Using a method based on the concept of "now of atoms" in the nuclear fuel 238U to describe its generation of derived nuclide under neutron flux’s reaction and the transformation of derived nuclide from generation to generation as well as fission prodtiction accompanied with each derived nuclide

expression of the instantaneous transformation for the fuel consumption and the total fission production is obtained.At the beginning stage

this ratio grows up from zero and increases gradually with reactor operating time corresponding to fifth unstable nuclide series constituted by 238U and its derived nuclide group being in the developing stage. As soon as the fifth unstable nuclide series reaches its equilibrium stage

the transforination ratio approaches its final value very close to unity.