Abstract
A new approach to calculate the burnup matrix using a simplified formula is proposed. We used the perturbation form of the burnup matrix and the Zassenhaus formula as the numerical procedure. The merit of the approach comes from the fact that it treats only the perturbed matrix whose eivenvalues are shifted so that the absolute values are much smaller than the original. The unperturbed matrix, which generally has large eigenvalues, is treated in advance and stored to be repeatedly used in each time step. The modified procedure leads to a drastic reduction of the computing task. A small test calculation was done to check the accuracy of the simplified formula. The accuracy of the simplified formula seems to be reasonable provided that the time step is appropriately set.