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An Experimental Study on Melt Fragmentation, Oxidation and Steam Explosion during Fuel Coolant Interactions
KTH, School of Engineering Sciences (SCI), Physics, Nuclear Power Safety.
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 [en]
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: urn:nbn:se:kth:diva-239899ISBN: 978-91-7873-057-5 (print)OAI: oai:DiVA.org:kth-239899DiVA, id: diva2:1268247
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
List of papers
1. On the influence of water subcooling and melt jet parameters on debris formation
Open this publication in new window or tab >>On the influence of water subcooling and melt jet parameters on debris formation
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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
National Category
Physical Sciences
Identifiers
urn:nbn:se:kth:diva-198956 (URN)10.1016/j.nucengdes.2016.09.017 (DOI)000387629900022 ()2-s2.0-84991702020 (Scopus ID)
Note

QC 20170113

Available from: 2017-01-13 Created: 2016-12-22 Last updated: 2018-12-05Bibliographically approved
2. On the fragmentation characteristics of melt jets quenched in water
Open this publication in new window or tab >>On the fragmentation characteristics of melt jets quenched in water
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
Keywords
Multiphase flow; Jet breakup; Fragmentation; Debris formation
National Category
Other Physics Topics
Research subject
Physics
Identifiers
urn:nbn:se:kth:diva-204647 (URN)10.1016/j.ijmultiphaseflow.2017.02.005 (DOI)000398752500019 ()2-s2.0-85013499337 (Scopus ID)
Note

QC 20170412

Available from: 2017-03-30 Created: 2017-03-30 Last updated: 2018-12-05Bibliographically approved
3. An experimental study on the intense intense heat transfer and phase change during melt and water interactions
Open this publication in new window or tab >>An experimental study on the intense intense heat transfer and phase change during melt and water interactions
2018 (English)In: Experimental heat transfer, ISSN 0891-6152, E-ISSN 1521-0480Article in journal (Refereed) Published
Place, publisher, year, edition, pages
Taylor & Francis Group, 2018
National Category
Other Physics Topics
Identifiers
urn:nbn:se:kth:diva-239897 (URN)10.1080/08916152.2018.1505786 (DOI)2-s2.0-85052292697 (Scopus ID)
Note

QC 20181214

Available from: 2018-12-05 Created: 2018-12-05 Last updated: 2018-12-14Bibliographically approved
4. Study on thermal fragmentation characteristics of a superheated alumina droplet
Open this publication in new window or tab >>Study on thermal fragmentation characteristics of a superheated alumina droplet
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2018 (English)In: Annals of Nuclear Energy, ISSN 0306-4549, E-ISSN 1873-2100, Vol. 119, p. 352-361Article in journal (Refereed) Published
Abstract [en]

In the frame of the European Commission FP7 SAFEST project, IRSN proposed to experimentally investigate the steam explosion triggering mechanisms of a superheated alumina droplet falling into water, through a set of experiments in the Micro Interactions in Steam Explosion Energetics facility (MISTEE) at KTH. Since thermal fragmentation is considered to be a likely process for the triggering of Steam Explosions in the KROTOS tests (performed at CEA) with alumina, the ability of a single droplet of such material to undergo thermally induced fine fragmentation is studied on the MISTEE facility with a close-up visualization. A series of experiments were conducted, where droplets of molten alumina were discharged into a water pool and potentially exposed to a small pressure wave. The intense interactions were recorded with a high-speed camera along with the pressure in the droplet vicinity. The ability of alumina to undergo thermal fragmentation is expected to be firstly contingent on the stability of the vapour film enshrouding the melt droplet. The water and melt temperatures may then play a crucial role on the vapour film stability, and therefore on the observation of a steam explosion. Indeed, under high to moderate water sub-cooling conditions, experimental observations indicate that fine fragmentation of the melt can occur when the droplet is exposed to even a weak pressure wave, in the range of 0.15 MPa. In contrast, melt fine fragmentation is suppressed at low water sub-cooling conditions (less than 30 °C), where the formation of a thick vapour film (and large wake) is observed, and which is probably too stable to be destabilized by the weak pressure wave. The effect of the melt temperature on thermal fragmentation is also assessed. This parameter influences the solidification of the droplet and the strength of the explosion as it determines the available heat energy. In the present conditions, fine fragmentation of melt occurred even at quite low melt superheat (≈60 °C). For a high melt superheat (above 200 °C) a very energetic spontaneous steam explosion was observed. A physical analysis on the debris particles acquired indicates a mass median diameter of ≈100 µm, comparable to the one observed in the KROTOS alumina experiments. The MISTEE experimental results are finally used to assess the heat and mass transfer modelling of the coolant during the fragmentation process in the FCI code MC3D.

Place, publisher, year, edition, pages
Elsevier, 2018
Keywords
Alumina, MC3D, Steam explosion, Thermal fragmentation
National Category
Energy Engineering
Identifiers
urn:nbn:se:kth:diva-228702 (URN)10.1016/j.anucene.2018.05.029 (DOI)000437819900032 ()2-s2.0-85047160015 (Scopus ID)
Funder
EU, FP7, Seventh Framework Programme
Note

QC 20180530

Available from: 2018-05-30 Created: 2018-05-30 Last updated: 2018-12-05Bibliographically approved

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