United States Nuclear Regulatory Commission - Protecting People and the Environment

Expanded Materials Degradation Assessment (EMDA): Aging of Reactor Pressure Vessels (NUREG/CR-7153, Volume 3)

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Publication Information

Manuscript Completed: October 2013
Date Published: October 2014

Prepared by Expert Panel:
Oak Ridge National Laboratory: Randy K. Nanstad,
Thomas M. Rosseel, and Mikhail A. Sokolov
ATI Consulting: William L. Server
Japan Central Research Institute of Electric Power Industry:
Taku Arai and Naoki Soneda
Electric Power Research Institute: Robin Dyle
The University of California, Santa Barbara: G. Robert Odette
U.S. Nuclear Regulatory Commission: Mark T. Kirk
Westinghouse: Brian N. Burgos and J. Brian Hall

On behalf of:
Oak Ridge National Laboratory
Managed by UT-Battelle, LLC

J. T. Busby, DOE-NE LWRS EMDA Lead.

P. G. Oberson and C. E. Carpenter, NRC Project Managers
M. Srinivasan, NRC Technical Monitor

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

Availability Notice

Abstract

In NUREG/CR-6923, "Expert Panel Report on Proactive Materials Degradation Assessment," referred to as the PMDA report, NRC conducted a comprehensive evaluation of potential aging-related degradation modes for core internal components, as well as primary, secondary, and some tertiary piping systems, considering operation up to 40 years. This document has been a very valuable resource, supporting NRC staff evaluations of licensees' aging management programs and allowing for prioritization of research needs.

This report describes an expanded materials degradation assessment (EMDA), which significantly broadens the scope of the PMDA report. The analytical timeframe is expanded to 80 years to encompass a potential second 20-year license-renewal operating-period, beyond the initial 40-year licensing term and a first 20-year license renewal. Further, a broader range of structures, systems, and components (SSCs) was evaluated, including core internals, piping systems, the reactor pressure vessel (RPV), electrical cables, and concrete and civil structures. The EMDA uses the approach of the phenomena identification and ranking table (PIRT), wherein an expert panel is convened to rank potential degradation scenarios according to their judgment of susceptibility and current state of knowledge. The PIRT approach used in the PMDA and EMDA has provided the following benefits:

  • Captured the status of current knowledge base and updated PMDA information,
  • Identified gaps in knowledge for a SSC or material that need future research,
  • Identified potential new forms of degradation, and
  • Identified and prioritized research needs.

As part of the EMDA activity, four separate expert panels were assembled to assess four main component groups, each of which is the subject of a volume of this report.

  • Core internals and piping systems (i.e., materials examined in the PMDA report) – Volume 2
  • Reactor pressure vessel steels (RPV) – Volume 3
  • Concrete civil structures – Volume 4
  • Electrical power and instrumentation and control (I&C) cabling and insulation – Volume 5

The present volume summarizes the results of the expert panel convened to evaluate agingrelated degradation of the RPV. The conceptual starting point for the evaluation of the RPV was found in the Materials Degradation Matrix (MDM) and the Issue Management Tables (IMTs) recently developed by the Electric Power Research Institute (EPRI). For EPRI, the MDM and IMT serve a similar role as the PMDA does for NRC, in that potential degradation scenarios are identified and evaluated to highlight knowledge gaps and prioritize research needs. Starting from the MDM and IMT, the EMDA panel independently determined whether degradation mechanisms for consideration should be added, removed, or modified. A consensus of the issues to be assessed was obtained through discussions among the members of the panel.

The technical issues evaluated by the panel for the RPV are summarized in the technical background assessments found in Chapters 2-6 of this report. These include

  • Environmental effects on fracture resistance
  • Thermal embrittlement of RPV steels
  • Long-term integrity of dissimilar metal welds
  • Fatigue mechanism/mode
  • Neutron embrittlement

The section on neutron embrittlement includes subsections assessing rate effects, effect of high fluence on alloys with high nickel content, attenuation, master curve fracture toughness, and thermal annealing, embrittlement beyond the beltline. Chapter 7 of this report summarizes the PIRT scoring for the RPV, and conclusions and recommendations are captured in Chapter 8.

The report concludes that although remarkable progress has been made in developing a mechanistic understanding of irradiation embrittlement, including the development of physically based and statistically calibrated models of Charpy V-notch-indexed transition-temperature shifts, important technical issues still need to be addressed to reduce the uncertainties in RPV material behavior. These include the effects of high fluence, prolonged irradiation expoure, and flux on the RPV material behavior evaluation process.

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