Company name

Japan Atomic Power Company

Date of occurrence

18.October.2007

Unit name

Tsuruga 2

Event

Flaws Identified in the Steam Generator Reactor Coolant
Inlet Nozzle Welds

International Nuclear Event Scale (INES)

0

Status of report

Final report

Status when event occurred
Considering the reported domestic and foreign events of stress corrosion cracking under the primarycooling water environment (hereafter referred to as “PWSCC”) in the portion made of Ni base alloy of thereactor coolant pressure boundary of the pressurized water reactor plant, during the period of the 16thplanned outage which commenced on August 26, 2007, the in-service inspection is being performed as aperiodic utility inspection. Also in the 16th planned outage, a material change (from 600 series Ni basealloy into 690 series Ni base alloy having better corrosion resistivity) of the nozzles(*1) by replacing thereactor vessel upper head is being implemented.
In addition, as an action for material surface stress relieving, it is planned to perform shotpeening work (*2)
for the steam generator reactor coolant inlet and outlet nozzles etc. in a sequence started with the 16thplanned outage.
For the flaw which was recently reported to have been identified in the surface of the weld portion between
the steam generator reactor coolant inlet nozzle (hereafter referred to as the “inlet nozzle”) and the safe
end(*3) in a domestic pressurized water reactor plant, it was in a location where the ultrasonic test(*4)
cannot be applied because of a limitation by its configuration, and so it was discovered by an eddy current
test(*5) performed as a pre-work checking activity for the shotpeening work.
Therefore, for the weld portions(*6) (a total of eight weld portions) of the inlet and outlet nozzles of foursteam generators (A~D), the eddy current test of the surfaces of these nozzle weld portions was being
performed since October 9, 2007 as a pre-work checking for the shotpeening work. In the course of thework, five locations with significant signal indication were identified in the inlet nozzle weld portion of thesteam generator B.
*1 Nozzle: A part which is provided to connect pipe etc. to the vessel body and structurally integrated withthe vessel body.
*2 Shotpeening work: A work to change tensile residual stress in a metal surface into compressive stressby striking the metal surface with metal balls at high speed.
*3 Safe end: A short pipe to connect a steam generator made of carbon steel with a pipe made of stainlesssteel.
*4 Ultrasonic test: A method to detect a defect in metal using ultrasonic wave. Ultrasonic wave created bya probe at the metal surface propagates inside the metal and the status of defect is identified using the

JANTI-OTO-008-002 attachment5

aspect of wave reflection.
*5 Eddy current test: A method to detect a flaw in a test object from the change of electromagnetic
induction in a material by causing the eddy current on the material surface.
*6 Weld portion: Steam generator reactor coolant inlet nozzle (carbon steel) has a weld portion between
the nozzle and the safe end (a short pipe) and a weld portion between this short pipe and the reactor
coolant loop pipe (stainless steel).
For the weld portion between nozzle and safe end, 600 series Ni base alloy was used as the weld metal,
and stainless steel was used as the weld metal for the weld portion between the safe end and the reactor
coolant loop pipe (stainless steel).
Subsequently, the eddy current test of the inlet and outlet nozzle weld portion surface was performed forthe other three steam generators, and thereby significant signal indication was identified at one location
for the steam generator A inlet nozzle and at 23 locations for the steam generator C inlet nozzle. Thesesignificant signal indications were all identified in the circumferential weld portion.
Further, no of significant signal indication was identified in the steam generator D inlet nozzle. Also, for theoutlet nozzles, no of significant signal indication was identified for any of the (A~D) steam generators.
For all of 29 locations where significant signal indication was identified by the eddy current test, theultrasonic test was performed, and the locations where flaw depth was possible to be clearly identified
were two locations in the steam generator B inlet nozzle and seven locations in the steam generator C
inlet nozzle.
Among these flaws of which depth were identified, both of the two locations of the steam generator B inlet
nozzle had approx. 12 mm depth (while length were approx. 19 mm and approx. 21 mm). Consideringsuch a flaw depth, the resultant thickness of the affected location (approx. 67 mm) was evaluated to be
below 75 mm, the value described in the application document for construction permit based on theElectricity Utilities Industry Law.
Also, for the flaws in the seven locations of steam generator C inlet nozzle, the maximum depth wasapprox. 13 mm (length: approx. 12 mm), therefore the thickness of the affected location (approx. 65 mm)
was evaluated to be below 75 mm, the value described in the application document for construction permit
based on the Electricity Utilities Industry Law.
There was no impact from this event on the environment.

