Numerical analysis of sub-atmospheric steam condensation in suppression tank with SIMMER IV code

One of the key safety components for nuclear fusion plants is the suppression tank, which is designed to protect the Vacuum Vessel (VV) against accidental pressurization events, e.g. Loss Of Coolant Accident (LOCA). In this framework the attention is focused on the Vacuum Vessel Pressure Suppression System (VVPSS), made of water tanks in which the pure steam, or eventually mixed with incondensable gases, is injected and consequently the overpressure is dumped profiting of Direct Contact Condensation (DCC). The design constraints of fusion reactor dictate that the pressure resulting (long-term) from any accidental or baking condition should be always kept lower than 0.15 MPa. The study of the phenomena evolving during DCC in LOCA conditions is the major novelty, especially in consideration of the lack of similar studies in the available literature. In this context, a wide series of experimental tests was carried out at Pisa University (UNIPI), Department of Civil and Industrial Engineering (DICI), in a Small Scale Test Facility (SSTF), designed and instrumented for investigating DCC at sub-atmospheric pressure, by varying water pool temperature, pressure and steam mass flow rate. The adoption and assessment of suitable numerical codes, to reliably simulate such a cutting-edge multiphase multicomponent scenario, have a crucial role for contributing to the phenomena understanding and for possible safety analysis of full-scale components. On this basis, a preliminary evaluation of the cartesian three-dimensional SIMMER IV code capabilities in simulating DCC at sub-atmospheric conditions was carried out, taking as reference one UNIPI test. SIMMER IV code was able to set up precise initial low-pressure boundary conditions and simulate superheated steam condensation in subcooled water pool, with condensation efficiency comparable to the experimental one. Moreover, SIMMER IV code predicted a longitudinal steam plume dimension and injected steam velocity consistent with experimental data.