Six factor formula

• , and are the average number of neutrons produced per fission in the medium.
• and are the microscopic fission and absorption cross sections for fuel, respectively.
• and are the macroscopic absorption cross sections in fuel and in total, respectively.
• is the macroscopic fission cross-section.
• is the number density of atoms of a specific nuclide.
• is the resonance integral for absorption of a specific nuclide.
• *
• is the average lethargy gain per scattering event.
• *Lethargy is defined as decrease in neutron energy.
• is the probability that a fast neutron is absorbed in fuel.
• is the probability that a fast neutron absorption in fuel causes fission.
• is the probability that a thermal neutron absorption in fuel causes fission.
• is the geometric buckling.
• is the diffusion length of thermal neutrons.
• * where is the diffusion coefficient.
• is the age to thermal.
• *
• * is the evaluation of where is the energy of the neutron at birth.

  Multiplication

• If is greater than 1, the chain reaction is supercritical, and the neutron population will grow exponentially.
• If is less than 1, the chain reaction is subcritical, and the neutron population will exponentially decay.
• If, the chain reaction is critical and the neutron population will remain constant.