Information Notice 2004-08, Reactor Coolant Pressure Boundary Leakage Attributable to Propagation of Cracking in Reactor Vessel Nozzle Welds

Reactor Coolant Pressure Boundary Leakage Attributable to Propagation of Cracking in Reactor Vessel Nozzle Welds

ML041130396
Person / Time
Issue date:	04/22/2004
From:	Beckner W D NRC/NRR/DIPM/IROB
To:
Dozier J I, NRR/IROB 415-1014
References
IN-04-008
Download: ML041130396 (7)

UNITED STATESNUCLEAR REGULATORY COMMISSIONOFFICE OF NUCLEAR REACTOR REGULATIONWASHINGTON, DC 20555-0001April 22, 2004NRC INFORMATION NOTICE 2004-08:REACTOR COOLANT PRESSURE BOUNDARYLEAKAGE ATTRIBUTABLE TO PROPAGATION

OF CRACKING IN REACTOR VESSEL NOZZLE

WELDS

Addressees

All holders of operating licensees for nuclear power boiling-water reactors (BWRs), exceptthose who have permanently ceased operations and have certified that fuel has been

permanently removed from the reactor vessel.

Purpose

The U.S. Nuclear Regulatory Commission (NRC) is issuing this information notice to alertaddressees to cracking identified in the nozzle-to-cap weld of control rod drive (CRD) return line

penetration N10 at Pilgrim Nuclear Power Station (Pilgrim Station). The NRC expects

recipients to review the information in this notice for applicability to their facilities and consider

actions, as appropriate, to avoid similar problems. However, suggestions contained in this

information notice do not constitute NRC requirements and, therefore, do not require any

specific action or written response.

Description of Circumstances

During a planned outage on September 30, 2003, the licensee for Pilgrim Station beganperforming drywell inspections to identify and make necessary repairs to reduce drywell

leakage. On October 1, 2003, the licensee's drywell inspections revealed leakage from the

nozzle-to-cap weld area of penetration N10. The licensee concluded that the leakage was a

contributor to the unidentified drywell leakage.The licensee used a Performance Demonstration Initiative (PDI) qualified manual ultrasonictesting (UT) technique to determine that the N10 nozzle-to-cap weld contained an unacceptable

flaw that was approximately 4.45cm (1.75 inches) long in the circumferential direction.

Observations by the nondestructive examination (NDE) inspector suggested that the flaw

initiated at the inner diameter (ID) of the weld, in the area of previous weld repairs. The

through-wall location appeared to be close to the centerline of the weld.Root Cause

The reactor pressure vessel (RPV) nozzle is made of SA-508, Class 2 low-alloy steel, while theCRD return line cap is made of Alloy 600. The subject weld is fabricated with Alloy 82/182 material, and the nozzle side of the weld is buttered with Alloy 182 material. Section 2.2.1.2 of the BWR Vessel and Internals Project report BWRVIP-49, "InstrumentPenetration Inspection and Flaw Evaluation Guidelines," states that there has been extensivelaboratory and field experience with stress corrosion cracking (SCC) of nickel based alloy, including wrought Alloy 600, Alloy 82 and Alloy 182 weld metal. Both Alloy 600 and Alloy 182 are potentially susceptible to SCC under normal water chemistry conditions in the BWR

environment. Alloy 600 is more resistant than Alloy 182 to crack initiation regardless of prior

fabrication history or metallurgical condition, particularly in the uncreviced condition. Consistent

with its higher chromium and lower carbon content, Alloy 82 weld metal is more resistant to

SCC than Alloy 182. Stress corrosion cracking in the base material is referred to as

intergranular SCC (IGSCC), while SCC in the weld material is referred to as interdendritic SCC

