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in connection with improvements to published regulatory guides, as well as items for inclusion in regulatory guides that are currently being developed. 

The NRC staff will revise existing guides, as appropriate, to accommodate comments and to reflect new inform ation or experience. W ritten comments

may be submitted to the Rules and Directives Branch, Office of Administration, U.S. Nuclear Regulatory Com mission, W ashington, DC 20555-0001.

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and 10, General.

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Documents Access and Management System (ADAMS) at http://www.nrc.gov/reading-rm /adam s.htm l, under Accession No. ML070380586.

U.S. NUCLEAR REGULATORY COMMISSION March 2007

REGULATORY GUIDE

OFFICE OF NUCLEAR REGULATORY RESEARCH

REGULATORY GUIDE 1.207

(Draft was issued as DG-1144, dated July 2006)

GUIDELINES FOR EVALUATING FATIGUE ANALYSES

INCORPORATING THE LIFE REDUCTION

OF METAL COMPONENTS

DUE TO THE EFFECTS OF THE LIGHT-WATER REACTOR

ENVIRONMENT FOR NEW REACTORS

A. INTRODUCTION

In Appendix A, “General Design Criteria for Nuclear Power Plants,” to Title 10, Part 50, of

the Code of Federal Regulations (10 CFR Part 50), “Domestic Licensing of Production and Utilization

Facilities” (Ref. 1), General Design Criterion (GDC) 1, “Quality Standards and Records,” requires, in part,

that structures, systems, and components that are important to safety shall be designed, fabricated,

erected, and tested to quality standards commensurate with the importance of the safety function

performed. In addition, GDC 30, “Quality of Reactor Coolant Pressure Boundary,” requires, in part,

that components that are part of the reactor coolant pressure boundary shall be designed, fabricated,

erected, and tested to the highest practical quality standards.

Augmenting those design criteria, 10 CFR 50.55a, “Codes and Standards,” endorses

the American Society of Mechanical Engineers (ASME) Boiler and Pressure Vessel Code (Ref. 2)

for design of safety-related systems and components. In particular, Section 50.55a(c), “Reactor Coolant

Pressure Boundary,” requires, in part, that components of the reactor coolant pressure boundary must meet

the requirements for Class 1 components in Section III, “Rules for Construction of Nuclear Power Plant

Components,” of the ASME Code, except as provided in that section.
RG 1.207, Page 2

Specifically, the ASME Class 1 requirements contain provisions, including fatigue design curves,

for determining a component’s suitability for cyclic service. These fatigue design curves are based on

strain-controlled tests performed on small polished specimens, at room temperature, in air environments. 

Thus, these curves do not address the impact of the reactor coolant system environment on the components

of the reactor coolant pressure boundary.

This regulatory guide provides guidance for use in determining the acceptable fatigue life

of ASME pressure boundary components, with consideration of the light-water reactor (LWR) environment. 

In so doing, this guide describes a method that the staff of the U.S. Nuclear Regulatory Commission

(NRC) considers acceptable to support reviews of applications that the agency expects to receive

for new nuclear reactor construction permits or operating licenses under 10 CFR Part 50; design certifications

under 10 CFR Part 52, “Early Site Permits; Standard Design Certifications; and Combined Licenses

for Nuclear Power Plants” (Ref. 3); and combined licenses under 10 CFR Part 52 that do not reference

a standard design. Because of significant conservatism in quantifying other plant-related variables

(such as cyclic behavior, including stress and loading rates) involved in cumulative fatigue life calculations,

the design of the current fleet of reactors is satisfactory.

This regulatory guide contains information collections that are covered by the requirements

of 10 CFR Parts 50 and 52, which the Office of Management and Budget (OMB) approved under OMB

control numbers 3150-0011 and 3150-0151, respectively. The NRC may neither conduct nor sponsor,

and a person is not required to respond to, an information collection request or requirement

unless the requesting document displays a currently valid OMB control number.

