ML19317F260

RO 287/77-10:on 770714,primary-to-secondary Sys Leakage Found in 3B once-through Sg.Caused by Leaking Tube W/ Circumferential Crack.Tube Plugged by Stablizing Rod Insertion.Instrumentation Installed to Identify Leak Causes

ML19317F260
Person / Time
Site:	Oconee
Issue date:	07/28/1977
From:	DUKE POWER CO.
To:
Shared Package
ML19317F257	List: ML19317F256 ML19317F260
References
RO-287-77-10, NUDOCS 8001090598
Download: ML19317F260 (5)

Text

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DUKE POWER COMPANY '

OCONEE UNIT 3 O

Report No.: R0-287/77-10 Report Date: July 28, 1977 Occurrence Date: July 14, 1977 Facility: Oconee Unit 3, Seneca, South Carolina Identification of Occurrence: Primary-to-secondary system leakage in "3B" once-through steam generator i

Conditions Prior to Occurrence: Unit at 97 percent full power Description of Occurrence:

On July 14, 1977, Oconee Unit 3 was shutdown to investigate a gradual increase in radiation levels as registered by air ejector monitor 3RIA-40. This increase, occurring over 17 days of operation, indicated a possible primary-to-secondary system leak occurring in the "3B" once-through steam generator (DTSG). The

. reactor was in hot shutdoun on July 15, 1977 and preparations were made to drain the Reactor Coolant System and investigate the source of the leak.

The Reactor Coolant System was cooled and drained and an internal inspection of the "3B" OTSG was initiated within 3 days. One leaking tube was removed from service. The Nuclear Regulatory Commission was informed of these actions in our letter of July 21, 1977 to Mr. E. G. Case, Acting Director, Office of Nuclear Reactor Regulation.

Apparent Cause of Occurrence:

Eddy current testing was performed and revealed one leaking tube, identified as tube number 02 in row 77. A fiber optics examination of the leaking tube was also made. The leaking tube was discovered to have a 600-900 circumferen-tial crack at the bottom of the upper tube sheet. The tube was plugged by insertion of a stabilizing rod. Eddy current examinations of approximately 120 other tubes revealed no additional tube leaks. One additional tube, tube 1 of row 77, located on the open tube lane, was also plugged and stabilized as a precautionary measure although the tube had no eddy current signal distortion.

With regard to this and previous Oconee steam generator tube leaks, evaluation by Duke and.the OTSG vendor, the Babcock & Wilcox Company, is continuing.

However, there is no evidence to date to indichte that the leaks have resulted froc tube wastage / thinning, chemical attack, or intergranular stress-corrosion cracking. Studies indicate, however, that leaks occurring in the tube lanes are caused by propagation of a local defect by high cycle fatigue due to vibration.

Analysis of Occurrence:

Primary-to-secondary system leakage, resulting from this occurrence, was calculated to be approximately 0.4 gallons per minute. This was' detected by installed radiation mcnitoring equipment. The leakage did not exceed 8 0 010 909 f

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the operational limits of Oconee Technical Specification 3.1.6.1, however, a reactor shutdown was initiated upon indication cf an increase of the radiation indication.

The calculated gaseous activity released to the environment via the air ejectors was approximately 2.03 curies. This amount is considered insigni-ficant in comparison to the station's annual release limit.

This report describes the tenth occurrence of steam generator tube leakage experienced c1 the three Oconee units. In the first three occurrences and the fifth, sixth, eighth and tenth occurrence, the defective tubes edged an open tube lane (a radial row with no tubes, used for inspection purposes).

In the fourth instance, the defective tube was angularly located approximately 1500 counterclockwise from the locations of the other defective tubes and was not adjacent to an open tube lane. In the seventh instance, the leaking tube was located approximately 450 clockwise from the open tube lane. In the ninth occurrence, the leaking tube was on the outer circumference of the tube lane next to the lane edging an open tube lane and therefore was subjected to approximately the same vibration as the tubes edging the open lane. Table 1 provides a summary of the major OTSG tube experience at Oconee.

It is considered that this incident did not affect the health and safety of the public.

Coyrective Action:

The leaking tube, tube 02 of row 77 was plugged with a stabilizing rod inserted in the top and explosively plugged at the bottom. Tube 01 of row 77 was also plugged in the same manner.