(IDSCC) because of the nature of the elongated grains (or dendrites) in the weld. Both

degradation mechanisms refer to essentially the same phenomenon in the base metal and weld

metal. The licensee concluded that the root cause of the cracking in the nozzle-to-cap weld of CRDreturn line penetration N10 was IDSCC, given that the flaw was completely contained within theweld. The licensee asserted that the IDSCC was induced by a combination of a crevice

condition and weld repair stresses resulting from previous local weld repairs.The licensee reviewed industry experience as part of its root cause evaluation. General Electric(GE) and utility personnel who comprised the root cause team for a 1997 event at Hope Creek

concluded that the through-wall leak in the core spray nozzle to safe-end weld was attributable

to IDSCC in the Alloy 182 material. The root cause team also concluded that the crack growth

rate was influenced by the presence of fabrication defects and weld repair stresses (i.e. the

leak was in the area of a previous local repair using Alloy 182).Corrective ActionThe Pilgrim Station licensee performed a weld overlay repair to stop the leakage. The

licensee's repair technique is an alternative to the requirements in Section XI, IWA-4000, of theBoiler and Pressure Vessel Code promulgated by the American Society of Mechanical

Engineers (ASME). The repair was based on the use of Code Case N-504-2, "AlternativeRules for Repair of Class 1, 2, and 3 Austenitic Stainless Steel Piping

" (with modifications), andCode Case N-638, Similar and Dissimilar Metal Welding Using Ambient Temperature MachineGTAW Temper Bead Technique.

" (See ADAMS Accession No. ML032870328.)Background

The N10 nozzle is a 10-cm (4-inch) diameter RPV penetration that was previously used toreturn CRD system flow to the reactor vessel. In 1977, the licensee modified the N10 nozzle to

prevent cracking attributable to the cyclic thermal stresses resulting from the return of cooler

water to the reactor vessel from the CRD system. That modification consisted of cutting and

isolating the existing CRD system return line to nozzle N10 and rerouting the CRD return line to

the CRD cooling water header. The modification also included removing the safe end and

thermal sleeve from nozzle N10 and installing an Alloy 600 cap. The final configuration of the nozzle was composed of an Alloy 82/182 nozzle-to-cap butt weld from the forged steel nozzle to the Alloy 600 cap. Radiographic examination following the modification identified defects in

the weld, which the licensee subsequently repaired. The final testing of the modification was

performed in 1977 using NDE and hydrostatic testing.The NRC subsequently issued Generic Letter (GL) 88-01, "NRC Position on IGSCC in BWRAustenitic Stainless Steel Piping," to address the subject of IGSCC cracking in BWR piping. During that same time period, GE recommended that BWR owners inspect nozzle-to-safe-end

welds containing Alloy 182 or a combination of Alloy 182 and Alloy 82 and, wherever practical, these inspections should be performed using automated UT scanning. Past inspections of

dissimilar metal piping welds at Pilgrim Station were completed using the guidance in GL 88-01, which was superseded by guidance in BWRVIP-75, "Technical Basis for Revisions to GenericLetter 88-01 Inspection Schedules.

" (See ADAMS Accession Nos. ML003688842 andML021350645.) In accordance with BWRVIP-75, the N10 nozzle-to-cap weld was classified as

a Category D weld, meaning that it is made of susceptible materials that have not been treated

with an IGSCC remedy and in which cracks have not been reported. The N10 nozzle is located

2.1m (84 inches) above the top of the active fuel and is not protected by hydrogen water

chemistry (HWC). (The purpose of HWC is to protect components from SCC.) Category D

welds have a 6-year inspection frequency. Prior to the Fall 2003 inspection, the licensee

performed its last inspection of the N10 nozzle-to-cap weld during the Spring 1999 refueling

outage. As part of that inspection, Inservice Inspection/ Nondestructive Examination personnel

reviewed data sheets, but did not discover any recordable indications of SCC. Other related generic communications involving weld inspections and degradation in BWRsystems include the following NRC information notices (INs):IN 1990-30:

"Ultrasonic Inspection Techniques for Dissimilar Metal Welds

"IN 1992-50:

"Cracking of Valves in the Condensate Return Lines of a BWR EmergencyCondenser System

"IN 1998-44:

"Ten-year Inservice Inspection (ISI) Program Update For Licensees That Intendto Implement Risk-Informed ISI of Piping

"Discussion

The licensee

's root cause for the cracking in nozzle N10 is consistent with the availableevidence and industry experience. The weld metal is susceptible to IDSCC, and there is

minimal protection (i.e., no HWC) from SCC mechanisms because of the location of the nozzle

cap and stagnant flow conditions. In conducting the Spring 1999 inspection, the licensee used manual ultrasonic inspectiontechniques with qualified inspectors. The 2003 examinations were performed to the updated

requirements of Appendix VIII to Section XI of the ASME Code and the PDI program.

