Simulating Dynamic Fracture in Oxide Fuel Pellets Using Cohesive Zone Models

Conference · Sat Aug 01 00:00:00 EDT 2009

OSTI ID:966177

Williamson, R L

It is well known that oxide fuels crack during the first rise to power, with continued fracture occurring during steady operation and especially during power ramps or accidental transients. Fractures have a very strong influence on the stress state in the fuel which, in turn, drives critical phenomena such as fission gas release, fuel creep, and eventual fuel/clad mechanical interaction. Recently, interest has been expressed in discrete fracture methods, such as the cohesive zone approach. Such models are attractive from a mechanistic and physical standpoint, since they reflect the localized nature of cracking. The precise locations where fractures initiate, as well as the crack evolution characteristics, are determined as part of the solution. This paper explores the use of finite element cohesive zone concepts to predict dynamic crack behavior in oxide fuel pellets during power-up, steady operation, and power ramping. The aim of this work is first to provide an assessment of cohesive zone models for application to fuel cracking and explore important numerical issues associated with this fracture approach. A further objective is to provide basic insight into where and when cracks form, how they interact, and how cracking effects the stress field in a fuel pellet. The ABAQUS commercial finite element code, which includes powerful cohesive zone capabilities, was used for this study. Fully-coupled thermo-mechanical behavior is employed, including the effects of thermal expansion, swelling due to solid and gaseous fission products, and thermal creep. Crack initiation is determined by a temperature-dependent maximum stress criterion, based on measured fracture strengths for UO2. Damage evolution is governed by a traction-separation relation, calibrated to data from temperature and burn-up dependent fracture toughness measurements. Numerical models are first developed in 2D based on both axisymmetric (to explore axial cracking) and plane strain (to explore radial cracking) assumptions. A 3D model is then developed, permitting simultaneous radial and axial fractures. Although fuel cracking is clearly three-dimensional, 2D models are of interest since they are simpler to implement, are much less computationally intensive, and have potential application to existing 2D fuel performance codes. Numerical issues related to cohesive zone models, such as mesh dependency and viscous stabilization, are addressed. Model results indicate that for typical oxide fuel properties, both axial and radial cracking occurs during initial heat-up, well before steady-state thermal gradients are established in the pellet. Cracking results in local stress relief and a shift in peak stress locations, leading to the initiation of new cracks. Continued growth of existing cracks, plus the initiation and growth of additional fractures, is observed during steady operation and power ramping. 3D models provide considerable insight into the progressive interactions between radial and axial cracking. Parametric studies demonstrate the effects of temperature dependent material properties on crack initiation and progression. Increasing fracture strength and toughness with temperature, leads to crack arrest in high temperature regions near the pellet’s symmetry axis.

• Research Organization: Idaho National Lab. (INL), Idaho Falls, ID (United States)
• Sponsoring Organization: DOE - NE
• DOE Contract Number: DE-AC07-99ID-13727
• OSTI ID: 966177
• Report Number(s): INL/CON-08-14787; TRN: US0904043
• Resource Relation: Conference: 20th International Conference on Structural Mechanics in Reactor Technology,Espoo, Finland,08/09/2009,08/14/2009
• Country of Publication: United States
• Language: English

Related Subjects

11 NUCLEAR FUEL CYCLE AND FUEL MATERIALS
97 MATHEMATICS AND COMPUTING
CREEP
FISSION
FISSION PRODUCTS
FRACTURE PROPERTIES
FRACTURES
FUEL PELLETS
OXIDES
REACTOR TECHNOLOGY
STABILIZATION
STRAINS
SWELLING
SYMMETRY
TEMPERATURE GRADIENTS
THERMAL EXPANSION
TRANSIENTS
cohesive zone
fracture modeling
oxide fuels