A major goal of fuel assembly design is a reliable prediction of the critical heat flux. Development and validation of CFD methods for predicting two-phase flows and CHF conditions is subject of ongoing research at AREVA-NP. This paper deals with numerical modeling of subcooled flow boiling of water in pipes and subchannels at flow conditions corresponding to the range of pressurized water reactor operation conditions. As simulation tool the commercial CFD software Star-CD is used in conjunction with its Extended Boiling Framework (EBF), a software package providing modeling tools for two-phase flow boiling. Substantial progress towards a CFD prediction of thermal hydraulic phenomena relevant for fuel assembly design could be achieved. In the future, modelling will be needed to be complemented by experimental validation data with sufficient local resolution, to validate CFD models for high pressure thermal hydraulic operating conditions.