Fast Reactor Fuel Burnup

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.