B. DISCUSSION

The ASME Section III design curves (Ref. 2), developed in the late 1960s and early 1970s,

are based on tests conducted in laboratory air environments at ambient temperatures. The original code

developers applied a margin of 2 on strain and a margin of 20 on cyclic life to account for variations

in materials, surface finish, data scatter, and environmental effects (including temperature differences

between specimen test conditions and reactor operating experience). However, the developers lacked

sufficient data to explicitly evaluate and account for the degradation attributable to exposure to aqueous

coolants. More recent fatigue test data from the United States, Japan, and elsewhere show that the LWR

environment can have a significant impact on the fatigue life of carbon and low-alloy steels, austenitic

stainless steel, and nickel-chromium-iron (Ni-Cr-Fe) alloys.

The staff evaluated two distinct methods for incorporating LWR environmental effects

into the fatigue analysis of ASME Class 1 components. The first method involves developing

new fatigue curves that are applicable to LWR environments. Given that the fatigue life of ASME Class 1

components in LWR environments is a function of several parameters, this method necessitates

the development of several fatigue curves to address potential parameter variations. Alternatively,

a single bounding fatigue curve could be developed, but this approach might be overly conservative

en for most applications. The second method involves using an environmental correction factor (F )

to account for LWR environments by correcting the fatigue usage calculated with the ASME “air” curves. 

This method affords the designer greater flexibility to calculate the appropriate impacts for specific

environmental parameters.
RG 1.207, Page 3

Th en e NRC staff has selected the F method as an acceptable method to properly incorporate

en the LWR environmental effects into fatigue analyses of ASME Class 1 components. The F method

is presented in NUREG/CR-6909, “Effect of LWR Coolant Environments on the Fatigue Life of Reactor

Materials” (Ref. 4). In particular, Appendix A to that report, “Incorporating Environmental Effects

into Fatigue Evaluations,” describes a method that the staff considers acceptable to incorporate the effects

of reactor coolant environments on fatigue usage factor evaluations of metal components. In addition,

NUREG/CR-6909 provides a comprehensive review of, and technical basis for, the method described

in this regulatory guide, including analyses of each parameter affecting the fatigue evaluations. 

In developing the underlying models, researchers from Argonne National Laboratory (ANL) analyzed

existing data to predict fatigue lives as a function of temperature, strain rate, dissolved oxygen level

in water, and sulfur content of the steel. The resultant method postulates a strain threshold, below which

en environmental effects on fatigue life do not occur. By definition, F is the ratio of fatigue life

of the component material in a room temperature air environment to its fatigue life in LWR coolant

at operating temperature. To incorporate environmental effects into the fatigue evaluation, the fatigue

usage is calculated using provisions set forth in Section III of the ASME Code, and the fatigue design

curve is multiplied by the correction factor.

The staff also reviewed the nonconservatism of the current ASME Code design curve in respect to

the existing fatigue data for austenitic stainless steels. Recent evaluations of stainless steel test data

indicate that the ASME curve is inconsistent with the appropriate test materials and conduct

of the fatigue test. Consequently, through this regulatory guide, the NRC staff endorses a new

stainless steel air design curve. Section 5.1.8 of NUREG/CR-6909 (Ref. 4) provides a comprehensive

en review of, and technical basis for, that new design curve. The F defined for stainless steel

in NUREG/CR-6909 should be used in conjunction with the new stainless steel air design curve

when evaluating the fatigue usage of ASME Class 1 components.

en In addition, the staff evaluated the incorporation of the F method in fatigue analyses

for Ni-Cr-Fe alloys (e.g., Alloy 600 and 690) and welds. Section 6 of NUREG/CR-6909 (Ref. 4) discusses

the technical basis for incorporating the environmental effects on nickel alloys and welds. In summary,

fatigue evaluations for Ni-Cr-Fe alloys are based on the fatigue design curve for austenitic stainless steels. 

