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On Fuel Coolant Interactions and Debris Coolability in Light Water Reactors
KTH, School of Engineering Sciences (SCI), Physics, Nuclear Power Safety.ORCID iD: 0000-0001-8001-9323
2015 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

During the case of a hypothetical severe accident in a light water reactor, core damage may occur and molten fuel may interact with water resulting in explosive interactions. A Fuel-Coolant Interactions (FCI) consists of many complex phenomena whose characteristics determine the energetics of the interactions. The fuel melt initially undergoes fragmentation after contact with the coolant which subsequently increases the melt surface area exposed to coolant and causes rapid heat transfer. A substantial amount of research has been done to understand the phenomenology of FCI, still there are gaps to be filled in terms of the uncertainties in describing the processes such as breakup/fragmentation of melt and droplets.

The objective of the present work is to substantiate the understanding in the premixing phase of the FCI process by studying the deformation/pre-fragmentation of melt droplets and also the mechanism of melt jet breakup. The focus of the work is to study the effect of various influential parameters during the premixing phase that determine the intensity of the energetics in terms of steam explosion. The study is based on numerical analysis starting from smaller scale and going to the large scale FCI. Efforts are also taken to evaluate the uncertainties in estimating the steam explosion loads on the reactor scale.

The fragmented core is expected to form a porous debris bed. A part of the present work also deals with experimental investigations on the coolability of prototypical debris bed. Initially, the phenomenology of FCI and debris bed coolability is introduced. A review of the state of the art based on previous experimental and theoretical developments is also presented.

The study starts with numerical investigation of molten droplet hydrodynamics in a water pool, carried out using the Volume Of Fluid (VOF) method in the CFD code ANSYS FLUENT. This fundamental study is related to single droplets in a preconditioning phase, i.e. deformation/pre-fragmentation prior to steam explosion. The droplet deformation is studied extensively also including the effect of the pressure pulse on its deformation behavior. The effect of material physical properties such as density, surface tension and viscosity are investigated. The work is then extended to 3D analysis as a part of high fidelity simulations, in order to overcome the possible limitations of 2D simulations.

The investigation on FCI processes is then continued to the analysis on melt jet fragmentation in a water pool, since this is the crucial phenomenon which creates the melt-coolant pre-mixture, an initial condition for steam explosion. The calculations are carried out assuming non-boiling conditions and the properties of Wood’s metal. The jet fragmentation and breakup pattern are carefully observed at various Weber numbers. Moreover, the effect of physical and material properties such as diameter, velocity, density, surface tension and viscosity on jet breakup length, are investigated.

After the fundamental studies, the work was extended to reactor scale FCI energetics. It is mainly oriented on the evaluation of uncertainties in estimating the explosion impact loads on the surrounding structures. The uncertainties include the influential parameters in the FCI process and also the code uncertainties in calculations. The FCI code MC3D is used for the simulations and the PIE (propagation of input errors) method is used for the uncertainty analysis.

The last part of the work is about experimental investigations of debris coolability carried out using the POMECO-HT facility at KTH. The focus is on the determination of the effect of the bed’s prototypical characteristics on its coolability, in terms of inhomogeneity with heap like (triangular shape) bed and the radial stratified bed, and also the effect of its multi-dimensionality. For this purpose, four particle beds were constructed: two homogeneous, one with radial stratification and one with triangular shape, respectively. The effectiveness of coolability-enhanced measures such as bottom injection of water and a downcomer (used for natural circulation driven coolability, NCDC) was also investigated. The final chapter includes the summary of the whole work.

Abstract [sv]

Under ett svårt haveri i en kärnkraftsreaktor kan en härdsmälta bildas och smältan växelverka på ett explosivt sätt med kylvattnet. En sådan FCI (Fuel-Coolant-Interaction) inbegriper flera fysikaliska processer vilkas förlopp bestämmer hur stor den frigjorda energin blir. Vid kontakt med vattnet fragmenteras först härdsmältan vilket i sin tur leder till att en större yta exponeras för kylvattnet och att värmeöverföringen från smältan snabbt ökar. Mycket forskning har ägnats åt att förstå vad som sker under en FCI men det finns fortfarande luckor att fylla vad beträffar t ex osäkerheter i beskrivningen av fragmentering av såväl smälta som enskilda droppar av smält material.