As stated in R0-269/77-2 cubmitted by my letter of January 31, 1977, the OTSC vendor, Babcock & Wilcox, is currently involved in a program to further evaluate the cause of the tube failures. This program includes a review of deposit samples, a computer evaluation of all available eddy current data, a review of visual observations from fiber-optics and videotapes, and macro-microscopic analysis of two of the defective tubes that were removed from the Oconee Unit 2 "2B" OTSG, one which was removed from the Unit 1 "lB" 0TSG, and one tube removed along with its stabilizing rod from the Unit 1 "13" 0TSG. The open lane flow characteristics are being analyzed and a detailed stress / vibration analysis is being conducted on the tubes in rows 75 and 77. A review has been made of the Oconee station operat1ng history, the Oconee steam generator manufacturing history and previously conducted OTSG analysis and testing results.

Additionally, an instrumentation package has been devised and installed on Oconee Unit 2 "2B" OTSG to assist in the identification of the cause of the tube leaks.

StlHilARY OF HA.JOR OTSG TUBE EXPERIENCE AT OCONEr? .D '

IANE i TUBES EXAMISED C_ENERATOR ROW TUBE ELEVATION DATE TUBE LEAKER CONDITION ACTION ROI (excludes lenker1 1-A 77 17 Tubesheet 10/31/76 Yes Yes Crack Plugged R0-269/76-17 15 1-A 77 18 Tubesheet 10/31/76 Yes No Distorted Eddy Plugged Current Signal 1-B 114 109 14th Plate 12/8/76 No Yes No Visual Nalled R0-269/76-19 139 Inspection 1-B 113 115 14th Plate 12/8/76 No No Distorted Eddy Plugged Current Signal /

.1-B 113 110 14th Plate 12/8/76 No No Similar to Nalled 114/109 1-B 75 18 Tubesheet 12/8/76 Yes No 3000 Crack Halled 1-B 75 12 Tubesheet 1/15/77 Yes Yes 3500 Crack Nailed R0-269/77-2 140 1-B 81 128 Tubesheet 1/15/77 No No Eddy Current Nailed Indication 1-B 32 13 14th Plate 2/28/77 No Yes Eddy Current Nailed R0-269/77-8 3%

Indication

)

1-B 33 14 14th Plate 2/28/77 No No Eddy Current Halled ,

Indication )

1-B 77 25 14th Plate 2/28/77 Yes No Eddy Current Removed Indication 1-B 2 7 13th Plate 2/28/77 No No Eddy Current Plugged Indication 1-B t 8 13th Plate 2/28/77 No No Eddy Current Plugged Indication 1-B 101 40 4th Plate 2/28/77 No No Eddy Current Plugged Indication

SUf! MARY OF MA10R OTSG TilllE EXPERIENCE AT OCONEE (Cof4T'D) ,

l.AN E # TULLES EXAMJNED GENERATOR ROW TUBE El.EVATIOfl 11 ATE TUBE I.EAKER CONDITION ACTION R0f (excludes lenkers 1-B 77 25 Tubesheet 3/22/77 Yes Yes Weld Crack Replug- R0-269/77-11 100 ged 1-B 77 3,5,8, ---

3/22/77 Yes No Distorted Plugged Eddy Current

. Signal 1-n 77 15 Tubesheet S/7/77 Yes Yes Crack Plugged R0-269/77-16 507 1-B 77 18 ---

5/7/77 Yes No --- Removed 1 - 11 17 5 ---

5/7/77 No No Distorted Plugged Eddy Current Signal 2-B 77 23 Tubesheet 12/4/76 Yes Yes 270" Crack, Removed R0-270/76-15 133 llo l e 2-B 77 27 15th Plate 12/4/76 Yes No Wear Appearance Removed 2-B 124 42 12th Plate 12/4/76 No No Eddy Current Plugged Indicatlon, 40-60%

2-B 118 52 12th Plate 12/4/76 No No Similar to Left 124/42, 15% 3 3-11 77 11 15th Plate 7/21/76 Yes Yes No Visual Plugged R0-287/76-10 9 Inspection 3-B 77 19 15th Plate 2/14/77 Yes Yes Eddy Current Nailed R0-287/77-2 142 45" Crack 3 - 11 77 12,13,14,15 2/14/77 Yes No Eddy Current Nailed 16,17,18,20, 21 3-11 75 2 2/14/77 Yes No Eddy Current Nailed 3-11 78 1 15th Plate 6/10/77 No Yes 900 Crack & 1/8" Nailed RO-287/77-8 133 longitudinal

SUMMARY

OF HA.JOR OTSG TilBE EXPERIEllCE AT OCONEE (CONT'D) ,

I.ANE I TilBES EXAHl(!FeDJ

GENERATOR ROW TUBE El.EVATION DATE TUBE 1.EAKER CONDITION ACTION Roi (excludes leakdrs)

Tubesheet Yes Yes 600-90" Crack Nalled R0-287/77-10 120 3-B 77 2 7/14/77 3-B -77 1 ---

7/14/77 Yes No Nalled .

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