E-Journal of Advanced Maintenance Vol.7-1 (2015) 66-73
Japan Society of Maintenology

Reconstruction of Grand Design for Fatigue Evaluation based on Fatigue
Failure Analysis of Japanese NPPs
Takao NAKAMURA1,*, Ryosuke FUJIKAWA1 , Mikiya MATSUSHITA1, Shigeki ABE1,
and Masayuki KAMAYA2
1
2Osaka University, 2-1 Yamadaoka, Suita, Osaka 565-0871, Japan
Institute of Nuclear Safety System, Inc., 64 Sata, Mihama-cho, Mikata-gun, Fukui, 919-1205, Japan

ABSTRACT
After Fukushima Daiichi NPP accident, the improvement of nuclear safety is highly requested in order toprevent re-occurrence of severe accident. New scheme of fatigue evaluation to confirm system safety in NPPs isenhanced to be established based on Defense in Depth concept. This study focuses on the direction toreconstruct Grand Design of fatigue evaluation to ensure system safety based on fatigue failure analysis ofJapanese NPPs. The issues of fatigue evaluation we have now is investigated in order to review Grand Designon the basis of new role of fatigue management in plant long term safe operating scheme that is required after
Fukushima daiichi NPP accident.
KEYWORDS

ARTICLE INFORMATION

defense in depth concept, environment assisted fatigue, fatigue
evaluation, fatigue failure analysis, grand design, NUCIA (nuclear
information archives), system safety, postulated crack growth curve

Article history:
Received 24 October 2014
Accepted 13 March 2015

1.

Introduction

2.

Grand Design for Environmental Fatigue Evaluation Method

The proper re-evaluation of safety countermeasures for NPP (Nuclear Power Plant) is required toprevent the recurrence of nuclear hazard after Fukushima Daiichi NPP accident occurred in March 11,
2011. Especially for the ageing plants, it is necessary to reply the demand of “nuclear safety” from thepeople not only by evaluating the degradation of materials but also by establishing the scheme which
can evaluate safety of plant total system realistically and quantitatively.
For this purpose, we need to reconstruct Grand Design of fatigue evaluation in order to develop
the method to predict reduction of safety margin resulted by long term operation in major components
quantitatively and to evaluate safety of total plant system.
This study analyzes domestic fatigue failure database registered in NUCIA (Nuclear Information
Archives) and proposes to expand the target area of Grand Design from low-cycle fatigue to all types
of fatigue failure that are postulated to occur in NPPs and constructs new scheme of Grand Design for
fatigue evaluation combining code system, maintenance program and system safety evaluation
methods based on Defense in Depth concept.
“Defense in Depth concept” is an idea to secure nuclear safety by setting multiple layers ofdefensive measures between nuclear hazards (radioactive material) and protective objects (people andenvironment) and reducing risk below acceptable level, then to achieve the purpose of nuclear safety
and to protect people and environment from nuclear hazards.[1]Reduction of fatigue life in light water reactor environment was first recognized in Japan andreported to the world nearly 30 years ago. Since then, the evaluation method used for fatigue
management in ageing plants has been progressed and published as “Environmental Fatigue
Evaluation Method for Nuclear Power plants (JSME S NF1-2009)”[2]. We have worked for about ten
years discussing Grand Design to establish environmental fatigue evaluation method. The purpose ofGrand Design is to clarify the total scheme to develop evaluation methods (codes system) and to draw
a road map of research and development.
The past activities in Grand Design of environmental fatigue [3] is to introduce environmental*

Corresponding author, E-mail: nakamura@see.eng.osaka-u.ac.jp

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

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T. Nakamura, et al./ Reconstruction of Grand Design for Fatigue Evaluation
based on Fatigue Failure Analysis of Japanese NPPs

effect to conventional fatigue evaluation method for NPP i.e., ASME Sec.Ⅲ codes as a newknowledge. The requirement to prevent low cycle fatigue failure was introduced into ASME codes
nearly 50 years ago. Design fatigue curve is used to calculate usage factor (UF) and it is supposed tostart fatigue fracture after taking various factor in the actual plant into consideration when UF equal
one that is calculated by design curve. In Japan, further improvement of evaluation method is
considered to be necessary to introduce environmental fatigue evaluation in design phase.
Furthermore we start to work for introducing ASME Sec Ⅺ Appendix L [4] which enables plants tocontinue operation even if UF exceeds one, by evaluating the assumed flaw which has a margin of
size in the detection limit of inspection and does not result in penetration or unstable fracture. Thismeans to introduce fatigue management into operation and is said to be the activities of new
framework to prevent fatigue failure.

