Nonlinear Analyses for Embedded Cracks Under Pressurized Thermal Shock: Comparisons with FAVOR and Weibull Stress Approaches (NUREG/CR-6956)

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Publication Information

Manuscript Completed: August 2007
Date Published: February 2008

Prepared by:
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

Prepared for:
Office of Nuclear Regulatory Research
U.S. Nuclear Regulatory Commission
Washington DC 20555-0001

NRC Job Code Y6951

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Abstract

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 K_J , 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), K_I values for flaws in RPVs. This study begins by comparing predictions of the macroscopic crack driving force ( K_J ) 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 K_I -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 K_J -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 K_I -T stress methodology, for defect assessments of curved, embedded flaws in RPVs under PTS conditions.

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