E-Journal of Advanced Maintenance Vol.2 (2010) 82-100
Japan Society of Maintenology

Environmental Fatigue Evaluation in PLM Activities of PWR plant*
Takao NAKAMURA 1,† and Shoichi MIYAMA 2
1

Osaka University, 2-1 Yamadaoka, Suita-city, Osaka 565-0871, JapanThe Kansai Electric Power Co., Inc. 8 Yokota, 13 Go-shi, Mihama-cho, Mikata-gun, Fukui,919-1141 Japan

2

ABSTRACT
Numbers of nuclear power plants have been operating for more than 30 years and some of them exceeding 40
years in Japan. In these ageing plants, fatigue evaluation has a significant measure in assuring the plant reliability.
The environmental fatigue in light water reactors, which was first recognized in the 1980’s in Japan, has beendrawing attention worldwide because it may cause remarkable effects on the life of important components. The
Japanese nuclear industries developed the world’s first practical methodology to evaluate environmental fatigue
and it has been utilized in plant life management (PLM) technical evaluation. The Japan Society of MechanicalEngineers (JSME) issued the codes regarding environmental fatigue, JSME S NF1-2006, and has redrafted themincorporating the latest knowledge in 2009. This paper introduces a few examples of environmental fatigue
evaluation, which was performed using the JSME codes as a part of PLM evaluation for Japanese PWR plant.
KEYWORDS

ARTICLE INFORMATION

Environmental Fatigue, Environmental Fatigue Life Correction Factor, Fen,
JSME Codes, Modified Rate Approach Method, Plant Life Management

Article history:
Received 18 May 2010
Accepted 31August 2010

1. Introduction
Over 10 years have passed since the first plant life management (PLM) technical evaluation(hereafter called PLM evaluation)i was implemented for a nuclear power plant which had marked its
30th year of operation. Nineteen plants have implemented the 30th year PLM evaluation in Japan andrecently the 40th year PLM evaluations were conducted for two plants.
Fatigue evaluation has significant meaning in assuring the integrity of ageing plant components.
The fact that the fatigue life may be reduced in the environment simulating light water reactor coolingwater was first found in Japan in the 1980’s. Then, the phenomenon has attracted world attention as an
event which could cause significant effects on the life of important components. The Japanese nuclear
industry developed the world’s first practical evaluation method for environmental fatigue ii and has
applied it to the PLM evaluation.
The Japan Society of Mechanical Engineers (JSME) issued the world first consensus code
regarding environmental fatigue evaluation method for Nuclear Power Plants, JSME S NF1-2006 [1].
Considering the latest knowledge, JSME has redrafted the code and issued 2009 version. This paper
introduces several examples of environmental fatigue evaluation based on the JSME Code, which wasincorporated in the PLM technical evaluation for a PWR plant.
*†

Japanese Original: Maintenology, Vol.9 No.2 (2010), pp.30-38 (Received 29 January 2010).
Corresponding author, E-mail: nakamura@see.eng.osaka-u.ac.jpiPlant Life Management Technical Evaluation (PLM evaluation): Technical evaluations including fatigue
evaluation, conducted to determine the effects of assumed aging phenomena which may occur in the structures,
systems and components constituting a nuclear power plant on the safety functions to contribute to safe andlong-term plant operations. The Rules on the Installation and Operation of Commercial Nuclear Power
Generating Facilities required nuclear facilities operating more than 30 years to implement technical evaluationsof ageing phenomena and to take appropriate maintenance actions to address them.

ii

Environmental fatigue: a sort of corrosion fatigue. In this paper, it refers to a reduced fatigue life of component
material under hot high-purity water condition, such as reactor coolant.

ISSN-1883-9894/10 © 2010 – JSM and the authors. All rights reserved.

