Technical Basis for Regulatory Guidance on the Alternate Pressurized Thermal Shock Rule, Final Report (NUREG-2163)

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Publication Information

Manuscript Completed: December 2016
Date Published: September 2018

Prepared by:
G. Stevens¹
M. Kirk
M. Modarres²

¹ Structural Integrity Associates, Inc.
11515 Vanstory Drive, Suite 125
Huntersville, NC 28078

² University of Maryland
Clark School of Engineering
College Park, MD 20742-7531

Mark Kirk, NRC Project Manager

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

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Abstract

During plant operation, the walls of reactor pressure vessels (RPVs) are exposed to neutron radiation, resulting in embrittlement of the vessel steel and weld materials in the area of the RPV adjacent to the core. If an embrittled RPV had a flaw of critical size and if certain severe system transients were to occur, the flaw could 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 of the internal RPV surface in combination with repressurization of the RPV. Advances in understanding material behavior, the ability to realistically model plant systems and operational characteristics, and the ability to better evaluate PTS transients to estimate loads on vessel walls led the U.S. Nuclear Regulatory Commission (NRC) to develop a risk-informed revision of the existing PTS Rule in Title 10 of the Code of Federal Regulations (10 CFR) 50.61a, "Alternate Fracture Toughness Requirements for Protection against Pressurized Thermal Shock Events."

This report explains the basis for the requirements that es tablish the entry conditions to permit the use of 10 CFR 50.61a and describes methods that licensees can use to meet the following four requirements:

(1)     criteria related to the date of construction and design requirements
(2)     criteria related to the evaluation of plant-specific surveillance data
(3)     criteria related to inservice inspection data and nondestructive examination requirements
(4)     criteria related to alternate screening criteria on embrittlement