Contents lists available at ScienceDirect Nuclear Engineering and Design journal homepage: www.elsevier.com/locate/nucengdes Effect of temperature gradient on chemical element partitioning in corium pool during in-vessel retention V.B. Khabensky a , V.S. Granovsky a , V.I. Almjashev a , S.A. Vitol a , E.V. Krushinov a , S.Ju. Kotova a , A.A. Sulatsky a , V.V. Gusarov b , S.V. Bechta c , M. Barrachin d , D. Bottomley e, ⁎ , M. Fischer f , S. Hellmann f,1 , P. Piluso g , A. Miassoedov h , W. Tromm h a Alexandrov Research Institute of Technology (NITI), Sosnovy Bor, Russia b Ioffe Institute, St. Petersburg, Russia c KTH, Stockholm, Sweden d Institut de Radioprotection et de Sûreté Nucléaire (IRSN), St Paul lez Durance, France e European Commission, Joint Research Centre Karlsruhe (JRC-Karlsruhe), Germany f AREVA GmbH, Erlangen, Germany g CEA Cadarache-DEN/DTN/STRI, France h Karlsruhe Institute of Technology Campus Nord, Karlsruhe, Germany ABSTRACT The paper presents some results of the ISTC (International Science and Technology Center)-financed project 'Investigation of Corium Melt Interaction with NPP Reactor Vessel Steel' (METCOR). In the METCOR experiments the metallic phase of a two-liquid system was produced by the interaction between hot suboxidized corium and cooled VVER vessel steel, with the steel being corroded. Models of corrosion mechanisms in the considered conditions are used to systematize data on the limiting temperature of corrosion/(dissolution) of the vessel steel. A considerable influence of thermal gradient condi- tions is shown, which has to be taken into account in the analysis of molten pool behaviour. 1. Introduction The concept of the in-vessel steel retention (IVR) is an essential component of the Severe Accident Management strategy (SAM) applied in a number of operating and developed NPPs with light-water reactors (Kymalainen et al., 1997; Theofanous et al., 1997; Rogov et al., 1996; Fischer and Levi, 2010; Oh et al., 1400; Dinh et al., 2004). The IVR efficiency is primarily determined by the conditions of thermal inter- action between the melt and the vessel, namely, by the departure from nucleate boiling (DNB) on the water-cooled external surface of the vessel. In their turn, the thermal interaction conditions are, in parti- cular, modified by the physicochemical processes in the corium melt. Previous OECD MAterial SCAling (MASCA) program studies showed that the structure of molten pool formed in the lower head of the vessel is influenced by the partitioning of the chemical elements between suboxidized corium melt (U, Zr)O 2-x and steel melt of in-vessel struc- tures. The masses, compositions and relative position of oxidic and metallic layers were then determined for close-to-thermal-equilibrium conditions (Asmolov et al., 2004, 2007). The ISTC METCOR project studied the corrosion of an externally- cooled VVER vessel steel interacting with a suboxidized corium melt. In addition to the data on the vessel steel corrosion kinetics and final corrosion depth (Bechta et al., 2004, 2006) the project provided quantitative information on the compositions of two-liquid oxidic-me- tallic systems, which also contained solid phases (Bechta et al., 2006, 2008). In contrast to MASCA melts, the formed system in the METCOR tests was kept in essentially thermal gradient conditions. Besides that, in MASCA the overall composition of the system was predetermined by the masses of molten components, while in METCOR the mass of steel interacting with molten corium was changing in the course of interac- tion, and it reached the maximum at the maximum depth of steel cor- rosion controlled by reducing the temperature profile of vessel steel. It was shown in (Bechta et al., 2004) that in the considered conditions corrosion follows the mechanism of dissolution (eutectic melting), when an interaction zone (IZ) is formed at the steel/oxide interface. From here it follows that when corrosion reaches the maximum depth and the system comes into a chemical equilibrium the corrosion front has a limiting temperature, at which steel dissolution stops. Previously https://doi.org/10.1016/j.nucengdes.2017.11.030 Received 21 May 2017; Received in revised form 15 November 2017; Accepted 18 November 2017 ⁎ Corresponding author (currently Invited Researcher at JAEA/CLADS, Iwaki, Japan). 1 Retired. E-mail address: dboksb3@gmail.com (D. Bottomley). Nuclear Engineering and Design 327 (2018) 82–91 0029-5493/ © 2017 Elsevier B.V. All rights reserved. T