82

T. Nakamura et al./ Environmental Fatigue Evaluationin PLM Activities of PWR plant

2. Outline of environmental fatigue evaluation methods
2.1. Objectives of environmental fatigue evaluation
The environmental fatigue evaluation was first introduced into the PLM technical evaluation forJapan’s light water reactors as new technical knowledge by referring to the U.S. NRC NUREG
reports[2] and results of domestic research projects.
Subsequently, the database containing the results of domestic tests was developed and thenAgency of Natural Resources and Energy, then Ministry of International Trade and Industry issued
the “Guidelines for the Evaluation of Environmental Fatigue Life”[3] in 2000 (hereafter called “MITI
Guideline”) and required the electric utilities to follow the Guideline. Since then, the utilities havebeen required to implement the environmental fatigue evaluation during their PLM evaluation inaccordance with MITI Guideline. On the other hand, Thermal and Nuclear Power Engineering Society(TENPES) organized a committee to issue practical environmental fatigue evaluation methods
utilizing MITI Guideline. This is the “Guidelines of Environmental Fatigue Evaluation for LWR”[4](hereafter called “TENPES Guidelines”, which is known as the world first practical guideline. By
incorporating the latest technical knowledge and operating experience, Japan Society of MechanicalEngineers (JSME) issued the codes regarding environmental fatigue, NF1-2006 in March 2006.
The JSME codes were referred to Atomic Energy Society of Japan’s “Code on Implementation
and Review of Nuclear Power Plant Ageing Management Programs”[5], which has been applied tooperating plants for the ageing management evaluation.
JSME’s “Code for Nuclear Power Generation Facilities, Rules on Design and Construction for
Nuclear Power Plants”[6] requires the implementation of fatigue evaluation for Class 1 components
during the design stage.
The evaluation according to JSME’s Code is intended to confirm that there are sufficient design
margins against fatigue failure throughout the service period. The fatigue evaluation is conductedassuming a 40 years of operation and setting the conditions with sufficient design margins against theassumed evaluation period and design loads.
On the other hand, the fatigue evaluation conducted as a part of PLM evaluation is intended toconfirm the degree of margins against fatigue from the viewpoint of ageing and incorporate theevaluation result into long-term maintenance programs. Considering the objective of the fatigue
evaluation, a 60 years of operation is assumed and realistic conditions based on the actual operating
experience are set considering environmental effects as well.
2.2. Characteristics of environmental fatigue evaluation methods
The environmental fatigue evaluation methods used in the PLM technical evaluation have major
three features as shown below[7],[8]:1) Environmental fatigue life correction factor, Fen is adopted as a measure to provide optimized
correction of environmental effects.
2) To reduce the workload of evaluation, three options to calculate Fen are provided, from acomplicated but refined and optimized method to a conservative but simplified method.
3) The stress analysis method specific to each component described in JSME’s Rules on Design
and Construction for Nuclear Power Plants is considered and the procedure to evaluate strains
rate applicable to each stress analysis method is developed so that the environmental fatigue
evaluation can be performed for all the components subject to the evaluation.
2.3. Environmental fatigue life correction factor
The environmental fatigue life correction factor Fen is defined as the value determined by
dividing the fatigue life in air, NA at a certain level of strain amplitude by the fatigue life in the reactor
cooling water environment, Nw at the same level of strain amplitude (see Fig. 1).

83

T. Nakamura et al./ Environmental Fatigue Evaluationin PLM Activities of PWR plant

Fen =

NA
NW

(1)

Strain amplitude

Ｓ－Ｎ curve（Design fatigue curve）
Current design curve(ASME & JSME)
in air

１／Ｆｅｎ

In reactor cooling water

Life（Cycles）
Fig. 1. Environmental fatigue life correction factor(Fen)

The cumulative usage factor, Uen considering environmental effects can be calculated by using
Fen as shown in the equation below:nUen = U × Fen = ∑ Ui × Fen , i

(2)

i =1

Where, Ui is the usage factor without considering environmental effects at the ith stress cycle
among total n stress cycles and Fen , i is the environmental fatigue life correction factor at the ith stresscycle among total n stress cycles.
Fen is a factor which depends on the strain rate, temperature, dissolved oxygen concentration andother parameters. Therefore, Fen can be obtained if these parameters are determined. However, Fenvalues do not depend on the strain amplitude. Since no environmental effect is seen in small strain
amplitudes, a lower threshold of Fen is provided for strain amplitudes.
The method to calculate the cumulative usage factor will be explained in Section 3.
2.4. Three options to calculate environmental fatigue life
To perform an optimized environmental fatigue evaluation and set the time segments for theevaluation, three types of evaluation methods are established.
1) Factor multiplication method: A conservative evaluation is performed according to the design
conditions (temperature, dissolved oxygen concentration, etc.) without defining the segments
to be evaluated for individual transients.
2) Simplified method: An evaluation is performed by considering the range where strains
continuously increase during a transient as a single segment.
3) Detailed method: An evaluation is performed by dividing the range where strains continuously
increase during a transient into incremental segments (refer to the modified rate approach
method shown in Fig.2).
2.5. Modified rate approach method
As shown in Fig. 2, the modified rate approach method calculates Fen during a transient bydividing the time history of strain, temperature and dissolved oxygen during a transient into84

