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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. Fri . "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 = {Fri Jul 01 00:00:00 EDT 2016},
month = {Fri Jul 01 00:00:00 EDT 2016}
}
  • 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
  • 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)
  • A newly developed two-phase mixture model is applied, in conjunction with a control-volume-based finite difference method, to numerically investigate boiling with thermal convection in a porous layer heated from below. The numerical procedure employs a fixed grid and avoids tracking explicitly the moving interface between the liquid and two-phase regions. Numerical results are obtained to shed light on the intricate interactions between boiling and natural convection as well as to explain experimental observations. Four distinct flow patterns that were observed in previous experiments are predicted. A quantitative comparison of the predicted and measured vapor volume fraction in the porous bedmore » shows good agreement. The numerical results also agree with published linear stability results. In addition, the present study documents the effects of important parameters such as Rayleigh number, bottom heat flux, and aspect ratio.« less