Summary of examination of cause
Since significant signal indication was identified in a total of 29 locations in the inlet nozzle weld portions ofsteam generators A, B and C, the investigation based on the causal factor analysis chart was performed.
(1) Observation by replica(*7), SUMP(*7) and etching(*9)
Among the locations identified with significant signal indication by eddy current test, one location (No.1) of
the steam generator A inlet nozzle weld portion and two locations (No.1 and No.2) of the steam generator
B inlet nozzle weld portion that were evaluated to have the deepest flaws in the five locations withindication were selected for the objects and the surface appearance verification by replica, the metal

JANTI-OTO-008-002 attachment5

texture observation by SUMP and etching were performed.
*7 Replica: An indirect observation method used when a surface to be observed can not be observed
directly. An organic material which has a high capability of shape following and a excellent removability isadhered to the surface to be observed, thereby a “replica” transferring the unevenness of surface is madeand used to observe indirectly the surface to be observed.
*8 SUMP: Film etc. is adhered to a surface of damaged portion to transfer its shape, which is observed bya microscope. By this method, metal texture investigation for a damaged portion become possible as well
as by the method using a sample cut out.
*9 Etching: A corroding process used for observation of a metal texture by optical microscope etc. In thisprocess a test surface after polishing is corroded with oxalic acid solution etc. to remove the surface layer
affected by polishing process, and the texture to be observed is exposed.
1) Steam generator A inlet nozzle weld portion
One location with significant signal indication was an association of longitudinal surface roughing (fineflaws).
Since the area having rough surface (fine flaws) is different in finished status indicating coarse finishingcompared to that of the surrounding area, the rough surface was considered to be resulted from finishingby a grinder.
Further, the maximum length of surface roughing (fine flaw) was approx. 5 mm and the cracking was along
the dendrite(*10) boundary indicating the branching characteristic to PWSCC.
*10 Dendrite: Column shaped crystal produced while melted metal becomes solid.
Since a trace of approx. 10 mm length thought to be that of rewelding was identified at the boundary of theinlet nozzle bevel buttering weld portion and the circumferential weld portion and the area around it wasseen with coarse finishing, the affected portion was considered to be finished by a grinder after therewelding.
The surrounding area without any rough surface (fine flaws) was seen with circumferential fine finishing,
and therefore it was considered to be finished with buffing after machining by a grinder.
2) Steam generator B inlet nozzle weld portion
For the signal indications of the two locations (No.1 and No.2) investigated this time, the signal indicationof No.1 consisted of two circumferential cracks of approx. 4 mm and approx. 6 mm length with an overall
length of approx. 9 mm. The signal indication of No.2 was a series of multiple longitudinal cracks ofapprox. 1 mm to approx. 4.5 mm length with an overall length of approx. 11 mm.
Around these cracks, multiple rough surfaces (fine flaws) were distributed.
The surrounding area with rough surface (fine flaws) was seen with coarse finishing, differing from thesurrounding area without any rough surface, and therefore it was considered to be finished by a grinder.
The rough surface (fine flaw) was along the dendrite boundary and the branching characteristic to the
PWSCC was identified.

