Information Notice 2010-07, Welding Defects in Replacement Steam Generators

Welding Defects in Replacement Steam Generators

ML100070106
Person / Time
Issue date:	04/05/2010
From:	David Beaulieu Office of New Reactors
To:
Beaulieu, D P, NRR/DPR, 415-3243
References
IN-10-007
Download: ML100070106 (5)

UNITED STATES

NUCLEAR REGULATORY COMMISSION

OFFICE OF NUCLEAR REACTOR REGULATION

OFFICE OF NUCLEAR MATERIAL SAFETY AND SAFEGUARDS

OFFICE OF NEW REACTORS

WASHINGTON, DC 20555-0001 April 5, 2010

NRC INFORMATION NOTICE 2010-07: WELDING DEFECTS IN REPLACEMENT STEAM

GENERATORS

ADDRESSEES

All holders of an operating license or construction permit for a nuclear power reactor issued

under Title 10 of the Code of Federal Regulations (10 CFR) Part 50, Domestic Licensing of

Production and Utilization Facilities, except those who have permanently ceased operations

and have certified that fuel has been permanently removed from the reactor vessel.

All holders of or applicants for an early site permit, standard design certification, standard

design approval, manufacturing license, or combined license issued under 10 CFR Part 52, Licenses, Certifications, and Approvals for Nuclear Power Plants.

All holders of or applicants for a license for a fuel cycle facility issued pursuant to

10 CFR Part 70, Domestic Licensing of Special Nuclear Material.

PURPOSE

The U.S. Nuclear Regulatory Commission (NRC) is issuing this information notice (IN) to inform

addressees to the problem of welding defects that were associated with the manufacturing of

replacement steam generators (RSGs). The NRC expects that recipients will review the

information for applicability to their facilities and consider actions, as appropriate, to avoid

similar problems. Suggestions contained in this IN are not NRC requirements; therefore, no

specific action or written response is required.

DESCRIPTION OF CIRCUMSTANCES

The licensee, Southern California Edison (SCE), contracted Mitsubishi Heavy Industries (MHI),

to manufacture four RSGs in Japan for installation at San Onofre Nuclear Generating Station

(SONGS) Units 2 and 3. MHI completed manufacturing and testing of the first two RSGs in

2008 and shipped them to SONGS Unit 2 for scheduled installation in October 2009. MHI was

scheduled to complete manufacturing and testing of the two RSGs for SONGS Unit 3 in 2009.

On March 18, 2009, MHI conducted a routine visual inspection after completion of the American

Society of Mechanical Engineers Boiler and Pressure Vessel Code (ASME Code),Section III

primary and secondary side hydrostatic pressure test on the SONGS Unit 3 B RSG and

discovered a 5-inch long surface flaw (crack) in the dissimilar metal weld between the divider

plate, made from Alloy 690, and the channel head, made from low-alloy steel (LAS). The flaw

formed between the LAS and the Alloy 152 butter. A photograph of flaw and a schematic

diagram of the weld joint are shown in the enclosure.

After discovering this weld flaw, MHI conducted additional inspections with dye penetrant test

(PT) and ultrasonic test (UT) examinations on all the divider plate to channel head welds in both

SONGS Unit 3 RSGs. With the expanded inspections, MHI found several small surface flaws in

the Unit 3 A RSG that was being manufactured at the same time. In addition, in both Unit 3 RSGs, UT examinations revealed that almost all of the Alloy 152 butter under the divider plate

and some stainless steel cladding adjacent to the divider plate had separated from the LAS

substrate. SCE performed the same PT and UT examinations on the two RSGs made for

SONGS Unit 2 by MHI in 2008 and did not find any indication of flaws.

SCE and MHI investigated the root cause of the cracking in the SONGS Unit 3 RSGs. SCE and

MHI determined that the separation in the SONGS Unit 3 RSGs followed the fusion line

between the Alloy 152 butter/stainless steel cladding and the LAS substrate. The weld joint was

prepared by removing the stainless steel cladding from the RSG surface using air carbon-arc

gouging (ACAG) and surface grinding to prepare for the deposition of Alloy 152 as a butter

pass. The root cause of the separation was associated with the ACAG technique. The ACAG

resulted in higher carbon content and areas of higher hardness in the vicinity of the fusion line

between the butter pass and the LAS substrate. During subsequent surface preparation by

grinding, MHI did not ensure that all of the surface carbonized material was removed. The

regions of higher hardness and variations in surface conditions led to unfavorable metallurgical

properties at the interface between the Alloy 152 butter and LAS substrate. Separation between

the Alloy 152 butter and LAS was subsequently identified during the inspections following the

primary side hydrostatic testing of the SONGS Unit 3 RSGs. The cladding in the SONGS Unit 2 RSGs was removed by machining without using ACAG. As a result, the RSGs for SONGS Unit

2 passed the ASME Code Section III secondary side hydrostatic pressure test without any

inspection issues.

SCE and MHI presented the results of their root cause investigation at a public meeting with the

NRC on September 15, 2009. The presentation slides are available through the NRCs

Agencywide Documents Access and Management System, under Accession No.

