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The Effective Convectivity Model for Simulation and Analysis of Melt Pool Heat Transfer in a Light Water Reactor Pressure Vessel Lower Head
KTH, School of Engineering Sciences (SCI), Physics, Nuclear Power Safety. (Division of Nuclear Power Safety)
2009 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Severe accidents in a Light Water Reactor (LWR) have been a subject of intense research for the last three decades. The research in this area aims to reach understanding of the inherent physical phenomena and reduce the uncertainties in their quantification, with the ultimate goal of developing models that can be applied to safety analysis of nuclear reactors, and to evaluation of the proposed accident management schemes for mitigating the consequences of severe accidents.  In a hypothetical severe accident there is likelihood that the core materials will be relocated to the lower plenum and form a decay-heated debris bed (debris cake) or a melt pool. Interactions of core debris or melt with the reactor structures depend to a large extent on the debris bed or melt pool thermal hydraulics. In case of inadequate cooling, the excessive heat would drive the structures' overheating and ablation, and hence govern the vessel failure mode and timing. In turn, threats to containment integrity associated with potential ex-vessel steam explosions and ex-vessel debris uncoolability depend on the composition, superheat, and amount of molten corium available for discharge upon the vessel failure. That is why predictions of transient melt pool heat transfer in the reactor lower head, subsequent vessel failure modes and melt characteristics upon the discharge are of paramount importance for plant safety assessment.  The main purpose of the present study is to develop a method for reliable prediction of melt pool thermal hydraulics, namely to establish a computational platform for cost-effective, sufficiently-accurate numerical simulations and analyses of core Melt-Structure-Water Interactions in the LWR lower head during a postulated severe core-melting accident. To achieve the goal, an approach to efficient use of Computational Fluid Dynamics (CFD) has been proposed to guide and support the development of models suitable for accident analysis.

 

The CFD method, on the one hand, is indispensable for scrutinizing flow physics, on the other hand, the validated CFD method can be used to generate necessary data for validation of the accident analysis models. Given the insights gained from the CFD study, physics-based models and computationally-efficient tools are developed for multi-dimensional simulations of transient thermal-hydraulic phenomena in the lower plenum of a LWR during the late phase of an in-vessel core melt progression. To describe natural convection heat transfer in an internally heated volume, and molten metal layer heated from below and cooled from the top (and side) walls, the Effective Convectivity Models (ECM) are developed and implemented in a commercial CFD code. The ECM uses directional heat transfer characteristic velocities to transport the heat to cooled boundaries. The heat transport and interactions are represented through an energy-conservation formulation. The ECM then enables 3D heat transfer simulations of a homogeneous (and stratified) melt pool formed in the LWR lower head. In order to describe phase-change heat transfer associated with core debris or binary mixture (e.g. in a molten metal layer), a temperature-based enthalpy formulation is employed in the Phase-change ECM (so called the PECM). The PECM is capable to represent natural convection heat transfer in a mushy zone. Simple formulation of the PECM method allows implementing different models of mushy zone heat transfer for non-eutectic mixtures. For a non-eutectic binary mixture, compositional convection associated with concentration gradients can be taken into account. The developed models are validated against both existing experimental data and the CFD-generated data. ECM and PECM simulations show a superior computational efficiency compared to the CFD simulation method. The ECM and PECM methods are applied to predict thermal loads imposed on the vessel wall and Control Rod Guide Tubes (CRGTs) during core debris heatup and melting in a Boiling Water Reactor (BWR) lower plenum. It is found that during the accident progression, the CRGT cooling plays a very important role in reducing the thermal loads on the reactor vessel wall. Results of the ECM and PECM simulations suggest a high potential of the CRGT cooling to be an effective measure for severe accident management in BWRs.

