Nonlinear Analyses for Embedded Cracks Under Pressurized Thermal Shock: Comparisons with FAVOR and Weibull Stress Approaches (NUREG/CR-6956)
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Manuscript Completed: August 2007
Date Published: February 2008
B. Wasiluk1, X. Qian 2 and R.H. Dodds, Jr.1
1Department of Civil and Environmental Engineering
University of Illinois at Urbana-Champaign
205 N. Mathews Avenue
Urbana, IL 61801
2Oak Ridge National Laboratory
P.O. Box 2008
Oak Ridge, TN 37831
S.N. Malik, NRC Project Manager
NRC Job Code Y6951
Office of Nuclear Regulatory Research
Thick-walled reactor pressure vessels (RPVs) can potentially experience rapid temperature and pressure changes under pressurized thermal shock (PTS) conditions. This work describes progress towards utilization of a Weibull stress approach for cleavage fracture assessment of RPVs subjected to PTS events. The Weibull stress approach couples the macroscopic crack driving force, J or KJ , with the local, crack-front conditions for cleavage characterized by the Weibull stress and requires realistic stress analyses. Extensive previous work focused on the conventional, linear-elastic stress-intensity factor (SIF), KI values for flaws in RPVs. This study begins by comparing predictions of the macroscopic crack driving force ( KJ ) made by the FAVOR (Fracture Analysis for Vessels – Oak Ridge) code with detailed, linear-elastic and elastic-plastic finite element analyses for circumferentially and axially embedded flaws located in a representative RPV and subjected to two well characterized transients, denoted here as transients A and B. These solutions provide needed benchmarks for future efforts to approximate the nonlinear material response near the crack front through a simpler, linear-elastic KI -T stress field imposed on a 2-D, smallscale yielding configuration. The postulated loadings considered here include a critical thermal transient with a small change of internal pressure (Transient A) and a mild thermal transient concurrent with significant re-pressurization (Transient B). The RPV models employ ferritic steel for the base material and austenitic steel for the cladding; the combination leads to pronounced mismatches in both the mechanical and thermal properties. The SIF computed from the linear-elastic analyses show lower values than the KJ -solutions obtained from the elastic-plastic analyses. Based on previous research for the Weibull stress approach applied to through-crack fracture specimens, the current study concludes with proposals for refined and simplified engineering procedures, as well as a KI -T stress methodology, for defect assessments of curved, embedded flaws in RPVs under PTS conditions.
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