Contents lists available at ScienceDirect Nuclear Engineering and Design journal homepage: www.elsevier.com/locate/nucengdes Experimental study of transient phenomena in the three-liquid oxidic- metallic corium pool V.I. Almjashev a, ⁎ , V.S. Granovsky a , V.B. Khabensky a , S.Yu. Kotova a , E.V. Krushinov a , A.A. Sulatsky a , S.A. Vitol a , V.V. Gusarov b , F. Fichot c , B. Michel c , P. Piluso d , R. Le Tellier d , M. Fischer e , C. Le Guennic f , N. Bakouta f a Alexandrov Research Institute of Technology (NITI), Sosnovy Bor, LR, Russia b Ioffe Institute, St. Petersburg, Russia c Institut de Radioprotection et de Sûreté Nucléaire (IRSN), St Paul lez Durance, France d CEA Cadarache-DEN/DTN/STRI, France e AREVA NP GmbH, Erlangen, Germany f EDF, Saclay, France ABSTRACT Non-steady physicochemical phenomena in the three-liquid molten pool of prototypic corium are studied in the context of in-vessel melt retention problem. Experiments are made on the Rasplav-3 test facility within the CORDEB program. Structure of the initial molten pool consists of the surface light melt of molten steel, the intermediate layer of oxidic melt separated from steel melt by the crust; and the bottom layer of heavy metallic melt. It is determined that the three-layer pool structure can stay stable for a certain period of time, but the partitioning of steel and oxidic melt components through the crust brings the possibility of transformation of the three-layer pool to a two-layer structure. 1. Introduction One of the phenomena limiting the in-vessel melt retention (IVR) during a severe accident with core meltdown in a Light Water Reactor (LWR) is the focusing effect along the top metallic layer of the oxidic- metallic pool, which determines the maximum heat flux applied to the vessel wall. Comparison of this flux with DNB (departure from nucleate boiling) on the outside of the water-cooled reactor vessel surface gives the safety margin. The most systematic study of this problem was made in (Theofanous et al., 1997). In a general case the intensity of focusing effect increases, when the depth of top metal layer or stainless steel (SS) mass decreases. An additional complexity in the problem solution was brought by the results of studies carried out within the OECD MASCA program. In (Asmolov et al., 2004, 2007), in particular, it was established that component partitioning in the system of the melted suboxidized UO 2 + ZrO 2 + Zr corium and SS in the miscibility gap produces a two- layer oxidic-metallic pool, and its metallic liquid, beside SS compo- nents, includes U, Zr and a small amount of O. Depending on the system composition, the metallic liquid density can be either lower or higher than the oxidic liquid density. Therefore, the oxidic and metallic liquids can take both bottom and top position in the molten pool. Other con- ditions being equal, the lower SS fraction in the system is, the larger is U fraction in the metallic liquid, and the higher is its density. Experi- mental data on the metallic liquid composition provided in (Asmolov et al., 2004, 2007) are close to the data of (Barrachin and Defoort, 2004; Salay and Fichot, 2004) calculated using the NUCLEA thermo- chemical database and minimization of the Gibbs energy. The MASCA data are used for the modelling of molten pool structure and assessment of risks related to the focusing effect produced by the top metallic layer (Zhang et al., 2010; Le Tellier et al., 2015). In principle, a multi-component system can have thermo- dynamically stable coexistence of two, three and more liquid phases. A most frequent case is the coexistence of metallic liquid with two oxidic liquid phases. Such stratification, for example, can be caused by the interaction of corium with certain oxidic materials (Gusarov et al., 2007; Asmolov et al., 2007). There are also publications on the strati- fication of metallic melt into three liquid phases (Konovalov et al., 2012). The non-equilibrium melt having two, three and more liquid phases is also possible in presence of thermal gradient maintained in the system (Gusarov et al., 2006), or because of the spatial separation of https://doi.org/10.1016/j.nucengdes.2018.03.004 Received 26 October 2017; Received in revised form 1 March 2018; Accepted 3 March 2018 ⁎ Corresponding author. E-mail address: vac@mail.ru (V.I. Almjashev). Nuclear Engineering and Design 332 (2018) 31–37 Available online 19 March 2018 0029-5493/ © 2018 Elsevier B.V. All rights reserved. T