Company nameHokkaido EPCO

Date of occurrence

April 9, 2008

Unit nameTomari 2

Event

Confirmation of Flaws at Welded Part of Primary Coolant
Inlet Nozzle of Steam Generator A and B

International Nuclear Event Scale (INES)

0

Status of report

Final report

Status when event occurred
During the 13th periodic inspection conducted from March 13, 2008, an eddy current test*1 (hereinafterreferred to as “ECT”) was conducted based on the NISA document “Inspection of the inner surface of
inlet/outlet nozzle welded parts of steam generators” dated on November 16, 2007. As a result,significant signals were indicated at three places with steam generator (hereinafter referred to as “SG”) A
and 10 places with SG-B at the welded parts of primary coolant inlet nozzle (hereinafter referred to as“inlet nozzle”). No significant signal was indicated at the primary coolant outlet nozzle of both SGs.
Ultrasonic test*2 (hereinafter referred to as “UT”) was conducted at areas where significant signals wereindicated with ECT and significant signals that can evaluate the depth of the flaw were indicated at one
place in SG-A and SG-B respectively.
Depths of the flaws were evaluated as approx. 7 mm and 5 mm in SG-A and SG-B respectively. The plate
thicknesses of the two areas which were subtracted the depths of the flaws from the plate thicknesses of
the relevant parts of approx. 78 mm, SG-A approx. 71 mm and SG-B approx. 73 mm, were judged below
the value of the application for construction plan approval of 75 mm stipulated by the Electricity Utilities
Industry Law.
<Time series>
March 31 to April 3: ECT conducted at SG-A and B inlet nozzle
April 3 to April 9: UT conducted at SG-A and B inlet nozzle
April 6 to April 9: ECT conducted at SG-A and B outlet nozzle*1: Eddy current test (ECT)
A method to detect a flaw by causing an eddy current to an object material, which is done by contacting acoil with high-frequency current to the object material such as a pipe, and a flaw is detected fromchanges in eddy current that occur around flaws.
*2: Ultrasonic test (UT)
A method to detect a flaw inside an object material using a characteristic of ultrasonic wave which thenpropagates through the object material and reflects at an area with discontinuity such as flaws.

Summary of examination of cause

An investigation of flaws at inlet nozzle welded part of SG was conducted based on factor analysis FT
diagram. As for flaw observations, a replica examination*3 was conducted on the indicated part No. 1 of
SG-A and a replica examination and SUMP observation*4 was conducted on the indicated part No. 5 of
SG-B.
*3: Replica examination
Observation to check the surface fabrication status by transcribing the condition of the surface to film.
*4: SUMP observation
Observation of a damaged surface using a film, etc. The damaged surface is polished and a film, etc. isplaced on the damaged surface to be transcribed, which is observed using a microscope.
(1) Replica examination resulta. Traces of buff-finishing *6 were observed above grinding work*5 (grinding) at the whole area of theindicated part No. 1 of SG-A, and clear traces of grinding work (grinding) were observed including thecracked part.
b. Traces of buff-finishing were observed above grinding work (grinding) at the whole area of theindicated part No. 5 of SG-B and traces of finishing work other than grinding work (polishing) wereobserved at the cracked part.
*5: Grinding work
A surface finishing work performed on a welded surface using a grinder (a machine tool to grind or topolish a material or tool by rotating a circular grinding stone in high speed). The finished surface isrougher when compared to that of buffing work.
*6: Buff work
A surface finishing work performed on a welded surface using circular sandpaper, etc. (buff) attached to a
tool. The finished surface is finer when compared to that of grinding work.
(2) SUMP observation result
SUMP observation was conducted on the indicated part No. 5 of SG-B.
a. Cracks were found at girth weld area and overlay weld area of 600 type nickel base alloy and were notfound at the safe-end of stainless steel and the stainless lining part of the inlet nozzle.
b. Cracks of approx. 1.8 mm to 5.0 mm in axial direction were found, where the whole length was approx.
7.6 mm.
c. Traces of repair welding of approx. 7 mm in diameter were found around the crack area.
d. The cracks have occurred along the dendrite border*7 and a similar facet to that of primary coolant
stress corrosion cracking*8 (hereinafter referred to as “PWSCC”) observed both domestically andinternationally at welded area using 600 type nickel base alloy was found.
*7: Dendrite border
A pillar-shape crystal (dendrite crystal) is formed at weld areas when melted metal hardens and theborder of this crystal texture is called the dendrite border.
*8: Primary coolant stress corrosion cracking (PWCC)
A stress corrosion cracking that occurs with 600 type nickel base alloy in the primary coolant environmentthat is characteristic of PWR plants (cracking occurs due to overlay of conditions of three elements;
material, environment and stress). As a result of SUMP observation, cracks in the indicated part No. 6 of

