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The effective convectivity model for simulation of molten metal layer heat transfer in a boiling water reactor lower head
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
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 (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. Vol. 2, 1523-1537 p.
Keyword [en]
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
URN: urn:nbn:se:kth:diva-164543ScopusID: 2-s2.0-84907988397ISBN: 9781617386084OAI: diva2:806336
International Congress on Advances in Nuclear Power Plants 2009, ICAPP 2009, Shinjuku, Tokyo, Japan, 10 May 2009 through 14 May 2009

QC 20150420

Available from: 2015-04-20 Created: 2015-04-17 Last updated: 2015-06-18Bibliographically approved
In thesis
1. The Effective Convectivity Model for Simulation and Analysis of Melt Pool Heat Transfer in a Light Water Reactor Pressure Vessel Lower Head
Open this publication in new window or tab >>The Effective Convectivity Model for Simulation and Analysis of Melt Pool Heat Transfer in a Light Water Reactor Pressure Vessel Lower Head
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.
TRITA-FYS, ISSN 0280-316X ; 2009:22
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
urn:nbn:se:kth:diva-10671 (URN)978-91-7415-381-1 (ISBN)
Public defence
2009-09-02, FA32, AlbaNova University Center, Roslagstullsbacken 21, Stockholm, 10:00 (English)

QC 20100812

Available from: 2009-06-16 Created: 2009-06-16 Last updated: 2015-06-18Bibliographically approved

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