Six factor formula

The six-factor formula is used in nuclear engineering to determine the multiplication of a nuclear chain reaction in a non-infinite medium.

six-factor formula: ${\displaystyle k=\eta fp\varepsilon P_{FNL}P_{TNL}}$[1]

Symbol	Name	Meaning	Formula	Typical Thermal Reactor Value
${\displaystyle \eta }$	Thermal Fission Factor (Eta)	The number of fission neutrons produced per absorption in the fuel.	${\displaystyle \eta ={\frac {\nu \sigma _{f}^{F}}{\sigma _{a}^{F}}}}$	1.65
${\displaystyle f}$	The thermal utilization factor	Probability that a neutron that gets absorbed does so in the fuel material.	${\displaystyle f={\frac {\Sigma _{a}^{F}}{\Sigma _{a}}}}$	0.71
${\displaystyle p}$	The resonance escape probability	Fraction of fission neutrons that manage to slow down from fission to thermal energies without being absorbed.	${\displaystyle p\approx \mathrm {exp} \left(-{\frac {\sum \limits _{i=1}^{N}N_{i}I_{r,A,i}}{\left({\overline {\xi }}\Sigma _{p}\right)_{mod}}}\right)}$	0.87
${\displaystyle \varepsilon }$	The fast fission factor (Epsilon)	total number of fission neutrons/number of fission neutrons from just thermal fissions	${\displaystyle \varepsilon \approx 1+{\frac {1-p}{p}}{\frac {u_{f}\nu _{f}P_{FAF}}{f\nu _{t}P_{TAF}P_{TNL}}}}$	1.02
${\displaystyle P_{FNL}}$	The fast non-leakage probability	The probability that a fast neutron will not leak out of the system.	${\displaystyle P_{FNL}\approx \mathrm {exp} \left(-{B_{g}}^{2}\tau _{th}\right)}$	0.97
${\displaystyle P_{TNL}}$	The thermal non-leakage probability	The probability that a thermal neutron will not leak out of the system.	${\displaystyle P_{TNL}\approx {\frac {1}{1+{L_{th}}^{2}{B_{g}}^{2}}}}$	0.99

The symbols are defined as:[2]

• ${\displaystyle \nu }$, ${\displaystyle \nu _{f}}$ and ${\displaystyle \nu _{t}}$ are the average number of neutrons produced per fission in the medium (2.43 for Uranium-235).
• ${\displaystyle \sigma _{f}^{F}}$ and ${\displaystyle \sigma _{a}^{F}}$ are the microscopic fission and absorption cross sections for fuel, respectively.
• ${\displaystyle \Sigma _{a}^{F}}$ and ${\displaystyle \Sigma _{a}}$ are the macroscopic absorption cross sections in fuel and in total, respectively.
• ${\displaystyle N_{i}}$ is the number density of atoms of a specific nuclide.
• ${\displaystyle I_{r,A,i}}$ is the resonance integral for absorption of a specific nuclide.
  • ${\displaystyle I_{r,A,i}=\int _{E_{th}}^{E_{0}}dE'{\frac {\Sigma _{p}^{mod}}{\Sigma _{t}(E')}}{\frac {\sigma _{a}^{i}(E')}{E'}}}$.
• ${\displaystyle {\overline {\xi }}}$ is the average lethargy gain per scattering event.
  • Lethargy is defined as decrease in neutron energy.
• ${\displaystyle u_{f}}$ (fast utilization) is the probability that a fast neutron is absorbed in fuel.
• ${\displaystyle P_{FAF}}$ is the probability that a fast neutron absorption in fuel causes fission.
• ${\displaystyle P_{TAF}}$ is the probability that a thermal neutron absorption in fuel causes fission.
• ${\displaystyle {B_{g}}^{2}}$ is the geometric buckling.
• ${\displaystyle {L_{th}}^{2}}$ is the diffusion length of thermal neutrons.
  • ${\displaystyle {L_{th}}^{2}={\frac {D}{\Sigma _{a,th}}}}$.
• ${\displaystyle \tau _{th}}$ is the age to thermal.
  • ${\displaystyle \tau =\int _{E_{th}}^{E'}dE''{\frac {1}{E''}}{\frac {D(E'')}{{\overline {\xi }}\left[D(E''){B_{g}}^{2}+\Sigma _{t}(E')\right]}}}$.
  • ${\displaystyle \tau _{th}}$ is the evaluation of ${\displaystyle \tau }$ where ${\displaystyle E'}$ is the energy of the neutron at birth.