the order of 10 ²² per cm ³ 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...