This document provides guidance for performing and validating the sequence of steady-state calculations leading to prediction in all types of operating commercial nuclear reactors, of the following: reaction-rate spatial distributions; reactivity; change of nuclide compositions with time. The document provides guidance for the selection of computational methods, criteria for verification and validation of calculation methods used by reactor core analysts, criteria for evaluation of accuracy and range of applicability of data and methods, and requirements for documentation of the preceding.