[en] Implementation of design procedure for prompt neutron (lp) lifetime calculation at the given spatial distribution of fission neutron warth is described. The procedure has been implemented in 26- and 21-group variants of the MMK-22 program, in the MKP subgroup program, and in the MKPA subgroup program with scattering anisotropy, and in the OMK-26 tprogram for 26-group calculation with thermalization. Estimation of lp by averaging of age t by fission events has been carried out on models of fast critical assemblies. Results of calculation of a large set of critical assembly models show great dependence of lp on the con stant system and the moderation model. Using the BNAB-78 constant system, subgroup approximation and detailed energy tracking, observed earlier systematic reduction of lp by calculation, if compare with an experiment, is removed