T. Nakamura et al./ Environmental Fatigue Evaluationin PLM Activities of PWR plantincremental segments and then calculating environmental fatigue life correction factor Fen according
to the segment followed by the integration of all the calculated environmental fatigue life correction
factors weighted with strain gains to obtain the total environmental effects. This method is a refined
approach which can evaluate environmental effects in the most optimized way. Since it takes a lot oflabor, however, it is necessary to study efficient ways of applying the evaluation method.
Maximum total stress intensity

εε max,A30

Δε k

Transient A

ε min,A

Δt k

t

0

0

Fig. 2. Modified rate approach methodm

Fen , det, A = ∑ Fen , k
k =1

30

Δεk
ε max, A − ε min, A(3)

Where Fen,det,A: environmental fatigue life correction factor for transient A obtained by using themodified rate approach method
Fen,k: environmental fatigue life correction factor in the kth segment among total m-number segments
Δεk: incremental strain in the kth segment among total m-number segments

3. Environmental fatigue evaluation as a part of PLM technical evaluation
3.1. Flow of fatigue evaluation
The flow of environmental fatigue evaluation in PLM technical evaluation is shown in Fig. 3.
This section explains the procedure of environmental fatigue evaluation conducted as a part of PLM
technical evaluation according to the flow.
1) Selection of components and locations subject to the evaluation
In the fatigue evaluation of PLM evaluation, components subject to the fatigue evaluation are
first selected. The components subject to the evaluation include those for which the fatigue evaluation
during operation although the code does not require the fatigue evaluation.
Among the components subject to the fatigue evaluation as part of PLM evaluation, those
exposed to reactor water will be selected for the environmental fatigue evaluation. Several
components subject to the fatigue evaluation are shown in Table 1.

85

T. Nakamura et al./ Environmental Fatigue Evaluationin PLM Activities of PWR plantFig. 3. Flow of environmental fatigue evaluation in PLM
Table 1 Selection of components in fatigue evaluation86

T. Nakamura et al./ Environmental Fatigue Evaluationin PLM Activities of PWR plant

Next, the locations to be evaluated will be selected. The Design and Construction Rule requires
the fatigue evaluation for all the components subject to the fatigue evaluation excluding the cases inwhich significant fluctuating loads do not act. On the other hand, in the fatigue evaluation of PLM
evaluation, the locations for which severe results of fatigue evaluation are expected with possibilitiesof structural discontinuities, thermal shock and thermal stratification will be selected.
2) Setting transient conditions
In performing the fatigue evaluation, transient conditions are set as the conditions to be used instress analysis (i.e., fatigue analysis).
The transient conditions are set considering the history of pressure and temperature (degrees andnumbers of changes) which are assumed under all the operating conditions subject to the evaluation
based on JSME’s Rules on Design and Construction.
Table 2 shows the comparison of evaluation conditions between the design phase and PLM
technical evaluation. Compared with the conditions used in the evaluation during the design phase,the PLM evaluation adopts realistic conditions in terms of the number of transients considering
operating experience while the same conservative transient curve as that used in the design phasebeing adopted.
The number of transients used in PLM evaluation is set estimating the number of transients
expected to occur during 60-year operating period considering the past operating records. However,for a transient which has never occurred at operating plants, the number is assumed to be once per 60
years. An example of setting transients is shown in Fig. 4.
Table 2 Comparison of evaluation conditions between design and PLM
Conditions for Evaluation
Transient Curve(Variations in temperature,pressure, etc.)

Number of Transients

OthersDesignDegree of variations is determined
to be sufficiently severe.
-Evaluation period : 40 years.
-The frequency of transients is set
with sufficient margin considered.

−−−

PLM
Design transient condition is usedin principle
if not available, such a condition
should be developed-Evaluation period : 60 years.
-Assumed based on operating
experience at the time ofevaluation.
Such events not required by
JSME’s Design & Construction
rules as thermal stratification is
also considered.

Fig. 4. Method to determine numbers of transients in PLM

87

T. Nakamura et al./ Environmental Fatigue Evaluationin PLM Activities of PWR plant3) Calculation of temperature and strain histories for components
To evaluate the cumulative usage factor, it is necessary to calculate the temperature and stress(strain) histories first on the concerned locations of a component subject to the evaluation in terms ofevery transient (refer to Fig. 5).