Syftet med detta arbete är främst att underbygga en bättre förståelse av den inledande delen av en FCI genom att studera dels hur enskilda droppar av smält material deformeras och splittras och dels hur en stråle av smält material fragmenteras. Vi studerar särskilt vilka parametrar som mest påverkar den energi som frigörs vid ångexplosionen. Problemet studeras med numerisk analys med början i liten skala och sedan i full skala. Vi söker också uppskatta de laster som explosionen utsätter reaktorns komponenter för.

En annan viktig fråga gäller kylbarheten hos den slaggansamling som bildas under reaktorhärden efter en FCI. Slagghögen förväntas ha en porös struktur och en del av avhandlingen redogör för experimentella försök som genomförts för att utvärdera kylbarheten i olika prototypiska slaggformationer.

I avhandlingens inledning beskrivs de fysikaliska processerna under en FCI och kylningen av en slaggansamling. Det aktuella kunskapsläget på dessa områden presenteras också utgående från tidigare experimentella och teoretiska studier.

Studierna i avhandlingen inleds med numerisk analys av hydrodynamiken för en enskild droppe smälta i en vattentank där VOF-metoden i CFD-programmet ANSYS FLUENT används. Denna grundläggande studie rör en enskild droppe under förstadiet till fragmentering och ångexplosion då droppen deformeras alltmer. Deformationen studeras ingående också med hänsyn tagen till inverkan av en tryckpuls. Inverkan av olika egenskaper hos materialet, som densitet, ytspänning och viskositet studeras också. Arbetet utvidgas sedan till en beskrivning i 3D för att undvika de begränsningar som finns i en 2D-simulering.

Studierna av FCI utvidgas sedan till en analys av fragmentering av en stråle smälta i vatten. Detta är en kritisk del av förloppet då smälta och vatten blandas för att ge utgångstillståndet för ångexplosionen. Beräkningarna genomförs under antagande att kokning inte sker och med materialegenskaper som för Wood´s metall. Mönstret för fragmentering och uppsplittring studeras ingående för olika Weber-tal. Dessutom studeras effekten på strålens uppsplittringslängd av parametrar som diameter och hastighet för strålen samt densitet, ytspänning och viskositet hos materialet.

Efter dessa grundläggande studier utvidgas arbetet till FCI-energier i reaktorskala. Här ligger tonvikten på utvärdering av osäkerheter i bestämningen av den inverkan explosionen har på omgivande konstruktioner och komponenter. Osäkerheterna inkluderar eventuell bristande noggrannhet hos såväl de viktiga parametrarna i FCI-processen som i själva beräkningarna.

Den sista delen av arbetet handlar om experimentella undersökningar av slaggformationens kylbarhet som genomförts i uppställningen POMECO-HT vid avdelningen för kärnkraftsäkerhet på KTH. Vi vill bestämma effekten av formationens prototypiska egenskaper på kylbarheten. För detta ändamål konstruerades fyra olika formationer: två homogena, en med radiell variation i partikelstorlek och en med triangulär variation. Vi undersökte också hur förbättrad kylning kan uppnås genom att tillföra kylvatten underifrån respektive via ett fallrör (kylning genom naturlig cirkulation).

I det avslutande kapitlet ges en sammanfattning av hela arbetet.

Place, publisher, year, edition, pages
Stockholm: KTH Royal Institute of Technology, 2015. , xxii, 85 p.
Series
TRITA-FYS, ISSN 0280-316X ; 2015:22
Keyword [en]
severe accident, fuel-coolant interactions, melt jet fragmentation, steam explosion, debris bed coolability, uncertainty analysis
National Category
Energy Engineering
Research subject
Energy Technology
Identifiers
URN: urn:nbn:se:kth:diva-166261ISBN: 978-91-7595-537-7 (print)OAI: oai:DiVA.org:kth-166261DiVA: diva2:810188
Public defence
2015-05-26, FA32, AlbaNova University Center, Roslagstullsbacken 21, Stockholm, 10:00 (English)
Opponent
Supervisors
Note

QC 20150507

Available from: 2015-05-07 Created: 2015-05-06 Last updated: 2015-05-07Bibliographically approved
List of papers
1. A numerical analysis on hydrodynamic deformation of molten droplets in a water pool
Open this publication in new window or tab >>A numerical analysis on hydrodynamic deformation of molten droplets in a water pool
2013 (English)In: Annals of Nuclear Energy, ISSN 0306-4549, E-ISSN 1873-2100, Vol. 53, 228-237 p.Article in journal (Refereed) Published
Abstract [en]

