A theoretical critical heat flux (CHF) model based on microlayer dryout and Helmholtz instability for subcooled tube flow under pressurized water reactor operation conditions is first extended to the conditions of saturated low-quality flow. Then the applicability of this extended theoretical CHF model to rod bundles is evaluated. The effects of grid spacers, cold wall, and axial heat flux nonuniformity on bundle CHFs are investigated. The extended CHF model is very accurate when compared with three other well-known CHF correlations on a data base of round tube CHF. In the simple case with uniform axial heat flux distribution, simple grid spacers, and no guide tubes in bundles, the theoretical CHF model gives good results. In other more complex cases, the cold-wall effects due to the existence of guide tubes, the effects of mixing vane grids, and the effects of nonuniform axial heat flux distributions on CHF are significant. The present model generally gives satisfactory results when compared with ∼1400 bundle CHF experimental data points although corrections for grid spacers, cold wall, and axial heat flux have not yet been considered.