JANTI-OTO-008-002 attachment5

Since a trace of approx. 7 mm diameter thought to be that of rewelding was identified at the boundary of
the inlet nozzle bevel buttering weld portion and the circumferential weld portion and the area around it
was seen with coarse finishing, the affected portion was considered to be finished by a grinder after therewelding.
The surrounding area without any rough surface (fine flaws) was seen with circumferential fine finishing,
and therefore it was considered to be finished with buffing after machining by a grinder.
(2) Investigation by sampling from the actual weld portion
From the steam generator C inlet nozzle weld portion, a part including the No.3 and No.4 locations withsignal indication among the 23 locations identified with significant signal indication by the eddy current test
for steam generator C was taken as a boat sample and a detailed examination was performed at a hotlaboratory.
1) Appearance observation
From the difference in color of the inner surface, the boundary between the safe end and thecircumferential weld was identified.
Deformation and scratching were not identified for the sample taken.
2) Surface observation
Signal indication of No.3 identified by the eddy current test was of longitudinal cracking, and around thiscracking, longitudinal and circumferential traces thought to be that made by grinding and buffing wereidentified.
Among these, the trace thought to be due to grinding was clear compared to the trace of grinding found forthe inlet nozzle weld portions of steam generators A and B.
3) Penetrant test
An indication of approx. 5.5 mm in the No.3 cracking portion and an indication of approx. 4.7 mm in theNo.4 cracking portion were identified.
No indications other than these were identified.
4) Fracture observation
The cracking of No.3 had a surface length of approx. 5.8 mm and the maximum depth of approx. 6.4 mm.
The cracking of No.4 had a surface length of approx. 5.8 mm, the maximum depth of approx. 3.4 mm andthe crack internal maximum length of approx. 9.3 mm.
Fracture along the dendrite boundary characteristic to the PWSCC was identified for both the No.3 andNo.4 cracking.
5) Fracture observation by SEM
Fracture along the dendrite boundary characteristic to the PWSCC was observed for both the No.3 andNo.4 cracking.
6) Analysis of deposit on the fracture
Using an energy dispersion detector (EDS), a chemical composition analysis of the deposit on the fracturesurface was performed. For both the No.3 and No.4 fracture, the main elements were the components of
the weld metal (Ni, Cr, Fe etc.), and no deposit of chlorine was identified.
7) Macroscopic and microscopic observation of the cross-section

JANTI-OTO-008-002 attachment5

Both the cracking No.3 and No.4 were identified to be the cracking along the dendrite boundary
characteristic to the PWSCC.
Further, no trace of rewelding was identified around the cracking of No.3 and No.4.
8) Hardness measurement
Hardness measurement was performed from the inner surface toward the depth direction. As a result, thenear surface layer of No. 3 was of HV 321 to HV 339 and that of No.4 was of HV 309 to HV 395, thus
hardening in the near surface region was verified.
This was considered to be due to the effect of machining by grinder etc.
For both the No.3 and No.4, any difference in hardness distribution along depth between the fracture andthe inside was not identified.
9) Chemical composition analysis
Using an energy dispersion detector (EDS), a chemical composition analysis was performed for the inlet
nozzle bevel buttering weld portion and the circumferential weld portion. As a result, the standard of 600
series Ni base alloy weld metal was met and no chlorine deposit on the surface was identified.
10) Measurement of residual stress
Residual stress in the circumferential weld portion surface was performed by X-ray residual stressmeasurement. The result was -591 MPa to -263 MPa in circumferential direction and -546 MPa to 244
MPa in longitudinal direction and thus the residual stress was verified to be compressive.
11) SMP observation
Near cracking No.3, four locations having fine cracking of approx. 240 μm to 4.5 mm were identified.
The amount of surface roughing (fine flaws) was less than that found for the inlet nozzle weld portion of
steam generators A and B.
Further, as far as the area where the SUMP observation was performed, no rewelding trace was identified.
12) Additionally performed microscopic cross-section observation
For the fine cracking found in the four locations near No.3 cracking, microscopic cross-sectional
observation was performed. Among the multiple fine cracking, the deepest one was of 0.8 mm depth and it
was identified to be a cracking along the dendrite boundary.
Further, no cracking having a depth greater than No.3 cracking was identified.
(3) Investigation of manufacturing and operation history
1) Investigation of manufacturing history
Steam generators of Tsuruga 2 were manufactured during the period of July 1982 to April 1985, andinstalled at the site during February to July 1985. The work records and inspection records for that period
were confirmed and a hearing from relevant persons was performed to investigate the manufacturinghistory.
As a result, no singularity in the manufacturing procedure of the inlet nozzle was identified, the welder had
enough experiences, and no record of rewelding performed was identified from the records investigatedthis time.
Further, as a result of hearing from relevant persons, the type of tools used for grinding and buffing of thecircumferential weld portion was confirmed to be possible to differ by the worker.

