The 12 th International Topical Meeting on Nuclear Reactor Thermal Hydraulics (NURETH-12) Log Number: 35 Sheraton Station Square, Pittsburgh, Pennsylvania, U.S.A. September 30-October 4, 2007. (1/22) ON THE SIMULATION OF TWO-PHASE FLOW PRESSURIZED THERMAL SHOCK (PTS) D. Lucas 1,* * * * , D. Bestion 2 1 Forschungszentrum Dresden-Rossendorf, e.V. (FZD), Institute of Safety Reseach P.O.Box 510 199, 01454 Dresden, Germany D.Lucas@fzd.de 2 Commissariat à l’Énergie Atomique (CEA), Centre d’Études Nucléaires de Grenoble 17 Rue des Martyrs, 38054 Grenoble, France dominique.bestion@cea.fr E. Bodèle 1 , M. Scheuerer 3 , F. D’Auria 4 , D. Mazzini 4 , B. Smith 5 , I. Tiselj 6 , A. Martin 7 , D. Lakehal 8 , J.-M. Seynhaeve 9 , R. Kyrki-Rajamäki 10 , M. Ilvonen 11 , J. Macek 12 3 Gesellschaft für Anlagen- und Reaktorsicherheit (GRS, Germany), 4 Università di Pisa (Italy), 5 Paul Scherrer Institute (PSI, Switzerland), 6 Jožef Stefan Institute (JSI, Slovenia), 7 Électricité de France (EDF), 8 ASCOMP GmbH (Switzerland), 9 Université Catholique de Louvain La Neuve (UCL, Belgium), 10 Lappeenranta University of Technology (LUT, Finland), 11 VTT Industrial Systems (Finland), 12 Nuclear Research Institute Rez plc (NRI, Czech Republic) ABSTRACT This paper reports some activity about the Pressurized Thermal Shock (PTS) performed within the European Integrated Project NURESIM. The PTS phenomenon is expected to take place in some water cooled nuclear reactors equipped with pressure vessels during selected accident scenarios. The PTS implies the formation of temperature gradients in the thick vessel walls with consequent localized stresses and the potential for propagation of possible flaws present in the material. Current generation Pressurized Water Reactors, PWR (including the Russian VVER types), are primarily affected by the phenomenon which is investigated within three broad areas: material damage originated by irradiation, thermal-hydraulics (including single and two-phase flow conditions in the region of the ‘shock’) and structural mechanics with main reference to fracture mechanics. The present paper, in the area of thermal-hydraulics, focuses on the study of two-phase conditions that are potentially at the origin of PTS. Within the above context, the paper summarizes recent advances in the understanding of the two-phase phenomena occurring within the geometric region of the nuclear reactor, i.e. the cold leg and the downcomer, where the ‘PTS fluid-dynamics’ is relevant. Available experimental data for validation of two-phase CFD simulation tools are reviewed and the capabilities of such tools to capture each basic phenomenon are discussed. Key conclusions show that several two phase mechanisms (or sub- phenomena) are involved and can individually be simulated at least at a qualitative level, but the capability to simulate their interaction and the overall system performance (case of two phase flow) is presently not available. * Corresponding Author