Thermal Utilization Factor
The "Thermal Utilization Factor" in an infinite medium, is the ratio of the number of thermal absorbed in a fissionable nuclide or in a nuclear fuel, as specified, to the total number of thermal neutrons absorbed.
Applicable units are those of quantitykind:ThermalUtilizationFactor
http://en.wikipedia.org/wiki/Four_factor_formula
The "Thermal Utilization Factor" in an infinite medium, is the ratio of the number of thermal absorbed in a fissionable nuclide or in a nuclear fuel, as specified, to the total number of thermal neutrons absorbed.
http://www.iso.org/iso/catalogue_detail?csnumber=31895