Overview of JSME flaw evaluation code for nuclear power plants

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


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 215Ću177´��CuŃip �base metal�

�7�

�CF� � 26 2 24´Si 2 61Ńi 1 301´��CuŃip �weld metal�

�8�

f � f0éxp�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


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


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', èlastic±plastic fail-

ure analysis', or a `two parameter approach' can be selected.

When `limit load analysis' or èlastic±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 èlastic±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


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).

[7] Ando K, Hirata T, Iida K. An evaluation technique for fatigue life of

multiple surface cracks (Part 2). J Soc Naval Arch Japan

1983;153(June):352±63 (in Japanese).

[8] Takahashi Y, Funada T. Status on the Revisions of Standards Related

to Fracture Toughness Evaluation for Light Water Reactors in Japan,

ICONE-7218, Seventh International Conference on Nuclear Engineer-

ing, Tokyo, Japan, April 19±23, 1999.

[9] Mishima Y, et al. PTS Integrity study in Japan. Int J Pressure Vessel

Pip 1994;58:91±101.


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

Documents

Overview of JSME flaw evaluation code for nuclear power plants