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Dynamics and preconditioning in a single-droplet vapor explosion
KTH, School of Engineering Sciences (SCI), Physics, Nuclear Power Safety.
KTH, School of Engineering Sciences (SCI), Physics, Nuclear Power Safety.
KTH, School of Engineering Sciences (SCI), Physics, Nuclear Power Safety.
2009 (English)In: Nuclear Technology, ISSN 0029-5450, E-ISSN 1943-7471, Vol. 167, no 1, 223-234 p.Article in journal (Refereed) Published
Abstract [en]

The present study aims to develop a mechanistic understanding of the thermal-hydraulic processes in a vapor explosion, which may occur in nuclear power plants during a hypothetical severe accident, involving interactions of high-temperature corium melt and volatile coolant. Dynamics of the hot liquid (melt) droplet and the volatile liquid (coolant) were investigated in the Micro-Interactions in Steam Explosion Experiments (MISTEE) facility by performing well-controlled, externally triggered, single-droplet experiments, using a high-speed visualization system with synchronized digital cinematography and continuous X-ray radiography, called Simultaneous High-speed Acquisition of X-ray Radiography and Photography (SHARP). After an elaborate image processing, the SHARP images depict the evolution of both melt material (dispersal) and coolant (bubble dynamics) and their microscale interactions. The analysis of the data shows a deficiency in using the bubble dynamics alone to provide a consistent explanation of the energetic behavior. In contrast, the SHARP data reveal a correlation between the droplet's dynamics in the bubble's first cycle and the energetics of the subsequent explosive evaporation in the bubble's second cycle. The finding provides a basis to suggest that a so-called melt-droplet preconditioning, i.e., deformation/prefragmentation of a hot melt droplet immediately following the pressure trigger, is instrumental to the subsequent coolant entrainment, evaporation, and energetics of the resulting vapor explosion.

Place, publisher, year, edition, pages
2009. Vol. 167, no 1, 223-234 p.
Keyword [en]
X-ray radiography, vapor explosion, preconditioning
National Category
Subatomic Physics
Identifiers
URN: urn:nbn:se:kth:diva-28207ISI: 000267406300020Scopus ID: 2-s2.0-68649118880OAI: oai:DiVA.org:kth-28207DiVA: diva2:385205
Conference
12th International Meeting on Nuclear Reactor Thermal Hydraulics Pittsburgh, PA, OCT 04, 2007
Note
QC 20110111(12th International Meeting on Nuclear Reactor Thermal Hydraulics Pittsburgh, PA, OCT 04, 2007)Available from: 2011-01-11 Created: 2011-01-11 Last updated: 2017-12-11Bibliographically approved
In thesis
1. An Experimental Study on the Dynamics of a Single Droplet Vapor Explosion
Open this publication in new window or tab >>An Experimental Study on the Dynamics of a Single Droplet Vapor Explosion
2010 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

The present study aims to develop a mechanistic understanding of the thermal-hydraulic processes in a vapor explosion, which may occur in nuclear power plants during a hypothetical severe accident involving interactions of high-temperature corium melt and volatile coolant. Over the past several decades, a large body of literature has been accumulated on vapor explosion phenomenology and methods for assessment of the related risk. Vapor explosion is driven by a rapid fragmentation of high temperaturemelt droplets, leading to a substantial increase of heattransfer areas and subsequent explosive evaporation of the volatile coolant. Constrained by the liquid-phase coolant, the rapid vapor production in the interaction zone causes pressurization and dynamic loading on surrounding structures. While such a general understanding has been established, the triggering mechanism and subsequent dynamic fine fragmentation have yet not been clearly understood. A few mechanistic fragmentation models have been proposed, however, computational efforts to simulate the phenomena generated a large scatter of results.

Dynamics of the hot liquid (melt) droplet and the volatile liquid (coolant) are investigated in the MISTEE (Micro-Interactions in Steam Explosion Experiments) facility by performing well-controlled, externally triggered, single-droplet experiments, using a high-speed visualization system with synchronized digital cinematography and continuous X-ray radiography, called SHARP (Simultaneous High-speed Acquisition of X-ray Radiography and Photography). After an elaborate image processing, the SHARP images depict the evolution of both melt material (dispersal) and coolant (bubble dynamics), and their microscale interactions, i.e. the triggering phenomenology.

The images point to coolant entrainment into the droplet surface as the mechanism for direct contact/mixing ultimately responsible for energetic interactions. Most importantly, the MISTEE data reveals an inverse correlation between the coolant temperature and the molten droplet deformation/prefragmentation during the first bubble dynamics cycle. The SHARP observations followed by further analysis leads to a hypothesis about a novel phenomenon called pre-conditioning, according to which dynamics of the first bubble-dynamics cycle and the ability of the melt drop to deform/pre-fragment dictate the subsequent explosivity of the so-triggered droplet.

The effect of non-condensable gases on the perceived mechanisms was investigated on the MISTEE-NCG test campaign, in which a considerable amount of non-condensable gases (NCG) are present in the film that enfolds the molten droplet. The SHARP images for the MISTEE-NCG tests were analyzed and special attention was given to the morphology (aspect ratio) and dynamics of the air/ vapor bubble, as well as the melt drop preconditioning and interaction energetics. Analysis showed twomain aspects when compared to the MISTEE test series (withoutentrapped air). First, the investigation showed that the meltpreconditioning still strongly depends on the coolant subcooling. Second,in respect to the energetics, the tests consistently showed a reducedconversion ratio compared to that of the MISTEE test series.

The effect of the melt material in the steam explosion triggerability was also summoned, since it would in principle directly implicate the melt preconditioning. Since a number of the thermo-physical properties of the material would influence the triggering process, we focused on the material properties by using the same dioxide material with difference concentrations, i.e. eutectic and non-eutectic. Unfortunately, due to the high melt superheat the possible differences were not perceived. Thus, inaddition to other materials, lower melt superheat tests were schedule inthe future.

Place, publisher, year, edition, pages
Stockholm: KTH, 2010. xx, 171 p.
Series
Trita-FYS, ISSN 0280-316X
Keyword
steam explosions, radiography, melt preconditioning, noncondensable gas
National Category
Subatomic Physics
Identifiers
urn:nbn:se:kth:diva-26014 (URN)978-91-7415-611-9 (ISBN)
Public defence
2010-11-23, seminar room F3, Lindstedtsvägen 26, KTH, Stockholm, 14:50 (English)
Opponent
Supervisors
Note
QC 20101110Available from: 2010-11-10 Created: 2010-11-09 Last updated: 2011-01-11Bibliographically approved

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