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Overview of JSME ¯aw evaluation code for nuclear power plants
Hideo Kobayashia, Koichi Kashimab,*
aTokyo Institute of Technology, 2-12-1 Ohokayama, Meguro-ku, Tokyo 152, JapanbCentral Research Institute of Electric Power Industry, 2-11-1 Iwado-kita, Komae-shi, Tokyo, 201-8511, Japan
Received 18 May 2000; revised 2 January 2001; accepted 2 January 2001
Abstract
A Japanese ¯aw evaluation code for nuclear power plant components has been developed at the Japan Society of Mechanical Engineers
(JSME). The code prescribes methods for the evaluation of ¯aws, which are detected during inservice inspection for pressure vessels and
pipes in nuclear power plants. This paper describes the basic ¯ow chart, methods of evaluation and allowable ¯aw sizes for acceptance
standards and criteria, including comparisons with the ASME Code Section XI. q 2001 Elsevier Science Ltd. All rights reserved.
Keywords: Flaw evaluation; Nuclear power plant; Inservice inspection; Fracture mechanics; ASME code
1. Introduction
In Japan, nuclear power components have been designed
and constructed in accordance with the Ministry of Interna-
tional Trade and Industry (MITI) Structural Technical Stan-
dard for Nuclear Power Plant Components [1], which
corresponds to the ASME Code Section III [2].
This standard has also been applied as the regulations,
which components should satisfy during operation. There-
fore, when a ¯aw is found in operating components such as
vessels or pipes, replacement or repair of the components is
required.
In the United States, ¯aw evaluation methods have been
developed and provided in the ASME Code Section XI [3],
and they have been applied to operating components. The
methods are based on fracture mechanics, and are able to
evaluate ¯aw behaviour such as crack growth caused by
fatigue or stress corrosion and failure.
At the present time, in October 2000, the operating dura-
tion of 20 plants among all the 51 Japanese nuclear power
plants exceeds 20 years. The importance of ¯aw evaluation
has been recognized under such circumstances, and the new
Japan Society of Mechanical Engineers (JSME) ¯aw
evaluation code has been established based on a draft Main-
tenance Standard in Japan developed by the Plant Operation
and Maintenance Standards committee.
JSME activities are in progress to establish codes related
to mechanical engineering, such as design and construction,
materials, welding and ®tness-for-service for nuclear power
plants.
The ®tness-for-service code comprises three parts,
inspection, ¯aw evaluation and repair/replacement. The
¯aw evaluation methods for class 1 nuclear power plant
components are now provided in the JSME ®tness-for-
service code.
This JSME Code is developed with close examination of
the ASME Code Section XI, and it is basically similar to
ASME Code Section XI in its structure but introduces
results of Japanese research projects on leak-before-break,
fracture toughness, etc. to the provisions of the evaluation
procedure, crack growth rate and acceptable ¯aw standards,
etc. and contains many original provisions.
2. Flaw evaluation in the JSME ®tness-for-service code
2.1. Flaw evaluation steps
The code is composed of a main part, appendices and
technical explanations. The main part includes the follow-
ing ¯aw evaluation steps for class 1 vessels and class1 pipes:
1. Flaw characterization: The shape and the dimensions of
the ¯aws detected by inspection are de®ned.
2. The ®rst step in ¯aw evaluation: The ¯aw size is
compared with the allowable ¯aw size in the acceptance
standards. A ¯aw whose size does not exceed the allow-
able ¯aw size can be acceptable for continued service. A
¯aw whose size exceeds the allowable ¯aw size shall be
assessed using the second step in ¯aw evaluation, as
International Journal of Pressure Vessels and Piping 77 (2000) 937±944
0308-0161/00/$ - see front matter q 2001 Elsevier Science Ltd. All rights reserved.
PII: S0308-0161(01)00016-3
www.elsevier.com/locate/ijpvp
* Corresponding author. Fax: 181-3-3430-2410.
E-mail address: [email protected] (K. Kashima).
described in point 3 below, or the components with the
¯aw shall be repaired or replaced.
3. The second step in ¯aw evaluation: A ¯aw that exceeds
the allowable size in the acceptance standards shall be
evaluated by analytical procedures to calculate its growth
until the end of the evaluation period (such as the next
inspection time or the end of service lifetime of the
component), which can be set in the evaluation. The
mechanisms of crack growth considered are fatigue and
stress corrosion cracking (SCC). If the predicted ¯aw size
at the end of the evaluation period does not exceed the
allowable ¯aw size in the acceptance criteria, the ¯aw is
acceptable for continued service during the evaluated
period. If the predicted ¯aw size at the end of the evalua-
tion period exceeds the allowable ¯aw size in the accep-
tance criteria, repair or replacement of the components
shall be made. Evaluation of the acceptability of the ¯aw
at the end of the evaluation period is based on fracture
analyses utilizing limit load analysis, elastic±plastic
failure analysis or a two-parameter approach.
