A technique is demonstrated which is an alternative to the necessity of using many groups in order to perform an accurate fuel-cycle analysis on a fast breeder. This is accomplished by the consistent use of a space-dependent spectrum computed at the beginning-of-life of the system. The information thus generated is useful throughout the life of the system and allows an accurate and efficient analysis with very few energy groups. This technique is developed concisely for a finite differenced system of equations by the method of weighted residuals.