Place, publisher, year, edition, pages
KTH: KTH , 2009. , xx, 76 p.
Series
TRITA-FYS, ISSN 0280-316X ; 2009:22
Keyword [en]
light water reactor, hypothetical severe accident, accident progression, accident scenario, core melt pool, heat transfer, turbulent natural convection, heat transfer coefficient, phase change, mushy zone, crust, lower plenum, analytical model, effective convectivity model, CFD simulation
National Category
Energy Engineering
Identifiers
URN: urn:nbn:se:kth:diva-10671ISBN: 978-91-7415-381-1 (print)OAI: oai:DiVA.org:kth-10671DiVA: diva2:224035
Public defence
2009-09-02, FA32, AlbaNova University Center, Roslagstullsbacken 21, Stockholm, 10:00 (English)
Opponent
Supervisors
Note

QC 20100812

Available from: 2009-06-16 Created: 2009-06-16 Last updated: 2015-06-18Bibliographically approved
List of papers
1. An approach to numerical simulation and analysis of molten corium coolability in a boiling water reactor lower head
Open this publication in new window or tab >>An approach to numerical simulation and analysis of molten corium coolability in a boiling water reactor lower head
2010 (English)In: Nuclear Engineering and Design, ISSN 0029-5493, E-ISSN 1872-759X, Vol. 240, no 9, 2148-2159 p.Article in journal (Refereed) Published
Abstract [en]

This paper discusses an approach for application of the computational fluid dynamics (CFD) method to support development and validation of computationally effective methods for safety analysis, on the example of molten corium coolability in a BWR lower head. The approach consists of five steps designed to ensure physical soundness of the effective method simulation results: (i) analysis and decomposition of a severe accident problem into a set of separate-effect phenomena, (ii) validation of the CFD models on relevant separate-effect experiments for the reactor prototypical ranges of governing parameters, (iii) development of effective models and closures on the base of physical insights gained from relevant experiments and CFD simulations, (iv) using data from the integral experiments and CFD simulations performed under reactor prototypic conditions for validation of the effective model with quantification of uncertainty in the prediction results and (v) application of the computationally effective model to simulate and analyze the severe accident transient under consideration, including sensitivity and uncertainty analysis. Implementation of the approach is illustrated on a so-called effective convectivity model for simulation of turbulent natural convection heat transfer and phase changes in a decay-heated corium pool. It is shown that detailed information obtained from the CFD simulations are instrumental to ensure the effective models capture safety-significant local phenomena, e.g. the enhanced downward heat flux in the vicinity of a cooled control rod guide tube.

National Category
Subatomic Physics
Identifiers
urn:nbn:se:kth:diva-24069 (URN)10.1016/j.nucengdes.2009.11.029 (DOI)000280657000010 ()2-s2.0-77955427390 (Scopus ID)
Note
QC 20110405Available from: 2010-08-12 Created: 2010-08-12 Last updated: 2017-12-12Bibliographically approved
2. The effective convectivity model for simulation of melt pool heat transfer in a light water reactor pressure vessel lower head. Part I: Physical processes, modeling and model implementation
Open this publication in new window or tab >>The effective convectivity model for simulation of melt pool heat transfer in a light water reactor pressure vessel lower head. Part I: Physical processes, modeling and model implementation
2009 (English)In: Progress in nuclear energy (New series), ISSN 0149-1970, E-ISSN 1878-4224, Vol. 51, no 8, 849-859 p.Article in journal (Refereed) Published
Abstract [en]

This paper, and its companion paper [Tran C.T., Dinh, IN. The effective convectivity model for simulation of melt pool heat transfer in a light water reactor pressure vessel lower head. Part II: Model assessment and application. Progress in Nuclear Energy (companion paper), in preparation] document the development, validation and applications of a simulation platform for computationally-effective, sufficiently-accurate numerical predictions of core melt-structure-water interactions in the light water reactor lower head during a postulated severe core-melting accident. The centerpiece of this work is the Effective Convectivity Model (ECM) for description of energy splitting in a core melt pool. Built on the concept of characteristic velocities in Effective Convectivity Conductivity Model and supported by the key findings obtained from Computational Fluid Dynamics (CFD) simulations of turbulent natural convection, heat transfer and phase changes in volumetrically heated liquid pools, the ECM is refined and extended to three-dimensions and phase changes to enable simulations of melt pool formation and corium coolability in complex geometry such as a Boiling Water Reactor (BWR) lower plenum.

