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An Experimental Study on the Dynamics of a Single Droplet Vapor Explosion
KTH, School of Engineering Sciences (SCI), Physics, Nuclear Power Safety.
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 [en]
steam explosions, radiography, melt preconditioning, noncondensable gas
National Category
Subatomic Physics
Identifiers
URN: urn:nbn:se:kth:diva-26014ISBN: 978-91-7415-611-9 (print)OAI: oai:DiVA.org:kth-26014DiVA: diva2:366932
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
List of papers
1. Simultaneous high speed digital cinematographic and X-ray radiographic imaging of a intense multi-fluid interaction with rapid phase changes
Open this publication in new window or tab >>Simultaneous high speed digital cinematographic and X-ray radiographic imaging of a intense multi-fluid interaction with rapid phase changes
2009 (English)In: Experimental Thermal and Fluid Science, ISSN 0894-1777, E-ISSN 1879-2286, Vol. 33, no 4, 754-763 p.Article in journal (Refereed) Published
Abstract [en]

As typical for the study of the vapor explosions, the qualitative and quantitative understanding of the phenomena requires visualization of both material and interface dynamics. A new approach to multifluid multiphase visualization is presented with the focus on the development of a synchronized highspeed visualization by digital cinematography and X-ray radiography. The developed system, named SHARP (simultaneous high-speed acquisition of X-ray radiography and photography), and its image processing methodology, directed to an image synchronization procedure and a separate quantification of vapor and molten material dynamics, is presented in this paper. Furthermore, we exploit an intrinsic property of the X-ray radiation, namely the differences in linear mass attenuation coefficients over the beam path through a multi-component system, to characterize the evolution of molten material distribution. Analysis of the data obtained by the SHARP system and image processing procedure developed granted new insights into the physics of the vapor explosion phenomena, as well as, quantitative information of the associated dynamic micro-interactions.

Keyword
Rapid multiphase multi-fluid interaction, Vapor explosion, Simultaneous, visualization, High speed imaging, Continuous X-ray, fuel-coolant interactions, neutron-radiography, vapor explosion, water, visualization, fragmentation, droplet, light
Identifiers
urn:nbn:se:kth:diva-18376 (URN)10.1016/j.expthermflusci.2009.01.011 (DOI)000265722300019 ()2-s2.0-72549099529 (Scopus ID)
Note
QC 20100525Available from: 2010-08-05 Created: 2010-08-05 Last updated: 2017-12-12Bibliographically approved
2. Dynamics and preconditioning in a single-droplet vapor explosion
Open this publication in new window or tab >>Dynamics and preconditioning in a single-droplet vapor explosion
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.

Keyword
X-ray radiography, vapor explosion, preconditioning
National Category
Subatomic Physics
Identifiers
urn:nbn:se:kth:diva-28207 (URN)000267406300020 ()2-s2.0-68649118880 (Scopus ID)
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
3. Triggering and energetics of a single drop vapor explosion: The role of entrapped non-condensable gases
Open this publication in new window or tab >>Triggering and energetics of a single drop vapor explosion: The role of entrapped non-condensable gases
2009 (English)In: Nuclear Engineering and Technology, 2009, Vol. 41, no 9, 1215-1222 p.Conference paper, Published paper (Refereed)
Abstract [en]

The present work pertains to a research program to study Molten Fuel-Coolant Interactions (MFCI), which may occur in a nuclear power plant during a hypothetical severe accident. Dynamics of the hot liquid (melt) droplet and the volatile liquid (coolant) were 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. The current study is concerned with 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. Energetics of the vapor explosion (conversion ratio) were also evaluated. The MISTEE-NCG tests showed two main aspects when compared to the MISTEE test series (without entrapped air). First, analysis showed that the melt preconditioning still strongly depends on the coolant subcooling. Second, in respect to the energetics, the tests consistently showed a reduced conversion ratio compared to that of the MISTEE test series.

Series
Nuclear engineering and technology, ISSN 1738-5733
Keyword
Fuel-Coolant Interactions, Non-Condensable Gases, Preconditioning, X-ray Radiography
National Category
Subatomic Physics
Identifiers
urn:nbn:se:kth:diva-28208 (URN)000272496800010 ()
Conference
7th International Topical Meeting on Nuclear Reactor Thermal Hydraulics, Operation and Safety (NUTHOS-7) Seoul, SOUTH KOREA, OCT 05-09, 2008
Note
QC 20110111Available from: 2011-01-11 Created: 2011-01-11 Last updated: 2011-01-11Bibliographically approved

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