A package of connected code systems for the neutronic calculations relevant in fuel management and core design of pressurized water reactors (PWRs) has been developed and applied for validation to the startup tests and first operating cycle of a 900MW(electric) PWR. The package includes the MARIA code system for the modeling of the different types of PWR fuel assemblies, the CARMEN code system for detailed few group diffusion calculations for PWR cores at operating and burnup conditions, and the LOLA code system for core simulation using onegroup nodal theory parameters explicitly calculated from the detailed solutions.