Study of thermal hydraulics of a hexagonal sub-assembly is very important to ensure safe and reliable operation of fast reactors. Identifying the dryout location in fuel sub-assembly (FSA) is a precursor for having safe Critical Heat Flux (CHF) margins, for a wide range of operating conditions. Although Computational Fluid Dynamics (CFD) studies are more accurate and detailed, multiphase CFD simulations involve different modeling parameters which extensively depends on empirical correlations. Furthermore, CFD study is computationally expensive and time consuming to perform a parametric study for higher bundle sizes, and over a wide range of operating conditions. As an alternative, reduced order models are simple and effective in performing thermal hydraulic studies for complex geometries with ease. Sub-channel analysis is a very popular approach as it models the complex geometries close to the realistic conditions. To this end, a sub-channel analysis code is developed by extending the standard single phase DIANA algorithm to two phase flow conditions. The present model is coupled with the film thickness model to predict the dryout location for a hexagonal sub-assembly. The mass, momentum and energy conservation equations are solved, using a mixture model, which is validated against available experimental data and found to be in good agreement. The occurrence of dryout depends on different geometric and operating conditions. The geometric parameters such as, pitch-to-diameter (P/D) ratio, rod-wall clearance etc., are chosen for the analysis. The effect of blockage ratio (b) on the dryout location in a hexagonal sub-assembly is systematically investigated in the range of 0.0 ≤ ≤ 0.3. It was noticed that, the presence of blockage in a sub-channel causes a reduction in the local coolant mass flux. As a result, the heat transfer rate decreases and hence the local clad temperature shoots up. In flow regimes with higher vapor quality, the presence of blockage leads to a disturbance in the continuously flowing liquid film on the heated fuel rod surface, resulting in an early occurrence of dryout. Non-uniform axial power distribution (APD) enhances the heat transfer from the fuel rod surface to the coolant. Hence, uniform and sinusoidal heat flux distributions are numerically investigated. It was observed that sinusoidal power distribution delays the occurrence of dryout, compared to the uniform APD. © 2016 Association for Computing Machinery Inc. All rights reserved.