JANTI-OTO-008-002 attachment5

2) Investigation of operational history
Operational history of Tsuruga 2 from the start of operation (February 17, 1987) to the start of 16th planned
outage was investigated and the result was as follows:a. Reactor coolant temperature and pressure
Tsuruga 2 has experienced since the start of operation one event of reactor automatic shutdown, six
events of manual shutdown, and three events of power suppression. None of the events accompaniedwith transients other than these were identified.
Further, it was confirmed that during these transients, abnormal temperature and pressure was notindicated.
b. Status of water chemistry control
Since the start of Tsuruga 2 operation, the water chemistry of reactor coolant has been kept to satisfy thevalues defined by plant’s safety preservation rules and no harmful material such as chlorine etc. wasidentified.
(4) Confirmatory test for surface machining condition (Mockup test)
After welding the inlet nozzle to the safe end, finishing of the inner and outer surfaces with grinding andbuffing was performed, and this could be a cause of hardening and high tensile residual stress. Further, in
the area where finishing with a grinder was performed for the actual weld portion, cracking and roughsurface (fine flaws) were identified. Considering these, a mockup test to simulate the surface finishingcondition of the actual weld portion was performed.
For testing, buildup welding by 600 series Ni base alloy was performed on a stainless steel plate, then it
was finished with a grinder and a buff under the finishing condition based on the result of hearing, andusing this as the test sample, surface condition observation, residual stress measurement, hardness
measurement and roughness measurement were performed.
1) Surface condition observation
From the samples (a total of ten species) made with variation of surface finishing condition by grinder andbuff, the surface replicas were taken and compared with that taken from the actual weld portions. The
result was as follows:a. For the replica taken from steam generator A inlet nozzle weld portion, the area of grinder machining
with rough surface (fine flaws) was similar to the finished status by air grinder (skill touch) of the test
sample No.6.
Also, it’s surrounding area finished by buffing and having no cracking was in the same condition as thecondition of sample No.4 finished by buffing after air grinder treatment (skill touch).
b. For the replica taken from the steam generator B nozzle weld portion, the area machined by grinder andhaving the cracking of No.1 and No.2 and the rough surface (fine flaws) around them was in a same
condition as the condition of the sample No.6 finished by air grinder (skill touch).
Also, it’s surrounding area finished by buffing and having no cracking was in the same condition as thecondition of the sample No.4 finished by buffing after air grinder treatment (skill touch).
c. For the replica of the boat sample taken from the steam generator C nozzle weld portion, generalfinishing condition of the sample remained a wide clear trace of grinder finishing in the surface finished by

JANTI-OTO-008-002 attachment5

buffing, and in a same condition as the sample No.9 finished by buffing after high frequency grinderfinishing.
2) Residual stress measurementa. For the surface (of the sample No.6) finished by air grinder (skill touch), residual stress was measured
and the result was 470 MPa as an average.
b. For the condition finished by buffing after air grinder finishing (sample No.4), residual stress wasmeasured and the result was -292.5 MPa as an average.
c. For the condition finished by buffing after high frequency grinder finishing (sample No.9), residual stress
was measured and the result was -211 MPa as an average which was a compressive stress as the actual
weld portion.
d. For the condition finished by buffing after high frequency grinder finishing (sample No.9), a local
residual stress was measured within a small area around the bottom of grinding groove, and thereby atensile stress was verified to exist.
Since there was a difference in residual stress by approx. 680 MPa between the surface finished by highfrequency grinder and the surface additionally finished by buffing, the residual stress at the bottom of
grinding groove in the area with deep groove made by high frequency grinder was considered possible tobe estimated by adding a correction by approx. 680 MPa to the value obtained from a general residual
stress measurement.
3) Hardness measurement
For the area finished by buffing after finishing by high frequency grinder, the hardness in near surface
layer was HV 283 to HV 384. By this result, as well as by the result of hardness measurement by sample
from actual weld, the hardening in the nearby surface layer was verified.
(5) Review of literature
Literature on cracking in 600 series Ni base alloy weld was reviewed. As the events of damage in a similar
portion, an event of leakage from the reactor vessel outlet nozzle weld at V.C. summer in the US(discovered in October 2000) and an event with ultrasonic test indication identified for the reactor vessel
outlet nozzle weld of Ringhals 3 and 4 in Sweden (discovered in August and September 2000) werereported.
Also, in Tsuruga 2, an event of cracking in the nozzle for pressurizer relief valve etc. occurred (discoveredin September 2003).
Cracking in all of these cases was longitudinal cracking, and cracking in the case of V.C. Summer in US
and Tsuruga 2 developed in the weld metal stopped at the boundary with low alloy steel and at theboundary with the safe end. The cause of cracking in both cases is considered to be PWSCC occurred
due to high tensile residual stress resulted from repair welding.
Further, on PWSCC caused in 600 series Ni base alloy weld, the 360°C temperature accelerated SCC
constant load test was performed and from it PWSCC is considered to be caused at a stress greater than
300 MPa.
(6) Other forms of damage

JANTI-OTO-008-002 attachment5

Also investigation for the possibility of ductile cracking and fatigue cracking was performed. Though, there
was no anomaly in usage environment condition and material used and thus these were not considered as
the cause. In addition, for cracking due to high temperature during manufacturing, what could be a cause
was not identified from the manufacturing history and inspection records.
(7) Summary of the investigation result
The rough surface (fine flaws) and cracking identified in the inlet nozzle weld portion of steam generator A,
B and C were all along the dendrite boundary and they were verified to exhibit PWSCC aspects.
Further, for the residual stress (circumferential) in surface layer due to finishing by grinder etc in the steam
generator C inlet nozzle weld portion, it was confirmed that summing the residual stress measured value
for the actual weld portion boat sample measured at the hot laboratory (-591 MPa to -263 MPa) and thecorrection value determined from the result of the mockup test (approx. 680 MPa), the surface residual
stress at the bottom of groove made by grinder was evaluated at approx. 90 MPa to approx. 420 MPa, andthus a tensile stress exceeding the PWSCC occurrence condition of approx. 300 MPa could exist.
Further, the residual stress in surface layer due to air grinder finishing (skill touch) in the area where
cracking and rough surface (fine flaws) were identified in the inlet nozzle weld portion of steam generators
A and B was verified to be equivalent with the stress in the bottom of groove made by high frequency
grinder from the result of the mockup test, therefore it was also considered to be equivalent with thesurface stress of cracking in steam generator C inlet nozzle weld portion during operation.
The operational stress (mainly due to internal pressure) caused inside the pipe material was larger in thecircumferential direction than in longitudinal direction and this was considered to be the reason that thedirection of cracking development was longitudinal.
From the above, these cracking and rough surface (fine flaws) was considered to be due to PWSCC.

Cause of event
During manufacturing of steam generator, the inlet nozzle was welded to the safe end using 600 series Nibase alloy weld metal and finishing by air grinder and buff was performed. For the area (steam generator
A and B SUMP investigation area) where finishing by air grinder (skill touch) was performed after
rewelding and the area (steam generator C boat sample investigation area) where the trace of finishing by
high frequency grinder remained locally, high residual stress in surface layer of the inner surface wascaused. PWSCC was considered to have occurred due to this high stress and developed in longitudinal
direction due to operational stress etc.

Measures to prevent recurrence
For steam generators B and C, the following recurrence prevention measures will be implemented:(1) The circumferential weld portion inner surface will be machined for entire circumference in order toremove shallow cracking, and then deeper cracking will be removed by grinder.
(2) Subsequently, for the area where deeper cracking was removed, buildup welding with 600 series Nibase alloy will be performed. Then, on the circumferential weld portion inner surface entire circumferencemachined to remove shallow cracking, buildup welding will be performed using 690 series Ni base alloy
which has a better corrosion resistivity.

JANTI-OTO-008-002 attachment5

(3) For the sake of caution, from the view point of reducing residual stress in the area where buildup
welding was performed with 690 series Ni base alloy, buff finishing will be performed for the area.
Also, for steam generator A of which circumferential weld portion was identified to have rough surface (fineflaws), removal of the rough surface will be performed based on considerations for the structural strengthof the portion, the function of stainless lining material (inner surface buildup portion) of the inlet nozzle
inner surface and its inspectability for the eddy current test from the inner surface side.
Then, the residual stress in the weld portion surface will be reduced by shotpeening etc.

JANTI-OTO-008-002 attachment5

Attachment (1/4)
Periodic inspection status at Tsuruga Unit 2(The cause and countermeasures of the flaws in the steam generator reactor
coolant inlet nozzle welds)
Location of occurrence

Steam generator schematic
Steam outlet

Reactor containment

Pressurizer
Steam

Steam
generator
Control rod

Steam generator
Feedwater inlet
Water

Structure of steam generator
reactor coolant inlet nozzle

FuelPartition plate
Reactorcoolant pump
Stainlessoverlay

Reactorvessel

ECT results (location withsignificant indication)
View from steam generator side
Steam generator(Head)

Nozzle(carbonsteel)

Nozzle(carbonsteel)

Nozzle bevel
buttering weld
(Ni base alloy)

Weld portion(Ni base alloy)
Safe end(stainlesssteel)
Weld portion(stainless steel)
Reactor coolant
pipe (stainlesssteel)

Significant signal
indications
identified by ECTReactor coolant
inlet nozzle

Safe end(stainlesssteel)
Weld portion(stainless steel)
Reactor coolant
pipe (stainlesssteel)

Reactor coolant
inlet nozzle

Reactor coolant
outlet nozzle[Dimensions are nominal size on
fabrication drawings]
Stainlessoverlay(Outside)

NozzleEvaluation results of depth and length indications by UT
Although significant indication found, length and depth
could not be identified.

(Inside)Steam generator A

ButteringbuildupWeld
portion
Safe
end
SUMP
observation

approx.79mm(Measured value)

Evaluation results of depth and length indications by UTMaximum depth: approx. 12 mm, Maximum length:
approx. 21 mm

Stainlessoverlay(Outside)

NozzleSteam generator B(Inside)ButteringbuildupWeld
portion
Safe
end
SUMP
observationapprox.79mm(Measured value)

Stainlessoverlay(Outside)

NozzleEvaluation results of depth indications by UTMaximum depth: approx. 13 mm, Maximum
length: approx. 12 mm

Evaluation results of length indications by
UT
Maximum depth: approx. 14 mm, Maximum
length: approx. 9 mm

(Inside)ButteringbuildupWeld
portionSafe
end
approx.78mm(Measured value)Cross
sectionSample taking
position(Value in the application for construction
plan approval: approx.75mm)

JANTI-OTO-008-002 attachment5

Sample taken were shipped to research laboratory organization anddetailed investigation such as fracture observation etc. were performed.

Steam generator CAttachment (2/4)
Cause investigation result
Steam generator A cracking of No.1
Cracking of No.1 was an association of longitudinal fine flaws of max. length approx.
5 mm.
In the area with fine flaws, a rewelding trace of approx. 10 mm diameter was
identified.
The area with no cracking was supposed to be finished by buffing.
Cracking was identified to have branching along crystal boundary.

View from steam generator side

Observation
direction

SUMP
observation
range

StainlessoverlayNozzle(Outside)

Cracking of No.1

SUMP observation result(Inside)Butteringbuildup

Butteringbuildup
Circumferential
weld portion

approx.
10mm

Circumferential
weld portion

Safe
end

approx.5mm

Trace of weldingoo0 sideCross section

90 sideArea seen with finefinishing condition

Trace of welding

Area seen withcoarse finishing

Steam generator B cracking of No.1 and No.2
Cracks of two locations were longitudinal with length of approx. 9 mm and approx. 11 mm.
Around these cracks, fine flaws were identified.
Between two locations of cracking, a rewelding trace of approx. 7 mm diameter was identified.
The area with no cracking was supposed to be finished by buffing.
Cracking was identified to have branching along crystal boundary.

View from steam generator side

SUMP observation
range
Observation
direction

Crackingof No.1

Stainlessoverlay

SUMP observation result
Cracking of No.2

NozzleCircumferential
weld portion

Butteringbuildup
Circumferential
weld portion

approx. approx.7mm
9mm
approx.11m

Butteringbuildup(Inside)(Outside)

Crackingof No.1

Safe
end

o

0 sideo

90 sideCross section
Area seen with finefinishing condition

JANTI-OTO-008-002 attachment5

Trace of welding

Area seen withcoarse finishing

Trace ofwelding

Crackingof No.2

Uneven polishing in preparative
stage for SUMP observation

Attachment (3/4)
Steam generator C cracking of No.3 and No.4
View from steam generator side

Observation
direction(Outside)
Butteringbuildup
Circumferential
weld portion

By penetrant test, cracking of No.3 and No.4 were identified with indication of approx. 5.5 mm and approx. 4.7mm, respectively.
Linear pattern was seen all over the sample surface and the pattern was more clear than that seen for steam
generator A and B.
By fracture observation, maximum depth was identified to be approx. 6.4 mm (No. 3) and approx. 3.4 mm (No.4).
Fracture branching along weld metal crystal grain boundary was identified.

StainlessoverlayNozzleCracking of No.1

Indication of No.3

(Inside)Observation resultof sample surface

Penetrant test resultof the sample taken
Indication of No.4

Safe
end
0osideCross section

90osideSampling position
90oside0osideFracture observation result
for cracking of No.3
Inner surface side

Fracture observation result
for cracking of No.4

Scanning type electron microscopic
observation result (magnified)

Inner surface side

Fracture
Fracture
Forcedfracture

Forcedfracture

Crystalline boundary
fracture
Inner surface

Length at surface:approx.5.8mm

Estimatedboundary between
the weld and thebuttering buildup
weld

Inner surface

Length at surface:approx.5.8mm

Inner surface

Inner surface

Forced fracture
Fracture
Fracture

Maximum depth:
approx.3.4mm

Maximum depth:
approx.6.4mm

Partially lost while
cuttingNozzle side

Estimated boundary
between the weld and thebuttering buildup weld

Forced fracture
Cutting margin

Maximum length: approx.9.3mm
CavityForced fracture

Cutting
marginSafe end side

Nozzle side

Safe end side

Confirmatory test for surface
machining condition

Evaluation by confirmatory test forsurface machining condition･ Approx. 680 MPa difference exist between the surface processed

Condition aroundcracking of steam
generator A and B
was simulated.

by grinder and the surface additionally finished by buffing.

･ In area of deeper grinding groove, residual stress at groove bottom
is considered possible to be estimated by adding a correction of
approx. 680 MPa to a general measured value.
･ Adding 680 MPa to residual stress measured value for the sample
taken from steam generator C, residual stress at grinding groove
bottom was estimated at 90~420 MPa tensile.

(Finished bygrinder-buff-grinde
r sequence)

Approx. 470 MPa as average (Tensile stress)
Condition aroundcracking of steam
generator C was
simulated.
(After coarser
grinding process,finished by buffing)

Approx. 210 MPa as average (Compressive stress)

JANTI-OTO-008-002 attachment5

Residual stressmeasurement
result for the
sample takenfrom steam
generator C (forreference)

263~591 MPa (Compressive stress)

Attachment (4/4)
Presumed cause
From the characteristics of fracture etc. the cause is presumed to be stress corrosion
cracking occurred and developed by coincidence of the three factors below.
Environment

Verification test of temperature effect on stress corrosion
cracking occurrence[Operated in primary water environment including hightemperature, high pressure etc.]

Material

Stress corrosion cracking occurrence test in hightemperature water for 600 series Ni base alloy[600 series Ni base alloy of which stress corrosion cracking has
been verified is used as the weld metal.]Stress

Evaluation by surface machining condition verification test
[Due to welding and grinding after it during steam generator
manufacturing, high tensile residual stress was caused insurface layer of nozzle inner surface.]

Stresscorrosioncracking

Material

Environment

StressMaterial, environ and stress are all in the region of stress corrosion cracking.

Countermeasures
Circumferential weld portion(600 series Ni base alloy)

Steam generator A
Channel head

Guiding arm

Buttering weld(600 series Ni base alloy)

Nozzle(carbon steel)

Safe end(stainlesssteel)

Inlet nozzle
Reactor coolant
inlet pipe
ECT indication length
Machining head

Stainlessoverlay

To be finished smoothly

Sealing plug

Remove the cracking identified in weld between inlet nozzle and safe
end.
Verify crack removal by eddy current test (ETC)
Considering future inspectability, finish inner surface smoothly.
Perform shotpeening to reduce residual stress in weld portion surface.
Steam generator B and C
Inlet nozzle

Circumferential weld portion(600 series Ni base alloy)

Buttering weld(600 series Ni base alloy)

Nozzle(carbon steel)

Safe end(stainlesssteel)

Safe end

Elbow

ECT indication length
CutoutAfter completion of inlet
nozzle to safe end weld
repair, recovered withnew elbow.
CutoutFull circumference buildup
repair welding(690 series Ni base alloy)

Stainlessoverlay

Local buildup repair welding(690 series Ni base alloy)

Remove the cracking identified in weld between inlet nozzle and safe end.
For the position locally machined, perform buildup welding using 600 series Ni base alloy.
Perform full circumferential surface buildup welding using 690 series Ni base alloy which has
better corrosion resistivity.

In performing strength evaluation required for construction plan application, evaluation of calculation condition for safeend thickness ( 75 mm) etc. to be performed according to current evaluation method.

JANTI-OTO-008-002 attachment5