ML092590470.

DISCUSSION

The welding defects identified at the fabrication facility on the RSGs for SONGS Unit 3 are

unlike the weldability issues that are typically observed in the welding of nickel-based alloys.

For the SONGS Unit 3 RSGs, surface preparation for the Alloy 152 butter was inadequate.

Contamination from the ACAG, used to remove the stainless steel cladding, resulted in elevated

hardness of the material adjacent to the fusion line that remained on the surface after the

grinding step. When the butter pass was applied, the metallurgical bond between the LAS and

Alloy 152 butter weld pass was not as strong as it should have been.

The fabricator followed approved welding procedures for dissimilar metal welding of Alloy 690 to

LAS and had recently built two RSGs for SONGS Unit 2 according to these procedures without

problems. However, for the SONGS Unit 3 RSGs, the fabricator requested and the licensee

approved a deviation to allow using an alternative method (in this case ACAG) to prepare the LAS surface for butter application. According to the ASME Code,Section IX, this deviation did

not require the requalification of the welding procedure because this aspect of the weld joint

preparation was not considered an essential variable.

ACAG could be used in the manufacturing or repair of any ASME Code Class 1, 2, or 3 component. The ACAG technique is an accepted procedure for removing metal. American

Welding Society (AWS) C5.3:2000, Recommended Practices for Air Carbon Arc Gouging and

Cutting, states that welding on a surface after ACAG may generally be performed with a

minimum of grinding or cleaning. However, the standard also notes the limitation that the

process increases the surface hardness on cast iron and air hardenable metals. This may be

objectionable.

ACAG is not specifically covered in Section III of the ASME Code; however, ASME Code,Section XI, IWA-4461 covers the qualification and use of a thermal removal process like ACAG.

In addition, 10 CFR 50.55a(b)(2)(xxiii) states:

The use of provisions to eliminate the mechanical processing of thermally cut

surfaces in IWA-4461.4.2 of Section XI, 2001 Edition through the latest edition

and addenda incorporated by reference in paragraph (b)(2) of 10 CFR 50.55a

are prohibited.

Although all specific requirements or standards were met, this event illustrates that control over

all aspects of welding ASME Code Class 1, 2, and 3 components can prevent welding defects

like those found in the RSGs for SONGS Unit 3 from occurring.

CONTACT

This IN requires no specific action or written response. Please direct any questions about this

matter to the technical contact listed below or the appropriate Office of Nuclear Reactor

Regulation (NRR) project manager.

/RA/ /RA/

Timothy J. McGinty, Director Daniel H. Dorman, Director

Division of Policy and Rulemaking Division of Fuel Cycle Safety and Safeguards

Office of Nuclear Reactor Regulation Office of Nuclear Material Safety and Safeguards

/RA/

Glenn M. Tracy, Director

Division of Construction Inspection

and Operational Programs

Patrick T. Purtscher, NRR

301-415-3942 E-mail: Patrick.Purtscher@nrc.gov

Enclosure: San Onofre Nuclear Generating Station (SONGS)

Replacement Steam Generator Welding Defects

Note: NRC generic communications may be found on the NRC public Website, http://www.nrc.gov, under Electronic Reading Room/Document Collections.

CONTACT

This IN requires no specific action or written response. Please direct any questions about this

matter to the technical contact listed below or the appropriate Office of Nuclear Reactor

Regulation (NRR) project manager.

/RA/ /RA/

Timothy J. McGinty, Director Daniel H. Dorman, Director

Division of Policy and Rulemaking Division of Fuel Cycle Safety and Safeguards

Office of Nuclear Reactor Regulation Office of Nuclear Material Safety and Safeguards

/RA/

Glenn M. Tracy, Director

Division of Construction Inspection

and Operational Programs

Patrick T. Purtscher, NRR

301-415-3942 E-mail: Patrick.Purtscher@nrc.gov

Enclosure: San Onofre Nuclear Generating Station (SONGS)

Replacement Steam Generator Welding Defects

Note: NRC generic communications may be found on the NRC public Website, http://www.nrc.gov, under Electronic Reading Room/Document Collections.

ADAMS ACCESSION NO.: ML100070106 TAC ME2447 OFFICE CVIB:DCI TECH EDITOR BC:CVIB:DCI D: DCI BC:CIB1:DE:NRO

NAME PPurtscher CHsu MMitchell MEvans DTerao

DATE 2/4/10 1/19/10 email 2/4/10 2/12/10 2/12/10

OFFICE PGCB:DPR PGCB:DPR BC:PGCB:DPR D:DCIP:NRO D:FCSS:NMSS D:DPR

NAME DBeaulieu CHawes MMurphy GTracy DDorman TMcGinty

DATE 2/16/10 2/18/10 3/1/10 3/30/10 3/19/10 4/05/10

OFFICIAL RECORD COPY

IN 2010-07 San Onofre Nuclear Generating Station (SONGS)

Replacement Steam Generator Welding Defects

SONGS Unit 3 B Replacement Steam Generator

Cold-Side Crack in Divider Plate-to-Channel Head Weld

Enclosure