Phenomenological and mechanistic modeling of melt-structure-water interactions in a light water reactor (LWR) severe accident

Sammanfattning: Severe accidents in light water reactors have been the fotalpoint of much research, performed in the last two decades,aimed at understarrding the inherent physical phenomena and toevaluate proposed accident management schemes for mitigatingthe consequences of such accidents. Severe accident progressionand consequences, av the reactor core overheats and melts, areintimately related to the interactions of the melt with coolant(water) and structures.The objective of this work is to address the modeling of thethermal hydrodynamic phenomena and interactions, occurringduring the progression of reactor severe accidents. The maintheme of the present work is to integrate phenomenologicalmodeling with mechanistic modeling. Integrated phenomenologicalmodels are developed to describe the accident scenarios, whichtonsist of many processes, while mechanistic modeling,including direct numerical simulation, is carried out todescribe separate effects and selected physical phenomena ofparticular importance.Modeling of the in-vessel melt-structure interactions is thetopic of the first chapter. In its first part, models aredeveloped for the core debris heat up and the formation of amelt pool in the lower head of reactor vessel and the resultantthermal loads on the vessel. The heat transport andinteractions, occurring in this scenario, are representedthrough energy-conservation formulation. In order to describethe phase change associated with core debris and vessel wallmelting, a temperature-based enthalpy method is employed andthe initial energy-conservation equation is modified. Naturalconvection heat transfer inside the decay-heated melt pool isaccounted for in this work by an effective diffusivityconvectivity model. Its application has also been extended tothe case of metallit layers, heated from below and cooled fromtop and sides. These models, implemented in a computer codenamed MVITA, have becn applied to predict the vessel thermalloads during core debris heat up and melting in the reactorlower head. It was found that the formation of melt pool isquite coherent and large melt pool volumes result coherently.If the reactor vessel is not cooled from outside, the vesselmelt-through is inevitable. With vessel external cooling, themelt pool can be retained inside the vessel for moderate powerdensities (or reactor power level), even though partial wallmelting may occur. Modeling of the trust formation has beenincluded in the MVITA code. We believe that it is the firsttime that the melt pool, trust layers, metallit layer and thevessel have been described in an integrated two-dimensionalfashion and the results obtained showed that the vessel thermalloads are reduced dur to the 2D heat diffusion in thevessel.The second part of the first chapter deals with the issue oflocal vessel failureand its modeling. Specifically, theprocess of melt discharge from the local failure of the vesseland the dynamics of the failure enlargement caused by the meltdischarge process are considered. A general model, integratingmajor physical aspects of melt discharge and failure siteenlargement, is developed. Analysis, based on this integratedmodel, was performed to study the effects of various parameteruncertainties. It was shown that significant narrowing of theuncertainty range of the process important parameters, such asthe final size of the failure in the vessel and the meandischarge rate of the core melt to the containment, could beachieved when employing the new understarrding of hole ablationphenomenology.In the second chapter, modeling efforts are directed towardsinvestigating the phenomenology of the mixing of a corium meltjet, discharging from the reactor vessel, with water, presentin the BWR containment lower drywell. Particular attention isfocused on the break-up of the melt jet in water, the behaviorof the fragments, as well as on the dynamics of the mixing zoneand its feedback to the break-up process. Numerical methods,which allow solution of the mathematical models of themultiphase system with minimal numerical diffusion, aredeveloped and used to investigate relevant interfacialphenomena, such as Kelvin-Helmholtz instabilities and jet/dropfragmentation in a flow field. The development of surfaceinstabilities and fragmentation of melt jet or drops were foundto be affected strongly by the property variations in themelt-coolant system caused, e.g., by the melt solidificationand water vaporization. It was found that break-up of ahigh-density melt drop in the water flow field is dominated bythe shear break-up mode, except when the melt drop hss highviscosity. The break-up of a heavy melt at the leading edge dueto Rayleigh-Taylor instabilities was found to be governed bythe jet-water density ratio and jet velocity. An integratedmodel of melt-water interactions is also developed in thiswork, which takes into account major physical phenomena andinter-relations. Results of the analysis using this modelindicate the sensitivity of the general behavior of the melt,drop, and water fields on a number of key factors, e.g., theheat transfer regimes (film boiling and radiation), finalfragment size, dynamits of water vaporisation and steamcondensation, etc.The third chapter of the dissertation is devoted to theproblem of ex-vessel debris bed coolability, i.e., behavior ofa core debris bed, located on the containment floor, and theablation of containment concrete basemat. The relevant physicalprocesses are described by a simple model, which takes intoaccount the dynamits of trust growth and the physical-chemicalaspects of malten torium-concrete interactions. Stability ofthe trust layer, separating the melt pool portion of the debrisbed and the molten concrete, as well as the heat transferthroughthis layer (which is similar to that of film boiling)were found to be important factors for concrete ablation.Coolability of an ex-vessel core debris bed is determined bythe bed geomettical tonfiguration and straclure, which definethe area available for cooling from outside. If such cooling isinsufficient, concrete melt-through is feasible.Keywords:light water reactor, severe accidents,melt-structure-coolant interaction, melt pool formation, holeablation, jet fragmentation, drop breakup, premixing, debriscoolability, natural convection, heat transfer, instabifities,multiphase flow, phase change.