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NKS Programme Area:NKS-R
Research Area:Severe accidents and Reactor Physics
Report Number:NKS-160
Report Title:Ex-Vessel Corium Coolability and Steam Explosion Energetics in Nordic Light Water Reactors
Activity Acronym:EXCoolSE
Authors:T.N. Dinh, W.M. Ma, A. Karbojian, P. Kudinov, C.T. Tran, C.R. Hansson
Abstract:This report presents advances and insights from the KTH’s study on corium pool heat transfer in the BWR lower head; debris bed formation; steam explosion energetics; thermal hydraulics and coolability in bottom-fed and heterogeneous debris beds. This report presents advances and insights from the KTH’s study on corium pool heat transfer in the BWR lower head; debris bed formation; steam explosion energetics; thermal hydraulics and coolability in bottom-fed and heterogeneous debris beds. Specifically, for analysis of heat transfer in a BWR lower plenum an advanced three-dimensional simulation tool was developed and validated, using a so-called effective convectivity approach and Fluent code platform. An assessment of corium retention and coolability in the reactor pressure vessel (RPV) lower plenum by means of water supplied through the Control Rod Guide Tube (CRGT) cooling system was performed. Simulant material melt experiments were performed in an intermediate temperature range (1300-1600K) on DEFOR test facility to study formation of debris beds in high and low subcooled water pools characteristic of in-vessel and ex-vessel conditions. Results of the DEFOR-E scoping experiments and related analyses strongly suggest that porous beds formed in ex-vessel from a fragmented high-temperature debris is far from homogeneous. Calculation results of bed thermal hydraulics and dryout heat flux with a two-dimensional thermal-hydraulic code give the first basis to evaluate the extent by which macro and micro inhomogeneity can enhance the bed coolability. The development and validation of a model for two-phase natural circulation through a heated porous medium and its application to the coolability analysis of bottom-fed beds enables quantification of the significant effect of dryout heat flux enhancement (by a factor of 80-160%) due to bottom coolant injection. For a qualitative and quantitative understanding of steam explosion, the SHARP system and its image processing methodology were used to characterize the dynamics of a hot liquid (melt) drop fragmentation and the volatile liquid (coolant) vaporization. The experimental results provide a basis to suggest that the melt drop preconditioning is instrumental to the subsequent coolant entrainment and resulting energetics of the so-triggered drop explosion. For steam explosion risk in reactors, a revisited study of the material property effect on steam explosion energetics showed that corium high density, high melting point and low conductivity are central to mechanisms in premixing that govern corium low explosivity.
Keywords:Severe Accident; Corium; Debris Bed; Coolability; Steam Explosion
Publication date:01 Mar 2008
ISBN:ISBN: 978-87-7893-225-9
Number of downloads:3751
Download:pdf NKS-160.pdf
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