Melt-water interactions in a reactor containment : from integrated assesment to phenomenological studies

Abstract: This thesis examines melt-water interactions for theex-vessel phase of a severe reactor accident. The studies startfrom the development of an assessment methodology that tiestogether the vessel melt release scenarios and the mainprocesses affecting the melt-water interaction effects in thereactor containment. For modelling the continuous physicalprocesses, the integrated assessment framework combinesdeterministic predictions with probabilistic treatment ofuncertainties; whereas the bifurcative melt-water interactioneffects are modelled by considering the physical limitingmechanisms. The methodology is applied to scope the keyuncertainties associated with the ex-vessel severe accidentphase in a Swedish design boiling water reactor; and to gearthe focus of the phenomenological studies discussed below.The melt jet-water interactions in a subcooled containmentwater pool are studied furst from the film boiling point ofview, this boiling mode being characteristic forhigh-temperature melt jets. High-temperature film boiling testsare analysed with the original vapor-film-unit model, and afterthat a new model is developed to include also radiative heattransfer and forced coolant convection. The model predictionsare used to estimate the boiling characteristics on the jetsurface, including vapor film conditions and the potentialconditions leading to a very thin vapor film (unstable filmboiling). The film boiling studies are followed by a closerexamination of the jet breakup behaviour, leading to anapproach that addresses and predicts the existence of asignificant vapor film and the influence of two-phase coolantflow characteristics.The melt droplet freezing in contact with water is examinedby numerical and analytical studies, and a subsequentcharacterization of different freezing categories. Thenumerical studies show that binary oxidic melt mixtures mayundergo a fast transition into the so-talled mushy zone (i.e.,temperature between solidus and liquidus). The significantinfluence of such effects on melt-water interactions isintroduced and explored by conducting tests in which binaryoxidic melt droplets fall into a subcooled water pool. Thetests show the influence of mushy zone and stable film boilingon melt droplet deformation and fragmentation. Suchdependencies are expected to be parficularly characteristic foroxidic core melt mixtures, hence the above studies indicate astrong need to examine the effects of melt droplet freezing andmushy melt properties on prototypical situations and on steamexplosion energetics, in particular.The most important contributions of this thesis are theintegrated assessment methodology for melt-water interactioneffects in a reactor containment, the multiphase flowperspective to melt jet breakup in water, the modelling of filmboiling heat transfer and stability on a vertical surface, aswell as the studies on the potential influence of binary oxidicmelt freezing characteristics on melt-water interactions andsteam explosion energetics. The above studies range fromassessment methods to deeper phenomenological investigations;and involve experimental work, model development, and reactorapplications.Keywords:boiling water reactor, severe reactor accident, ex-vesselmelt progression, fuel-coolant interactions, melt-waterinteractions, jet breakup, jilm boiling, melt droplet, dropletjFeezing, droplet fiagmentation, steam explosion, safetyassessment.