Advanced Nuclear Technology January 2017

ISSUE STATEMENT

The ASME Code Section III design rules for piping and components have analytical requirements to prevent fatigue failures during service for a 40-year design life. The design analysis procedures for computing a cumulative fatigue usage factor (CUF) incorporate design cyclic stresses and fatigue design reference curves to safeguard against crack initiation for the materials of construction. There has been extensive technical debate within ASME on the best method for updating the fatigue design curves to explicitly account for environmental factors that can affect the fatigue life and the CUF calculations. As a result, the ASME Code process has not been able to obtain a consensus position on the issue, and comprehensive code revision has been slow in develop-ment. The U.S. NRC has published regulatory guidance based on laboratory results obtained by their contractors and collaborators to address environmental effects; the docu-ments include Regulatory Guide 1.207 and NUREG/CR-6909. This guidance has resulted in difficulties in dem-onstrating acceptable fatigue usage for both new plant and operating plant license renewal applications designs. License renewal approval with these requirements enforced do not appear to have been restrained, but relied upon more expen-sive options. These options included fatigue monitoring and performing complex 3-D elastic-plastic finite element analy-sis to more accurately evaluate component stresses and strains. New plants can also analyze the problem in more detail or modify the designs of adversely affected compo-nents. Plants seeking long term operation, e.g., 80 year oper-ating life, will have significant difficulty meeting these EAF requirements because little fatigue margin will remain to use for that period of time. Since there has not been an identified EAF piping or component failure in the field during fleet operation to date, it is believed that the current regulations are overly conservative. This conservatism exists in the origi-nal plant design as required by ASME Section III Code’s fatigue design curve and the Code’s evaluation methodology.

DRIVERS

Regulatory Impact on New Plant Designs and License Renewal PlantsRegulatory Guide 1.207 and its referenced NUREG/CR-6909 requires a complex methodology and places con-servative requirements on component fatigue evaluation resulting in compliance difficulty for both new plants and

operating plants over a plant’s extended operating period (e.g., potential 80-year life). To obtain an acceptable CUF often requires changes that increase design, construction, and operational costs without meaningful safety benefits. Affected items in the design may include materials selection, piping thickness, fitting tolerances, as well as number and locations of piping supports. License renewal plants address the issue by performing more detailed elastic-plastic finite element analysis to obtain more accurate stress and strains and/or enhanced fatigue monitoring in the field. Addition-ally, for license renewal and long term operation, there is uncertainty as to the requirements that have been imposed by NRC because the scope of locations requiring environ-mental fatigue evaluation is open to interpretation.

Overall Cost to Plant OwnersThe impacts on plants will directly affect the overall cost to plant owners in the form of design, re-analysis, and hard-ware modifications. The application of environmentally assisted fatigue factors results in significantly higher CUFs for both new and existing plants. More detailed and com-plex calculations are often needed to meet requirements specified in NUREG/CR-6909. For some utilities, addi-tional fatigue monitoring may be required to demonstrate acceptable usage for the operating life of the plant. For new plants with early design certification, any significant changes to the design would require a re-evaluation of applicable components, which can be costly and may affect license amendments and approval schedules.

Global InterestSeveral non-U.S. organizations and utilities have been actively engaged in environmentally assisted fatigue issues because 1) regulatory bodies worldwide tend to take similar approaches, and 2) the ASME Code is often used as the gov-erning design and operational basis for nuclear power plants.

RESULTS IMPLEMENTATION

Because this issue affects both operating and new plants, several EPRI programs have combined resources and will share final results. EPRI will support effective ASME code revisions that resolve the fatigue issue and could also seek approval directly from the NRC to expedite implementation of new guidance. These actions will include:• Publication of reports and related documents that form

the technical basis of Code modifications to obtain code approval and regulatory acceptance.

IN USE: ENVIRONMENTALLY-ASSISTED FATIGUE

EPRI | Nuclear Sector Roadmaps January 2017

• Development of guidance and possible Code Cases that provide evaluation procedures for assessing fatigue envi-ronmental factors that are accepted by regulatory authorities.

• Support of ASME Section III and XI Code revisions that permanently include Environmentally Assisted Fatigue (EAF) procedures within the body of the code.

• Development of an understanding of new procedures to provide consistency of application by nuclear plant ven-dors, construction firms, and utilities (new and operating plant owners).

• Publication of EAF guidance documents within the pur-view of NEI 03-08 and seek NRC endorsement of this guidance.

• Coordination of international activities on EAF to reach a common objective within the various international regulations.

Regulatory bodies will participate in the development pro-cess and consider Code changes related to environmentally assisted fatigue via their membership in ASME Section III and XI standards committees. NRC may also be requested to evaluate new and revised EPRI guidance for performing fatigue evaluations applicable to new plant design and revised plant operating licenses through a safety evaluation report.

PROJECT PLAN

The work is divided into analytical efforts and testing. Ana-lytical efforts are focused on better defining the scope of locations that require EAF analysis and identifying possible changes with the basis to ASME Section III fatigue evalua-tion methodology that incorporates more appropriate levels of analytical conservatism. The testing activities involve gen-eration of data that provides for a better understanding of the mechanistic nature of EAF and EAF data that better represent actual plant operating conditions which lead to component fatigue degradation. Both efforts are driven by the knowledge gap prioritization and roadmap first described in the 2011 Technical Report Environmentally Assisted Fatigue Gap Analysis and Roadmap for Future Research — Gap Analysis Report (Product ID 1023012) with a revision provided by 2012 Technical Report Environmentally Assisted Fatigue Gap Analysis and Roadmap for Future Research — Roadmap (Product ID 1026724).

