[en] The Dynamic Flow Regime Model (DFRM) provides relations for heat, mass, and momentum transfer in two-phase flow based on simple transfer rates and the interfacial area. A dynamic equation follows the evolution of the interfacial area, taking into account generation by boiling, loss by condensation, and transformation by bubble break up and merging. THe model was motivated by the inconsistency and inadequacy of previous, nonmechanistic correlations. Key phenomena were observed in experiments and captured in a microscale simulation model, the BUBBLE code. Once the phenomena were quantitatively reproduced, a microscale description, the DFRM, was developed that could be used in thermal-hydraulic codes. The DFRM is now in use in advanced versions of VIPRE, for subchannel analysis, and PORTHOS, for steam generator simulations. A similar approach is being taken to produce accurate, reliable models for Critical Heat Flux (CHF). Existing correlations have considerable scatter, can lead to aphysical predictions, and are not based on a mechanistic understanding. Exploratory experiments have revealed that flow dynamics associated with the boiling process at individual nucleation sites are responsible for expelling liquid from the wall region. A heuristic model of this expulsion process, coupled with the DFRM, has produced a correlation for CHF that involves only a few empirical constants whose agreement with data is comparable to other correlations using ten or more parameters. A microscale model of the boiling phenomena has been developed to follow the expulsion phenomena on a first principles basis. The initial results are encouraging and suggest a means for incorporating surface conditions into the prediction. Coupled with macroscale flow calculations to account for the effect of grid spacers, a complete understanding is sought that will reduce the error in predicting CHF onset and improve the design of future fuel assemblies