Fig. 5. Calculation of histories in temperature and generated stresses(strains) in each transient4) Calculation of peak stress intensity
Considering the history of stresses in each transient, the peak stress intensity is calculated for the
stress cycles using the equation (4).

Sa1 = ( A max − B min) / 2
Sa 2 = ( C max − B min) / 2(4)

Sa: Peak stress intensity representing a half of the difference between the maximum and minimum
stress values in a stress cycle

Fig. 6. Calculation of peak stress intensity for the stress cycles

88

T. Nakamura et al./ Environmental Fatigue Evaluationin PLM Activities of PWR plant5) Calculation of cumulative usage factor
The number of allowable cycles in terms of the peak stress intensity N* is determined byreferring to the design fatigue curve specified in the Design and Construction Rule, and then the ratioof N* to the number of actual transient cycles N is determined. The usage factor for each stress cycle iscalculated by using the equation (5) to confirm that the summed cumulative usage factor (U) does notexceed 1 (refer to Fig. 7).

Fig. 7. Calculation of usage factor (U)U=N
N1 N 2N3Nn= * + * + * +L+ * < 1*N
N1 N 2
N3
Nn(5)

The environmental fatigue evaluation methods will be referred to in the following section bytaking some components as an example.
3.2. Codes and standards applicable to environmental fatigue evaluation
When Japanese plants first introduced environmental fatigue evaluation in PLM evaluation, there
was no applicable code or standard and therefore the evaluation was performed by referring to theknowledge obtained from recent research programs or NUREG reports.
Since then, the database containing the results of domestic fatigue tests has been developed.
Meanwhile, MITI Guideline and TENPES Guidelines have been established and applied to theenvironmental fatigue evaluation.
More recently, The Japan Society of Mechanical Engineers (JSME) issued the code regarding
environmental fatigue evaluation method for Nuclear Power Plants, JSME S NF1-2006, which iscurrently applied to the environmental fatigue evaluation in lieu of MITI Guideline.
TENPES Guidelines and JSME’s Code on environmental fatigue evaluation have common
characteristics described in Section 2.2. However, a major difference between these guidelines is that
JSME’s code specifies the new equations to calculate environmental fatigue life correction factor(Fen) based on the database containing the results of recent domestic fatigue tests.
In this section, the equations to calculate Fen for austenitic stainless steel material which is ofsignificance in the fatigue evaluation under the PWR plant environmental conditions, which aredescribed in TENPES Guidelines and JSME Codes, are compared in Table 3 and Fig. 8.
In JSME’s Codes, data on lower saturated strain rates are added and the threshold for caststainless steel has been modified as shown in Fig. 8. The equation to calculate Fen under the PWR
plant environment remains the same in JSME Codes 2009 as that in the 2006 version.
On the other hand, the U.S. NRC issued NUREG/CR-6909[9] in 2007 summarizing the research
results implemented by Argonne National Laboratory. Based on the NUREG report, an evaluation
method using an environmental fatigue life correction factor (Fen) for the newly constructed plants
was shown in Regulatory Guide 1.207[10].

89T. Nakamura et al./ Environmental Fatigue Evaluationin PLM Activities of PWR plant

ASME Code Committee addressed the environmental fatigue evaluation method as an ASME
Sec. III Code Case. The committee is discussing the design fatigue curve considering environmental
effects and the criteria to be applied when the cumulative usage factor U exceeds 1 as well as theevaluation method using an environmental fatigue life correction factor (Fen).
Table 3 Comparison of Fen equation between TENPES guidelines and JSME codes
in austenitic stainless steel

Fig. 8. Comparison of Fen between TENPES guidelines and JSME codes in cast stainless steel

90

T. Nakamura et al./ Environmental Fatigue Evaluationin PLM Activities of PWR plant

3.3. Evaluation of vessels
As an example of evaluation of vessels, the result of environmental fatigue evaluation for thepressurizer conducted as part of PLM evaluation by JSME codes is summarized below:
1) Selection of locations subject to the evaluation
In the evaluation during the design phase, nozzles and skirt welds are selected. For the PLM
evaluation, spray line nozzles and surge line nozzles where a relatively high level of thermal stressoccurs due to the actuation of spray and the flow in the surge line are selected. In addition, thelocations in contact with reactor cooling water and for which the cumulative usage factor U becomesthe largest are selected as the target of environmental fatigue evaluation (refer to Fig. 9).

