Six factor formula
|Symbol||Name||Meaning||Formula||Typical Thermal Reactor Value|
|Thermal Fission Factor (Eta)||The number of fission neutrons produced per absorption in the fuel.||1.65|
|The thermal utilization factor||Probability that a neutron that gets absorbed does so in the fuel material.||0.71|
|The resonance escape probability||Fraction of fission neutrons that manage to slow down from fission to thermal energies without being absorbed.||0.87|
|The fast fission factor (Epsilon)||
|The fast non-leakage probability||The probability that a fast neutron will not leak out of the system.||0.97|
|The thermal non-leakage probability||The probability that a thermal neutron will not leak out of the system.||0.99|
The symbols are defined as:
- , and are the average number of neutrons produced per fission in the medium (2.43 for Uranium-235).
- 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 number density of atoms of a specific nuclide.
- is the resonance integral for absorption of a specific nuclide.
- (often referred to as worm-bar or squigma-bar) is the average lethargy gain per scattering event.
- Lethargy is defined as decrease in neutron energy.
- (fast utilization) 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.
- is the age to thermal.
- is the evaluation of where is the energy of the neutron at birth.
The multiplication factor, k, is defined as (see Nuclear chain reaction):
If k is greater than 1, the chain reaction is supercritical, and the neutron population will grow exponentially.
If k is less than 1, the chain reaction is subcritical, and the neutron population will exponentially decay.
If k = 1, the chain reaction is critical and the neutron population will remain constant.