AUTHOR=Delacroix Jules , Journeau Christophe , Piluso Pascal 

TITLE=High-Temperature Characterization of Melted Nuclear Core Materials: Investigating Corium Properties Through the Case Studies of In-Vessel and Ex-Vessel Retention

JOURNAL=Frontiers in Energy Research

VOLUME=Volume 10 - 2022

YEAR=2022

URL=https://www.frontiersin.org/journals/energy-research/articles/10.3389/fenrg.2022.883972

DOI=10.3389/fenrg.2022.883972

ISSN=2296-598X

ABSTRACT=During a severe accident in a nuclear reactor, the molten core – or corium – may be relocated into the reactor vessel lower plenum in case of core support plate failure. The severe accident management strategy for In-Vessel Retention – or IVR – consists in stabilizing corium within the reactor pressure vessel by external cooling of the vessel lower head. If now, the vessel fails due to excessive thermal loading on its walls, the Ex-Vessel Retention – or EVR – strategy is adopted. In this case, the core melt stabilization can be achieved by effective corium spreading, either in the reactor vessel cavity or in a dedicated “core-catcher”, and cooling by water. The success of both strategies highly depends on corium behavior at high temperatures, conditioning vessel integrity for IVR, and promotion of spreading for EVR.  This involves a variety of fundamental mechanisms closely related to heat and mass transfer regimes prevailing at system scale, which requires further analytical and experimental insight to determine the primary mechanisms and feed the modelling tools allowing the numerical simulations of severe accidents scenarios.
Within the framework of corium characterization at high temperatures, the present article aims at filling the lack of such fundamental data as density, surface tension, liquidus and solidus temperatures, and viscosity. In order to measure accurately these properties at high temperatures, the VITI facility is designed with various configurations. Concerning IVR, the influence of density and surface tension is particularly highlighted through VITI-SD and VITI-MBP configurations, and practical applications of experimental results are finally discussed, in link with the focusing effect issue at the thin upper metallic layer of the corium pool. Concerning EVR, the properties of interest are solidus/liquidus temperatures and dynamic viscosity, and typical experimental results obtained through VITI-VPA and VITI-GFL configurations are discussed in view of characterizing corium spreading.