Modeling and analysis of heat and mass transfer processes during in-vessel melt progression stage of light water reactor (LWR) severe accidents

Abstract: The last two decades of nuclear industry are marked byconsiderable attention tonuclear power plant accidents with core meltdown, knownalsoas severe accidents(SA). Two wellknown severeaccidents, TMI (USA, 1979) and Chernobyl (USSR, 1986), igniteda new wave of publit cancern about safety of existing nuclearpower plants. Significant research efforts have been investedfrom governments and nuclear utilities to investigate phenomenaassociated with severe accidents. New concepts of severeaccident manage ment schemes have been proposed, andconsiderable improvement in understanding of crucial andchallenging phenomena has been achieved.This thesis summarizes the work performed by author duringthe period of time from 1994 to 1998, and includes 16 papers,devoted to investigation of different phenomena related to thein-vessel melt progression (IVMP) t g s a e o a severeaccident. The thesis includes introduction and three technicalchapters, relevant to different issues of the IVMP, such asMelt-Vessel Interactions (MVIs), Fuel-Coolant Interactions(FCIs) and turbulent natural convection heat transfer indecay-heated oxidic and metallit pools.Thefirsttechnical chapter is dedicated to analysis ofturbulent natural convection heat transfer.Applicability of modern turbulente modeling methods isreviewed and examined. It is found, that, due to anisotropy ofturbulent transport in the buoyancy-induced stratificationconditions, the commonly used low-Reynolds-number (LRN) K - emodels (KEMS) are incapable of predicting both the lotal andintegral heat transfer characteris ties of naturally convectingflows in internally-heated liquid pools, The method of DirectNumerical Simulation (DNS) is then applied to investigate thecharacter of turbulente in internally-heatedunstably-stratified fluid layer.The DNS method is further employed for investigation ofnatural convection heat transfer in amoben metal layer, overlaying an oxidic (decay-heated)liquid pool in a prototypic reactor case. Physical situation ofinterest involves both Rayleigh-Bénard (RB) and verticalwall boundary layer natural convection flows (VW). The methodsuccessfully predicts the heat transfer conditions for theseparate-effect experiments on Rayleigh Bénard convection,when a good agreement with classical (experimental) heattransfer correlations is achieved. This approach is then usedto analyze the lotal heat transfer and interactions of twodifferent types of natural convection flows in case of "mixednatural convection".Thesecondtechnical chapter is devoted to the developmentof CFD methods forintense multiphase interactions.Applications, germane to MV1 and FCI issues of a severeaccident, require resolution of multiphase flow interfaces in amultidimensional geometry. Novel methods, such as (i) the"Lotal-Homogeneous-Slip"(LHS)model of film boiling,(ii)the "Multiphase Eulerian Lagrangian Transport" model(MELT-3D)for multidimensional multiphase mixing, and(iii) the "Pseudo-Confined Jet" (PSCJ) modeling approach forsimulation of free-surface impinging jets, are developed andvalidated. Additionally, Lattice-Boltzmann Method (LBM), whichhas recently emerged as a powerful tool for direct numericalsimulation of multiphase-multicomponent flows, is developed andapplied for simulation of dispersed flows. The intensemultiphase interac tion CFD methods, developed in this study,provide valuable information about several key-phenomena of asevere accident .In thelasttechnical chapter, the methods, developed forturbulent natural convection heat transfer and intensemultiphase interactions, are applied forprocesses assessment of severe accident.Investigation of non-prototypical conditions in simulantand real natural convection heat transfer experiments isperformed with application of DNS method. In particular, thefollowing non-prototypicalities are identified andinvestigated: (i) simulant fluid properties (Pr number andtemperature-dependent viscosity effects), (ii) geomet ritaleffects (2D vs. 3D, the slice-thickness-effect for semicircularcavity tonfiguration), {iii) boundary, transient and heatingconditions (side-wall vs. internal heating, Lorentz forteeffects, applicability of the transient cooldown approach). Inaddition, reactor-scale simulations are performed for moltenmetal layer natural convection. Furthermore, the LHS model isutilized to simulate film boiling under prototypical(high-temperature) FCI conditions. Finally, the coolant/jetproperty effects (i.e., coolant viscosity and density, andjet/coolant velocity) are analyzed with the MELT-3D method.The thesis is aimed at obtaining insights into thefundamental phenomena of in-vessel melt progression phase of asevere accident, thereby to improve the current knowledge base,and to reduce uncertainties in reactor safety assessments.Keywords:severe accident, in-vessel melt progression,melt, torium, heat transfer, natural convection, forcedconvection, turbulente, multiphase multicomponent Aow, intensemultiphase interactions, film boiling, jet impingement,numerical simulation, di rett numerical simulation.