3.

New Scheme of Fatigue Management

4.

Analysis of Fatigue Failure Database in Japanese NPPs

The Fukushima Daiichi NPP accident showed that the past safety design based on postulated
events was not sufficient. In conventional fatigue design, fatigue failure in the pressure boundary will
not happen if the design transient conditions which have sufficient margin are given in the assumed
period of operation and the fatigue design which is based on enough margins is performed.
In the new scheme, it is required to clarify the margin from the situation which loses safety of
total system of NPP quantitatively by predicting fatigue degradation of material, considering
maintenance, and performing overlapping evaluation of other phenomena even if beyond design event
occurs. It is necessary to develop evaluation method which can predict accumulation of fatigue
damage as the plant is ageing in addition to conventional fatigue evaluation. To do this, it is necessary
to evaluate the margin to fatigue failure realistically in addition to the evaluation based on UF. As
mentioned above, the situation where UF is equal to one is equivalent to the starting point of fatigue
failure, but it is said that fatigue crack development consists of two stages, crack initiation and crack
propagation. Most of fatigue life is related to crack propagation from about 10μm to 3mm.Therefore,
if crack propagation can be quantified, the fatigue life to failure may be predicted [5],[6],[7].
It becomes possible to introduce the role of inspection and maintenance to fatigue life
management by quantifying crack propagation. ASME Sec. III codes now work to extend and apply
crack propagation evaluation to a design phase. This approach may intend to change design
philosophy and implicitly permit existence of a flaw under detection limit in design phase. Kamaya,
et.al.[6] propose postulated crack growth curve to apply this crack propagation evaluation from crack
initiation to fatigue failure for all the plant life. So, it becomes possible to utilize the prediction modelof fatigue crack propagation as input of the maintenance program and system safety evaluation of
NPP and to superpose other damage factor on fatigue evaluation [8],[9],[11],[12]. Postulated crack growth
curve intends to reinforce maintenance program and introduce system safety evaluation based on theapplication of crack growth model, but it doesn’t allow crack initiation in design phase.
4.1 Purpose and procedure of analysis
The purpose of analysis is to expand the scope of fatigue failure in Grand Design from low-cycle
fatigue to all kinds of fatigue failure. The fatigue failures occurred in domestic NPPs are extracted
from failure database registered in NUCIA and analyzed the relation of fatigue mechanism, type ofcomponents, significance of safety functions, and duration of operation, etc., then the analyticalresults are summarized based on types of components and mechanism of fatigue failure[10].The
summary of analytical results is shown in Table.1.
4.2 Ratio of Fatigue Failure Occurrence classified by Mechanism and Types of Component
Almost 220 fatigue failures are extracted from NUCIA database. (Target period is since 1966 tothe end of March, 2012) About 80% of total failures were caused by fluid induced vibration fatigue
and mechanical vibration fatigue. Classified failures according to type of components, they become inorder of piping, vessels, pumps, and valves as shown in Fig.2. The summary of analytical results inTable 1 indicates that significant components (classes 1 and 2) except these four types of componentsthat affect plant safety are emergency diesel generator and instrument air compressor.

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T. Nakamura, et al./ Reconstruction of Grand Design for Fatigue Evaluation
based on Fatigue Failure Analysis of Japanese NPPs

4.3 Fluid Induced Vibration Fatigue failure
Among fatigue failures, fluid induced vibration fatigue failures were the largest in number ofoccurrence. The components that occurred failures are classified to many types as shown in Fig.3. But
almost all of them belong to class 3 and other components that are less significant in safety function
as shown in Fig.4. The occurrences in significant components of safety functions are limited to pumps,
piping and valves as shown in Table 1.
4.4 Mechanical Vibration Fatigue Failure
The second largest fatigue failures in the number of occurrence were mechanical vibration
fatigue failures. Almost all of them belong to piping, pumps, valves and no failure occurs in vessels asshown in Fig.5. The occurrence of mechanical vibration fatigue failure in significant components
are limited to piping, pumps and valves as shown in Fig.5 and 6.
Table 1. Summary of fatigue failure analysis in Japanese NPPs
Component

Fluid vibration
fatigue

Mechanicalvibration
fatigue

Fretting

High-cycle
Low-icycle
thermalfatiguefatigue

53(9)

×Heater, etc.