SG-B were detected, which also had a similar facet to that of PWSCC as in the indicated part No. 5 of
SG-B.
(3) Manufacture and operation history investigation result
a. Manufacture history investigation
SG of Unit 2 of Tomari NPP was manufactured during the period of July 1985 to March 1989 and thehistory of manufacturing was investigated through test inspection records of that time and interviews to
persons involved.
(a) Investigation of test inspection record- The welded part of inlet nozzles of SGs were manufactured according to the procedure stipulated by the
manufacturer and no peculiar situation was found in the work procedure and term of weld work in eitherof the inlet nozzles.
- It was confirmed that the welders had sufficient experience.
- No record of repair welding was found.
(b) Investigation through interviews with persons involved- Although there is a possibility of various kinds of grinders being used by workers, buffing work
(reciprocated three times) was performed after grinding work (grinding) as the standard finishing method.
- It was confirmed that there is a possibility of repair welding performed after buffing work in some cases,
such as when a significant signal was indicated through penetrant test (hereinafter referred to a “PT”) by
the manufacturer.
- It was confirmed that there is a possibility of grinding work performed (polishing) when repair welding
was performed.
*9: Penetrant test (PT)An observation method used to visually check the flaws open on the surface of a test sample. High
penetration liquid with visible dye is penetrated into the surface, which remaining liquid on the surface iswiped off and the indicated pattern by the penetrated liquid is observed using a developer.
b. Operation history investigation
The operation history from commencement of operation (April 12, 1991) to 13th periodic test (parallel off
on March 13, 2008) of Tomari NPP Unit 2 was investigated.
(a) Primary coolant temperature/pressure
Two events of manual reactor shut-down were found as events with transient conditions that occurred
during the operation of plant. No abnormal transition in temperature/pressure that causes excessive
stress was found in said transient conditions.
(b) Water quality control status
The water quality after the commencement of operation has been satisfying the water quality standard
stipulated by the safety preservation rules and it was confirmed that hazardous components such as
chloride ion were controlled.
(4) Welded part material confirmation result
It was confirmed from the weld record and mill sheet that the components of the material were according
to the SG manufacturer’s specifications and was of 600 type nickel base alloy.
(5) Design document confirmation result

It was confirmed from the strength calculation document of the application documents for license of
construction plan that considerations to ductile cracking and fatigue cracking were made in the design.
(6) Document investigation result
As a result of document investigation regarding cracking in welded part using 600 type nickel base alloy,
damage examples of similar areas were confirmed as described below.
a. Welded part of reactor vessel outlet nozzle
Leakage was reported at V. C. Summer NPP, U.S. (detected in October 2000) and flaw indication wasreported at Units 3 and 4 of Ringhals NPP, Sweden (detected in August and September, 2000). The
cracks detected at both NPPs were cracking in the axial direction along the dendrite border, wherein theevent in V.C.Summer NPP reported that the crack has advanced through the weld metal and almost
stopped at the borders of low-alloy steel and safe-end. The cracking was assumed to be caused by
PWSCC that occurred due to high tensile residual stress from repair welding work.
b. Pressurizer nozzle welded part
Leakage was reported at Unit 2 of Tsuruga NPP of Japan Atomic Power Company (hereinafter referred toas “Tsuruga NPP Unit 2”) (leakage in pressurizer relief valve nozzle and flaw indication in safety valve
nozzle, detected in September 2003). The crack that was detected was cracking in the axial direction
along the dendrite border, which was reported that the crack has advanced through the weld metal andalmost stopped at the borders of low-alloy steel and safe-end. The cracking was assumed to be caused
by PWSCC that occurred due to high tensile residual stress from repair welding work.
c. Welded part of SG inlet nozzle
Flaw indications were reported at Unit 2 of Mihama NPP of Kansai Electric Power Company (hereinafterreferred to as “Mihama NPP Unit 2”) (detected in September 2007) and Tsuruga NPP Unit 2 (detected in
October 2007), respectively. Cracks detected in both NPP were cracking in the axial direction along thedendrite border. Cracking in the case of Mihama NPP Unit 2 was assumed to be caused by PWSCC that
occurred due to high tensile residual stress in the inner top surface area caused by machining work
performed during the manufacturing process. Also as for the case of Tsuruga NPP Unit 2, cracking was
assumed to be caused by PWSCC that occurred due to high tensile residual stress in the inner top
surface area caused by grinding work (polishing), etc. after repair welding performed during the
manufacturing process. Furthermore, a confirmation test of surface treatment status (hereinafter referred
to as “mockup test”) and a sampling investigation on actual equipment were conducted in theinvestigation of Tsuruga NPP Unit 2 and surface residual stress was estimated.
- Results of surface observation and X-ray residual stress measurement with 10 kinds of test samples
according to the assumed surface treatment procedure were reported.
- As for the residual stress (circumferential direction) of the surface area of welded part of Tsuruga NPP
Unit 2 SG-C inlet nozzle caused by finishing work such as grinding, the surface residual stress at floors ofgrinder grooves is estimated as approx. 90 MPa to 420 MPa when the correction value obtained from themockup test result (approx. 680 MPa) is added to the residual stress value of the boat sample measured
in a hot laboratory (- 591 MPa to - 263 MPa). The surface residual stress of test sample No. 6 ismeasured as approx. 470 MPa on average.
d. Stress in occurrence of PWSCC

