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On the fragmentation characteristics of melt jets quenched in water
KTH, School of Engineering Sciences (SCI), Physics, Nuclear Power Safety.
KTH, School of Engineering Sciences (SCI), Physics, Nuclear Power Safety.ORCID iD: 0000-0001-7816-8442
KTH, School of Engineering Sciences (SCI), Physics, Nuclear Power Safety.
2017 (English)In: International Journal of Multiphase Flow, ISSN 0301-9322, E-ISSN 1879-3533, Vol. 91, p. 262-275Article in journal (Refereed) Published
Abstract [en]

Experiments were carried out to investigate the characteristics of jet breakup and debris formation after melt jets fall into a subcooled water pool, which may occur in industrial processes such as the interactions of molten corium with coolant during a severe accident of light water reactors. A high-speed visualization system developed previously at KTH was used to capture the jet fragmentation phenomenon. Molten metal (Woods metal or tin) and mixture of binary oxides (WO3-Bi2O3 or WO3-ZrO2) were employed separately as melt materials to address different breakup mechanisms (e.g., hydrodynamic vs. thermodynamic fragmentation) and material effect. Moreover, the parameters related to melt and water conditions, including superheat, jet diameter and velocity of melt as well as subcooling of water, were scrutinized for their influences on jet fragmentation characteristics. The experimental data were acquired on the melt jet fragmentation patterns, breakup length, droplet size spectrum, droplet breakup and solidification as well as debris morphology, which can be useful for validation of the codes used for the prediction of debris formation phenomena.

Place, publisher, year, edition, pages
Elsevier, 2017. Vol. 91, p. 262-275
Keywords [en]
Multiphase flow; Jet breakup; Fragmentation; Debris formation
National Category
Other Physics Topics
Research subject
Physics
Identifiers
URN: urn:nbn:se:kth:diva-204647DOI: 10.1016/j.ijmultiphaseflow.2017.02.005ISI: 000398752500019Scopus ID: 2-s2.0-85013499337OAI: oai:DiVA.org:kth-204647DiVA, id: diva2:1085913
Note

QC 20170412

Available from: 2017-03-30 Created: 2017-03-30 Last updated: 2018-12-05Bibliographically approved
In thesis
1. An Experimental Study on Melt Fragmentation, Oxidation and Steam Explosion during Fuel Coolant Interactions
Open this publication in new window or tab >>An Experimental Study on Melt Fragmentation, Oxidation and Steam Explosion during Fuel Coolant Interactions
2018 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Nordic type boiling water reactors (BWRs) adopt reactor cavity flooding as a severe accident mitigation strategy (SAMS) to achieve core melt fragmentation and long-term cooling of decay heat generating core debris. The qualification of this SAMS needs to address two main severe accident issues: debris bed coolability and steam explosion. 

 

Since the coolability of a debris bed is determined by the bed’s  properties including debris particle’s size distribution and morphology as well as the bed’s configuration and inhomogeneity, it is important to investigate the mechanisms of melt jet breakup and resulting fragmentation in water which affect debris bed’s properties. Hence, the first part of this thesis is concerned with characterization of melt jet breakup and resulting debris particles.  A series of jet breakup experiments have been conducted in small scale with simulant binary oxide melt mixtures of WO3-Bi2O3, WO3-ZrO2 and Wood's metal. The experiments reveal significant influence of melt superheat, water subcooling, melt jet diameter and material properties on debris size and morphology. Specifically, transition in debris size and morphology is found to occur at a specific water subcooling range. The difference in debris properties at varied melt release conditions is attributed to the competition between liquid melt hydrodynamic fragmentation and thermomechanical fracture of quenched particles.

 

The second part of this thesis work is dedicated to provide a new understanding of steam explosion (SE) with the support of small-scale experiments at the level of droplets. Self- and externally-triggered SE experiments are conducted with simulant binary oxide melt mixtures in the temperature range of 1100 to 1500°C. The dynamics of steam explosion process is recorded using a sophisticated simultaneous visualization system of videography and X-ray radiography. Further, the influence of melt composition on steam explosion is summoned.  The results reveal that a droplet of eutectic composition is more explosive than a droplet of non-eutectic composition since latter may form a mushy zone which thereby limits the amount of melt actively participating in a steam explosion. To reduce the temperature difference between simulant melt and corium, investigation was extended to perform high temperature (˃2000°C) melt experiments. For this purpose, steam explosion of a molten Al2O3 droplet was investigated, and the experimental results confirmed that Al2O3 melt can undergo spontaneously triggered steam explosion at a high melt superheat and high subcooling. Within the context the effects of melt superheat and water subcooling were obtained.

 

The third part of this thesis is concerned with the oxidation of metallic melt representing unmixable metallic liquid of molten corium, which interactions with water can be spatially and chronologically separated from the oxidic corium FCI. The objective of the study  is to provide new insights into the characteristics of oxidation of Zr droplet falling in a water pool through a series of small-scale experiments. The dynamics of droplet and bubbles were recorded by high-speed cameras, and the spatial distributions of the elements in the quenched droplet (debris) were acquired by Energy- Dispersive X-Ray Spectroscopy (EDS). The results have shown noticeable influence of generated hydrogen and oxidation heat on droplet behavior and cooling rate. Water subcooling had significant influence on oxidation kinetics, and the oxygen content of the solidified particle increased with decreasing subcooling. Incomplete oxidation of Zr happened before melt crystallization and cooling down in all experiments.  

Place, publisher, year, edition, pages
Stockholm: KTH Royal Institute of Technology, 2018. p. 99
Series
TRITA-SCI-FOU ; 2018:54
Keywords
severe accident, fuel coolant interactions, melt jet breakup, debris properties, steam explosion, oxidation, high-speed visualization
National Category
Other Physics Topics
Research subject
Physics
Identifiers
urn:nbn:se:kth:diva-239899 (URN)978-91-7873-057-5 (ISBN)
Public defence
2018-12-20, FA31, Roslagstullsbacken 21, Stockholm, 14:00 (English)
Supervisors
Note

QC 20181205

Available from: 2018-12-05 Created: 2018-12-05 Last updated: 2018-12-11Bibliographically approved

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Bechta, SevostianMa, Weimin

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