Fig. 9. Selection of components and locations in fatigue evaluation

2) Setting transient conditions
The estimated number of transients after going through a 60 years of operation is used for theevaluation. Since thermal stratification appears and disappears in the pressurizer following theactuation of spray or flow in the surge line, stresses generated from thermal stratification, which are
not included in the design evaluation, are considered in the environmental fatigue evaluation as part ofPLM evaluation.
Based on these conditions for the evaluation, the histories of temperature and stress arecalculated using FEM models.

91

T. Nakamura et al./ Environmental Fatigue Evaluationin PLM Activities of PWR plant3) Calculation of environmental fatigue life correction factor and usage factor
To make a stress cycle, transients with larger stresses are selected in descending order. Usagefactors are calculated for the stress cycle, and then the usage factor is multiplied with anenvironmental fatigue life correction factor (Fen) to calculate the usage factor in environment (Uen).
The calculated usage factors for the pressurizer surge line nozzle by JSME codes are shown in Fig.
10.
Fig. 11 illustrates an example of the calculation of environmental fatigue life correction factor.
- Examples of Calculation (Pressurizer Surge Line Nozzle)
Stress Cycle

Transient 1

Peak Stress
Intensity

Number ofTransients

Allowable
CyclicNumberSa(N/mm)

N

Transient 2

N*

Usage
Factor by
DesignCurve
U

Fatigue
Life
Correction
FactorFen

Usage
FactorinEnvironment
UFen

D

F

345.5

1

33500

0.000038.203

0.00024

C

D

271.1

1

1220000.00001

11.3010.00009

C

A

232.1

3

3020000.00001

12.7370.00013

E

B

189.0

1

-

0.00000

1.000

0.00000

-

-

-

-

-

-

-

-

Operating Mode I
A: Large stepwise load reduction from 100% power
B:one loop shutdown / one loop start-up
Operating Mode II
C:Loss of offsite power
D:Abnormal depressurization of primary cooling system
E: False ECCS start-up during power operation
F: False start-up of one reactor coolant pump

Fig. 10. Uen calculation results in pressurizer surge line nozzle (Example of vessel evaluation)

92

T. Nakamura et al./ Environmental Fatigue Evaluationin PLM Activities of PWR plant

TransientNo. ofIncremental
segment

Increase
In strain
(%)

D

36

0.021

D
D

37

Time
(sec)

Strain rate(%/sec)Temperature
(°C)

Fen k

40

0.000530

252.6

9.605

0.028

50

0.000567

254.1

9.604

38

0.034

100

0.000335

255.2

10.399

F
F

2

0.028

1

0.0277

280.5

5.176

3

0.015

1

0.0146

274.1

5.720

F

4

0.009

1

0.0091

271.0

6.206

F

5

0.006

1

0.0058

269.1

6.729

F

6

0.005

2

0.0027

267.7

7.840

F

7

0.003

3

0.0009

266.0

9.628

F

8

0.001

2

0.0003264.4

11.315Fen

8.203

Fig. 11. Example of Fen calculation with modified rate approach method

93

T. Nakamura et al./ Environmental Fatigue Evaluationin PLM Activities of PWR plant

To calculate the environmental fatigue life correction factor Fen, the modified rate approach
method, that is a detailed evaluation method, is used. The procedure of calculating Fen when transients
D and F are combined is explained below.
In transient D, strain changes in case of spurious open of pressurizer relief valve, which is theseverest event of primary cooling system depressurization, are analyzed. Reactor trip occurs following
the event, which results in temporary outsurge in the pressurizer. When outsurge turns into insurge, alarge change in strain occurs. Subsequently, temperature and pressure becomes lower as the primarycooling system is cooled down. When the cause that depressurizes primary cooling system are
eliminated by operator’s actions 10 minutes later, the surge becomes zero and strain turn into a large
negative value temporarily followed by an increase in strain while the plant condition is stabilized.
In transient F, a RCP out of service starts inadvertently, which causes the injection of cold
coolant in the shutdown loop into the core. This event causes the insertion of positive reactivity,
which results in increases in the reactor power and primary cooling water temperature followed byinsurge in the pressurizer leading to an increased strain. Subsequently, reactor trip occurs, whichcauses outsurge in the pressurizer, resulting in a rapid decline of strain.
The time history in strain in transients D and F are calculated and divided into small segments.
For the sections within the range of strain increase in which maximum and minimum peak stressintensities occur, Fen, k for individual segments is weighing integrated with its strain value to calculate
Fen. The calculated Fen is multiplied with the usage factor U obtained from the cyclic peak stressintensity, which is calculated by combining the maximum and minimum peak stress intensities toderive the environmental usage factor Uen.
4) Results of environmental fatigue evaluation
Cumulative usage factor is calculated by summing usage factor with environmental effect in all
stress cycles.
Fig. 12 illustrates the result of environmental fatigue evaluation for the pressurizer.

