Transient stratification modelling of a corium pool in a LWR vessel lower head

Abstract In the context of light water reactor severe accidents analysis, this paper is focused on one key parameter of in-vessel corium phenomenology: the immiscible phases stratification and its impact on the heat flux distribution at the corium pool lateral boundary with the so-called focusing effect related to a “thin” top metal phase and the potential vessel failure at that point. More particularly, based on the limited knowledge of the stratification transient phenomenon derived from the MASCA-RCW experiment, a basic model is proposed that can be used for corium in lower head sensitivity analyses. It has been implemented in the PROCOR platform developed at CEA Cadarache. A short parametric study on a simple hypothetical transient is presented in order to highlight the different focusing effect “modes” that can be encountered based on this in-vessel corium pool model. An early mode may occur during the formation of the top metal layer while two other modes may appear later during the thinning of this top metal layer because of thermochemically induced mass transfers. Some associated relevant parameters (model or scenario-dependent) and modelling issues are mentioned and illustrated with some results of a Monte-Carlo based sensitivity calculation on the transient behaviour of the corium in the lower head of a 1650 MWe GenIII PWR. Within the limiting modelling hypotheses, the thermal modelling of the steel layer for small (centimetre) heights and the mass diffusivity (limited in this case to the uranium diffusivity in the oxidic layer) are main sensitive parameters.

[1]  O. Kymäläinen,et al.  In-vessel retention of corium at the Loviisa plant , 1997 .

[2]  J. M. Seiler,et al.  A phenomenological analysis of melt progression in the lower head of a pressurized water reactor , 2014 .

[3]  D. Dropkin,et al.  Natural-Convection Heat Transfer in Liquids Confined by Two Horizontal Plates and Heated From Below , 1959 .

[4]  Karine Froment,et al.  Consequences of material effects on in-vessel retention , 2007 .

[5]  Fabrice Gaudier.  URANIE: The CEA/DEN Uncertainty and Sensitivity platform , 2010 .

[6]  O. Kymäläinen,et al.  In-vessel coolability and retention of a core melt , 1997 .

[7]  Jean-Marie Seiler,et al.  Simulation of MCCI with the TOLBIAC-ICB code based on the phase segregation model , 2006 .

[8]  Jean-Marie Seiler,et al.  Theoretical Analysis for Corium Pool with Miscibility Gap , 2003 .

[9]  S. V. Beshta,et al.  Distribution of components between immiscible melts of a system under nonisothermal conditions , 2006 .

[10]  Florian Fichot,et al.  Validation of ASTEC V2 models for the behaviour of corium in the vessel lower head , 2014 .

[11]  Marc Barrachin,et al.  Corium phase equilibria based on MASCA, METCOR and CORPHAD results , 2008 .

[12]  Clotilde Villermaux.  Modélisation physique et numérique de la convection naturelle dans une couche de fluide de faible rapport d'aspect dans le cadre des études d'accidents graves de réacteurs à eau sous pression , 1999 .

[13]  Jon C. Helton,et al.  Survey of sampling-based methods for uncertainty and sensitivity analysis , 2006, Reliab. Eng. Syst. Saf..

[14]  Kyoung-Ho Kang,et al.  CORIUM BEHAVIOR IN THE LOWER PLENUM OF THE REACTOR VESSEL UNDER IVR-ERVC CONDITION: TECHNICAL ISSUES , 2012 .

[15]  Lorenzo Moneta,et al.  ROOT - A C++ framework for petabyte data storage, statistical analysis and visualization , 2009, Comput. Phys. Commun..

[16]  T. C. Chawla,et al.  Heat Transfer From Vertical/Inclined Boundaries of Heat-Generating Boiling Pools , 1982 .