The ¯ow chart of the ¯aw evaluation method is shown in
Fig. 1. Some main features of the JSME Code, speci®cally
the allowable ¯aw size, the crack growth rate and the failure
analysis, are described in the following sections.
2.2. Allowable ¯aws for the ®rst step
2.2.1. Allowable ¯aw size in acceptance standards
The JSME Code de®nes allowable ¯aws for linear ¯aws,
surface and subsurface planar ¯aws of vessels and pipes. In
the development process, the ASME Code Section XI was
examined. However, developments were also based on
many studies in Japan. Therefore, the methodology and
the corresponding results have now become fairly different
from those of the ASME Code Section XI.
The JSME Code de®nes allowable ¯aws only for
inservice inspection (ISI), because preservice inspection
(PSI) is done for obtaining the reference data for ISI.
The allowable ¯aws are determined considering the mini-
mum sizes of ¯aws that can be detected by non-destructive
H. Kobayashi, K. Kashima / International Journal of Pressure Vessels and Piping 77 (2000) 937±944938
Fig. 1. Flow chart of ¯aw evaluation method.
examination (UT) for both vessels and pipes. It was reported
that experimental data showed that the minimum detectable
size was the greater of 1.5 mm in depth or 4% of the thick-
ness of a plate for arti®cial defects made by electric
discharge machining [4]. Furthermore, a project is now in
progress at Japan Power Engineering and Inspection
Corporation (JAPEIC) to validate that this observation is
also applicable for actual fatigue cracks. That the capability
of non-destructive examination is quantitatively considered
in the determination of the allowable ¯aws is one of the
original features of the JSME Code.
For vessels, the allowable ¯aws are determined by linear
elastic fracture mechanics for thicknesses from 100 to
300 mm, based on a postulated initial ¯aw with the depth
of t/4 (t: thickness) and the aspect ratio of 1/6 [5] and corre-
sponding safety margins. This methodology is the same as
that used in the ASME Code Section XI. However, for
thicknesses less than 100 mm, the allowable ¯aws are deter-
mined to be the lesser of the allowable ¯aw size at the
thickness of 100 mm and that determined for pipes of the
same thickness. This is because when a plate becomes thin-
ner, failure tends to be in the plane stress mode, making the
application of linear elastic fracture mechanics less reason-
able, and also because the portions of vessels corresponding
to thickness less than 100 mm are the junctions of vessels to
the end of pipe branch connection. In addition, for thick-
nesses more than 300 mm, the allowable ¯aw size at
300 mm is adopted, based on the fact that the stress intensity
factor becomes almost constant for thicknesses more than
300 mm. Fig. 2(a) shows the allowable ¯aw sizes for
vessels.
For pipes, the allowable ¯aws are determined differently
from those of the ASME Code Section XI. Because aus-
tenitic stainless steels and ferritic steels used for the primary
coolant system of Japanese nuclear power plants have high
fracture toughness, it is judged that determination of
common allowable ¯aw sizes using net section stress
criteria for both materials is reasonable. The allowable
¯aws are determined so that the net stress at a section
with a ¯aw does not exceed 1.1 times the stress at a section
without a ¯aw. The allowable ¯aw size thus determined can
be simply expressed by:
a=t � 0:12a=l 1 0:06 �1�
Where, a, l and t are allowable ¯aw depth, allowable ¯aw
length and pipe thickness, respectively. Allowable ¯aws are
determined for pipes whose thickness is more than 7 mm
and less than 80 mm. For thickness greater than 80 mm, the
allowable ¯aw size at 80 mm is used. Fig. 2(b) shows the
allowable ¯aw size for pipes.
The safety margins of these allowable ¯aw sizes are
quantitatively evaluated by fracture mechanics. It has been
proved that it takes several decades for allowable ¯aws to
propagate by 1 mm in the thickness direction, and that the
margin to the failure load ranges from two to six.
2.2.2. Limitations of application
A number of limitations are imposed for application of
the allowable ¯aws in the JSME Code, which are not
included in the ASME Code Section XI. Firstly, the aspect
ratio of the allowable ¯aws is limited to be more than 0.06.
