Benchmark exercise TH-27 on natural convection with steam injection and condensation inside the extended THAI facility

Abstract An international double-blind and blind code benchmark was conducted in the frame of the German THAI program. The basis for the benchmark was test TH-27 which was performed as the commissioning test for the extended, two-vessel THAI+ test facility. The test provides code validation data for simulating flow and mixing inside a containment test facility under typical accident conditions which are of practical interest to nuclear reactor safety. The TH-27 benchmark addresses the phenomena of gas mixing, steam condensation and stratification behavior of light gases under typical containment flow conditions. Double-blind, blind and open simulations were performed by thirteen participants using lumped parameter (LP) models (ASTEC, COCOSYS, MELCOR) as well as CFD codes, namely FLUENT, CFX, GASFLOW and GOTHIC. The challenge of the double-blind benchmark was to generate a model and simulate a long lasting transient without previous model calibration based on knowledge gained from earlier tests being performed in the facility. The experimental measurements allow quantifying transport mechanisms and flow conditions between the two vessels as well as inhomogeneities of the gas mixtures of the vessels. Overall, the TH-27 benchmark demonstrated the high prediction quality of LP and CFD codes provided that dedicated nodalization guidelines are considered. On the other hand the benchmark revealed noticeable user influence for LP codes, especially due to different nodalization schemes and partial lack of special treatment regarding plumes or upward directed jets.