ML20085M441
| ML20085M441 | |
| Person / Time | |
|---|---|
| Site: | Browns Ferry  |
| Issue date: | 02/07/1974 |
| From: | Gilleland J TENNESSEE VALLEY AUTHORITY |
| To: | Knuth D US ATOMIC ENERGY COMMISSION (AEC) |
| Shared Package | |
| ML20085M433 | List: |
| References | |
| 10CFR-050.55E, 10CFR-50.55E, NUDOCS 8311080574 | |
| Download: ML20085M441 (10) | |
Text
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. O ! 'i- ' TENNESSEE VALLEY AL s THORITY CHATTANOOGA, TENNESSEE ;'
37401 February 7, 1974 4,viansany0 num 05ETN!sEs"Hi Dr. Donald F. Knuth, Director Directorate of Regulstory Operations U.S. Atomic Energy Commission Washington, DC 20545
Dear Dr. Knuth:
On December 3, 1973, TVA made initial report of a possible design deficiency discovered at unit 2 of our Drowns Ferry Nuclear Plant. We submitted an interim report on this failure on January 4, 1974, J. E. Gilleland to F. E. Kruesi. The deficiency involved failure of connector E9 (incorrectly reported as G9) in electrical penetration EF. Enclosed is a final report on this deficiency. Very truly yours, l
/J.E.Gilleland Assistant to the Manager of Power Enclosure CC (Enclosure):
Mr. Norman C. Moseley, Director Directorate of Regulatory Operations U.S. Atomic Energy Commission Region II - Suite 818 230 Peachtree Street, NW. l Atlanta, Georgia 30303 i h s i
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BROWNS FERRY NUCLEAR PLANT UNIT 2 ELECTRICAL PENETRATION EF - CONNECTOR E9* FAILURE FINAL REPORT On December 3, 1973, TVA made an initial report to AEC-DRO Inspector W. S. Little of the failure of connector E9 in penetra' tion EF at the Brouns Ferry Nuclear Plant. This report was designated as potentially reportable and was made in compliance with paragraph 50.55(e) of 10CFR50. An interim report was submitted on January 4,1974. This is the final report for this occurrence. Description of Connector Electrical penetration EF is n canister type, containing nine connectors, with eight connectors grouped around one in the center. The center connector outside the drywell, designated as E9* failed. Failure of the connector was discovered on November 1, 1973. The first evidence of the problem had appeared about two weeks earlier when the circuit br.eakers to the recircula-tion pump motor heaters were found to be tripped. The connector was supplied by Physical Sciences Corporation, and carried manufacturer's part number 6502-1. It has 35 pins of number 8AWG size and rating of 600 volts ac.. As shown in Figure 1, the pins are in the fixed portion (penetration half) of the connector; the female receptacles are in the movable portion (cable half). Twenty-seven of the pins are used for nine 3-phase, 480-volt, ungrounded loads as listed in Table 1. l
, Examination of Failed Connector i After the failure, the connector was examined as follows, revealing the t
{ follos ing information: l 1. The penetration half of the connector had no failure. .h
- 2. Some of the 35 female receptacles. in the cable half of the connector were short-circuited to ground.
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- 3. The potting compound around the cable studs in the cable half of the connector was chipped away, and the quality of the potting worknanchip was judged to be excellent.
- 4. Material was successively removed from the cable half of the connec-tor until only the bulkhead, the Teflon block, and the female receptacles inside the Teflon block remained. Figure 2 shows the bulkheed after the cable studs were cut off. There was no evidence of failure in this area.