Secondly, the allowable ¯aws are only applied to fatigue
and not to SCC. In the case of SCC, it is probable that
multiple ¯aws initiate at an identical section and these can
coalesce to a larger ¯aw. Furthermore, the allowable ¯aws
are not applicable to the inside nozzle radius. Here, there are
stress concentrations at the region that cause larger stress
intensity factors compared with those for the same ¯aw at a
cylindrical portion.
2.3. Crack growth evaluation
Crack growth evaluation is prescribed in the JSME Code
for vessels and pipes in order to determine the anticipated
¯aw sizes at the end of the evaluation period. Evaluation
procedures similar to the ASME Code Section XI are
employed, but with some differences.
Major differences from the ASME Code Section XI are
the introduction of new reference crack growth curves of
H. Kobayashi, K. Kashima / International Journal of Pressure Vessels and Piping 77 (2000) 937±944 939
Fig. 2. Allowable ¯aw sizes for vessels and pipes.
fatigue and SCC for austenitic stainless steels in BWR
environments as shown in Figs. 3 and 4. The reference
fatigue crack growth curves in air for ferritic and aus-
tenitic stainless steels and the reference fatigue crack
growth curves in LWR environment conditions for ferri-
tic steels are similar to those in the ASME Code
Section XI.
Other differences from the ASME Code Section XI
are the numbers of transients to be considered, the order
of these transients, and the coalescence condition with
sub-critical crack growth. These differences are to elim-
inate excessive conservatism. In the JSME Code, the
numbers of transients to be considered can be based
on past operating experience. It is not necessary to
consider the order of the transients based on analyses
showing that there is little difference in crack growth
among several cases where the order of the transients is
varied. The crack coalescence rules are also based on
experimental results [6,7].
Consideration of SCC crack growth for ferritic steel pipes
is not required because no experiences of such cracking
have been reported and little possibility of SCC crack
growth is expected under operating plant conditions as it
will only occur for high stress intensity factor regions in
laboratory experiments.
2.4. Failure analysis of ferritic vessels
2.4.1. Evaluation procedure
The procedure of ¯aw evaluation of ferritic vessels,
shown in Fig. 5, and its analytical methods are basically
the same as those provided in the ASME Code Section
XI. The safety factors of 10 and 2 are in compliance with
those speci®ed in IWB-3610 of the ASME Code for normal
and upset conditions, and emergency and faulted conditions
in Appendix A of the ASME Code. One of the major differ-
ences in the ¯aw evaluation of ferritic vessels between the
JSME Code and the ASME Code Section XI is the fracture
toughness requirements, which are described below.
2.4.2. Fracture toughness applied to the evaluation
Reference fracture toughness values, KIa and KIc, in the
JSME Code have been established based on the results of a
study on fracture toughness for Japanese pressure vessel
materials [8]. The equations to obtain fracture toughness
are available for both base metal and weld metal corre-
sponding to material toughness indices, which are the refer-
ence nil-ductility transition temperature RTNDT obtained
from one-pass bead drop weight tests (DWTs), RTNDT by
two-pass bead DWTs and 50% shear fracture by Charpy
impact tests. The following equations show fracture tough-
ness obtained from RTNDT based on one-pass bead DWTs.
(unit: MPa���mp �
KIa � 29:46 1 15:16 exp�0:0274�T 2 RTNDT���base and weld metal�
�2�
KIc � 33:46 1 65:29 exp�0:0332�T 2 RTNDT�� �base metal��3�
KIc � 32:55 1 32:64 exp�0:0378�T 2 RTNDT�� �weld metal��4�
Fracture toughness based on crack arrest KIa in the JSME
Code, obtained from RTNDT by two-pass bead DWTs, is
identical to that speci®ed in Appendix A of the ASME
Code Section XI, and is higher than that based on one-
pass bead DWTs. The KIa from one-pass bead is the same
as the alternative reference fracture toughness KIR included
in the ASME Code Case PWTs N-610, which is based on
the Japanese study on fracture toughness for pressure vessel
materials.
2.4.3. Estimation method against neutron irradiation
embrittlement
The estimation method against neutron irradiation
embrittlement in the JSME Code allows use of the following
Eqs. (5)±(9), which were established as a part of the
Japanese study on Pressurized Thermal Shock [9] concern-
ing the irradiation embrittlement issue. The equations are
based on irradiated toughness data from Japanese pressure
vessel materials, taking account of the difference especially
H. Kobayashi, K. Kashima / International Journal of Pressure Vessels and Piping 77 (2000) 937±944940
Fig. 3. Reference fatigue crack growth rate for austenitic stainless steels in
primary BWR water environments (tr� 1000 s).
in chemical content in weld metal from those materials in
the US.
