Burnup equations based upon isotopic analyses of the fuel are derived for fast reactors. Initial concentrations of U235, U236, U238, Pu239, Pu240 and Pu241 were considered. Relative reaction rates were used to simplify the isotopic network into an analyzable form. The resulting equations are valid when:

  1. the total neutron flux is in the range 1014–1017 n/cm2.
  2. initial Pu241 does not exceed total initial uranium, and
  3. a two-week cooling-off period occurs before the isotopic analysis is performed.
The success of this method depends upon the availability of accurate fast-neutron cross sections.