However, the existing fatigue data for Ni-Cr-Fe alloys and their welds are not consistent with the current

ASME Code fatigue design curve for austenitic stainless steels. The data are either comparable

or slightly conservative with the updated ANL model for austenitic stainless steels. Thus, the new

fatigue design curve proposed for austenitic stainless steels adequately represents the fatigue behavior

of Ni-Cr-Fe alloys and their welds. Therefore, the new design curve for austenitic stainless steels

may also be used for Ni-Cr-Fe alloys and their welds, and the staff finds it acceptable to use the new

austenitic stainless steels air design curve in Ni-Cr-Fe alloys environmental fatigue evaluations. 

Con en sequently, Section 6 of NUREG/CR-6909 presents the respective F equations to be used

for Ni-Cr-Fe alloys and their welds.

Section 7 of NUREG/CR-6909 (Ref. 4) evaluates the ASME design curve margins. In conducting

that evaluation, the researchers reviewed data available in the literature to assess the subfactors

(excluding environment) that are necessary to account for the effects of various uncertainties and differences

between actual components and laboratory test specimens. The researchers also performed statistical

analyses using Monte Carlo simulations to develop fatigue design curves, using the “95/95 criterion.” 

In other words, the curves should provide 95% confidence that the fatigue life of 95% of the population

will be greater than that predicted by the design curves. The NRC deems this criterion acceptable

because the fatigue design curves are based on crack initiation, rather than component failure and,

therefore, additional margin exists between crack initiation and actual component failure.
RG 1.207, Page 4

The results of the Monte Carlo simulations indicate that for both carbon and low-alloy steels

and austenitic stainless steels, the current ASME Code procedure of adjusting the mean test data

by a factor of 20 for cyclic life is conservative compared to the 95/95 criterion. The results also indicate

that a minimum factor of 12 for cyclic life of both carbon and low-alloy steels and austenitic stainless

steels will satisfy the 95/95 criterion. Figures 9, 10, and 37 of NUREG/CR-6909 (Ref. 4) present

the resultant new air design curves, using margins of 12 for cyclic life and 2 for stress, for carbon steel,

low-alloy steel, and austenitic stainless steel, respectively. This regulatory guide uses these new

air design curves; thus, an applicant that chooses to adopt the procedure discussed in this guide

to determine the fatigue life of stainless steels should use these air design curves. However, the existing

ASME air design curves for carbon and low-alloy steels may also be used with the procedure in this guide

to determine the fatigue life of those materials, since their use will yield conservative results.

The NRC en reviewed and found acceptable several methods for calculating F . Only the types

of stress cycles or load set pairs that exceed strain threshold criteria for carbon and low-alloy steels,

en austenitic stainless steel, and Ni-Cr-Fe alloys need to be considered for F calculations. The evaluation

options depend on the complexity of the analyzed transient condition and the detail of the evaluation. 

For example, in an evaluation in which the results of detailed transient analyses are available

to determine the necessary parameters (strain rate, temperature, and others), the “modified rate approach”

(presented and referenced in Section 4.2.14 of NUREG/CR-6909, Ref. 4) is an acceptable method

en for determining the F values. This method involves a strain-based integral for evaluating conditions

en for which temperature and strain rate change, resulting in the variation of F over time. This detailed

en approach calculates the F values based on the strain history for each load set in the fatigue analysis

evaluation, considering the effects of strain rate and temperature variations for each incremental segment

en in the strain history. Such results may be used to reduce the conservatism in the calculated F values. 

For a simplified calculation yielding a more conservative result for a complex or poorly defined set

of transients, the temperature is equal to the average temperature in the transient or segment. 