Motivated by basic understanding on the single-droplet steam explosion phenomenon, the present study is concerned with the simulation of the molten droplet preconditioning (deformation/pre-fragmentation) during the initial phase of the fuel coolant interaction (FCI) by using the CFD code FLUENT through the Volume of Fluid (VOF) method. Due to the complexity of the problem, only the hydrodynamics of the multiphase system, without heat transfer, is considered. Calculations were performed for various configurations (melt-coolant two-phase system, and melt-coolant-vapor three-phase system) demonstrating that the current approach is capable of capturing the interfaces between the phases (melt, water and vapor) and predicting the droplet behavior with a reasonable mass conservation. A melt droplet disturbed by an external trigger is also analyzed, as well as the effect of the pressure pulse magnitude on the severity of droplet deformation. The MISTEE experimental data was used as a benchmark for the validation of the performed calculations. The results of the separate-effect study quantitatively demonstrate the importance of droplet velocity and melt properties on the melt droplet preconditioning.

Keyword
fuel coolant interaction, steam explosion, melt droplet, volume of fluid method, droplet deformation
National Category
Physical Sciences
Research subject
SRA - Energy
Identifiers
urn:nbn:se:kth:diva-106263 (URN)10.1016/j.anucene.2012.09.008 (DOI)000315550500030 ()2-s2.0-84870234260 (Scopus ID)
Funder
StandUp
Note

QC 20130405

Available from: 2012-12-01 Created: 2012-12-01 Last updated: 2017-12-07Bibliographically approved
2. An experimental study on pressure drop and dryout heat flux of two-phase flow in packed beds of multi-sized and irregular particles
Open this publication in new window or tab >>An experimental study on pressure drop and dryout heat flux of two-phase flow in packed beds of multi-sized and irregular particles
2012 (English)In: Nuclear Engineering and Design, ISSN 0029-5493, E-ISSN 1872-759X, Vol. 242, 369-378 p.Article in journal (Refereed) Published
Abstract [en]

This paper is concerned with debris bed coolability in a postulated severe accident of light water reactors, where the debris particles are irregular and multi-sized. To obtain and verify the friction laws predicting the hydrodynamics of the debris beds, the drag characteristics of air/water single- and two-phase flow in a particulate bed packed with multi-sized spheres or irregular sand particles were investigated on the POMECO-FL test facility. The same types of particles were then loaded in the test section of the POMECO-HT facility to obtain the dryout heat fluxes of the particulate beds heated volumetrically. The effective (mean) particle diameter is 2.25 mm for the multi-sized spheres and 1.75 mm for the sand particles, determined from the Ergun equation and the measured pressure drop of single-phase flow through the packed bed. Given the effective particle diameter, both the pressure drop and the dryout heat flux of two-phase flow through the bed can be predicted by the Reed model. The experiment also shows that the bottom injection of coolant improves the dryout heat flux significantly and the first dryout position is moving upward with increasing bottom injection flowrate. Compared with top-flooding case, the dryout heat flux of the bed can be doubled if the superficial velocity of coolant injection is 0.21–0.27 mm/s. The experimental data provides insights for interpretation of debris bed coolability (how to deal with the multi-sized irregular particles), as well as high-quality data for validation of the coolability analysis models and codes.

National Category
Energy Engineering
Identifiers
urn:nbn:se:kth:diva-53264 (URN)10.1016/j.nucengdes.2011.11.006 (DOI)000301084800038 ()2-s2.0-84355161430 (Scopus ID)
Note
QC 20120104Available from: 2011-12-26 Created: 2011-12-26 Last updated: 2017-12-08Bibliographically approved
3. 3D simulations of the hydrodynamic deformation of melt droplets in a water pool
Open this publication in new window or tab >>3D simulations of the hydrodynamic deformation of melt droplets in a water pool
2015 (English)In: Annals of Nuclear Energy, ISSN 0306-4549, E-ISSN 1873-2100, Vol. 75, 123-131 p.Article in journal (Refereed) Published
Abstract [en]

