In this paper fast-fission effects in lattices with cluster elements are treated by dividing the cluster into concentric annuli and homogenizing each ring by means of the subcell method. Each annular zone is analyzed separately, and the influence of neighboring zones is described by in- and out-currents at the zone boundaries. Consistency of cross sections is checked in the simple case of an isolated rod in air. Backscattering and cluster-cluster interaction effects are taken into account and an extensive comparison is made between theoretical and experimental fission ratios. Finally, a new definition of ε is proposed, which is more consistent with the definition of p and combines all fast events in ε . EPSICH, a program based on this theory, calculates a complete case in a few tenths of a second on a CDC-1604, which justifies its use for optimalization studies.