- 5. A cut was next made through the Teflon block and parallel to the bulkhead. Figure 3 shows the appearance of the remaining portions '
of several female receptacles after the remaining Teflon was removed. Cause of Occurrence The pattern of pitting on the mating side (Teflon block side) of the bulkhead face and on the pins showed that intense arcs had developed between some of the female receptacles and the bulkhead. Refer to Figures 3 and 4 (Figure 4 is the same as Figure 3 except that the pin designations have been added). There was no evidence of arcing over or through the ceramic beads. Calcula-tions showed that the maximum electric stress in the ceramic bead was less than 15.7 volts per mill, which is well below the dielectric strength of the insulating materials used in the connector. The only remaining location in which an are could be initiated was in the air-filled voia region separating the receptacle from the bulkhead shown in Figure 1. Under ideal conditions with a minimum clearance of approximately 1/16 inch between the receptacle and the ceramic bead, it would not have been likely for arcing to occur. However, any significant reduction in the minimum clearance brings the end surface of the female receptacle into close proximity with the bulkhead, making dielectric breakdown of the air-filled gap highly probsble. Refer to detail A on Figure 1. It.is possible that some reduction in the normal minimum clearance may occur when the cable connector is mated and the backing ring tightened. The pressure from the tightened backing ring will be trans-mitted through the pin in the penetration half of the connector to the female receptacic and will tend to slide it further down over the cable stud toward the bulkhead. Several of the receptacles were, in fact, closer to the
3 bulkhead than 1/16 inch. The likelihood of breakdown in this region is increased by contamination but the inaccessibility of the voids makes visual inspection impossible. It has been concluded, therefore, that the connector failure was due to dielectric breakdown in air in the minimum clearance between the female receptacle and the bulkhead, caused by close proximity and possible contamination. Safety Implications of occurrence Equipment items 1 through 4, 6, and 8 through 11, listed in Table 1, are not essential for safe shutdown. However, equipment items 5 and 7 are important to safe shutdown in the degree described below: A. Valve FCV-68-3 (equipment item 5) in recirculation loop A must be closed in order that the LPCI system can be effective in case a LOCA were initiated by failure of primary system lines connected to recirculation pump 1B. The LPCI system is redundant to the core spray system. FCV-68-3 uses pins M, N, and D. B. Valve FCV-71-2 (equipment item 7) is one of two redundant valves that are closed in case of a break outsida containment in the steam line to the RCIC turbine. The other valve is outside the drywell. FCV-71-2 uses pins T, U, and V. The power source to each of the above valves is 480-volt, 3-phase ungrounded delta which would allow the motor to function with one grounded phase. However, if a second phase were grounded, the circuit breaker would trip and the valve would be disabled in the position achieved at the time of failure. This failure would be annunciated in the control room when the circuit breaker tripped, and thus does not represent an undetectable failure. l l Corrective Action The failed cable half of the connector was replaced with one of similar design that had successfully passed the inspection and checkout procedures. After review of the occurrence, TVA decided to replace the 35-pin cabic half of all connectors of the type that failed. The new cable-half assemblies
N. O O 4 , I were designed and are being fabricated by Culton Industries of Buena Park, California. Gulton Industries is the present owner of Physical Sciences Corporation. The slip-socket assemblies that form the female receptacles will be replaced with a threaded socket that screws onto the cable stud. This will prevent the receptacle from sliding down over the cabic stud toward the bulkhead. In addition, the outside edges of the female receptacle near . the bulkhead will be beveled to increase the minimum clearance. A contract was placed on December 27, 1973, with Gulton Industries for 20 replacement 35-pin connectors incorporating the above modifications. The confirming requisition, number 85467, was issued on January 7,1974. The contract requires the contractor to terminate 8-foot number 10ANG pigtails to each pin of the cable half of the connector and seal each connector with potting material. Each pigtail will be identified with a pin number by.a surface stamp in the insulation or by a tightly secured tag. Culton will perform tests on the completed assemblies. Included in these tests are the following: t
- 1. Dielectric strength test -.Each conductor shall be tested at 2200 volts ac, 60 Hz, for a duration of one minute.
- 2. Insulation resistance test - Each conductor shall be tested and must have an insulation resistance of not less than 1 x 108 ohms at 500 volts de and at 70*F.
The contractor has promised delivery to the project by March 13, 1974. The cable half of the connectors of the type that failed on unit 2 will be replaced before fuel loading, and those on unit I will be replaced at. the first refueling outage. Unit 3 has a different penetration design of an entirely different concept. L l'
- Incorrectly shown as G9 in'the initial and interim reports.
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.. n n O O Table 1 Pin Assignments for Unit 2 Penetration EF, Connector E9 Equipment Item Number Pin Assignments Equipment Description 1 A,B,C Valve FCV-68-1 2 D, II, F Floor drain sump pump 2A 3 G, II, I Spares 4 J,K,L 480-volt 3-phase ** power outlet 5 M, N, 0 Valve FCV-68-3 6 P,R,S Valve FCV-68-80 7 T,U,V Valve FCV-71-2 8 W,X,Y Recirculatien pump motor 2A heater 9 Z,a,b Recirculation pump notor 2B heater 10 c, d, e Equipment drain sump pump 2A 11 f,g,h,j,k Spares
**The power outlet was erroneously listed as single phase in the initial and interim reports, u !
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