CFD based buoyant stratification factor for augmenting lumped parameter hydrogen concentration predictions

Hydrogen distribution inside nuclear reactors containment following postulated design basis accident (DBA) and beyond design basis accident (BDBA) is a challenge and has become more evident after the Three-Mile-Island (TMI) in 1979 in USA, Chernobyl disaster 1989 and the recent Fukushima hydrogen explosion in 2011. The assessment of hydrogen transport behavior in typical multi-compartment containment geometries of the NPPs was initially based on the simplified assumption of uniform mixing of hydrogen in the entire containment atmosphere. This approach was subsequently evolved into a lumped parameter simulation considering a network of compartments interconnected by junctions. Yet, it was not satisfactory enough as the stratification of hydrogen within a compartment could not be predicted. Computational Fluid Dynamics (CFD) codes have the potential to address the local stratification issues by performing more detailed spatial calculations after considering the buoyancy terms on a highly localized basis. BARC has proposed a new approach where the lumped parameter calculation is improved using stratification factors obtained by CFD modelling. The present paper highlights the evolution of stratification factor for augmenting the lumped parameter predictions using two approaches. In the first approach, a stratification factor has been established on the basis of review of experimental data available in open literature. Since the experiments are almost always performed in smaller geometries as compared to the actual containment scale, a scaling assisted CFD simulation methodology has been used to derive these factors for nuclear reactor in the next approach. (author)