Experimental and numerical investigation of molten corium behavior in lower head under external subcooling and boiling conditions P. Pandazis a,⇑ , T. Hollands a , X. Gaus-Liu b , A. Miassoedov b a Gesellschaft für Anlagen- und Reaktorsicherheit (GRS) gGmbH, Boltzmannstraße 14, 85748 Garching, Germany b Karlsruhe Institute of Technology, Hermann-von-Helmholtz-Platz-1, 76344, Eggenstein-Leopoldshafen, Germany article info Article history: Received 28 November 2017 Received in revised form 8 June 2018 Accepted 11 June 2018 Available online xxxx Keywords: IVR External cooling LIVE ATHLET-CD Lower head simulation abstract The in-vessel melt retention by flooding the reactor vessel externally is regarded as an effective severe accident management (SAM) strategy. According to this strategy, the corium will be stabilized within the lower head, by transferring the decay heat through the wall into the containment via external cooling. One key question of this strategy is how the melt pool heat transfer reacts to different external cooling conditions. In this paper, the melt’s thermal–hydraulic behavior under different external cooling condi- tions is studied experimentally in two LIVE tests performed in the frame of the LIVE program investigat- ing late in-vessel melt pool behavior and calculated with the lower head module AIDA of ATHLET-CD. One LIVE test was performed under nucleate boiling condition, the other under sub-cooling condition. Melt temperature, heat flux along the curved vessel wall and the crust behavior are described in transient and steady states. The simulation results have been compared with the experimental results. The results have been demonstrated the applicability of ATHLET-CD to investigate the SAM strategy in-vessel melt retention by external cooling. Furthermore, on the basis of the experimental results the modelling of heat transfer between corium and coolant has been improved. Ó 2018 Published by Elsevier Ltd. 1. Introduction One of the main goals of severe accident management (SAM) strategies is to avoid the release of radioactive fission products into the environment through stabilizing the degraded core within the containment. In the late phase of an in-vessel severe accident in light water reactors the molten core fuel together with the core internals (corium) are relocated in the lower plenum of the reactor pressure vessel (RPV). Without any counter measure the thermo- chemical attack of the relocated corium could lead to failure of the RPV wall. The SAM strategy of in-vessel melt retention has been developed to avoid the failure of the RPV wall and with this to avoid of radioactive fission product release into the contain- ment. During this measurement the RPV wall has been external cooled by flooding of the reactor cavity depending on the concept, passively or actively. The decay heat of the corium can be removed through the lower head wall into the coolant and transferred to the cavity. The success of the strategy is strongly dependent on the ther- mal load of the lower head characterized by the mass, the com- position and the included decay heat in the corium as well as on the thermal hydraulic behavior of the melt under different cooling conditions. The relocated mass and the included decay heat are dependent on the course of the accident scenario and can be usually calculated with severe accident codes. The time of a possible lower head failure is also dependent on the cooling conditions which are characterized by complex multiphase heat transfer processes between the formed crust and the lower head wall and between the wall and the surrounding coolant. Accord- ing to experimental observations there are currently three differ- ent molten pool configurations assumed in the late phase and each of these has a strong influence on the crust formation and the heat flux distribution along the lower head wall. In the sim- plest case a homogeneous corium pool is developed. In more complex cases the stratification of the corium pool is assumed in two or three layers due to the density difference of the non- mixable components and as a result of chemical reactions. In case of a stratified corium configuration the heat flux could focus on a smaller part of the lower head wall, resulting a high local heat load on the lower head wall. However, because of the geometry of a typical lower head, the convective processes within the corium pool and the inhomogeneous surrounding temperature and void distribution lead to an inhomogeneous heat flux distribution along the lower head wall also in the case of a homogeneous corium pool (Sehgal et al., 2013). https://doi.org/10.1016/j.anucene.2018.06.020 0306-4549/Ó 2018 Published by Elsevier Ltd. ⇑ Corresponding author. E-mail address: peter.pandazis@grs.de (P. Pandazis). Annals of Nuclear Energy xxx (2018) xxx–xxx Contents lists available at ScienceDirect Annals of Nuclear Energy journal homepage: www.elsevier.com/locate/anucene Please cite this article in press as: Pandazis, P., et al. Experimental and numerical investigation of molten corium behavior in lower head under external subcooling and boiling conditions. Ann. Nucl. Energy (2018), https://doi.org/10.1016/j.anucene.2018.06.020