The international OECD/NRC BWR Full-size Fine-Mesh Bundle Tests (BFBT) benchmark, based on the NUPEC database, encourages advancement in sub-channel analysis of two-phase flow in rod bundles, which has great relevance with regard to the nuclear reactor safety margin evaluation. This benchmark specification is being designed so that it can systematically assess and compare the participants’ numerical models for the prediction of detailed sub-channel void distributions and critical powers to full-scale experimental data on a prototypical BWR rod bundle. Currently the numerical modelling of sub-channel void distribution has no theoretical approach that can be applied to a wide range of geometrical and operating conditions. In the past decade, experimental and computational technologies have tremendously improved the study of the two-phase flow structure. Over the next decade, it can be expected that mechanistic approaches will be more widely applied to the complicated two-phase fluid phenomena inside fuel bundles. The development of truly mechanistic models for critical power prediction is currently underway. These models must include processes such as void distribution, droplet deposition, liquid film entrainment and spacer grid behaviour. The benchmark specification requires participants to explain their modelling correlations between the measured critical power and the two-phase flow dominant processes.