The phase and mass flux distribution is analyzed in the fuel bundle of a boiling water reactor (BWR). The numerical predictions of phase distribution, obtained with the multifield two-phase flow model implemented in a computational fluid dynamics code, are compared with detailed void measurements. The present model takes into account the detailed geometry of the assembly and the spatial distribution of heat sources. The influence of spacers is modeled by introducing pressure loss and turbulence sources in the momentum and turbulence equations, respectively. The model has been applied for simulation of bubbly two-phase flow for both subcooled and saturated nucleate boiling in a seven-rod bundle and a typical BWR fuel assembly. The predictions are in good agreement with tomographic measurements performed in the FRIGG loop at Westinghouse Atom.