As for the occurrence of PWSCC at the welded part using 600 type nickel base alloy, a constant load
stress corrosion cracking (SCC) test is conducted and PWSCC occurred at the stress of approx. 300
MPa or more in the primary coolant environment.
(7) Estimation of surface residual stress of Tomari NPP Unit 2 SG inlet nozzle
Comparisons were made with inner surface conditions that were checked through replica examinationand stress measurement results of the mockup test and sampling investigation on actual equipment
conducted with Tsuruga NPP Unit 2 in order to estimate the surface residual stress of Tomari NPP Unit 2.
- Clear traces of grinding work (grinding) were observed including the cracked part at the indicated partNo. 1 of SG-A, where the surface was in a similar condition as to that of the boat sample of the actual
equipment of Tsuruga NPP Unit 2 SG-C. Also, finishing traces that are assumed to be of grinding work(polishing) after repair welding were observed at the cracked part of the indicated part No. 5 of SG-B,
where the surface was in a similar condition as to that of the mockup test piece No. 6.
- From the conditions above, the surface residual stress of the welded part in concern of the Tomari NPP
Unit 2 is assumed to have exceeded 300 MPa, which is the stress that causes PWSCC.
(8) Other damage investigation result
The possibility of ductile cracking and fatigue cracking was investigated but no abnormality was observedin the use environment condition, used material, etc. hence, these are not the causes of the event inconcern. Moreover, no evidence of cause was found from the records and actual equipment investigation
results regarding improper welding or welding defect during the manufacturing phase.
(9) Summary of investigation resultsa. Through the SUMP observation results, cracks were observed along the dendrite border that have asimilar facet to that of PWSCC observed both domestically and internationally at welded areas using 600
type nickel base alloy.
b. Through the replica examination results, traces of grinding work (grinding and polishing) wereobserved where cracks were detected.
c. It was confirmed that there is a possibility of grinding work performed (polishing) when repair welding
was performed after buffing work.
d. Through the replica examination and document investigation results, a tensile residual stressexceeding approx. 300 MPa that is considered susceptible to the occurrence of PWSCC was assumed tohave occurred at the welded part in concern of Tomari NPP Unit 2.
e. From the results above, as similar to that of Tsuruga NPP Unit 2, there is a possibility that PWSCC
have occurred at Tomari NPP Unit 2 also due to high tensile residual stress at the inner surface wheretraces of grinding work (grinding) and buff finishing grinding work (grinding) were remained and at the
inner surface where grinding work (polishing) at the repair welding location was performed.

Cause of event
PWSCC was assumed to have occurred at the manufacturing of SG due to high tensile residual stress at
the areas where traces of grinding work (grinding) were remained after the inlet and safe-end werewelded using 600 type nickel base alloy and then finishing was performed by grinding work (grinding)and buffing work, or at the areas finished by grinding work (polishing) after finishing was performed bygrinding work (grinding) and buffing work and then some repair welding was performed on innersurfaces. Furthermore, it is assumed that the cracking has advanced in the axial direction since thestress at the welded area in concern is larger at the circumferential direction than in axial direction during
the operation.