Cumulative Usage Factor
Location

Fatigue Evaluation
by Design Curve
U

Environmental
Fatigue Evaluation
UFen

Spray Line Nozzle

0.015

0.548

Surge Line Nozzle

0.001

0.001

Fig. 12. Example of calculation results of cumulative usage factor in pressurizer

94

T. Nakamura et al./ Environmental Fatigue Evaluationin PLM Activities of PWR plant

3.4. Evaluation of piping
As an example of evaluation of piping, the environmental fatigue evaluation by JSME codes forthe pressurizer surge line piping conducted as part of PLM technical evaluation is summarized below:
1) Selection of locations subject to the evaluation
The entire surge line piping is subject to FEM analysis. The locations showing a large stresschange and in contact with reactor cooling water, are selected as the target of evaluation.
2) Setting transient conditions
Like the evaluation of vessels, the estimated number of transients after going through a 60 yearsof operation is used for the evaluation. Since thermal stratification appears and disappears in thepressurizer surge line piping following the flow in the surge line, stresses generated from thermal
stratification, which are not included in the design evaluation, are considered in the environmental
fatigue evaluation as part of the PLM evaluation (refer to Fig. 13).

Fig. 13. Determination of conditions for evaluation

Based on these conditions for the evaluation, the histories of temperature and stress arecalculated using FEM models.
Since the stress in piping is normally calculated from the absolute sum of stresses in individual
terms used in the equation (6), the history of strains in terms of all the stresses to be used in
calculating Fen is not available. Therefore, the histories of strain rate and temperatures are calculated
by focusing on the dominant term of stress according to each stress cycle.

Sp =Eα ΔT 2
K 1C1P 0 D 0 K 2C 2 Mis K 3 Eα ΔT 1++
+ K 3C 3 Eab αaTa − αbTb +
2t
Zi
1.4
0.7
Thermalexpansionterm

ΔT1 Term

Ta-Tb Term

(6)

ΔT2 Term

Refer to JSME’s Code for Nuclear Power Generation Facilities, Rules on Design andConstruction of Nuclear Power Plants, Volume I, Section 5 PPB-3532 for the definition of symbols
used in the equation (6).
3) Calculation of environmental fatigue life correction factor and cumulative usage factor
Tables 4 and 5 shows the methods of calculating the environmental fatigue life correction
factor (Fen) and cumulative usage factor (Uen) for pressurizer surge line piping by JSME codes.

95

T. Nakamura et al./ Environmental Fatigue Evaluationin PLM Activities of PWR plant
Table 4 Example of determination method in strain rate and temperature for pressurizer surge line piping
Predominant Stress Term

Strain Rate

Thermal expansion term

Temperature

Value at start-up(slow and conservative)

325 ºC(Temperature parameter of Fen T*
at 325 ºC or more is constant.)

ΔT1 Term

Calculated from time history ofstrain obtained by ΔT1 Term

Time history of temperature at inner sideof pipe for the predominant transientsamong the transient combinations

Ta-Tb Term

Calculated from time history ofstrain obtained by Ta-Tb Term

Time history of temperature at inner sideof pipe for the predominant transientsamong the transient combinations

ΔT2 Term

Calculated from time history ofstrain obtained by ΔT2 Term

Time history of temperature at inner sideof pipe for the predominant transientsamong the transient combinations

Thermal Stratification
Stress (calculated with
FEM model)

Calculated from time history ofstrain obtained by thermal
stratification stressMaximum temperature of the transientcombinations

Table 5 Example of calculation results in Fen and Uen for pressurizer surge line piping
Stress Cycle
Transient1

PredominantStress Term

Peak
StressIntensity

Transient
2

U

Fatigue
Life
Correction
FactorFen

Usage Factor
withEnvironmental
EffectUen

NumberofTransient

Allowable
CyclicNumberUsage
FactorIn Air

N

N*

Sa
(MPa)
A

A

Thermalstratification

639.2

1

1550

0.00065

12.9010.00832

B

B

Thermalstratification

457.3

1

5340

0.000199.785

0.00183

C

C

Thermalexpansion

264.2

1

72800

0.00001

11.9320.00016

･

･

･

･

･

･

･

･

･

A: Reactor trip from 100% rated power(accompanied by inadvertent cooling and safety injection)