DRTNDT�8C� � �CF�´f 0:2920:04´log f �base metal� �5�
DRTNDT�8C� � �CF�´f 0:2520:10´log f �weld metal� �6�
�CF� � 216 1 1210´P 1 215´Cu177´��������Cu´Nip �base metal�
�7�
�CF� � 26 2 24´Si 2 61´Ni 1 301´��������Cu´Nip �weld metal�
�8�
f � f0´exp�20:24 a 0=25:4� �9�Where; [CF], chemistry factor; Cu, Si, Ni and P, chemical
content in wt% for copper, silicon, nickel and phosphorus;
f, neutron ¯uence ( £ 1019 n/cm2, E $ 1 MeV�; f0, neutron
¯uence at inside surface of a vessel ( £ 1019 n/cm2,
E $ 1 MeV�; a 0, distance to the point of evaluation from
inside surface of a vessel.
2.5. Failure analysis of pipes
2.5.1. Major characteristics of failure analysis of pipes
In the JSME Code, failure analysis methods are estab-
lished based on the material properties and fracture tough-
ness of Japanese ferritic and austenitic pipe materials.
The limit load failure mode and the elastic±plastic failure
mode are prescribed due to the thin thickness of the pipe
walls and the high toughness for Japanese ferritic and aus-
tenitic pipe materials even for older plants. The safety
factors are speci®ed similar to the ASME Code Section
XI IWB 3640 and 3650 [3].
One of the major differences from the ASME Code
Section XI is the ¯ow stress of 2.7Sm for both ferritic and
austenitic stainless steels based on the investigation of mean
actual ¯ow stress data. This makes the allowable ¯aw sizes
in the acceptance criteria for the limit load failure and
H. Kobayashi, K. Kashima / International Journal of Pressure Vessels and Piping 77 (2000) 937±944 941
Fig. 4. Reference SCC crack growth rate for sensitized 304 stainless steel in BWR water environments.
elastic±plastic failure mode identical between ferritic and
austenitic stainless steels except for a few points.
Another major difference from the ASME Code is the
deletion of linear elastic failure mode criteria, the provision
of a limitation to the allowable length for circumferential
¯aws and the consideration of thermal expansion stresses
for limit load evaluation as well as for elastic±plastic failure
analysis. Deletion of the linear elastic failure mode is based
on the investigation of the fracture toughness and of the
thickness of the pipes mentioned above. In order to prevent
double-ended pipe fracture, the circumferential ¯aw length
in terms of angular extent is limited to less than 608, in
addition to the ¯aw length limitation for longitudinal
¯aws. Thermal expansion stresses are considered with a
safety factor of unity for failure evaluation due to their
nature being similar to the primary stresses. The procedures
are not speci®ed in detail in the JSME ®tness-for- service
code. It can be calculated by the equations for thermal
expansion stress speci®ed in the class one piping design
rule of MITI Noti®cation 501 [1]. The safety factors are
set to be the same as those of the ASME Code.
2.5.2. Selection of failure analysis criteria
Fig. 6 shows the selection procedures of pipe failure
analysis. For austenitic stainless steel pipes, the failure
analysis is selected based on the three material categories,
`base metals other than cast stainless steels', `base metals of
cast stainless steels' and `welds'. If the ¯aws are located in
the welds and their direction is parallel to the welds, elastic±
plastic failure analysis is selected. If the ¯aws are located in
H. Kobayashi, K. Kashima / International Journal of Pressure Vessels and Piping 77 (2000) 937±944942
Fig. 5. Flaw evaluation procedure of ferritic vessels.
the welds and their direction is perpendicular to the welds,
then the failure analysis is selected depending on the base
metal. Thus either `limit load analysis', `elastic±plastic fail-
ure analysis', or a `two parameter approach' can be selected.
When `limit load analysis' or `elastic±plastic failure
analysis' is selected and the resulting evaluation does not
meet the allowable conditions, re-evaluation by the `two
parameter approach' is permitted. Cast stainless steel
pipes are categorized by the ferrite numbers into `less than
20%' and `from 20 to 23.5%' to take into account the possi-
bility of degradation of fracture toughness due to exposure
at high temperature by long-term operation.
For ferritic pipes, failure analysis is selected using a
selection factor, which is the same parameter as Screening
Criteria (SC) in the ASME Code. The selection factor
depends on the fracture toughness of the pipe materials,
¯aw size, shape and loading conditions. The selection
criteria are also the same as the ASME Code, except that
the linear elastic failure approach is deleted. When the `limit
load failure analysis', or `elastic±plastic failure analysis' is
selected and the resulting evaluation does not meet the
allowable conditions, re-evaluation by the `two parameter
approach' is permitted similar to that for the austenitic
stainless steel pipes.
