The effective neutron multiplication factor k can be described using the product of six probability factors that describe a nuclear system. These factors, traditionally arranged chronologically with regards to the life of a neutron in a thermal reactor, include the probability of fast non-leakage P F N L , the fast fission factor ε ε , the resonance escape probability p , the probability of thermal non-leakage P T N L , the thermal utilization factor f , and the neutron reproduction factor η η (also called the neutron efficiency factor). The six-factor formula is traditionally written as follows: