Company name

Kyushu EPCO

Date of occurrence

18, December, 2007

Unit name

Genkai 1
Inspection of the Inner Surfaces of Welded Locations ofEvent

the Primary Coolant Outlet and Inlet Nozzles of SteamGenerators

International Nuclear Event Scale (INES)

Not applicable

Status of report

Final report

Status when event occurred
The eddy current flow detection test (hereinafter referred to as “ECT”) was conducted on the innersurfaces of welded locations of the primary coolant outlet and inlet nozzles of all steam generators(hereinafter referred to as “SG”) in the 25th periodic inspection in accordance with the “Implementation ofinspections of inner surfaces of welded locations of outlet and inlets nozzles of steam generators”(November 13, 2007) instructed by the Nuclear and Industrial Safety Agency (NISA) and the Ministry ofEconomy, Trade and Industry (METI).
As a result of ECT, significant signal indications were found at three locations with respect to the welded
locations of the inlet nozzle of A-SG. No abnormality was found in the outlet nozzle of A-SG and outlet
and inlet nozzles of B-SG.
The ultrasonic test (hereinafter referred to as “UT”) was conducted for the parts showing significant signal
indications in ECT of the welded locations of the inlet nozzle of A-SG and the depth of the flaws wasidentified, but no significant signal indication was found.
Furthermore, a full investigation of these locations was conducted and as a result, extremely shallow
cracks on the surfaces were identified.
Moreover, it was confirmed that these locations satisfied the design board thickness, even when thesecracks are taken into consideration.
Summary of examination of cause
1. Inspection results(1) ECT result
Significant signal indication was found in the axial direction at three locations of the welded locations of
the inlet nozzle of A-SG; locations at 9 degrees in circumferential direction (No.1), at 130 degrees (No.2)
and 344 degrees (No.3). (Circumferential direction angle is measured setting the starting point (0degrees) at the highest point of the nozzle, then going around clockwise viewing the nozzle from thewater chamber.)(2) UT result
UT was conducted for the parts showing significant signal indications in ECT of the welded locations of
the inlet nozzle of A-SG. However, no significant signal indication was found nor was it estimated that theJANTI-OTO-008-002 attachment4

depth of indications is extremely shallow.
(3) Crack depth inspection result
As for the three locations where significant signal indication was found in ECT results, grinding of 0.6mm, 0.5 mm and 0.8 mm was performed at locations No.1, No.2 and No.3 respectively. ECT wasconducted after the grinding and since no significant signal indication was found, it was concluded thatthe cracks were extremely shallow about 1 mm in depth. So as to assure the procedure, an additional
grinding was performed to have sufficient margin in the detection accuracy of ECT.
It was confirmed that the board thickness after the additional grinding satisfied the design board
thickness of 75 mm, which is the indicated value in the application form for the construction planapproval.
2. Cause investigation
Cause of cracks at the welded locations of the inlet nozzle of A-SG with respect to the locations where
significant signal indication was found from ECT was investigated.
As for cracks of the welded locations of the nozzle, it was confirmed through SUMP observation that thecracks have occurred along the dendrite border and have characteristics of stress corrosion cracking in
the primary water environment (hereinafter referred to as “PWSCC”).
Machining marks made at the production phase were observed on the inner surfaces of welded locations
welded with 600 type nickel base alloy.
Through interview inspections regarding machining conditions and through mockup tests conducted toinvestigate the cause of Mihama Unit No. 2, it was estimated that the residual stress has exceeded 300
MPa, which is the threshold level in occurrence of PWSCC.
It was confirmed that there was no repair welding at the production phase or peticuliar
production/operation history.
Cause of event
It is assumed that PWSCC occurred due to the primary water environment and high stress from the highresidual stress, which occurred on inner surfaces of the welded location, since machining (grinding) wasconducted on the welded location of the inner nozzle welded with 600 type nickel alloy at the production
phase of the replacement SG.
Measures to prevent recurrence(1) Removal of cracks
As for locations where significant signal indication was found in ECT, grinding was performed until nosignificant signal indication was found in ECT for the depth investigation and, furthermore, an additional
grinding was conducted considering the investigation accuracy of ECT.
(2) Ultrasonic shot peening (USP) treatment
As a preventive maintenance measure, USP was applied to mitigate residual stress, which was planned
to be conducted in this periodic inspection.

JANTI-OTO-008-002 attachment4

JANTI-OTO-008-002 attachment4

General chart of inlet nozzle of steam generator of Genkai Nuclear Power Plant Unit 1
Schematic System Illustration

Schematic Illustration of A Steam Generator

Locations InspectedNozzle(low-alloy steel)PressurizeControlrod

Stainless overlay(stainless steel)SteamrPowergenerator

Water

Safe end(stainless steel)

FuelWater

Steamgenerator

Water

Reactor vessel Primary coolantpump

Water Chamber
Side

Condenser
To discharge channel
Cooling water (murky water)
Circulation pump

About 940mm

Feed-water pump
Pipe
Arrangement
Side
Weld overlay part(600-series nickel-based alloy)

PrimaryCoolant InletAngle measured in clockwise
direction at 0°by seeing nozzle
from water chamber

Circumferential weld part(600-series nickel-based alloy)

Primary
Coolant Outlet

Inspection Results of Welded Locationsof Outlet and Inlet Nozzles

Results of Eddy Current Flaw Detection Testof Inlet Nozzle of A Steam Generator

Eddy current flaw
*2
detection test
Circumferential
Axial direction
direction
lengthposition

Water Chamber Side

Inspected locations

Weld overlay part
Circumferential
weld partA
steam
generator*1 80.5mm(minimumvalue of actual
measurement)

Pipe Arrangement Side(): This means the grinding depth where significant signal indications could not be identified in theeddy current flaw detection test conducted after grinding.
*1: The design board thickness entered in the application for approval of construction plan is 75 mm.

B
steam
generator

Inlet side

Ultrasonic flaw
*3
detection test
Indicated depth

No.1

9°

5mm

Not identifiable

No.2

130°4mm

Not identifiableNo.3

344°4mm

Not identifiable

Outlet side－

No indication from eddy current flaw detection test

Inlet side－

No indication from eddy current flaw detection test

Outlet side－

No indication from eddy current flaw detection test

*2 Eddy current flaw detection test: This is a method for detecting flaws of objects to be tested fromchanges in electromagnetic induction during the passage of an eddy
current on the surface of the material.
*3 Ultrasonic flaw detection test:
This is a method for detecting flaws of material bodies by utilizing thenature of echoing flaws in a discontinuous manner during thetransmission of ultrasonic sound wave through the material body.

JANTI-OTO-008-002 attachment4