Keyword
Severe accident, Core melt pool, Heat transfer, Natural convection, Phase change, Effective convectivity, fluid layer, phase-change
National Category
Other Electrical Engineering, Electronic Engineering, Information Engineering Energy Engineering
Identifiers
urn:nbn:se:kth:diva-18846 (URN)10.1016/j.pnucene.2009.06.007 (DOI)000270636000010 ()2-s2.0-68749118116 (Scopus ID)
Note
QC 20100525Available from: 2010-08-05 Created: 2010-08-05 Last updated: 2017-12-12Bibliographically approved
3. The effective convectivity model for simulation of melt pool heat transfer in a light water reactor pressure vessel lower head. Part II: Model assessment and application
Open this publication in new window or tab >>The effective convectivity model for simulation of melt pool heat transfer in a light water reactor pressure vessel lower head. Part II: Model assessment and application
2009 (English)In: Progress in nuclear energy (New series), ISSN 0149-1970, E-ISSN 1878-4224, Vol. 51, no 8, 860-871 p.Article in journal (Refereed) Published
Abstract [en]

The paper reports detailed assessments and representative application of the effective convectivity model (ECM) developed and described in the companion paper (Tran and Dinh, submitted for publication). The ECM capability to accurately predict energy splitting and heat flux profiles in volumetrically heated liquid pools of different geometries over a range of conditions related to accident progression is examined and benchmarked against both experimental data and CFD results. Augmented with models for phase changes in binary mixture, the resulting PECM (phase-change ECM) is validated against a non-eutectic heat transfer experiment. The PECM tool is then applied to predict thermal loads imposed on the reactor vessel wall and Control Rod Guide Tubes (CRGTs) during core debris heatup and melting in the BWR lower plenum. The reactor-scale simulations demonstrate the PECM's high computational performance, particularly needed to analyze processes during long transients of severe accidents. The analysis provides additional arguments to support an outstanding potential of using the CRGT cooling as a severe accident management measure to delay the vessel failure and increase the likelihood of in-vessel core melt retention in the BWR.

Keyword
Severe accident, Melt pool formation, Natural convection, Corium, coolability, Effective convectivity, Validation, natural-convection
National Category
Energy Engineering Other Electrical Engineering, Electronic Engineering, Information Engineering
Identifiers
urn:nbn:se:kth:diva-18847 (URN)10.1016/j.pnucene.2009.06.001 (DOI)000270636000011 ()
Note
QC 20100525Available from: 2010-08-05 Created: 2010-08-05 Last updated: 2017-12-12Bibliographically approved
4. Simulation of core melt pool formation in a reactor pressure vessel lower head using an effective convectivity model
Open this publication in new window or tab >>Simulation of core melt pool formation in a reactor pressure vessel lower head using an effective convectivity model
2009 (English)In: Nuclear engineering and technology : an international journal of the Korean Nuclear Society, ISSN 1738-5733, E-ISSN 2234-358X, Vol. 41, no 7, 929-944 p.Article in journal (Refereed) Published
Abstract [en]

The present study is concerned with the extension of the Effective Convectivity Model (ECM) to the phase-change problem to simulate the dynamics of the melt pool formation in a Light Water Reactor (LWR) lower plenum during hypothetical severe accident progression. The ECM uses heat transfer characteristic velocities to describe turbulent natural convection of a melt pool. The simple approach of the ECM method allows implementing different models of the characteristic velocity in a Mushy Zone for non-eutectic mixtures. The Phase-change ECM (PECM) was examined using three models of the characteristic velocities in a mushy zone and its performance was compared. The PECM was validated using a dual-tier approach, namely validations against existing experimental data (the SIMECO experiment) and validations against results obtained from Computational Fluid Dynamics (CFD) simulations. The results predicted by the PECM implementing the linear dependency of mushy-zone characteristic velocity On fluid fraction are well agreed with the experimental correlation and CFD simulation results. The PECM was applied to simulation of melt pool formation heat transfer in a Pressurized Water Reactor (PWR) and Boiling Water Reactor (BWR) lower plenum. The Study suggests that the PECM is an adequate and effective tool to compute the dynamics of core melt pool formation.

