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Title: Multiphysics modeling of two-phase film boiling within porous corrosion deposits

Abstract

Porous corrosion deposits on nuclear fuel cladding, known as CRUD, can cause multiple operational problems in light water reactors (LWRs). CRUD can cause accelerated corrosion of the fuel cladding, increase radiation fields and hence greater exposure risk to plant workers once activated, and induce a downward axial power shift causing an imbalance in core power distribution. In order to facilitate a better understanding of CRUD's effects, such as localized high cladding surface temperatures related to accelerated corrosion rates, we describe an improved, fully-coupled, multiphysics model to simulate heat transfer, chemical reactions and transport, and two-phase fluid flow within these deposits. Our new model features a reformed assumption of 2D, two-phase film boiling within the CRUD, correcting earlier models' assumptions of single-phase coolant flow with wick boiling under high heat fluxes. This model helps to better explain observed experimental values of the effective CRUD thermal conductivity. Finally, we propose a more complete set of boiling regimes, or a more detailed mechanism, to explain recent CRUD deposition experiments by suggesting the new concept of double dryout specifically in thick porous media with boiling chimneys. - Highlights: • A two-phase model of CRUD's effects on fuel cladding is developed and improved. • Thismore » model eliminates the formerly erroneous assumption of wick boiling. • Higher fuel cladding temperatures are predicted when accounting for two-phase flow. • Double-peaks in thermal conductivity vs. heat flux in experiments are explained. • A “double dryout” mechanism in CRUD is proposed based on the model and experiments.« less

Authors:
;
Publication Date:
OSTI Identifier:
22572327
Resource Type:
Journal Article
Resource Relation:
Journal Name: Journal of Computational Physics; Journal Volume: 316; Other Information: Copyright (c) 2016 Elsevier Science B.V., Amsterdam, The Netherlands, All rights reserved.; Country of input: International Atomic Energy Agency (IAEA)
Country of Publication:
United States
Language:
English
Subject:
42 ENGINEERING; CLADDING; CORROSION; FILM BOILING; FINITE ELEMENT METHOD; HEAT FLUX; HEAT TRANSFER; NUCLEAR FUELS; POROUS MATERIALS; POWER DISTRIBUTION; PWR TYPE REACTORS; THERMAL CONDUCTIVITY; TWO-PHASE FLOW