Measures to prevent recurrence
1. The whole circumference of the inner surface of SG inlet nozzle welded part shall be cut by a cutting
device and after the shallow cracks are removed, deeper cracks shall be removed partially by grinding
work (grinding) as necessary. PT shall be used to confirm the removal of cracks, etc.
2. After the removal, overlay welding shall be performed using 600 type nickel base alloy as for areas
where deep cracks are removed and another overlay welding shall be performed throughout the wholecircumference of the inner surface of the welded part using 690 type nickel base alloy, which is superior
in corrosion resistivity as a recovery measure. As a precautionary measure, buffing work shall be
performed on areas in concern to reduce residual stress.

<Attachment>
The Periodic Inspection Status of Tomari NPP Unit 2
Cause and Countermeasures for Flaws at Welded Part of Primary
Coolant Inlet Nozzle of Steam Generator
Place of occurrence

Reactor containment vessel
Pressurizer
Steam generator
Lower head
(Carbon steel)

To steamturbineWelded part ofprimary coolant
piping(Stainless steel)

Feedwater

Stainless lining(Stainless steel)
Safe end(Stainless steel)

ReactorvesselPrimary
coolantpump

Approx. 940 mm
Primary coolant
piping (Stainless steel)

Overlay weld area
(Nickel-base alloy 600)

Tube sheet

The inspection status ofprimary coolant inlet
nozzle of steam generator
Viewed from the steam generatoro(Topmost shall be 0 )Girth weld area
(Nickel-base alloy 600)

Divider plate
Primary
coolant inlet
nozzlePrimary coolant
outlet nozzle: The flow of primary coolant

ECT results (Areas ofsignificant indication)
Cross sectional diagram

The area inspected during
this periodic inspection: Areas where evaluated from theultrasonic test result to be thinnerthan the wall thickness registered in
the application for construction plan
approval

Inspection Status of Steam Generator A(Inner side): Areas where depthsof flaws were notdetected by theultrasonic test
Stainless lining

Lower head(Maximum length)
No. 1: Approx. 13 mm*(Maximum depth)
No. 1: Approx. 7 mm

(Outer side)

Overlay
weld area*: The value was evaluatedconsidering several
adjacent ECT signal
indications as acontinuous flaw.

Girth weld
area
(Approx.
78 mm)

Safe end(The construction plan approval
application registry value: 75 mm)
Stainless lining

Inspection Status of Steam Generator B(Inner side)(Maximum length)
No. 5: Approx. 10 mm*(Maximum depth)
No. 9: Approx. 5 mm

Lower head

(Outer side)

Overlay
weld area*: The value was evaluatedconsidering several
adjacent ECT signal
indications as acontinuous flaw.

Girth weld
area

Detailed observation(No. 5)(Approx.
78 mm)

Safe end(The construction plan approval
application registry value: 75 mm)

Detailed observation of indicated part No. 5 of weldedpart of inlet nozzle in steam generator-B
Replica examination

SUMP observation result
SketchOverlay
welded partApprox.
5.0 mmApprox. Approx.
7.6 mm 1.8 mm

Approx. 7 mm

Girthwelding

Repair welding traces

Traces of
buffing work

Cracks have occurredalong the dendrite
border that has asimilar facet to that ofstress corrosion
cracking in the primarycoolant environment.

Surface treatment statusconfirmation test
Reproduction test of surface treatment traces in Unit
2 of Tsuruga NPP of Japan Atomic Power Company[Surface treatment
status]Grinding work
(grinding)+
Buffing work+
Grinding work(polishing)(Elastic grinding stone)

Finishing traces that are
assumed to be ofgrinding work (polishing)◦◦

Facet equivalent to replica examination result ofTomari NPP Unit 2.
Tensile residual stress was confirmed, which
has a possibility of causing stress corrosion
cracks in the primary coolant environment.

Assumed cause

Environment: Primary coolant water quality
environment in hot temperature

Material: 600 type nickel base alloysusceptible to stresscorrosion cracking
Stress:Tensile residual stressby grinding work

It is assumed that three factors
have overlaid and the stresscorrosion cracking occurred in
the primary coolant environment

Countermeasures◦ Cut out the area in concern throughout the whole circumference including
cracks with a cutting device.
◦ Confirm that the cracks are removed through penetrant test (PT).

Stainless liner

Lower head

◦ If there are any remaining cracks, partially remove the cracks with a grinder.
◦ Confirm that the cracks are removed through penetrant test (PT).
◦ Perform overlay repair welding using 600 type nickel base alloy.

Overlay weld
area
Girth weld
area

◦ Perform overlay welding throughout the whole circumference using 690 typenickel base alloy, which is superior in corrosion resistivity.
◦ Perform buffing work as a precautionary measure to reduce residual stress.

Safe end