Company nameKansai EPCO

Date of occurrence

25.September.2007

Unit name

Mihama 2

Event

Flaws Found in Steam Generator A Reactor Coolant Inlet
Nozzle Weld

International Nuclear Event Scale (INES)

0

Status of report

Final report

Status when event occurred
During the 24th planned outage, considering the stress corrosion cracking events in domestic and foreign
plants, shot-peening (*1) work to reduce residual stress in the surface of the steam generator (hereafterreferred to as “S/G”) reactor coolant inlet and outlet nozzle welds (total four welds) where 600 series Nibase alloy had been used was to be performed as a preventive maintenance task. To check the surface
conditions of the nozzle weld portions prior to the work, eddy current test(*2) (hereafter referred to as the“ECT”) was performed for the S/G nozzle weld portion surfaces and significant indications were identified
for 13 locations on the inlet nozzle weld portion of S/G-A. Also, visual checking of these locations having
significant signals was performed and a flaw was identified at one location. For the outlet nozzle of S/G-A
and the inlet and outlet nozzles of S/G-B, no significant signal was identified.
Further, for the locations found with significant ECT indication, a penetrant test (hereafter referred to as“PT”) was performed and as a result significant penetration indicating patterns (maximum length: approx.
17 mm) were identified. In addition an ultrasonic test (hereafter referred to as “UT”) was performed andthereby the maximum flaw depth was evaluated at approx. 13 mm (for the location of the maximum
length), resulting in the residual thickness of the affected location (approx. 68 mm) evaluated to be below
the thickness described (75 mm) in the application for construction permit based on the Electricity Utilities
Industry Law.
*1: Shot-peening
A technique to reduce residual stress in a weld portion surface by peening the weld portion surface withshot which consists of small metal balls.
*2: Eddy current test (ECT)A technique to detect a flaw in a test subject using an electromagnetic induction change caused in amaterial while eddy currents being made in the material.
*3: Penetrant test (PT)A technique to observe a flaw as a penetrant indication pattern developed with a developer agent. In thisprocess to make a flaw opening on the test subject surface easy to check visually, a highly permeableliquid including visible dye is used to penetrate the flaw, then the excess liquid is removed and thedeveloper is applied.
*4: Ultrasonic test (UT)
A technique to detect defects inside a material using ultrasonic wave which then propagates in thematerial and reflects any material discontinuity such as a defect.

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Summary of examination of cause(1) Observation results by SUMP(*5) and etching(*6)[Weld portion]
Among the 13 locations found with significant signal indication for the S/G-A inlet nozzle, the No.4
location of indication which was in the weld portion and evaluated to have the maximum depth wasobserved for its metal texture by means of SUMP and etching, and the result was as follows:
・ The cracking was found in the circumferential weld and battering portion, but not reaching the safeend and the stainless overlay portion. The cracking was an associate of multiple cracking having
approx. 3~5 mm length in the axial direction, and the overall length was approx. 17 mm with the
maximum opening width of approx. 30 μm.
・ The cracking was along the boundary of dendrite (column like shaped crystal made in melted metal
while being cooled).
・ In the buttering portion, three layers of weld metal were identified. Also in the circumferential weld twobeads of TIG weld metal were identified. Any clear trace of rewelding was not found in the buttering
portion and the weld portion.
・ Any ductile cracking, fatigue cracking or corrosion was not found. Also it was verified that the flaw was
not caused from a weld defect such as blow hole etc.
[Safe end]
Among the 13 locations found with significant signal indication for the S/G-A inlet nozzle, the No.13
indication found in the safe end was observed for its metal texture by SUMP and etching, and cracking inthe safe end (SUS316 base metal) was identified at approx. 3 mm from the boundary with thecircumferential weld. This cracking was multiple cracks with branching along the grain boundaries.
*5: SUMP
Film is adhered on failure portion surface in order to transfer the surface shape, and this is observed by
microscope. By this, investigation of metal texture of the failure portion becomes possible, as well as by
cutting out of metal sample.
*6: Etching
A method of metal texture observation by optical microscope in which the object test metal surface ispolished and corroded using oxalic acid solution etc, through this process, the surface layer affected bypolishing process is removed, and the texture to be observed is revealed.
(2) Investigation by cutting[Weld portion]
a. Appearance observation
Appearance observation and replica observation of the cracking was performed and the result was asfollows:
・ Cracking No.4 consisted of multiple longitudinal cracks on the surface with an overall length of approx.
17 mm. The cracking did not reach the stainless overlay. Because the area around the cracking No.4

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had been buffed prior to SUMP in the field, the surface was in a state polished to that of a mirror
surface.
・ Cracking No.6 consisted of two fine longitudinal cracks having surface lengths of approx. 2.5 mm andapprox. 2 mm. In addition, in the area nearby the cracking No.6, equally spaced circumferentialtraces of machining having approx. 0.4 mm width were identified.
・ Cracking No.10 consisted of two longitudinal fine cracks having a surface length of approx. 2.6 mm
and approx. 2.0 mm. Because the area around the cracking No.10 had been buffed for SUMP ofother cracking, the surface was in a state polished to that of a mirror surface.
・ Cracking No.21 consisted of two fine longitudinal cracks having surface lengths of approx. 2.6 mm andapprox. 1.3 mm. In addition, in the area nearby the cracking No.21, equally spaced circumferentialtraces of machining having approx. 0.4 mm width were identified.
・ Also in the general portion of the weld metal, equally spaced circumferential machining traces ofapprox. 0.4 mm were identified.
・ Any trace of rewelding or buffing during manufacturing was not identified for the cracking portion as
well as the general portion.
b. Fracture observation
After opening the cracks, fracture appearance observation, SEM (scanning electron microscopy)
observation, and cross-sectional microscopic observation were performed and the result was as follows.
Though, since buffing for cracking No.4 was performed before SUMP in the field, the status of thefracture in the most outer surface layer could not be verified.
・ Cracking No.4 consisted of multiple cracks of approx. 17 mm overall length on the surface, had the
maximum depth of approx. 11.5 mm, which crossed most of the buttering portion and thecircumferential weld portion, and propagated along the dendrite boundary. However, it did not reachthe low alloy steel and the stainless overlay portion. For the safe end (SUS316 base metal) side,
verification was not possible because of the notch introduced to open the fracture. As a result of SEM
observation, fracture along the dendrite boundary was observed in the buttering portion and thecircumferential weld portion.
・ Cracking No.6 consisted of two cracks, one having approx. 2.5 mm surface length with approx. 3 mm
maximum depth, and one having approx. 2 mm surface length with approx. 2.5 mm maximum depth,and developed along the dendrite boundary in the circumferential weld. However, it did not develop to
the safe end (SUS316 base metal) side. As a result of SEM observation, fracture along dendrite
boundary in the circumferential weld similar to the case of No.4 was identified.
・ For cracking No.10 and No.21, because of difficulty in forced opening, cross-sectional microscopic
observation for multiple cross-sections was performed. As a result, fractures were identified to havethe maximum depth of approx. 1.8 mm and approx. 0.47 mm respectively, and to have developed
along the dendrite boundary.
・ As a result of cross-sectional microscopic observation, deformed texture considered to be due tomachining effect was identified in the most outer surface layer of the weld (general portion).
c. Analysis of surface deposit

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Main elements were the components of the weld metal (Ni, Cr, Fe etc.) and harmful elements such as
chlorine were not found.
d. Measurement of residual stress
By X-ray the residual stress measuring method, the residual stress was measured for the surface layer ofthe buttering and the weld portion. As a result, the residual tensile stress in the buttering portion wasapprox. 280 MPa (circumferential) and approx. 73 MPa (longitudinal), while the circumferential weld wasapprox. 482 MPa (circumferential) and approx. 346 MPa (longitudinal).
e. Hardness measurement
As a result of hardness measurement, in the surface layer with approx. 0.1 mm depth, increased
hardness was identified (circumferential weld: Max. approx. 400 Hv, buttering: Max. approx. 470 Hv).
This was considered to be due to machining effect.
f. Chemical composition analysis
For the circumferential weld and the buttering portion as well, the composition was equivalent to that in
the mill sheets.
[Safe end]a. Appearance observation and PT
Appearance observation and replica observation were performed for the cracking and the result was asfollows:
・ Cracking No.13 mainly consisted of two cracks having surface length of approx. 3 mm and of approx.
4 mm, respectively, in longitudinal direction and in circumferential direction. Because the area aroundthe cracking No.13 had been buffed prior to SUMP in the field, the surface was in a state polished tothat of a mirror surface.
・ Cracking 27 consisted of multiple cracks in longitudinal or circumferential direction with an overallsurface length of approx. 3 mm. The area around the crack No.27 was in a state polished to that of amirror surface as the case of the crack No. 13.
・ The cracks in the safe end were all found in a region of 3~7 mm from the weld portion and no cracking
was found in other regions.
・ Equally spaced circumferential machining traces were identified.
・ Any trace of rewelding or buffing during manufacturing was not identified for the cracking portion as
well as the general portion.
b. Fracture observation
After opening the cracks, fracture appearance observation, SEM (scanning electron microscopy)
observation, and cross-sectional microscopic observation were performed and the result was as follows.
Although, there was buffing performed before SUMP in the field, the conditions of the most outer surface
layer of the fracture could not be verified.
・ Cracking No.13 mainly consisted of two fine cracks. One with approx. 17 mm surface length had the

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maximum depth of approx. 0.9 mm, and the other with approx. 4 mm surface length had the
maximum depth of approx. 0.7 mm. Further as a result of the fracture SEM observation, fracture
along the austenite grain boundary(*7) was identified.
・ Cracking No.27 was of approx. 3 mm surface length with the maximum depth of approx. 0.3 mm.
Further as a result of the fracture SEM observation, fracture along the austenite grain boundary as
the case of cracking No.13 was identified.
・ As a result of the cross-sectional microscopic observation, deformed texture and sliding lines thought
to be affected by machining were identified in the general portion of the sefe end.
*7: Austenite grain boundary
This refers to a boundary between crystal grains having a honeycomb like shape, which is seen for
austenite stainless steel represented by SUS316.
c. Analysis of surface deposit
Main elements were the components of the base metal of the safe end (Ni, Cr, Fe etc.) and harmful
elements such as chlorine were not found.
d. Measurement of residual stress
By X-ray residual stress measuring method, the residual stress of the safe end was measured for thesurface layer within 0.2 mm. As a result, the residual stress in the surface layer was approx. 571 MPa(circumferential) and approx. 356 MPa (longitudinal), while the residual stress in the region deeper than
0.1 mm was of compression.
e. Hardness measurement
As a result of the hardness measurement, the surface layer (within approx. 0.02 mm depth) had anincreased hardness of the maximum 450 Hv compared with the inside (approx. 250 Hv). Further, theregion found with significant hardness increase was approx. 0.1 mm depth of the surface, as the case of
the weld portion.
f. Chemical composition analysis
The composition was equivalent to that in the mill sheets.
g. Measurement of sensitization degree
For the safe end (SUS316 base metal), sensitization degree measurement in the general portion, nearcracking No.13 and near cracking No.27 was performed by EPR (Electrochemical potentiokinetic
reactivation) analysis. As a result, the EPR values were below the detectable limit and no symptom of
sensitization was identified. Also, from the result of TEM (transmission electron microscopy)/ EDS
analysis, symptoms of sensitization (reduction in Cr concentration) were not identified.
(3) Investigation result of manufacturing and operation history

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a. Investigation of manufacturing history
The affected S/G was manufactured during October 1991~February 1994, and installed during February
1994~March 1994. Through the records of work and inspection at that period and through the hearing
from the relevant persons, the manufacturing history was investigated.
As a result, any singularity in the manufacturing procedure of the affected nozzle, the welder had enoughexperience, and thus reweld was not considered to have been performed based on the records and thehearing. Further, the hearing was performed from the workers in that period because the inner surface
was machined after welding the safe end and so machining conditions might be somehow different
depending on the worker. As a result, no singularity was identified and so the machining was considered
to be done normally. Whether buffing after surface machining was performed or not was unable to beverified.
b. Investigation of operational history
The welding of the affected portion was performed during the S/G replacement work at the 14th planned
outage (paralleled on September 26, 1994) and so the operational history through the 24th planned
outage (off line on July 20, 2007) was investigated. The result was as follows:
1) Reactor coolant temperature and pressure
The events which occurred during that period that were accompanied with transients were not identified
except for two events of reactor manual shutdown, one event of reactor automatic shutdown, and two
events of power suppression. Also it was confirmed that these events including the transients did notshow any abnormal temperature and pressure change (for the period of September 1994~ March 1998,
confirmed based on the operation log).
2) Control of water chemistry
It was confirmed that the water chemistry in that period satisfied the plant’s safety preservation rules andharmful elements such as chlorine was controlled.
(4) Mockup test of machining
After welding the safe end, machining of the inner surface was performed. This could be a cause ofincreased hardness and high residual stress and therefore a mockup test was performed. The mockup
test subject was an actual size cylinder of SUS316L of which the inner surface was padded bycircumferential buildup welding of 600 series Ni base alloy. After it was machined with the condition
based on the hearing result, observation of surface condition, hardness measurement and residual
stress measurement were performed.
a. Observation of surface condition
On the surface after machining, equally spaced circumferential machining traces by a bit similar to ones
seen for the actual case were verified. Further, it was confirmed that the machining traces disappeared
when a simple buffing was performed.
b. Hardness measurement[Weld portion]
The hardness of the surface was approx. 300~400 Hv or so, showing hardness increase compared with

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the initial hardness of 180 Hv. These results were of the same order as the measured value (368 Hv) for
the actual safe end portion.
[Safe end portion]
The hardness of the surface was approx. 280~430 Hv, showing hardness increase compared with theinitial hardness of 240 Hv. These results were of the same order as the measured value (395 Hv) for theactual weld portion.
c. Measurement of residual stress[Weld portion]
Residual stress in the weld portion surface was approx. 600 MPa (circumferential) and approx. 580 MPa(longitudinal) as the maximum.
Further, when a standard buffing was performed, the residual stress became compressive(circumferential: approx. -153 MPa, longitudinal: approx. -428 MPa).
[Safe end portion]
Residual stress in the surface of was approx. 310 MPa (circumferential) and 360 MPa (longitudinal) as
the maximum.
Further, when a standard buffing was performed, the residual stress shifted toward compression(circumferential: approx. 4 MPa, longitudinal: approx. -226 MPa).
(5) Investigation of literature
As a result of literature research on cracking in 600 series Ni base alloy welds, it was identified that as
the events with damage in a similar portion of the reactor vessel outlet nozzle weld an event of leakage at
V.C. Summer in the US (found in October 2000) and an event of UT indication at Ringhals 3 and 4 inSweden (found in August 2000, and in September 2000, respectively) were reported. Also as a case of
the pressurizer nozzle, an event of leakage at Tsuruga 2 (leakage from pressurizer relief valve nozzle
and UT indication in safety valve nozzle, found in September 2003) was reported. These crackings were
all circumferential, and in cases of V.C. Summer and Tsuruga 2, cracking developed through the weld
metal and stopped at the boundary with low alloy steel and at the boundary with a safe end. The causesof these were all considered to be stress corrosion cracking (hereafter referred to as “PWSCC”) causedby high residual stress due to performance of rewelding.
Further, regarding PWSCC in 600 series Ni base alloy weld, a 360 °C temperature accelerated SCC
constant load test was performed and it was concluded that PWSCC occurs at a stress greater than 300
MPa.
[Safe end portion]
As a result of the literature research on cracking in the safe end portion (SUS316 base metal), it was
identified that the possibility of SCC occurrence is considered small even for a case of sensitizedstainless steel because the PWR primary water chemical environment is controlled with a low
concentration limit for dissolved oxygen and chloride ion.
On the other hand, for the BWR plant’s shroud and recirculation piping, generation of transgranular
cracking in the machined surface layer caused by welding and machining, and as a result of it,
generation and development of interglanular cracking were reported.

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For cold worked non-sensitized stainless steel, studies on SCC generation and development under PWR
environment is progressing and generation of SCC presently has not been verified. However, regarding
the development of SCC, it has been known from the test result with a CT coupon (given precracking)that the development rate increases with hardness. From this, cold worked non-sensitized stainless steel
is considered to have a susceptibility to SCC under a PWR environment. Further, the development rate isevaluated to be lower under a low oxygen concentration environment (equivalent to PWR environment)
than under a high oxygen concentration environment (equivalent to BWR environment).
(6) Other forms of damage
Also investigation for the possibility of ductile cracking, fatigue cracking, and corrosion was performed.
However, there was no anomaly in the usage environment and used materials and thus these are notconsidered to be the cause.
Also scratches made during manufacturing or periodic tests were not considered to be the cause basedon the records and actual plant investigation result. Further, regarding welding deficiency and defect inweld during manufacturing, nothing to be the cause was found from the records and actual plantinvestigation results.
(7) Summery of the investigation results[Weld portion]
・ It was verified that the cracking in the 600 series Ni base alloy weld portion was intergranular cracking
along the dendrite boundary and exhibited PWSCC aspects.
・ On the inner surface of the 600 series Ni base alloy weld portion, trace of machining during
manufacturing was identified, and residual stress exceeding the threshold value for PWSCC
generation of approx. 300MPa was identified.
・ It was confirmed that there was no rewelding during manufacturing, there was no singularity found in
the manufacturing history and operational history, and there were no harmful elements.
[Safe end portion]
・ Cracking in the safe end portion (SUS316 base metal) was very minor intergranular cracking andfound only nearby the weld portion boundary. (The initiation point of cracking generation was not able
to be identified.)
・ In the depth range of 0.1 mm from the inner surface, a remarkable increase in hardness thought to bean effect due to machining and high residual tensile stress were identified.
・ It was confirmed that there was no rewelding during manufacturing, there was no singularity found in
the manufacturing history and operational history, and there were no harmful elements.

Cause of event[Weld portion]
Though the initiation point of cracking was not identified, it is presumed that during manufacturing of thereplacement S/G, the machining of the 600 series Ni base alloy weld portion was performed, thereby highresidual stress was caused in a very thin surface layer on the inner surface, and this high stress caused
PWCSS. Further, the generated cracking is considered to have developed due to stress etc. during

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operation.
[Safe end portion]
During manufacturing of replacement of the S/G, after welding the inlet nozzle and the safe end using
600 series Ni base alloy weld metal, machining was performed, and thereby a hardness increase and ahigh residual stress in a very thin surface layer on the inner surface nearby the safe end weld werecaused.
Although the cracking initiation point was not identified, intergranular cracking nearby the weld isconsidered to have developed due to stress during operation etc.

Measures to prevent recurrence[Weld portion]
The portion cut out will be recovered using 690 series Ni base alloy weld which has better corrosion
resistivity. In addition, for caution’s sake, a buffing finish will be applied in order to reduce residual stressin the affected portion.
[Safe end portion]
The safe end portion (SUS316 base metal) will be replaced with a new one and finished with buffing.
Further, a study on cracking generation in the safe end will be performed to enhance relevant knowledge.

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Status of Mihama Unit 2 planned outage(Cause and Countermeasures of the Flaw in Steam Generator A
Reactor Coolant Inlet Nozzle Weld portion)
Location of flaw
Structure of Steam Generator A

System schematic diagram
Reactor containment
Pressurizer Steamgenerator

To steam turbine
Feedwater

ReactorvesselReactor coolant
pump

Scope of steam generatorthreplacement during 14
planned outage (1991~2004)
Lower head(low alloy steel)

Tube plate
Reactor coolant
inlet nozzle

Stainless overlay(Stainless steel)approx.