Citation Formats

Jin, Miaomiao, E-mail: mmjin@mit.edu, and Short, Michael, E-mail: hereiam@mit.edu. Multiphysics modeling of two-phase film boiling within porous corrosion deposits. United States: N. p., 2016. Web. doi:10.1016/J.JCP.2016.03.013.
Jin, Miaomiao, E-mail: mmjin@mit.edu, & Short, Michael, E-mail: hereiam@mit.edu. Multiphysics modeling of two-phase film boiling within porous corrosion deposits. United States. doi:10.1016/J.JCP.2016.03.013.
Jin, Miaomiao, E-mail: mmjin@mit.edu, and Short, Michael, E-mail: hereiam@mit.edu. 2016. "Multiphysics modeling of two-phase film boiling within porous corrosion deposits". United States. doi:10.1016/J.JCP.2016.03.013.
@article{osti_22572327,
title = {Multiphysics modeling of two-phase film boiling within porous corrosion deposits},
author = {Jin, Miaomiao, E-mail: mmjin@mit.edu and Short, Michael, E-mail: hereiam@mit.edu},
abstractNote = {Porous corrosion deposits on nuclear fuel cladding, known as CRUD, can cause multiple operational problems in light water reactors (LWRs). CRUD can cause accelerated corrosion of the fuel cladding, increase radiation fields and hence greater exposure risk to plant workers once activated, and induce a downward axial power shift causing an imbalance in core power distribution. In order to facilitate a better understanding of CRUD's effects, such as localized high cladding surface temperatures related to accelerated corrosion rates, we describe an improved, fully-coupled, multiphysics model to simulate heat transfer, chemical reactions and transport, and two-phase fluid flow within these deposits. Our new model features a reformed assumption of 2D, two-phase film boiling within the CRUD, correcting earlier models' assumptions of single-phase coolant flow with wick boiling under high heat fluxes. This model helps to better explain observed experimental values of the effective CRUD thermal conductivity. Finally, we propose a more complete set of boiling regimes, or a more detailed mechanism, to explain recent CRUD deposition experiments by suggesting the new concept of double dryout specifically in thick porous media with boiling chimneys. - Highlights: • A two-phase model of CRUD's effects on fuel cladding is developed and improved. • This model eliminates the formerly erroneous assumption of wick boiling. • Higher fuel cladding temperatures are predicted when accounting for two-phase flow. • Double-peaks in thermal conductivity vs. heat flux in experiments are explained. • A “double dryout” mechanism in CRUD is proposed based on the model and experiments.},
doi = {10.1016/J.JCP.2016.03.013},
journal = {Journal of Computational Physics},
number = ,
volume = 316,
place = {United States},
year = 2016,
month = 7
}
  • Two geometries of porous deposits are known to form on heat transfer surfaces. When boiling is not present, a homogeneous layer typically forms. When boiling is present, a heterogeneous porous layer with densely populated chimneys is generated. Typical dimensional characteristics of the deposits are 25 {mu}m thick, 2.5-{mu}m chimney diameter, 3 x 10{sup 9} chimney/m{sup 2}, 0.6 porosity, and 0.5-{mu}m particle diameter. Observations indicate that porous layers with chimneys give much better heat transfer performance than the layers without chimneys. This study considers both types of layers and constructs models for the thermal performance of each under boiling conditions.
  • A model for acidity within pores within corrosion products on anodically-dissolving UO{sub 2} was developed using Comsol Multiphysics 3.2 to complement ongoing electrochemical measurements. It was determined that a depression of pH within pores can be maintained if: electrochemically measured dissolution currents used in the calculations are attenuated to reflect very localized pores; corrosion potentials exceed -250 mV (vs. SCE); and pore depths are >1 {mu}m for 300 mV or >100 {mu}m for -50 mV (vs. SCE). Mixed diffusional-chemical equilibria control is suggested through deviations in the shapes between pH-potential and pH-pore depth plots. (authors)
  • The two-phase boundary layer theory was adopted to investigate subcooled free-convection film boiling over a body of arbitrary shape embedded in a porous medium. A general similarity variable which accounts for the geometric effect on the boundary layer length scale was introduced to treat the problems once for all possible two-dimensional and axisymmetric bodies. By virtue of the generalized transformation, the set of governing equations and boundary conditions for an arbitrary shape reduced into the one for a vertical flat plate already solved by Cheng and Verma. Thus, the numerical values furnished for a flat plate may readily be translatedmore » for any particular body configuration of concern. Furthermore, an explicit Nusselt number expression in terms of the parameters associated with the degrees of subcooling and superheating has been established upon considering physical limiting conditions.« less
  • The buildup of corrosion product deposits (CRUD) on the fuel cladding of the boiling water reactor (BWR) before and after zinc injection has been investigated by applying local experimental analytical techniques. Under the BWR water chemistry conditions, Zn addition together with the presence of Ni and Mn induce the formation of (Zn,Ni,Mn)[Fe{sub 2}O{sub 4}] spinel solid solutions. X-ray absorption spectroscopy (XAS) revealed inversion ratios of cation distribution in spinels deposited from the solid solution. Based on this information, a two-site ferrite spinel solid solution model is proposed. Electron probe microanalysis (EPMA) and extended X-ray absorption fine structure (EXAFS) findings suggestmore » the zinc-rich ferrite spinels formation on BWR fuel cladding mainly at lower pin. - Graphical Abstract: Analysis of spinels in corrosion product deposits on boiling water reactor fuel rod. Combining EPMA and XAFS results: schematic representation of the ferrite spinels in terms of the end members and their extent of inversion. Note that the ferrites are represented as a surface between the normal (upper plane, M[Fe{sub 2}]O{sub 4}) and the inverse (lower plane, Fe[MFe]O{sub 4}). Actual compositions red Black-Small-Square for the specimen at low elevation (810 mm), blue Black-Small-Square for the specimen at mid elevation (1800 mm). The results have an impact on the properties of the CRUD material. Highlights: Black-Right-Pointing-Pointer Buildup of corrosion product deposits on fuel claddings of a boiling water reactor (BWR) are investigated. Black-Right-Pointing-Pointer Under BWR water conditions, Zn addition with Ni and Mn induced formation of (Zn,Ni,Mn)[Fe{sub 2}O{sub 4}]. Black-Right-Pointing-Pointer X-Ray Adsorption Spectroscopy (XAS) revealed inversion of cations in spinel solid solutions. Black-Right-Pointing-Pointer Zinc-rich ferrite spinels are formed on BWR fuel cladding mainly at lower pin elevations.« less
  • This paper analyses both theoretically and experimentally the problem of film boiling from a body embedded in a liquid-saturated porous medium. Two body geometries are investigated thoroughly: a horizontal cylinder and a sphere. The theoretical model relies on the Brinkman-extended flow model to describe the flow field inside the thin vapor layer occupying the neighborhood near the heated surface. The theoretical model also includes an improved formulation of the effective conductivity in the vicinity of the heater as a function of the vapor layer thickness and the geometry of the porous medium material. Solutions are obtained for the vapor layermore » thickness and the local Nusselt number as a function of angular position. Numerical solutions are also obtained for the overall heat transfer rates from the surface to the fluid for a given vapor superheat. Experimental data for a 12.70 mm stainless steel cylindrical heater embedded in a 3-mm glass particle porous medium were obtained under steady-state operation. The experimental data obtained are compared with the theoretical analysis. The comparison shows that there is a good agreement between theory and experiments. The theoretical model is also compared with the experimental data obtained by other investigators for a spherical geometry. Excellent results are obtained in such comparison.« less