Cracks to welds on the nozzle stubs in the steam generator
Eddy Current Tests (ECT) were conducted at Mihama Unit 2 during outage in September 2007 and also at Tsuruga Unit 2during outage inOctober 2007 to verify the integrity of the inner surfaces of the welds (600 Series nickel-based alloy) on the primary coolant inlet nozzle stubs
in the steam generators. The tests identified significant signal indications.

The subsequent Ultrasonic Tests (UT) confirmed cracks measuring

up to 13mm in depth.
In response to these two incidents, the Nuclear and Industrial Safety Agency (NISA) instructed operators of Pressurized Water Reactors(PWR) on October 18, 2007 to step up their monitoring of any leakage from the welds on the inlet / outlet nozzle stubs in steam generatorsthat use the same 600 Series nickel-based alloy. It also instructed them, on November 16 of the same year, to inspect the inner surfaces of thewelds on the inlet / outlet nozzle stubs in steam generators during the next outage. In view of the facts that all the identified cracks ran in theaxial direction, and that 600 Series nickel-based alloy is highly ductile, it has been assessed that, even if the cracks aggravated to cause minor
leakage, this would not cause immediate damage to piping*.
PWR operators conduct inspections according to the plans. The table below shows the current status (as of March 3, 2008) of inspections
on primary inlet nozzle stubs, where 600 Series nickel-based alloy is used for welding.

The cracks are so far attributed to primary water stresscorrosion cracking (PWSCC) under the primary coolant environment, whereby 600 Series nickel-based alloy, sensitive to stress corrosion cracks,
was subject to residual tensile stress generated at the time of welding.

No cracks have been found in the welds of outlet nozzle stubs in steam

generators.
Operator
Hokkaido
EPCO
KansaiEPCO

Power
Station**TomariMihama

UnitNO.
12

SGECT result

22
ATBA
TBA
13 locations＊＊＊

2Takahama

B
A
B

No indication
3 locations
2 locations

C4 locations

A

7 locations

B

16 locations

2

3

JANTI-OTO-008-002

UT result

Cause

Inspection plan＊Details

TBATBA
Up to 17mm in length and13mm in depth―
Not detected
Up to 7mm in length and
6mm in depth
Up to 14mm in length and
8mm in depth
Up to 28mm in length and
9mm in depth
Up to 38mm in length and15mm in depth

TBA
TBA
PWSCC

8.2008〜
3.2008〜

TBA
TBA

―

Attachment 1

PWSCC

―

Attachment 2

PWSCC

―

Attachment 3―

Operator

Shikoku
EPCO

Power
Station**Unit
NO.

SG

ECT result

UT result

Cause

Inspection plan＊

Details

TBA―

2008.8〜（USPimplemented）（USPimplemented）
2008.9〜2008.9〜

TBA
―

TBA
TBA

―

Attachment 4

2008.5〜(USPimplemented)(SGR to be
implemented)
2008.11〜

TBA
―

TBA

―

Attachment 5C

9 locations

4134TBA―

Up to 14mm in length and
9mm in depth
TBA
―Ohi

34―――Ikata

4343TBA
TBA

TBA
TBA

TBA
TBA

34A
B
443 locations
No indication
TBA
No indication

13―

23A
B

1Genkai
KyushuEPCO

Not detected
―
TBA
―

PWSCC
―
TBA
―

――

TBA
Not detected
Up to 21mm in length and12mm in depth
Up to 14mm in length and
13mm in depth―TBA

SendaiJapan
AtomicPowerCompany＊

Tsuruga

2TBA
1 location
5 locations

C23 locations

DNo indication

PWSCC

―――

：Quoted from NISA reference materials dated February 5, 2008.

＊＊：The Power Stations not listed in this column use 690 Series nickel-based alloy with better anti-SCC properties.
＊＊＊：One of the locations had a crack on the safe end (SUS316).

It is suspected to have been an intercrystalline crack.

TBA: The results of future inspections will be inserted here as they become available.
― : Not applicable
USP implemented: Ultrasonic Shot Peening has been implemented to enhance anti-SCC properties.
SGR to be implemented:

JANTI-OTO-008-002

The steam generator will be replaced with one that uses the 690 Series nickel-based alloy in the next outage.