Coolability of particulate beds in severe accidents: Status and remaining uncertainties

Abstract Particulate debris beds may form during different stages of a severe core melt accident; e.g. in the degrading hot core, due to thermal stresses during reflooding, in the lower plenum, by melt flow from the core into water in the lower head, and in the cavity by melt flow out of a failing RPV into a wet cavity. Deep water pools in the cavity are used in Nordic BWRs as an accident management measure aiming at particulate debris formation and coolability. It has been elaborated in the joint work of the European Severe Accident Research Network (SARNET) in Work Package (WP) 11.1 that coolability of particulate debris, reflooding of hot debris as well as boil-off under decay heat (long-term coolability), is strongly favoured by 2D/3D effects in beds with non-homogeneous structure and shape. Especially, water inflow from the sides and via bottom regions strongly improves coolability as compared to 1D situations with top flooding, the latter being in the past the basis of analyses on coolability. Data from experiments included in the SARNET network (DEBRIS at IKE and STYX at VTT) and earlier ones (e.g. POMECO at KTH) have been used to validate key constitutive laws in 2D codes as WABE (IKE) and ICARE/CATHARE (IRSN), especially concerning flow friction and heat transfer. Major questions concern the need of the explicit use of interfacial friction to adequately treat the various flow situations in a unified approach, as well as the adequate characterization of realistic debris composed of irregularly shaped particles of different sizes. Joint work has been supported by transfer of the WABE code to KTH and VTT. Concerning realistic debris, the formation from breakup of melt jets in water is investigated in the DEFOR experiments at KTH. Present results indicate that porosities in the debris might be much higher than previously assumed, which would strongly support attainment of coolability. Calculations have been performed with IKEJET/IKEMIX describing jet breakup, mixing and settling of resulting particles. Models about debris bed formation and porosity are developed at KTH. The codes have been applied to reactor conditions for analysing the potential for coolability in the different phases of a severe accident. Calculations have been performed with WABE (MEWA) implemented in ATHLET-CD and with ICARE/ICATHARE for degraded cores and debris beds in the lower plenum, under reflooding and boil-off. Ex-vessel situations have also been analysed. Strong effects of lateral water inflow and cooling by steam in hot areas have been demonstrated. In support, some typical basic configurations have been analysed, e.g. configurations with downcomers considered as possible AM measures. Melt pool formation or coolability of particulate debris is a major issue concerning melt retention in the core and the lower head. Present conclusions from those analyses for adequate modelling in ASTEC are outlined as well as remaining uncertainties. Experimental and analysis efforts and respective continued joint actions are discussed, which are needed to reach resolution of the coolability issue.

[1]  Walter Widmann,et al.  Validation and application of the WABE code: Investigations of constitutive laws and 2D effects on debris coolability , 2006 .

[2]  D. Magallon,et al.  Characteristics of Corium Debris Bed Generated in Large-Scale Fuel Coolant Interaction Experiments. , 2006 .

[3]  David A. Petti,et al.  A Scenario of the Three Mile Island Unit 2 Accident , 1989 .

[4]  T. G. Theofanous.  On the proper formulation of safety goals and assessment of safety margins for rare and high-consequence hazards , 1996 .

[5]  Günter Lohnert,et al.  Simulation of melt jet breakup and debris bed formation in water pools with IKEJET/IKEMIX , 2006 .

[6]  Franz Mayinger,et al.  Heat transport and void fraction in granulated debris , 2006 .

[7]  D. Magallon,et al.  High pressure corium melt quenching tests in FARO , 1995 .

[8]  J. M. Seiler.  Analytical model for CHF in narrows gaps on plates and in hemispherical geometries , 2006 .

[9]  T. G. Theofanous,et al.  On the measurement and mechanism of dryout in volumetrically heated coarse particle beds , 1991 .

[10]  Stefan Holmström,et al.  Dryout heat flux experiments with deep heterogeneous particle bed , 2006 .

[11]  T. Ginsberg,et al.  Experimental and analytical investigation of quenching of superheated debris beds under top-reflood conditions. Final report , 1986 .

[12]  Weimin Ma,et al.  A scoping study of debris formation in DEFOR experimental facility , 2007 .

[13]  Nils Reinke,et al.  Formation, characterisation and cooling of debris: Scenario discussion with emphasis on TMI-2 , 2006 .

[14]  Manfred Groll,et al.  Basic investigations on debris cooling , 2006 .

[15]  Michel Quintard,et al.  The impact of thermal non-equilibrium and large-scale 2D/3D effects on debris bed reflooding and coolability , 2006 .

[16]  G. Berthoud,et al.  SILFIDE experiment: Coolability in a volumetrically heated debris bed , 2006 .

[17]  T. R. Schmidt,et al.  DCC-1/DCC-2 degraded core coolability analysis , 1985 .

[18]  Weimin Ma,et al.  A scoping study of debris bed formation in the DEFOR test facility , 2009 .

[19]  Michael Buck,et al.  Modelling of the late phase of core degradation in light water reactors , 2007 .

[20]  G. Hofmann.  On the Location and Mechanisms of Dryout in Top-Fed and Bottom-Fed Particulate Beds , 1984 .

[21]  I Huhtiniemi,et al.  Results of recent KROTOS FCI tests : Alumina versus corium melts , 1999 .