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On the influence of water subcooling and melt jet parameters on debris formation
KTH, School of Engineering Sciences (SCI), Physics, Nuclear Power Safety.
KTH, School of Engineering Sciences (SCI), Physics, Nuclear Power Safety.ORCID iD: 0000-0002-0683-9136
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
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2016 (English)In: Nuclear Engineering and Design, ISSN 0029-5493, E-ISSN 1872-759X, Vol. 309, p. 265-276Article in journal (Refereed) Published
Abstract [en]

Breakup of melt jet and formation of a porous debris bed at the base-mat of a flooded reactor cavity is expected during the late stages of a severe accident in light water reactors. Debris bed coolability is determined by the bed properties including particle size, morphology, bed height and shape as well as decay heat. Therefore understanding of the debris formation phenomena is important for assessment of debris bed coolability. A series of experiments was conducted in MISTEE-jet facility by discharging binary-oxide mixtures of WO3-Bi2O3 and WO3-ZrO2 into water in order to investigate properties of resulting debris. The effect of water subcooling, nozzle diameter and melt superheat was addressed in the tests. Experimental results reveal significant influence of water subcooling and melt superheat on debris size and morphology. Significant differences in size and morphology of the debris at different melt release conditions is attributed to the competition between hydrodynamic fragmentation of liquid melt and thermal fracture of the solidifying melt droplets. The particle fracture rate increases with increased sub cooling. Further the results are compared with the data from larger scale experiments to discern the effects of spatial scales. The present work provides data that can be useful for validation of the codes used for the prediction of debris formation phenomena.

Place, publisher, year, edition, pages
Elsevier, 2016. Vol. 309, p. 265-276
National Category
Physical Sciences
Identifiers
URN: urn:nbn:se:kth:diva-198956DOI: 10.1016/j.nucengdes.2016.09.017ISI: 000387629900022Scopus ID: 2-s2.0-84991702020OAI: oai:DiVA.org:kth-198956DiVA, id: diva2:1065124
Note

QC 20170113

Available from: 2017-01-13 Created: 2016-12-22 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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Manickam, LouisKudinov, PavelMa, WeiminBechta, SevostianGrishchenko, Dmitry

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