Definitions, units and constants

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  • ZA defines the element and isotope as 1000 \times Z + A, where Z is the proton number and A is the mass number. For example, the ZA for U-235 is 92235.


Quantity Unit Notes
Distance cm
Area cm2
Volume cm3
Time s depends on the case
Energy MeV
Microscopic cross section b barn = 10-24 cm2
Macroscopic cross section 1/cm
Mass g
Mass density g/cm3
Atomic density 1024/cm3 ( = b-1cm-1)
Power W
Power density kW/g
Particle flux 1/cm2s
Reaction rate 1/cm3s reaction rate density
Burnup MWd/kgU per total initial heavy metal
Burn time days


Constant Value Units Notes
INFTY 1037 N/A Used to denote infinity
ZERO 10-37 N/A The smallest nonzero absolute value