The c en alculated F values are then used to incorporate environmental effects into ASME fatigue

usage factor evaluations.
RG 1.207, Page 5

C. REGULATORY POSITION

This section describes the methods that the staff considers acceptable for use in performing

fatigue evaluations, considering the effects of LWR environments on carbon and low-alloy steels,

austenitic stainless steels, and Ni-Cr-Fe alloys. Specifically, these methods include calculating the fatigue

usage in air using ASME Code analysis procedures, and then employing the environmental correction

en factor (F ), as described in NUREG/CR-6909 (Ref. 4). In particular, Appendix A to that report

includes detailed descriptions and additional guidance concerning the overall method and all the required

calculations.

1. Carbon and Low-Alloy Steels

The following procedure should be used to calculate the environmental fatigue usage of carbon

and low-alloy steel components in LWR environments.

1.1 Fatigue Usage in Air

Calculate the fatigue usage in air using ASME Code analysis procedures and the fatigue air

curves provided in NUREG/CR-6909, Appendix A, Figures A.1 and A.2 (updated ANL model curves).

1 en .2 Environmental Correction Factor (F )

C en alculate the environmental correction factor, F , using Equation A.2 of NUREG/CR-6909

for carbon steels, or Equation A.3 of NUREG/CR-6909 for low-alloy steels. Equations A.4 through A.7

of NUREG/CR-6909 should be used to calculate the parameters used in Equations A.2 and A.3. 

Equation A.8 of NUREG/CR-6909 defines the strain threshold.

1.3 Environmental Fatigue Usage

Calculate the environmental fatigue usage using Equation A.20 of NUREG/CR-6909.

2. Austenitic Stainless Steels

The following procedure should be used to calculate the environmental fatigue usage

of austenitic stainless steel components in LWR environments.

2.1 Fatigue Usage in Air

Calculate the fatigue usage in air using ASME Code analysis procedures and the new stainless

steel fatigue air curve provided in NUREG/CR-6909, Appendix A, Figure A.3 (proposed design curve).

2 en .2 Environmental Correction Factor (F )

For en all types of austenitic stainless steels (e.g., Types 304, 310, 316, 347, and 348), calculate F

using Equation A.9 of NUREG/CR-6909. Equations A.10 through A.12 of NUREG/CR-6909 should be

used to calculate the parameters used in Equation A.9. Equation A.13 of NUREG/CR-6909 defines

the strain threshold.

2.3 Environmental Fatigue Usage

Calculate the environmental fatigue usage using Equation A.20 of NUREG/CR-6909.
RG 1.207, Page 6

3. Ni-Cr-Fe Alloys

The following procedure should be used to calculate the environmental fatigue usage

for Ni-Cr-Fe alloy components in LWR environments (e.g., Alloy 600 and 690).

3.1 Fatigue Usage in Air

Calculate the fatigue usage in air using ASME Code analysis procedures and the new stainless

steel fatigue air curve provided in NUREG/CR-6909, Appendix A, Figure A.3 (proposed design curve).

3.2 Environmental Correction Factor (Fen)

For en all types of Ni-Cr-Fe alloys (e.g., Alloy 600 and 690), calculate F using Equation A.14

of NUREG/CR-6909. Equations A.15 through A.17 of NUREG/CR-6909 should be used to calculate

the parameters used in Equation A.14. Equation A.18 of NUREG/CR-6909 defines the strain threshold.

3.3 Environmental Fatigue Usage

Calculate the environmental fatigue usage using Equation A.20 of NUREG/CR-6909.

D. IMPLEMENTATION

The purpose of this section is to provide information to applicants and licensees regarding

the NRC staff’s plans for using this regulatory guide. This regulatory guide only applies to new plants,

and no backfitting is intended or approved in connection with its issuance.

Except in those cases in which an applicant or licensee proposes or has previously established

an acceptable alternative method for complying with specified portions of the NRC’s regulations,

the methods described in this guide will be used in evaluating submittals in connection with applications

for construction permits, standard plant design certifications, operating licenses, early site permits,

and combined licenses.

