Discrete Modeling of EarlyLife Thermal Fracture in Ceramic Nuclear Fuel
Abstract
Fracturing of ceramic fuel pellets heavily influences performance of light water reactor (LWR) fuel. Early in the life of fuel, starting with the initial power ramp, large thermal gradients cause high tensile hoop and axial stresses in the outer region of the fuel pellets, resulting in the formation of radial and axial cracks. Circumferential cracks form due to thermal gradients that occur when the power is ramped down. These thermal cracks cause the fuel to expand radially, closing the pellet/cladding gap and enhancing the thermal conductance across that gap, while decreasing the effective conductivity of the fuel in directions normal to the cracking. At lower length scales, formation of microcracks is an important contributor to the decrease in bulk thermal conductivity that occurs over the life of the fuel as the burnup increases. Because of the important effects that fracture has on fuel performance, a realistic, physically based fracture modeling capability is essential to predict fuel behavior in a wide variety of normal and abnormal conditions. Modeling fracture within the context of the finite element method, which is based on continuous interpolations of solution variables, has always been challenging because fracture is an inherently discontinuous phenomenon. Work is underway atmore »
 Authors:
 Idaho National Lab. (INL), Idaho Falls, ID (United States)
 Duke Univ., Durham, NC (United States)
 Publication Date:
 Research Org.:
 Idaho National Lab. (INL), Idaho Falls, ID (United States)
 Sponsoring Org.:
 USDOE
 OSTI Identifier:
 1177218
 Report Number(s):
 INL/CON1431355
TRN: US1600123
 DOE Contract Number:
 DEAC0705ID14517
 Resource Type:
 Conference
 Resource Relation:
 Conference: 2014 Water Reactor Fuel Performance Meeting/Top Fuel/LWR Fuel Performance Meeting (WRFPM 2014), Sendai (Japan), 1417 Sep 2014
 Country of Publication:
 United States
 Language:
 English
 Subject:
 21 SPECIFIC NUCLEAR REACTORS AND ASSOCIATED PLANTS; 11 NUCLEAR FUEL CYCLE AND FUEL MATERIALS; THERMAL FRACTURES; NUCLEAR FUELS; FINITE ELEMENT METHOD; FUEL PELLETS; CRACKS; WATER MODERATED REACTORS; COMPUTERIZED SIMULATION; CERAMICS; THERMAL CONDUCTION; THERMAL CONDUCTIVITY; FUEL CANS; WATER COOLED REACTORS; TEMPERATURE GRADIENTS; FRACTURING; FRACTURE MECHANICS; RANDOMNESS; THERMAL STRESSES; Discrete Element Method; Extended Finite Element Method
Citation Formats
Spencer, Benjamin W., Huang, Hai, Dolbow, John E., and Hales, Jason D. Discrete Modeling of EarlyLife Thermal Fracture in Ceramic Nuclear Fuel. United States: N. p., 2015.
Web.
Spencer, Benjamin W., Huang, Hai, Dolbow, John E., & Hales, Jason D. Discrete Modeling of EarlyLife Thermal Fracture in Ceramic Nuclear Fuel. United States.
Spencer, Benjamin W., Huang, Hai, Dolbow, John E., and Hales, Jason D. 2015.
"Discrete Modeling of EarlyLife Thermal Fracture in Ceramic Nuclear Fuel". United States.
doi:. https://www.osti.gov/servlets/purl/1177218.
@article{osti_1177218,
title = {Discrete Modeling of EarlyLife Thermal Fracture in Ceramic Nuclear Fuel},
author = {Spencer, Benjamin W. and Huang, Hai and Dolbow, John E. and Hales, Jason D.},
abstractNote = {Fracturing of ceramic fuel pellets heavily influences performance of light water reactor (LWR) fuel. Early in the life of fuel, starting with the initial power ramp, large thermal gradients cause high tensile hoop and axial stresses in the outer region of the fuel pellets, resulting in the formation of radial and axial cracks. Circumferential cracks form due to thermal gradients that occur when the power is ramped down. These thermal cracks cause the fuel to expand radially, closing the pellet/cladding gap and enhancing the thermal conductance across that gap, while decreasing the effective conductivity of the fuel in directions normal to the cracking. At lower length scales, formation of microcracks is an important contributor to the decrease in bulk thermal conductivity that occurs over the life of the fuel as the burnup increases. Because of the important effects that fracture has on fuel performance, a realistic, physically based fracture modeling capability is essential to predict fuel behavior in a wide variety of normal and abnormal conditions. Modeling fracture within the context of the finite element method, which is based on continuous interpolations of solution variables, has always been challenging because fracture is an inherently discontinuous phenomenon. Work is underway at Idaho National Laboratory to apply two modeling techniques model fracture as a discrete displacement discontinuity to nuclear fuel: The extended finite element method (XFEM), and discrete element method (DEM). XFEM is based on the standard finite element method, but with enhancements to represent discontinuous behavior. DEM represents a solid as a network of particles connected by bonds, which can arbitrarily fail if a fracture criterion is reached. This paper presents initial results applying the aforementioned techniques to model fuel fracturing. This work has initially focused on early life behavior of ceramic LWR fuel. A coupled thermalmechanical XFEM method that includes discontinuities in both temperature and displacement fields at crack locations has been developed and is being applied to thermal fracture of LWR fuel. A DEM model of coupled heat conduction and solid mechanics has been developed and used to simulate random initiation and propagation of thermally driven cracks during initial power cycles. This DEM model predicts the formation of realistic radial cracking patterns during power rise and circumferential cracks as power is ramped down. These initial results are very encouraging, and these techniques are expected to provide improved understanding of fuel behavior in a wide variety of conditions.},
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place = {United States},
year = 2015,
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A discrete element Model (DEM) representation of coupled solid mechanics/fracturing and heat conduction processes has been developed and applied to explicitly simulate the random initiations and subsequent propagations of interacting thermal cracks in a ceramic nuclear fuel pellet during initial rise to power and during power cycles. The DEM model clearly predicts realistic earlylife crack patterns including both radial cracks and circumferential cracks. Simulation results clearly demonstrate the formation of radial cracks during the initial power rise, and formation of circumferential cracks as the power is ramped down. In these simulations, additional earlylife power cycles do not lead to themore »

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