Keyword
Heat Transfer, Effective Convectivity Model, Characteristic Velocity, Phase Change, Mushy Zone, Core Melt Pool, Severe Accident
National Category
Energy Engineering Other Electrical Engineering, Electronic Engineering, Information Engineering
Identifiers
urn:nbn:se:kth:diva-24070 (URN)000270560100007 ()2-s2.0-70350161939 (Scopus ID)
Note

QC 20100812

Available from: 2010-08-12 Created: 2010-08-12 Last updated: 2017-12-12Bibliographically approved
5. The effective convectivity model for simulation of molten metal layer heat transfer in a boiling water reactor lower head
Open this publication in new window or tab >>The effective convectivity model for simulation of molten metal layer heat transfer in a boiling water reactor lower head
2009 (English)In: International Congress on Advances in Nuclear Power Plants 2009, ICAPP 2009, Atomic Energy Society of Japan , 2009, Vol. 2, 1523-1537 p.Conference paper, Published paper (Refereed)
Abstract [en]

The paper is concerned with development of models for assessment of Control Rod Guide Tube (CRGT) cooling efficiency in Severe Accident Management (SAM) for a Boiling Water Reactor (BWR). In case of core melt relocation under a certain accident condition, there is a potential of stratified (with a metal layer atop) melt pool formation in the lower plenum. For simulations of molten metal layer heat transfer we are developing the Effective Convectivity Model (ECM) and Phase-change ECM (PECM). The models are based on the concept of effective convectivity previously developed for simulations of decay-heated melt pool heat transfer. The PECM platform takes into account mushy zone convection heat transfer and compositional convection that enables simulations of non-eutectic binary mixture solidification and melting. The ECM and PECM are validated against various heat transfer experiments for both eutectic and non-eutectic mixtures, and benchmarked against CFD-generated data including the local heat transfer characteristics. The PECM is applied to heat transfer simulation of a stratified heterogeneous debris pool in the presence of CRGT cooling. The PECM simulation results show no focusing effect in the metal layer on top of a debris pool formed in the BWR lower plenum and apparent efficacy of the CRGT cooling which can be served as an effective SAM measure to protect the vessel wall from thermal attacks and mitigate the consequences of a severe accident.

Place, publisher, year, edition, pages
Atomic Energy Society of Japan, 2009
Keyword
Accidents, Binary mixtures, Boiling water reactors, Cooling, Debris, Eutectics, Heat convection, Lakes, Liquid metals, Metals, Mixtures, Nuclear power plants, Solidification, Accident conditions, Boiling water reactor (BWR), Control rod guide tubes, Heat transfer simulation, Modeling for simulations, Molten metal layers, Severe accident management, Solidification and melting, Heat transfer
National Category
Physical Sciences
Identifiers
urn:nbn:se:kth:diva-164543 (URN)2-s2.0-84907988397 (Scopus ID)9781617386084 (ISBN)
Conference
International Congress on Advances in Nuclear Power Plants 2009, ICAPP 2009, Shinjuku, Tokyo, Japan, 10 May 2009 through 14 May 2009
Note

QC 20150420

Available from: 2015-04-20 Created: 2015-04-17 Last updated: 2015-06-18Bibliographically approved
6. Critical Heat Flux Correlations
Open this publication in new window or tab >>Critical Heat Flux Correlations
(English)Manuscript (preprint) (Other academic)
National Category
Energy Engineering Other Electrical Engineering, Electronic Engineering, Information Engineering
Identifiers
urn:nbn:se:kth:diva-24073 (URN)
Note
QC 20100812Available from: 2010-08-12 Created: 2010-08-12 Last updated: 2010-08-12Bibliographically approved

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