Simulation of LOCA-typical containment conditions with COCOSYS on the basis of THAI-test TH-29.3

In case of a postulated loss-of-coolant accident (LOCA) in a light water reactor, core degradation might occur if emergency measures fail. During this process a large amount of hydrogen might be generated by the oxidation of the fuel rod cladding. If the hydrogen leaks into the containment of a pressurized water reactor and if passive autocatalytic recombining fails, it might form a combustible mixture with air. In case of ignition, pressure peaks can occur that are relevant for containment integrity. During LOCA the third relevant component of the containment atmosphere is steam. Test TH-29.3 aims at expanding knowledge regarding the interaction of steam and light gas (e.g. hydrogen) and was conducted in the THAI test facility. During the test steam and helium (as ta substitute for hydrogen) are injected into the test vessel. A steam-helium cloud expands downwards and a helium-rich layer forms at the boundary of the cloud. The test is simulated by the use of the lumped parameter code COCOSYS (Containment Code System) which itself is part of the software package AC developed by Gesellschaft für Anlagen- und Reaktorsicherheit (GRS) gGmbH.