Nuclear Engineering and Design 265 (2013) 210–221 Contents lists available at ScienceDirect Nuclear Engineering and Design j ourna l h om epa ge: www.elsevier.com/locate/nucengdes Experiments and MPS analysis of stratification behavior of two immiscible fluids Gen Li a,∗ , Yoshiaki Oka a , Masahiro Furuya b , Masahiro Kondo b a Cooperative Major in Nuclear Energy, Waseda University, 3-4-1, Okubo, Shinjuku-ku, Tokyo 169-8555, Japan b Nuclear Technology Research Laboratory, Central Research Institute of Electric Power Industry (CRIEPI), 2-11-1 Iwado-kita, Komae, Tokyo 201-8511, Japan h i g h l i g h t s • Improving numerical stability of MPS method. • Implicitly calculating viscous term in momentum equation for highly viscous fluids. • Validation of the enhanced MPS method by analyzing dam break problem. • Various stratification behavior analysis by experiments and simulations. • Sensitivity analysis of the effects of the fluid viscosity and density difference. a r t i c l e i n f o Article history: Received 1 May 2013 Accepted 2 September 2013 a b s t r a c t Stratification behavior is of great significance in the late in-vessel stage of core melt severe accident of a nuclear reactor. Conventional numerical methods have difficulties in analyzing stratification process accompanying with free surface without depending on empirical correlations. The Moving Particle Semi- implicit (MPS) method, which calculates free surface and multiphase flow without empirical equations, is applicable for analyzing the stratification behavior of fluids. In the present study, the original MPS method was improved to simulate the stratification behavior of two immiscible fluids. The improved MPS method was validated through simulating classical dam break problem. Then, the stratification processes of two fluid columns and injected fluid were investigated through experiments and simulations, using silicone oil and salt water as the simulant materials. The effects of fluid viscosity and density difference on stratification behavior were also sensitively investigated by simulations. Typical fluid configurations at various parametric and geometrical conditions were observed and well predicted by improved MPS method. © 2013 Elsevier B.V. All rights reserved. 1. Introduction Stratification behavior of fluids is an important phenomenon in nature and traditional industry. It is also of great significance in the late in-vessel stage of core melt severe accident of a nuclear reactor. After water is boiled off from the Reactor Pressure Vessel (RPV), a molten corium pool may form in the RPV lower head due to the decay heat released from fission products. In that case, a substantial amount of molten corium is collected into the lower head, as happened in the TMI-2 accident. Following the principle of phase equilibrium, the molten corium may separate into non- miscible layers. Most available studies on the in-vessel retention of molten corium usually consider the two-layer pool as the final configuration, that is, the oxide layer under the light metal layer. ∗ Corresponding author. Tel.: +81 3 5286 8225; fax: +81 3 5286 8225. E-mail addresses: ligen@fuji.waseda.jp, leageon@gmail.com (G. Li). The thermo-hydraulics of the molten pool greatly influence the distribution of thermal load across the vessel wall, and it has been widely studied through experiments using simulant materi- als, such as COPO (Kymäläinen et al., 1994), ACOPO (Theofanous et al., 1997; Theofanous and Angelini, 2000), SIMECO (Sehgal et al., 1998), RASPLAV-SALT (Asmolov et al., 2003) and BALI (Bonnet and Seiler, 1999). The main objective of these studies is to investigate the heat flux distribution surrounding a volumetrically heated melt pool at a wide range of Rayleigh number. In addition, the chemi- cal behavior of oxide corium in contact with structural materials has been experimentally investigated by MASCA (Tsurikov et al., 2007) using the prototypic core materials. It focuses on the effects of iron content and oxidized degree of molten corium on the con- figurations of the pool. Post-test results indicate that the molten pool, initially a uniform mixture of components, separates into two layers with different densities. MAAP (Suh and Henry, 1996a,b), MELCOR (Gauntt et al., 2005) and PECM (Tran and Dinh, 2009a,b) codes analyzed heat transfer 0029-5493/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.nucengdes.2013.09.006