ー△SG tube△Nozzle,connection

VesselSummary

×・The failure probability of vessel by fretting and high thermal fatigue
Heateris low due to design countermeasures
tube, etc.

Piping68(23)○
Small pipe
weld○
Small pipe
weldー・All failures caused by fluid and mechanical vibration fatigue were
found by leak.
△○
Elbow(Low ・Countermeasues in design stage(Specific frequency evaluation)
Bending
melting
and monitoring of vibration, pressure perturbation and leakage ispipe,elbow,
metal
also effective.
connector
crack) ・Guidelinesfor high-cycle thermal fatigue is standardized, but farther
improvement of fatigue management is needed.

Valves21(15)○
Stem, stem
pin, etc.

○
Stem, etc.

ー△Valve sheet

×
Body

Pumps

29
(12)

○
Shaft, etc.

○
Shaft,etc.

○
Shaft

×

ー

Others47(10)

×
DG,Nozzle
deflector

×
DG,Compressor springーー×Boiler tube,
etc.

94(21)

85(29)

14(3)

20(17)

9(1)Number ofoccurrences

Fig.1

・Most of fluid and mechanical vibration failures were found by
overhall in periodic inspection.
・Countermeasues in design stage(Specific frequency evaluation) andmonitoring of vibration, etc. during operation is also effective.
・Fretting fatigue failurea were found by overhall in periodic inspection
・Reflection of technical knowledge obtained in operational experience
to design and inspection is effective.

Fig.2

Ratio of fatigue failure occurrenceclassified by fatigue mechanism

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wemarks ○:A lot of failure
ー in class1,2△:A few failure in class 1,2
×:bo failure in class 1,2－:bo failure
bumber is occurrence of failueand () is occurrence in class 1,2

Ratio of Fatigue Failure Occurrenceclassified by Damaged component

T. Nakamura, et al./ Reconstruction of Grand Design for Fatigue Evaluation
based on Fatigue Failure Analysis of Japanese NPPs

Fig.3
Ratio of Fatigue Failure classified byDamaged component in Fluid Induced Vibration

Fig.4
Ratio of Fluid Induced Vibration Fatigue
Failure classified by Significance of Safety
FunctionsFig.5
Ratio of Fatigue Failure Occurrenceclassified by Damaged component in
Mechanical Vibration

Fig.6
Ratio of Mechanical Vibration Fatigue
Failure classified by Significance of Safety
Functions

4.5 Fretting Failure
Almost all of the failures that were classified to fretting belong to fretting wear and corrosion.
The failures classified to fretting fatigue are only two failures in pump shaft and one failure in steam
generator tube (vessel). They are classified to class 1 components with high significance of safetyfunctions as shown in Table 1.
4.6 Piping Failure
Classifying the ratio of piping failures according to type of fatigue failure mechanism, it becamein order of mechanical vibration, fluid induced vibration, high-cycle thermal fatigue as shown in Fig.7,
and few low-cycle failures occurred. The ratio of components with high significance of safetyfunctions is large comparing to other components as shown in Fig.8 and Table 1. Almost all ofhigh-cycle thermal fatigue failures were occurred in the components with high significance of safetyfunctions.
Figs.9 and 10 show that the duration time of operation up to the occurrence of fluid andmechanical vibration fatigue failure and the number of occurrence in every 5 years by them are major
in piping failure. The number of failure occurrence is large in the beginning of plant life and remains
stable as the duration time increases. The big change is not seen by ageing. In addition, Fig.10 shows
two mountains of failure number in the 1980s and the 2000s in the generation, but further examinationis necessary for the latter mountain because more plants operate in this period.
4.7 Pump Failure
As for the fatigue failure of the pumps, machine vibration and fluid induced vibration fatigue
account for most as shown in Fig.11. In addition, the ratio of components which has high significanceof safety functions is bigger than other components as shown in Fig.12, 13 and 14 show the change offailure occurrence number by fluid induced vibration and mechanical vibration fatigue that accountfor most of the fatigue damage of pumps. A tendency is shown in Fig.14 that the failure number
increases in the 2000s. Fig 13 shows a peak in about 25 years from the starting period of plant
operation, and further examination is also necessary to clarify the cause.