B: False ECCS start-up during power operation
C: False start-up of one reactor coolant pump
Location

Pressurizer
Line Piping

Surge

Cumulative Usage Factor
Fatigue Evaluation
By Design Curve

Environmental
Fatigue Evaluation

0.003

0.013

96

T. Nakamura et al./ Environmental Fatigue Evaluationin PLM Activities of PWR plant

3.5. Evaluation of pumps
The evaluation of pumps by JSME codes is illustrated below by taking reactor coolant pumps(RCP) as an example.
1) Selection of locations subject to the evaluation
Although pumps are not required in the fatigue evaluation specified by JSME’s Rule on Design
and Construction of Nuclear Power Plants, fatigue evaluation is performed as part of PLM evaluation
for pumps subject to remarkable transients. In addition, the locations which are in contact with reactor
water are selected for the environmental fatigue evaluation.
Fig. 14 illustrates the locations in RCP, which are fabricated from cast stainless steel andexpected to show a severe evaluation result for environmental fatigue.
2) Setting transient conditions
Like the evaluation of vessels, the estimated number of transients after going through a 60 yearsof operation is used for the evaluation.
3) Calculation of environmental fatigue life correction factor and cumulative usage factors
Based on these conditions for the evaluation, the histories of temperature and stress arecalculated using FEM models.
Fig. 15 illustrates the method to calculate the environmental fatigue life correction factor Fen and
Table 6 shows the result of fatigue evaluation by JSME codes.

Fig. 14. Location selected in the fatigue evaluation of RCP

97

T. Nakamura et al./ Environmental Fatigue Evaluationin PLM Activities of PWR plantFig. 15. Example of Fen calculation in RCP outlet nozzle
Table 6 Example of Uen calculation in RCP outlet nozzle

98

T. Nakamura et al./ Environmental Fatigue Evaluationin PLM Activities of PWR plant

3.6. Example of environmental fatigue evaluation for PWR plant
Table 7 shows the results of environmental fatigue evaluation implemented as part of PLM
evaluation for a representative PWR plant by JSME codes.

As shown in this table, all the results of environmental fatigue evaluation for the
representative components meet the allowable limits. All the results are obtained from theevaluation using the detailed method. For the locations not showing a severe result, however,a more simplified method is applicable to enhance the efficiency of the evaluation. JSME’s
environmental fatigue evaluation methods offer a practical approach to perform efficientevaluation of all the components subject to the evaluation in conducting the PLM evaluation
for an operating plant.
Table 7 Example of environmental fatigue evaluation results in typical PWR plant- Results of PLM Evaluation on Typical PWR plant
Component
Reactor Vessel
Pressurizer
Main Coolant Pipe

RCS Piping
Loop Residual Heat Removal
No.1 Inlet Valve(Gate Valve)
Letdown Shut-off Valve(Ball Valve)
Reactor Coolant Pump
RHR Pump

Cumulative
Usage Factor by
Design Curve(Note 1)
0.000
0.015
0.001
0.001

Cumulative
Usage Factor withEnvironmental
Effects
0.000
0.548 (Note 2)
0.001
0.001

0.033

0.147

0.002

0.013 (Note 2)

0.080

0.410 (Note 2)

Body

0.002

0.058

Body

0.018

0.225

Locations Subject
to Evaluation
Coolant Outlet Nozzle
Spray Line Nozzle
Surge Line Nozzle
Crossover Leg
Pressurizer Surge Line
NozzlePressurizer Surge Line
PipingPressurizer Spray Line
PipingOutlet Nozzle
0.043
0.807
Outlet Nozzle
0.003
0.012
Lower Core Support
0.010
0.111
Core Support Structures
Plate
Feedwater Inlet Nozzle
0.002
0.354 (Note 2)
Steam Generator
Around Tube Plate
0.080
0.180
RHR Heat Exchanger
Tube Plate
0.126
0.443
(Note 1) These values represent those obtained for the locations subject to the environmental fatigue evaluation
as shown in the right column although the PLM report describes the maximum value including
locations in contact with reactor cooling water.
(Note 2) These values show analytical results including stresses due to thermal stratification.