Topic of present study emerges from the hypothetical nuclear severe accident in light water reactor (LWR) where molten fuel core may interact with a reactor pool causing steam explosion. Fundamental research is important to substantiate the knowledge of steam explosion energetics. In order to get the basic understanding of single droplet behavior during steam explosion phenomena, the present study is carried out to deal with hydrodynamic deformation of a droplet during preconditioning, the initial phase of fuel coolant interactions. The three-dimensional simulations are carried out with the help of volume of fluid (VOF) method using the CFD code ANSYS FLUENT. The deformation modes and deformation rate of a droplet initialized with different Weber numbers (We) are analyzed, followed by the separate-effect studies of droplet density and viscosity. As a next part, a droplet is exposed to high-intensity pressure pulses in order to study the deformation/breakup of the droplet at sudden accelerations. Finally, the deformation rates of the droplets starting solidification are analyzed, where the droplets are modeled with a modified surface tension.

Place, publisher, year, edition, pages
Elsevier, 2015
Keyword
Droplet deformation, Fuel coolant interactions, Nuclear severe accident, Reactor safety, Steam explosion, Volume of fluid method
National Category
Physical Sciences
Research subject
Energy Technology
Identifiers
urn:nbn:se:kth:diva-151364 (URN)10.1016/j.anucene.2014.07.046 (DOI)000347493400015 ()2-s2.0-84906544883 (Scopus ID)
Note

QC 20140919

Available from: 2014-09-18 Created: 2014-09-18 Last updated: 2017-12-05Bibliographically approved
4. A numerical simulation of jet breakup in melt coolant interactions
Open this publication in new window or tab >>A numerical simulation of jet breakup in melt coolant interactions
2015 (English)In: Annals of Nuclear Energy, ISSN 0306-4549, E-ISSN 1873-2100, Vol. 80, 467-475 p.Article in journal (Refereed) Published
Abstract [en]

During a hypothetical severe accident of a light water reactor (LWR), molten corium could fall in the form of jet into a water pool. The jet fragmentation is crucial process during fuel coolant interactions (FCI) which fragment into droplets and disperse in the coolant, and it may cause a steam explosion. This paper deals with a study of computational fluid dynamics on the melt jet falling into a water pool in order to get qualitative and quantitative understanding of initial premixing phase of FCI. The preliminary objectives to pursue are modeling of jet fragmentation and estimation of the jet breakup length. A commercial CFD code ANSYS FLUENT 14.0 is used for the 2D numerical analysis employing Volume of Fluid (VOF) method. The problem and simulation conditions are similar to the jet breakup tests carried out at KTH (Manickam et al., 2014). Initially, a fragmentation/breakup pattern of the jet is discussed. Further, the effect of jet diameter and the jet injection velocity on the jet breakup length is studied, with a wide range of ambient Weber number (Wea) from 1.25 to 1280. The numerical results compared with the experimental data are in a reasonable agreement. The impacts of physical properties of melt (density, viscosity and surface tension) on the jet breakup lengths are also investigated and presented. Finally the droplet size distributions are obtained based on the simulation results. These preliminary data may be helpful to substantiate the understanding of the phenomena during melt jet interactions with coolant.

Place, publisher, year, edition, pages
Elsevier, 2015
Keyword
Fuel–coolant interactions; Melt jet instability; Melt jet fragmentation; Jet breakup length
National Category
Other Engineering and Technologies
Research subject
Energy Technology
Identifiers
urn:nbn:se:kth:diva-164057 (URN)10.1016/j.anucene.2015.02.038 (DOI)000352041200052 ()2-s2.0-84925488544 (Scopus ID)
Projects
This study is supported by the research programs of APRI8, ENSI and NKS.
Note

QC 20150519

Available from: 2015-04-13 Created: 2015-04-13 Last updated: 2017-12-04Bibliographically approved
5. Uncertainty analysis in quantification of steam explosion energetics in a reference Nordic BWR
Open this publication in new window or tab >>Uncertainty analysis in quantification of steam explosion energetics in a reference Nordic BWR
(English)Manuscript (preprint) (Other academic)
National Category
Other Engineering and Technologies
Research subject
Energy Technology
Identifiers
urn:nbn:se:kth:diva-166279 (URN)
Note

QS 2015

Available from: 2015-05-07 Created: 2015-05-07 Last updated: 2015-06-02Bibliographically approved

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