Weld portion(Stainless)Partition plate
Reactor coolant
outlet nozzle
Reactor coolant flow direction

Safe end portion(Stainless steel)approx.

Reactor coolant pipe
(Stainless steel)approx.

Buttering portion(600 series Ni basealloy)
Circumferential weld
portion(600 series Ni base alloy)

Weld portion

Object location of theinspection this time

Inspection results
PT indication pattern
View from steam generator side
(Top regarded as 0 degree)

Spread ofpenetrant
approx.
17mm(maximumlength)Result of depth evaluation
by ultrasonic wave test
approx.13mm(maximum depth)

ECT and PT results(locations with significant indication)

Cross
Stainlessoverlay(Inside)(Outside)Weld portion

Lower
head

Buttering
portionCircumfer
Safe end-entialportionweld
portionapprox.

Locations observed
in detail(Value described in the application for
construction permit: approx.75mm)

Locations where flaw depth was not detectable byultrasonic wave test.

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Observation results for the weld portion
Multiple cracks of approx. 3 to 5 mm length existed
intermittently in longitudinal direction.
Cracking was along dendrite boundary in the weld portion.

Cross

Weld
portion270 degree side

Buttering
portion0 degree side

Stainless overlay(Inside)

Lower
head

Buttering
portionButtering portion

Circumferential weld
portionStainless overlay

Etching observation result for No.4
(location with the maximum depth)

Stainless overlay(Outside)

SUMP observation results

Safe end
portionapprox.

Circumferential
weld portion

Circumferential
weld portion

Safe end portion
Safe end portion

Fracture observation
Inner surface side

Fracture

Stainlessoverlay

Replica observation resultof weld portion

Maximum depth
approx.11.5mm

Lower head

Trace of machining was
found.

Maximum depth of approx. 11.5 mm,
developing along dendrite boundary.

Safe end observation result
Multiple cracking was found with branching along grainboundary of metal crystal.
Etching observation result for No.13
Stainless overlay

180 degree side

Weld
portionButtering
portionCircumferential
weld
portion

Lower
head

Circumferential
weld portion

Stainless overlay(Inside)(Outside)

270 degree side

SUMP observation results

Buttering portion

Safe end
portionCircumferential weld portion
Safe end
portion

Safe end
portionFracture observation
Inner surface side

Fracture

Maximum
depth
approx.0.9mm

Maximum depth was approx. 0.9 mm, developing
along austenite crystal grain boundary.

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Replica observation resultof safe end

Trace of machining was
found.

Machining[Machining during manufacturing]

Machining trace resulted by reproducibility test

Area machined with machining apparatus to remove
raggedness of the inner surface due to welding etc.
Stainless overlay

Bit sending direction

Weldreinforcement
Weld portion

Bit rotates.

Buttering
portionCircumferential weld
portion

Safe end
Metal bit

For machining,
metal bit isrotatedcircumferentially.

Machining trace similar to that of replicaof the actual portion was seen.
Residual stress measurement result
Large circumferential tensile residual stress
was identified.

Presumed cause
Weld portion
Stress corrosion
cracking underreactor coolant
environment

Material

StressEnvironment

Environment: High temperature reactor coolant waterchemistry environment
Material: 600 series Ni base alloy susceptible to stresscorrosion cracking
Stress: Tensile residual stress due to welding andmachining

By coincidence of the three factors, stress corrosion cracking was presumed tobe caused under reactor coolant environment.

Safe end portion
High tensile residual stress was caused in very thin surface layer of inner surface. Nearby the weld portion,
cracking was presumed to have developed along austenite crystalline boundary due to stress during
operation etc.

Countermeasures[New safe end to be welded][Pipe (elbow) portion to be replaced]
Safe end portion(new one) to bewelded.

Steam generator side
Buff finishing
to be performed(for reducing residual stress)

Pipe (elbow) portion to
be replaced with new one

Loop piping side

Weld portion material to be changed
600 series Ni base alloy ⇒ 690 series Ni base alloy(For improvement in resistivity to stress corrosion cracking)

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Steamgeneratorside