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T. Nakamura, et al./ Reconstruction of Grand Design for Fatigue Evaluation
based on Fatigue Failure Analysis of Japanese NPPs

Fig.7
Ratio of Fatigue Failureclassified by Fatigue Mechanism
in Piping

Fig.8
Ratio of Fatigue Failureclassified by Significance of Safety
Function in piping

Fig.9 Duration of Operation up to Fluidand Mechanical Vibration Fatigue
Failure in piping

Fig.10
Number of Fluid and MechanicalVibration Fatigue Failure in every 5 years
in pipingFig.11 Ratio of Fatigue Failure Occurrenceclassified by Fatigue Mechanism in pumps

Fig.12
Ratio of Fatigue Failure classified
by Significance of Safety in pumps

Fig.13 Duration of Operation up to Fluid andMechanical Vibration Fatigue Failure
Occurrence in pumps

Fig.14 Number of occurrence in every 5
years by Fluid and Mechanical Vibration
Fatigue Failure in pumps

4.8 Valve Failure
As for the fatigue failure of valves, fluid induced vibration, mechanical vibration, and high-cycle
thermal fatigue account for most as shown in Fig.15. In addition, the ratio of components with highsignificance of safety functions is relatively bigger than other components.
4.9 Vessel Failure
As for the fatigue failure of vessels, fluid induced vibration, high-cycle thermal fatigue and70

T. Nakamura, et al./ Reconstruction of Grand Design for Fatigue Evaluation
based on Fatigue Failure Analysis of Japanese NPPs

fretting account for most as shown in Fig.16. The mechanism of fatigue failure that occurs in thecomponents with high safety significance is high-cycle thermal fatigue as shown in Table 1 andFig.17.

Fig.15 Ratio of Fatigue Failure Occurrenceclassified by Fatigue Mechanism in ValvesFig.16

Ratio of Failure Occurrence classified by
Fatigue Mechanism in Vessels

Fig.17 Ratio of Fatigue Failure Occurrenceclassified by Significance of Safety in Vessels

4.10 Summary of Analysis
Table 1 shows the analytical result arranged by the mechanism of fatigue failure and the type ofcomponents. In the netted column of Table 1, a lot of failure occurred in the components with highsafety significance and the countermeasure are mainly examined for the root cause extracted frompast recurrence preventive measures. In addition, the influence of individual failure to plant safety isinvestigated. The result clarifies that there are high association with significance of safety functions
and the component failures with high significance of safety functions in the netted column are decided
to be the target of Grand Design examination. As for the low-cycle fatigue failure, no fatigue failure
occurred in the classes 1 and 2 components except one case damaged by low melting point metal
crack. It is thought that the rule of fatigue evaluation in design and construction code functions
effectively. However, as the targeted components for the fatigue evaluation has extremely highimportance and from the viewpoint of Defense in Depths concept, it is necessary to wrestle for the
maintenance of the fitness for service rule, the improvement of inspection and monitoring program
and the development of system safety evaluation technique for the prevention of accident occurrence
and relaxation of accident influence in addition to the preventive countermeasures that are taken at thedesign stage until now for low-cycle fatigue failure.

5.