Analytical ActivitiesEPRI is engaging the ASME Code working groups to develop new procedures that incorporate environmental fac-tors for fatigue where required. The basic strategy is to develop Code Cases that can be adopted by licensees, fol-lowed by Code revisions. An EPRI-led expert panel was used to identify and prioritize EAF knowledge gaps. Members of

this panel also participated in a voluntary effort to test cur-rent ASME Code Cases with guidance for performing envi-ronmental fatigue evaluations and provided comments and lessons learned. This effort provided input to update EPRI guidance for performing environmental fatigue evaluations as well as recommended changes to the Code Cases.

EPRI Technical Report, Guidelines for Addressing Environ-mental Effects in Fatigue Usage Calculations (1025823) was published in 2012. EPRI has also documented a methodol-ogy for identifying the most limiting locations for EAF; a report (Technical Report 1024995) was published in 2012 summarizing the results. Additionally, a report (Technical Report 3002003922) was published that defines potential, generic approaches to environmental fatigue assessment for those plants in both the BWR and PWR fleets that have yet to seek license renewal approval or have not met their license renewal commitment on fatigue evaluation.

In 2013 through 2015, efforts were undertaken to evaluate and reduce sources of excessive conservatism in EAF calcula-tions. Reports documenting these results will be produced and, where appropriate, submitted to the ASME Code and NRC for review and approval. Specifically, EPRI is presently investigating two areas of the ASME Code Section III fatigue procedures (NB-3200 and NB-3600) that may lead to a more appropriate level of conservatism in the design methodology. If the proposed approaches are significant and acceptable the impact of new EAF requirements in imple-menting the methodology will be minimized.

The U.S. NRC will issue Revision 1 to NUREG/CR-6909 in the Spring of 2017 and has issued Regulatory Guide 1.207.

Finally, EPRI has been actively involved with the NRC in developing a probabilistic fracture mechanics code titled extremely low probability of rupture (xLPR). The develop-ment of this code was initiated by NRC to address leak-before-break issues associated with primary piping that may exhibit PWSCC degradation. The NRC has also directed code development to address the issues of piping fatigue. This latter development could be beneficial in helping iden-tify the appropriate levels of conservatism when applying ASME fatigue design rules in conjunction with fatigue envi-ronmental effects as specified in NUREG/CR-6909. Addi-tionally, to date the ASME Code has not incorporated prob-abilistic analysis in establishing its requirements. EPRI may develop in the future a probabilistic approach to EAF using xLPR and document the identified requirements and meth-odology in a Code Case. Some work laying the foundation for this effort has been produced and published in recent ASME conferences and will be finalized going forward. Also, the chair of the ASME EAF Working Group has pub-licly indicated that he would like to steer his committee in the direction of using probabilistic analysis to set margin in the Code.

Advanced Nuclear Technology January 2017

Testing ActivitiesFurther research is necessary to close the gap between pre-dicted environmental effects on fatigue life and industry operating field experience. EPRI testing will be performed that identify reasons for the disparity between EAF predic-tion of fatigue failure and what has been seen in the field heretofore. Testing will encompass fatigue usage (cycling occurring prior to crack initiation), fatigue loading cycles more appropriate to what actual plant transients exhibit, fatigue crack growth (crack extension after a fatigue crack initiates), and hopefully fatigue testing of a prototypical component to provide more realistic data needed to close the gap. The experimental effort began in the 4th quarter of 2013 with research focused on addressing high priority knowledge gaps and providing the inputs to subsequent test-ing on a component-type test fixture. It is anticipated that these tests will be funded through 2019.

RISKS

• Additional environmental fatigue data may not support the hypothesis that the current EAF methodology is unnecessarily conservative to the extreme.

• The ASME Code committees may not approve Code Cases or Code changes based on EPRI recommendations.

• NRC may not agree to the proposed EPRI guidance, Code Cases, or revised ASME Code procedures.

RECORD OF REVISION

This record of revision will provide a high level summary of the major changes in the document and identify the Road-map Owner.

revision description of change

0 Original Issue: August 2011 Roadmap Owner: Letitia Midmore

1 Revision Issued: August 2012 Roadmap Owner: Shannon Chu and Jean Smith

Changes: Project plan updated to remove discussion of a completed project and reframe the descriptions in terms of short-term and long-term activities.

2 Revision Issued: August 2013 Roadmap Owner: Jean Smith

Changes: Updated flow chart to remove an unnecessary milestone and to revise the start date of EAF specimen testing and the end date of NRC revision of documents.

3 Revision Issued: January 2014 Roadmap Owner: Jean Smith

Changes: Numerous editorial changes were made to the project plan to reflect the completion of work activities and to show proposed work. The flowchart reflects completed milestones and the initiation of laboratory efforts.

4 Revision Issued: August 2014 Roadmap Owner: Jean Smith

Changes: Minor editorial changes for schedule tense based on progressive elaboration of project tasks. Updated flowchart to reflect completed milestone.

5 Revision Issued: December 2014 Roadmap Owner: Matthew O’Connor

Changes: Updated flow chart to reflect current schedule.

6 Revision Issued: August 2015 Roadmap Owner: David A. Steininger

Changes: Extensive changes to write-up to reflect new approach to issue and appropriate changes to flowchart

7 Revision Issued: December 2016 Roadmap Owner: David A. Steininger

Changes: Changes to write-up to reflect current approach.

EPRI | Nuclear Sector Roadmaps January 2017