United States Nuclear Regulatory Commission - Protecting People and the Environment

A Generalized Procedure for Generating Flaw-Related Inputs for the FAVOR Code (NUREG/CR-6817, Rev. 1)

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Publication Information

Manuscript Completed: March 2013
Date Published: August 2013

Prepared by:
F. A. Simonen, S. R. Doctor, G. J. Schuster, and P. G. Heasler

Pacific Northwest National Laboratory
P.O. Box 999
Richland, WA 99352

D. Jackson and W. E. Norris, NRC Project Managers

NRC Job Code Y6604 and N6398

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

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Abstract

The U.S. Nuclear Regulatory Commission (NRC) supported research to re-evaluate the technical basis for Title 10 of the Code of Federal Regulations (CFR) 50.61, "Fracture toughness requirements for protection against pressurized thermal shock events." During plant operation, the walls of reactor pressure vessels (RPV) are exposed to neutron radiation, resulting in a localized embrittlement of the vessel steel and weld materials in the core area. If an embrittled RPV had an existing flaw of critical size and certain severe system transients were to occur, this flaw could very rapidly propagate through the vessel, resulting in a through-wall crack that could challenge the integrity of the RPV. The severe transients of concern, known as pressurized thermal shock (PTS), are characterized by a rapid cooling (i.e., thermal shock) of the internal reactor pressure vessel surface in combination with re-pressurization of the RPV.

Specifically, the crack-like flaws of concern are those near the inner vessel surface. This report documents research performed at Pacific Northwest National Laboratory (PNNL) to assess fabrication flaws in RPVs. The research generated data on flaws in RPVs in terms of flaw densities, flaw locations, and flaw sizes (through-wall depth dimensions and lengths). This report describes data from the research, results from an expert judgment elicitation on RPV fabrication practices, and probabilistic models that characterize flaws that may exist in RPVs. A procedure is described for generating flaw-related parameters for use as inputs to probabilistic fracture mechanics calculations. The report addresses the treatment of welding flaws using the flaws per unit area concept and describes additional details of the flaw model used in the FAVOR(a) probabilistic fracture mechanics code.

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