Direction to reconstruct Grand Design

The trend of domestic and overseas code and research activity in fatigue evaluation were
investigated in chapter 2. Then the new scheme of fatigue management is clarified in chapter 3 tointroduce the role of inspection and maintenance to fatigue management by quantifying crack
propagation. Postulated fatigue growth curve is proposed to apply crack propagation evaluation as
input of maintenance program and system safety evaluation. In chapter 4, the analytical result ofdomestic fatigue failure database shows the significance of individual failure to plant safety andclarifies the target to reconstruct Grand Design of fatigue management. Based on Defense in Depth
concept, three stages of countermeasure are set to prevent fatigue failure against all types fatigue
mechanism and reduce risk of nuclear hazards. The interim proposal of Grand Design for fatigue

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T. Nakamura, et al./ Reconstruction of Grand Design for Fatigue Evaluation
based on Fatigue Failure Analysis of Japanese NPPs

evaluation is summarized in Table 2 based on these results and is organized of the matrix for threestages, 1st stage: code system, 2nd stage: maintenance program and 3rd stage: system safety
evaluation and five types of fatigue mechanism.
Table. 2. Interim Proposal of Grand Design based on Domestic Fatigue Failure

Low cycle
fatigueHigh cycle
thermal fatigue(1st stage)
Development of voluntary consensus code

(2nd stage)
Improvement of maintenance program(3rd stage)
System safety evaluation

・Revise Design fatigue curve
・Introduce Environmental effect into Design code
・Introduce Flaw tolerance concept
・Improve Aging management program
・Establish System safety evauation guidelines

・Apply postulated crack growth curve
to inspection and monitoring
・Evaluate realistic fatigue
accumulation based on fatigue
monitor and operation data

・Develop probabilistic fatigue
evaluation model by postulated crack
growth curve
・Reflect the effect of maintenace
and inspection appropriately

・Revise Guidelines to evaluate high-cycle
thermal fatigue and accumulate technicalknowledge based on update research result

・Countermeasure in design stage(Design consideration such as piping
layout and limitation of temperature
difference)・Establish inspection and montoring
program based on update research
result・Develop probabilistic evaluation
method applying crack initiation andpropagation model based on update
research result

・Countermeasure in Design stage(Management of fatigue threshold by
the evaluation of specific frquency,etc.)・Application and preparation ofmonitoring technique on vibration,pressure perturbation, leak, etc.)

・Apply random failure model insted ofaging degradation model to systemsafety evaluation and reflect theeffect of maintenance andinspection appropriately

Fluid vibration
fatigue
・Integrate technical information and establish
guidelines to utilize update knowledge such
Mechanicalvibration fatigue as past design modification and countermeasures
Fretting

6.

obtained by industry group

Conclusion

Japan has offered many research results related to fatigue evaluation including environmental
effect so far. We now start a new activity to establish the management system of evaluating aging
plant synthetically from a view point of system safety. In fatigue management, we will study thescheme that can evaluate safety margin by new measure of crack propagation analysis and Grand
Design is going to be improved based on this study.
New scheme of Grand Design is proposed based on Defense in Depth concept in order to ensure
system integrity for fatigue failure in NPPs. Further activity is highly requested to establish code
system, improve maintenance program and develop system safety evaluation method based on
Grand Design with the co-operation of related organizations.

Acknowledgement
This work has been performed as a part of the Japan Ageing Management Program on System
Safety (JAMPSS) sponsored by NISA and Nuclear Regulation Authority (NRA).

References[1] AESJ-SC-TR005(ANX):2013 “Defense in Depth concept” Standard Committee Technical Report(2013)[2] JSME, “Environmental Fatigue Evaluation Method for Nuclear Power plants”, JSME S NF1-2009 (2009).
[3] T. Nakamura, et.al, “Establishment of Voluntary Consensus Codes and the Role of Code Engineers”, Journalof Nuclear Science and Technology, Vol. 48, No.7 ,p.1-14 (2011).
[4] ASME Sec. XI Rules for Inservice Inspection of Nuclear Power Plant Components, App.L-3420(2011)[5] NUREG/CR-6909, Rev.1 “Effect of LWR Coolant Environments on the Fatigue life of Reactor Materials
Draft Report for Comment” ,Office of Nuclear Regulatory Research (2014)[6] T. Nakamura.et.al. “Reexamination of Grand Design for Fatigue Evaluation”, 1st ICMST Conference 2012,
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[9] S. Abe, et.al. “Approach to establish micro crack growth prediction model based on surface observations offatigue test specimen”, 10th technical conference 2013 Osaka, JSM.
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