REGULATORY ANALYSIS / BACKFIT ANALYSIS

The regulatory analysis and backfit analysis for this regulatory guide are available in Draft

Regulatory Guide DG-1144, “Guidelines for Evaluating Fatigue Analyses Incorporating the Life

Reduction of Metal Components Due to the Effects of the Light-Water Environment in New Reactors”

(Ref. 5). The NRC issued DG-1144 in July 2006 to solicit public comment on the draft of this

Regulatory Guide 1.207.
All NRC regulations listed herein are available electronically through the Electronic Reading Room on the NRC’s 1

public Web site, at http://www.nrc.gov/reading-rm/doc-collections/cfr/. Copies are also available for inspection

or copying for a fee from the NRC’s Public Document Room at 11555 Rockville Pike, Rockville, Maryland; the PDR’s

mailing address is USNRC PDR, Washington, DC 20555; telephone (301) 415-4737 or (800) 397-4209;

fax (301) 415-3548; email PDR@nrc.gov.

Copies may be purchased from the American Society of Mechanical Engineers, Three Park Avenue, NewYork, NY 2

10016-5990; phone (212) 591-8500; fax (212) 591-8501; www.asme.org.

Copies are available at current rates from the U.S. Government Printing Office, P.O. Box 37082, Washington, DC 3

20402-9328 (telephone 202-512-1800); or from the National Technical Information Service (NTIS) by writing NTIS

at 5285 Port Royal Road, Springfield, Virginia 22161, online at http://www.ntis.gov, by telephone at (800) 553-NTIS

(6847) or (703)605-6000, or by fax to (703) 605-6900. Copies are also available for inspection or copying for a fee

from the NRC’s Public Document Room (PDR), which is located at 11555 Rockville Pike, Rockville, Maryland; the

PDR’s mailing address is USNRC PDR, Washington, DC 20555-0001. The PDR can also be reached by telephone

at (301) 415-4737 or (800)397-4209, by fax at (301)415-3548, and by email to PDR@nrc.gov. NUREG/CR-6909

is also available through the NRC’s Agencywide Documents Access and Management System (ADAMS)

at http://www.nrc.gov/reading-rm/adams.html, under Accession #ML070660620.

Draft Regulatory Guide DG-1144 is available electronically under Accession #ML060970173 in the NRC’s

4

Agencywide Documents Access and Management System (ADAMS) at http://www.nrc.gov/reading-rm/adams.html.

Copies are also available for inspection or copying for a fee from the NRC’s Public Document Room (PDR), which is

located at 11555 Rockville Pike, Rockville Maryland; the PDR’s mailing address is USNRC PDR, Washington, DC

20555-0001. The PDR can also be reached by telephone at (301) 415-4737 or (800) 397-4209, by fax at (301) 415-3548,

and by email to PDR@nrc.gov.

RG 1.207, Page 7

REFERENCES

1. U.S. Code of Federal Regulations, Title 10, Energy, Part 50, “Domestic Licensing of Production

and Utilization Facilities,” U.S. Nuclear Regulatory Commission, Washington, DC.1

2. ASME Boiler and Pressure Vessel Code, Section III, “Rules for Construction of Nuclear Power

Plant Components,” American Society of Mechanical Engineers, New York, NY, 1992.2

3. U.S. Code of Federal Regulations, Title 10, Energy, Part 52, “Early Site Permits; Standard

Design Certifications; and Combined Licenses for Nuclear Power Plants,” U.S. Nuclear

Regulatory Commission, Washington, DC.1

4. NUREG/CR-6909, “Effect of LWR Coolant Environments on Fatigue Life of Reactor Materials”

(Final Report), ANL-06/08, U.S. Nuclear Regulatory Commission, Washington, DC,

February 2007.3

5. Draft Regulatory Guide DG-1144, “Guidelines for Evaluating Fatigue Analyses Incorporating

the Life Reduction of Metal Components Due to the Effects of the Light-Water Reactor

Environment for New Reactors,” U.S. Nuclear Regulatory Commission, Washington, DC,

July 2006.4