For the evaluation of the ferritic pipes, the elastic±plastic
fracture toughness JIC can be determined from experiments
using the same heat, from lower bound JIc data for similar
materials, from conversion from the Charpy absorbed
energy of the same heat, or by the value shown in Table
1. In determining JIc by conversion from the Charpy
absorbed energy, JIc � 1:296CVN (JIc: [KJ/mm2], CVN:
[J]) is prescribed for circumferential ¯aws, but cannot be
applied for longitudinal ¯aws.
For elastic±plastic failure analysis of circumferential
H. Kobayashi, K. Kashima / International Journal of Pressure Vessels and Piping 77 (2000) 937±944 943
Fig. 6. Procedure of selection of fracture evaluation criteria.
¯aws for both ferritic and austenitic stainless steel pipes,
allowable ¯aw sizes are evaluated by a simpli®ed method
using the Z factor as a load multiplier on the applied load in
the limit load evaluation equation. Z factors are de®ned as
the ratio of plastic collapse load to elastic±plastic failure
load and are based on the fracture toughness of domestic
materials with limitations of the circumferential ¯aw angle.
The values of Z factors for austenitic stainless and ferritic
steel pipes are as follows:
For austenitic stainless steels: (circumferential ¯aws)
TIG and SMAW : Z � 0:292´log{�OD�=25} 1 0:986
SAW and cast steels with ferrite contents less than20% :
Z � 0:350´log{�OD�=25} 1 1:215
For ferritic steels: (circumferential ¯aws)
Z � 0:2885´log{�OD�=25} 1 0:9573
where (OD) is nominal diameter of pipes in mm. For
longitudinal ¯aws, Z factors are under preparation.
3. Conclusions
The JSME ¯aw evaluation code described in this paper is
the ®rst one established in Japan which can be applied to
¯aws in the components of current operating nuclear power
plants, maintaining consistency with the MITI Structural
Technical Standard for Nuclear Power Components [1].
The new JSME Code is desired to be used for those ¯aws
found in the nuclear power plants in the near future to
enhance the operating availability and to clarify the safety
margins to failure. The code would not have been estab-
lished if there had not been either progress in fracture
mechanics, experience of the use of the ASME Code
Section XI in the US, or the results of Japanese research
and development.
The JSME Code of ®tness-for-service will continue to
extend its scope systematically to include ¯aw evaluation
for other classes of components, inservice inspection and
repair/replacement.
Acknowledgements
The authors wish to express their gratitude to Dr
Yasuhide Asada, who is the chair of the JSME Main
Committee on Power Generation Facility Code for review
and approval of this code. The authors also acknowledge Dr
Tai Asayama, Mr Hiroaki Eto, Dr Kunio Hasegawa, Mr
Masao Honjin, Dr Masaaki Kikuchi, Mr Koji Koyama,
and Mr Tomonori Nomura, who are the members of the
Subgroup on Fitness-for-Service of Subcommittee on
Nuclear Power for the support of preparation for this paper.
References
[1] MITI Noti®cation No. 501, Technical standard for nuclear power plant
components, 1980.
[2] ASME Boiler & pressure vessel code, Section III, 1998.
[3] ASME Boiler & pressure vessel code, Section XI, 1998.
[4] Iida K. Present Situation of ISI Performance in Japan, Fourteenth Inter-
national Conference on NDE in the Nuclear and Pressure Vessel Indus-
tries, Stockholm, Sweden, September 24±27, 1996.
[5] Inservice Inspection of Light Water Cooled Nuclear Power Plant
Components, JEAC 4205-1996, 1996 (in Japanese).
[6] Iida K, Ando K, Hirata T. An evaluation technique for fatigue life of
multiple surface cracks (Part 1). J Soc Naval Arch Jpn
1980;148(June):284±93 (in Japanese).
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[8] Takahashi Y, Funada T. Status on the Revisions of Standards Related
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H. Kobayashi, K. Kashima / International Journal of Pressure Vessels and Piping 77 (2000) 937±944944
Table 1
Elastic±plastic fracture toughness JIC for ferritic pipes
Type of steels Temperature T (8C) JIC (kJ/m2)
Circumferential ¯aw Longitudinal ¯aw
Group 1
STS410,STS480,SFVC2B, SGV410,SGV480 TIG,SMAW,SAW
Materials for which the lowest service temperature is below 208C
T $ 20 134 67
10 # T , 20 109 54
Group 2
STPT480, and other than those of Group 1 T $ 40 114 57
10 # T , 40 63 31