United States Nuclear Regulatory Commission - Protecting People and the Environment

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

This NUREG-series publication was issued for public comment. The comment period is now closed.

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Publication Information

Manuscript Completed: November 2014
Date Published:
February 2015

Prepared by:
Gary L. Stevens, Mark T. Kirk, and Mohammad Modarres*

*University of Maryland

Gary L. Stevens, NRC Project Manager

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

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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 certain severe system transients were to occur, the flaw could rapidly propagate through the vessel, resulting in a through-wall crack and, thereby, challenging 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. Advancements in understanding and knowledge of materials 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 that was published in Section 50.61a, "Alternate Fracture Toughness Requirements for Protection against Pressurized Thermal Shock Events," of Title 10, "Energy," of the Code of Federal Regulations (10 CFR 50.61a).

This report explains the basis for the requirements that establish the entry conditions to permit use of 10 CFR 50.61a and describes methods by which the following four requirements can be met: (1) criteria relating to the date of construction and design requirements, (2) criteria relating to evaluation of plant-specific surveillance data, (3) criteria relating to inservice inspection (ISI) data and non-destructive examination (NDE) requirements, and (4) criteria relating to alternate limits on embrittlement.

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