VALIDATION OF CFD-CODES FOR NATURAL CONVECTION AND CONDENSATION PHENOMENA IN CONTAINMENTS WITH GERMAN THAI-EXPERIMENTS

摘要

In the case of a severe accident in a PWR the flammability of the hydrogen/steam/air-mixture and the resulting pressure loads in the case of combustion are mainly influenced by the local and global distribution of these gases within the PWR containment. A reliable prediction of gas distribution in case of a severe accedent is necessary for the development of mitigation strategies. CFD codes have in principal the capability to predict local gas concentrations and distribution processes within NPP containments, but validation is needed to ensure that the implemented models are able to simulate all relevant phenomena (gas mixing, build-up and erosion of stabile light gas layers, steam condensation processes,…) expected under accident conditions. Relevant thermal-hydraulic phenomena within the containment are represented with increasing complexity by the German THAI experiments TH-21 (natural convection), TH-22 (natural convection and dissolution of a light gas cloud) and TH-24 (natural convection, steam cloud dissolution and condensation processes) which were performed by Becker Technologies (Germany). All experiments are used by the German Gesellschaft fur Anlagen- und Reaktorsicherheit (GRS) gGmbH for the verification and validation of Ansys CFX in projects sponsored by the German Federal Ministry of Economics and Technology (BMWi). This paper is mainly focused on the validation of Ansys CFX with the last experiment TH-24 in the series, while only some general conclusions from the earlier analyses of TH-21 and TH-22 are presented. In this last experiment a natural convection was induced by heating and cooling of different parts of the outer THAI vessel wall. A steam-stratification was built up by injecting steam at a higher elevation in the THAI vessel. This steam stratification was afterwards dissolved by the natural convection flow. This experiment was simulated with Ansys CFX-14.5, applying the SST (Shear Stress Transport) turbulence model and additional wall- and volume-condensation models. Mesh sensitivity studies were performed to ensure compliance with the CFD best-practice-guidelines. The results show that the CFX simulation is able to predict the gas concentrations and the temperature distribution in the THAI vessel in good agreement with experimental data. Good agreement between CFX simulations and experimental results were also found for the earlier experiments TH-21 and TH-22.