Information Notice 2005-02, Pressure Boundary Leakage Identified on Steam Generator Bowl Drain Welds

Pressure Boundary Leakage Identified on Steam Generator Bowl Drain Welds

ML050100104
Person / Time
Site:	Catawba
Issue date:	02/04/2005
From:	Hiland P NRC/NRR/DIPM/IROB
To:
Hodge, CV, NRR/DIPM/IROB, 415-1861
References
IN-05-002
Download: ML050100104 (4)

UNITED STATES

NUCLEAR REGULATORY COMMISSION

OFFICE OF NUCLEAR REACTOR REGULATION

WASHINGTON, D.C. 20555 February 4, 2005 NRC INFORMATION NOTICE 2005-02: PRESSURE BOUNDARY LEAKAGE IDENTIFIED

ON STEAM GENERATOR BOWL DRAIN WELDS

ADDRESSEES

All holders of operating licenses for pressurized-water nuclear power reactors, except those

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 (IN) to alert

addressees to cracking and leakage indications found on steam generator (SG) bowl drain

welds. It is expected that recipients will review the information for applicability to their facilities

and consider actions, as appropriate, to avoid similar problems. However, suggestions

contained in this information notice are not NRC requirements; therefore, no specific action or

written response is required.

BACKGROUND

On September 16, 2004, Duke Energy Corporation, the licensee for Catawba Unit 2, identified

boric acid deposits from pressure boundary leakage in the vicinity of an SG bowl drain while

conducting bare metal visual examinations of Alloy 600/82/182 components during the plants

refueling outage. These components are inspected because the Alloy 600/82/182 materials are

susceptible to primary water stress corrosion cracking (PWSCC). Catawba Unit 2, which began

commercial operation in 1986, is a four-loop pressurized-water reactor (PWR) unit designed

and fabricated by Westinghouse Electric Company. The four SGs (2A, 2B, 2C, and 2D) are

Westinghouse model D5. At 100% power, the average primary coolant temperature is 588 EF

in the cold leg and 617 EF in the hot leg. This event is not applicable to Catawba Unit 1 because the SGs were replaced with ones that do not have bowl drains.

A similar event occurred in 2001 when the licensee identified boric acid deposits on the 2B SG

drain line nozzle weld. Dye penetrant testing of the surrounding Alloy 82/182 weld metal

confirmed cracklike indications. The licensee repaired the 2B SG drain line nozzle with Alloy

690/52/152 materials, which are more resistant to PWSCC. Visual and surface inspections of

the remaining SG drain lines did not reveal any indications at that time.

DESCRIPTION OF CIRCUMSTANCES

Catawba Unit 2 was completing reactor coolant pressure boundary examinations during its

cycle 13 refueling outage. The examinations of the SG bowl drains were part of the licensees

Alloy 600 program. These areas are inspected because the SG bowl drains are constructed

with Alloy 600/82/182 materials, which are susceptible to PWSCC. The licensee previously

identified locations with Alloy 600/82/182 materials and, under its Alloy 600 program, removes

insulation and conducts bare metal visual examinations of these locations each refueling

outage.

Three of the four Catawba Unit 2 drain lines were fabricated from stainless steel nozzles

attached at the bottom of the SG vessel using 82/182 weld materials in a J-groove weld

configuration. Leakage was confirmed in the 2C SG drain line nozzle weld by the presence of

boric acid deposits. The amount of boric acid deposited was approximately 0.25-0.50 cubic

inch (4.1-8.2 cubic centimeters). The licensee also identified much smaller amounts of boric

acid on the 2D SG drain line. To determine the extent of the problem, the licensee performed

dye penetrant testing on the 2A and 2D SG drain lines. The licensee determined from the dye

penetrant examination that the ASME code acceptance limits were satisfied for the 2A SG bowl

drain line weld. However, the dye penetrant examination did confirm cracking in the Alloy

82/182 weld in the 2D SG drain line. The licensee submitted an event notification on

September 20, 2004, pursuant to 10 CFR 50.72(b)(3)(ii)(A) (event #41048). Dye penetrant

testing of the 2C SG drain line weld was not performed since leakage was confirmed through

the presence of boric acid deposits. The licensee repaired all three drain lines, which will

prevent any exposure of Alloy 600/82/182 materials to the reactor coolant water.

DISCUSSION

The design of the Catawba Unit 2 SG bowl drain allows primary water to directly contact the

Alloy 82/182 weld materials that attach the stainless steel drain line nozzle. Consistent with

industry operating experience, it seems likely that the most recent indications are a result of

PWSCC. There was no evidence of boric acid corrosion of the surrounding ferritic material. It

also appears that the safety significance of this particular event is low, given the leakage- limiting configuration of the drain lines. Some licensees have eliminated the potential for this

issue by modifying the SG bowl drain in such a way that the susceptibility to PWSCC has been

lowered or eliminated. Replacement SGs, which are in use at many plants, do not typically

have bowl drains.

Bare metal visual examinations can be used to identify evidence of leakage. It is important to

note that the problem would have gone unnoticed if the insulation had not been removed for the

inspections. This information notice requires no specific action or written response. If you have any

questions about the information in this notice, please contact the technical contact listed below

or the appropriate Office of Nuclear Reactor Regulation (NRR) project manager.

/RA/

Patrick L. Hiland, Chief

Reactor Operations Branch

Division of Division of Inspection Program Management

Office of Nuclear Reactor Regulation

Allison Black, NRR

301-415-3697 E-mail: AKB1@nrc.gov

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

ML050100104 DOCUMENT NAME: E:\Filenet\ML050100104.wpd

OFFICE OES:IROB:DIPM TECH EDITOR EMCB EMCB EMCB

NAME CVHodge PKleene AKBlack SMCoffin WHBateman

DATE 01/31/2005 12/14/2004 12/23/2004 12/23/2004 12/23/2004 OFFICE DLPM/PDII-1 OES:IROB:DIPM C:IROB:DIPM

NAME SEPeters TReis PLHiland

DATE 02/01/2005 02/04/2005 02/04/2005