05000368/LER-1979-086-03, B Shutdown Cooling Heat Exchanger Radiation Monitor Indication Found Increased. Caused by Small Vol Tube Leak in Heat Exchanger Tube Q74. Heat Exchangers & Tubes Examined.Tube Plugged

B Shutdown Cooling Heat Exchanger Radiation Monitor Indication Found Increased. Caused by Small Vol Tube Leak in Heat Exchanger Tube Q74. Heat Exchangers & Tubes Examined.Tube Plugged

ML19345B309
Person / Time
Site:	Arkansas Nuclear
Issue date:	11/21/1980
From:	Shively C ARKANSAS POWER & LIGHT CO.
To:	NRC OFFICE OF INSPECTION & ENFORCEMENT (IE REGION IV)
Shared Package
ML19345B307	List:
References
LER-79-086-03X, LER-79-86-3X, NUDOCS 8011280052
Download: ML19345B309 (4)

LER-1979-086, B Shutdown Cooling Heat Exchanger Radiation Monitor Indication Found Increased. Caused by Small Vol Tube Leak in Heat Exchanger Tube Q74. Heat Exchangers & Tubes Examined.Tube Plugged

Event date:
Report date:
3681979086R03 - NRC Website

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REVISED REPORT - PREVIOUS REPORT DATE 11/12/79 NRC,ORu ss u. s. hucLE An REcuL ATonv Comm $sioes LICENSEE EVENT REPORT EXHIBIT A

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[Hi] I During Mode 6 operation, it was discovered that the "B" shutdown cooling i i

mm I (SDC) heat exchAar.aer, 2E35B. radiation monitor indication had increased. I FET711 The heat exchancer was immcdiately isolated on both the service water & I i re: ctor coolant sides.

Interpretation of the SDC activity increase as iti n s t re ated to plant conditions indicated a small volume tube leak in the heat o a g i exchanger. Activity release was analyzed to be <MPC.

There have been noi i

i similar occurrences.

Reportable per T.S. 6.9.1.9.d.

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1 The "B" SDC Heat Exchanger was taken out of service & visual inspection i

. o i i i revealed that tube #074 was the leaker (an outer opricheral tuhpl.

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,ssu,o 8 LYJ@l Media Account. Arkansas Gazette, 10/17/79 1 ii i iiiii;iiii e

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NAME OF PREPAnER-Chris N. Shively W%MM

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LER 50-368/79-086 Rev. 1 Unit II in Rows C through Y (0.098" thickness) were examined at a 200 KHz test frequency.

The test instrumentation sensitivity. was adjusted to allow calibration of the test equipment to a tube standard containing known defects.

Due to the initial inspection results,100% of the tubes in each heat exchanger were examined.

Tables I & II list tubes which exhibited tube wall degradation.

Twelve tubes were plugged in 2E35A and 19 tubes were plugged in 2E358.

Five of the tubes plugged in the "B" heat exchanger were plugged for precautionary measures, as they surrounded the " leaker" (Tube 74Q).

Additional Eddy current examinations were conducted on the "A" heat exchanger for further information on the SDC condition.

The tube support plate signals received during examination are large in magnitude relative to a signal generated from a small volume defect.

To optimize the phase separation between tube flaws and support plate signals, a test frequency of 150 KHz was used.

At this frequency a defect signal will tend to distort the con-figuration of the tube support plate signal.

Further, to enhance the ability to detect flaws, an electronic device known as a signal subtraction unit was employed.

This unit subtracts a stored clean support plate signal from the distorted signal being analyzed, leaving the defect signal which can then be analyzed to determine the depth of the flaw.

Rows F through Y were inspected, because of the greater U-bend radius which allowed for probing through the bend.

No additional defects were identified.

Following the examination of 2E35B, a section of tube # 74Q containing the fracture surface and a section of tube # 32Y which contained two defect indications w'ere extracted from the heat exchanger for failure analysis. Metallurgical analysis of the presence of periodic markings, or arrest marks, on the fracture surface of 74Q with a small adjacent region of ductile fracture, indicate fatigue as the mode of final failure.

Most of the fracture surface contained little or no fractografic information because of wear damage which occurred during or after the fracture; therefore, root cause of the failure could not be established.

Tube #32Y contained three chloride stress corrosion cracks which initiated in a region of localized intergranular attack.

Transgranular cracks then begin to penetrate the tube wall and are highly branched.

The localized intergranular attack is involved in the initiation orocess of the cracks, originating from a combination of possible factors (material sensitization, presence of chlorides in cooling water, stress in tubes, and operating in a temperature range conducive to chloride stress cracking).

An engineering evaluation is being conducted to determine if tube bundle replacement using an alloy which is not susceptable to chloride stress corrosion cracking should be pursued.

Presently, the chloride levels in the cooling water are being maintained as low as practical utilizing a wet layup technique with demineralized water.

1

LER 50-368/79-086 Rev. 1 TABLE I SHUTDOWN COOLING HEAT EXCHANGER 2E-35-A LINE R0W LOCATION

% DEGRADATION 63 0

5B+8 72 10 L

4T13*

20 74 N

78 70 76 N

78 55 76 N

6B 80 73 P

78 76 75 P

7T 55 75 P

7B 55 75 P

68 65 75 P

5B 55 18 Q

7T-1 20 50 Q

7T+4 30 72 Q

7B 50 74 Q

ST 55 74 0

7T 55 74 Q

6B 55 74 Q

78 55 70 S

7T 55 70 5

78 80 27 V

3T-10 27

• Line 10 Row L anomaly is 6 inches long.

12 Tubes Plugged.

Location interpretation: Number indicates the support ! starting with the tube sheet which is designated as 0 Letter is T for Top Half or B for Botton Half of bundle, and the i # is inches away from indicated support.

LER 50-368/79-086 Rev. 1 TABLE II SHUTDOWN COOLING HEAT EXCHANGER 2E-35-B LINE R0W LOCATION

% DEGRADATION 10 C

6T-8 89 18 C

6B-6 93 36 C

5B+6 92 36 C

5B+5 78 36 C

5B-5 88 68 C

ST-8 94 72 C

ST+13 70 14 E

ST-5 90 21 F

6B-13 94 29 F

4T+15 79 77 F

5B-13 95 79 F

5B-13 88 48 G

1B+1 81 10 Q

68-6 86 74 Q

8B+1 20 74 Q

7T 100 32 Y

3T+3 83 32 Y

3T+2 70 32 Y

3T-5 87 14 Tubes Plugged + 5 Tubes Surrounding 74Q