Enhanced ultrasonic examinations using PDI-qualified inspectors have improved the capability

to detect flaws related to SCC mechanisms, including those that occur entirely within the weld

metal. With respect to future inspections of this weld, after the qualified ISI examination of the nozzleN10 weld, which is scheduled for the 2009 outage, the weld will be examined in accordance

with the schedule for Category E welds in BWRVIP-75. BWRVIP-75 defines Category E welds

as those that have weld overlay repairs made with an IGSCC-resistant, nickel-based alloy (such

as Alloy 52) and have received one qualified ISI since the initial post-overlay examination. After

the initial examination, Category E welds with weld overlays are successively examined in

accordance with BWRVIP-75, and related NRC comments, in order to ensure that there is no

new cracking or crack growth. The Category E welds are then examined at a rate of 25 percent

of the population every 10 years for normal water chemistry. The staff and the licensee discussed expanding the scope of the Fall 2003 inspection to includeall other Category D welds. The licensee used the following factors to consider this expanded

scope based on the attributes of the cracked N10 weld:- weld at a reactor vessel nozzle- Category D weld

- low HWC protection

- dissimilar metal weld (Alloy 82/182)

- significant weld repair during original installation

- ID grinding and/or radiographic defectsThe other Category D welds were, for example, protected by HWC, had improved inspections inthe past (i.e., automated UT, rather than manual UT), had no weld repairs, and had no

radiographic defects. Therefore, the licensee did not expand the scope of the inspection.The leakage from the penetration N10 nozzle-to-cap weld and other leak sources in the drywellwas less than the limit allowed by the plant

's technical specifications (TS) for unidentifiedleakage and total leakage (combined unidentified and identified). The staff found that the

licensee had mitigating procedures, routine inspection activities, operable leakage detection

equipment and TS requirements designed to detect low levels of leakage from the reactor

coolant system (RCS) and minimize the potential that a flaw could remain undetected. On that

basis, the staff determined, qualitatively, that the N10 pressure boundary leakage was of very

low safety significance. Generic Implications

Based on the information currently available, such as other capped BWR CRD return lines andprior industry experience with IDSCC, the degradation that occurred at Pilgrim Station may be

relevant to other BWR facilities. The licensee for Pilgrim Station used guidance from

BWRVIP-75 to determine the appropriate inspection method and frequency for this weld. This information notice does not require any specific action or written response. If you haveany questions about the information in this notice, please contact the technical contact identified

below or the appropriate project manager in the NRC

's Office of Nuclear Reactor Regulation(NRR)./RA/William D. Beckner, Chief

Reactors Operations Branch

Division of Inspection Program Management

Technical Contact:

Andrea D. Lee, NRRJerry Dozier, NRR(301) 415-2735(301) 415-1014 Email: adw1@nrc.gov Email: jxd@nrc.govAttachment: List of Recently Issued NRC Information Notices

ML041130396OFFICEOES:IROB:DIPMTech EditorOCIO:IRSD:PSSEMCB:DELPD4:DLPMNAMEIJDozier JWF*PKleene*PAGarrity*ADLee*ABWang*DATE04/20/200403/01/200403/01/200403/03/200404/07/2004OFFICEBC:EMCB:DEOES:IROB:DIPMSC:OES:IROB:DIPMC:IROB:DIPMNAMEWHBateman*CDPetrone*CJacksonWDBecknerDATE04/15/200404/15/200404/22/200404/22/2004 / /

______________________________________________________________________________________OL = Operating License

CP = Construction PermitAttachment LIST OF RECENTLY ISSUEDNRC INFORMATION NOTICES

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