3. Conclusion
This paper discussed the environmental fatigue evaluation implemented for an operating plant
and the evaluation methods utilized.
The environmental fatigue evaluation is implemented as part of the PLM technical evaluationaccording to JSME’s Environmental Fatigue Evaluation Method JSME-S-NF1-2006.
The result of the environmental fatigue evaluation has revealed that the cumulative usage factor

99

T. Nakamura et al./ Environmental Fatigue Evaluationin PLM Activities of PWR plantfor important components at an operating plant does not exceed unity even if a 60-year of operation isassumed and that no feedback from the fatigue evaluation should be taken into long-term maintenanceprograms. Since the evaluation was conducted using conservative design transient conditions, it isexpected that a lower cumulative usage factor will be obtained under actual operating conditions.
Considering the above results, it is concluded that the integrity of Japanese PWR plantcomponents can be assured against fatigue by ageing even if environmental effects are considered. In
addition, the applicability and usefulness of JSME’s environmental fatigue evaluation method in PLM
evaluation for operating plants has been demonstrated.
On the other hand, the application of JSME’s environmental fatigue evaluation method in thedesign phase results in more severe results. Then further improvement and optimization of evaluation
methods and conditions are necessary[11],[12]. In this respect, JSME Main committee on Power
Generation Facilities Codes, Subcommittee on Nuclear Power, Task Group on Fatigue Evaluation is
working on the improvement of the evaluation methods and conditions.

Acknowledgement
I would like to express my sincere gratitude and respect to those who have been making anutmost effort to develop the environmental fatigue evaluation methods and apply the methods tooperating plants, represented by the members of JSME Main committee on Power Generation
Facilities Codes, Subcommittee on Nuclear Power, Task Group on Fatigue Evaluation and Subgroupon Environmental Fatigue.

References[1] JSME Codes for Nuclear Power Generation Facilities, Environmental Fatigue Evaluation Method (2006)(JSME S NF1-2006), issued in March 2006.
[2] The reports released by the U.S. Nuclear Regulatory Committee (NRC), in particular reports summarizing
the research results implemented by external organizations under the contract with NRC, marked with
NUREG/CR- numbers, including NURG/CR-6583(1998), 5704 (1999), 6909 (2007) .
[3] Guidelines of Environmental Fatigue Evaluation (Sept. 2000): 2000, Agency of National Resources andEnergy , Ministry of International Trade and Industry, (in Japanese).
[4] Guidelines of Environmental Fatigue Evaluation for LWR, Thermal and Nuclear Power Engineering Society,2002.
[5] Code on Implementation and Review of Nuclear Power Plant Ageing Management Programs， Atomic
Energy Society of Japan (AESJ-SC-P005:2008), issued in February 2009, (in Japanese).
[6] Code for Nuclear Power Generation Facilities, Rules on Design and Construction for Nuclear Power Plants(2008 version) (JSME S NC1-2008), issued in 2008, (in Japanese).
[7] T.Nakamura, Y.sugie, Development of Voluntary Consensus Codes and Role of Code Engineers: Example ofEnvironmental Fatigue Evaluation Method for Light Water Reactors, Journal of Atomic Energy Society ofJapan, Vol. 9, No.1, (2010), pp1-12,,(in Japanese).
[8] M.Higuchi, T.Nakamura, Y.sugie, Development of Environmental Fatigue Evaluation Method for Nuclear
Power Plants in JSME Code, Journal of Japan Society of Mechanical Engineers, Vol. 76, No.762, (2010), (inJapanese).
[9] O.K.Chopra, "Effect of LWR Coolant Environments on the Fatigue Life of Reactor Materials,"
NUREG/CR-6909 (ANL-06/08), U.S.NRC, 2007.
[10] Regulatory Guide 1.207, "Guidelines for Evaluating Fatigue Analyses Incorporating the Life Reduction ofMetal Components Due to the Effects on the Light-Water Environment in New Reactors", U.S.NRC, 2007.
[11] T. Nakamura, M. Iwasaki, S. Asada, Optimization of Environmental Fatigue Evaluation (Step-1)
PVP2007-26247, 2007ASME PVP Conference, San Antonio, Texas, USA, July 2007.
[12] Y. Nomura, K. Tsutsumi, T. Inoue, S. Asada, T. Nakamura, Optimization of Environmental Fatigue
Evaluation (Step-2) PVP2009-77115, 2009ASME PVP Conference, Prague, Czech Republic, July 2009.

100