ML20084M068

AO 4-73-8:on 730713,smoke & Overheating Observed in Area of Grounding Transformer Bank Cubicle Bus 4B Following Annunciator Alarm.Caused by Phase a of Startup Transformer Connecting Cable 4 Grounded.Transformer Replaced

ML20084M068
Person / Time
Site:	Turkey Point
Issue date:	07/20/1973
From:	Schmidt A FLORIDA POWER & LIGHT CO.
To:	Oleary J US ATOMIC ENERGY COMMISSION (AEC)
Shared Package
ML20084M072	List: ML20084M068 ML20084M077
References
AO-4-73-8, NUDOCS 8306010242
Download: ML20084M068 (4)

Text

,.. .. . ,

eo sox aion wAvt.FtonioA 33101 s O O r.

f A QD&  %

,"T3j LJ aA TE >i !?/ff b.. -

, FLORtOA POWER & LIGHT COMPAt.Y July 20, 1973 .

f 'l S '

Mr. John F. O' Leary, Director s '4[,1 j fd hf fis .

Directorate of Reactor Licensing p; c(fff [4 i }', " Ld ,e U. S. Atomic Energy Commission -

-}

Washington, D. C. 20545 3 di 4 e[ 'k )h/

y :..

3 i TURKEY POINT UNIT NO. 4 *1 DOCKET NO. 50-251 4" ~ _NU 37 ABNORMAL OCCURRENCE NO. 4-73-8 INOPERABLE INTAKE COOLING WATER PUMPS

Dear Mr. O' Leary:

I. INTRODUCTION This report is submitted in accordance with Technical

. Specification 6.6.2.a for Turkey Point Unit No. 4, Operating License No. DPR-41. This Abnormal Occurrence Report No. 4-73-8, describes an abnormal occurrence which was identified on July 14, 1973. The Directorate of Regulatory Operations, Region II, was notified on July 14, 1973.

II. DESCRIPTION OF OCCURRENCE On July 13, 1973, Turkey Point Unit Fo. 4 was operating at a power level of 200 mwe with steady load conditions. About

8

55 p.m., an annunciator alarm indicated an electrical ground had occurred on either No. 4A or 4B, 4160 volt electrical systems. Operators observed smoke and overheating in the area of the No. 4B bus grounding transformer bank cubicle.

Efforts were directed toward identifying the location of the ground and isolating the ground by removing operating equipment from. service, one component at a time. Unit No. 4 turbine-generator load was reduced and the unit was removed from service at 9:04 p.m. The reactor was then maintained at critical, zero-load condition. The No. 4B startup transformer breaker was opened at 9:10 p.m. which de-j energized No. 4B, 4160 volt bus and cleared the electrical l ground condition. As an additional precautionary safety

measure, the 4160 volt breakers connected to No. 4B bus were opened, including No. 4B and 4C Intake Cooling Water Pump notor breakers. y

_ )

,fl p

8306010242 730720 PDR ADOCK 05000250

{b) .g71

, S pg COPi SENT REGION [..

n -- .a 2 Kr. John F. g ary, July 20, 1973

-- Director .

W

• On July 14, 1973, about 2:55 p.m., it was recognized that de-energizing No. 4B, 4160 volt bus had removed both No. 4B and 4C Intake Cooling Water pumps from. service. Accordingly, cooldown of No. 4 reactor was started to bring the reactor to cold shutdown condition.

III. ANALYSIS OF OCCURRENCE An investigation by plant personnel to determine the cause of the electrical ground on No. 4B bus revealed that one phase (A Phase) of the cable connecting the secondary winding of~No. 4 startup transformer to No. 4B bus was grounded due to a failure of the cable insulation. This failure caused excessive current flow in the 25 KVA grounding transformer bank associated with No. 4B bus. According to plant records, this is the first cable insulation failure for this-type of cable. Limiting conditions for Operation of Unit No. 4 require that two intake cooling water pumps must be operable or the reactor shall be placed in the cold shutdown condition. During the interval before No. 4 reactor cooldown was started, this requirement was not met.

IV. CORRECTIVE ACTION TO PREVENT RECURRENCE The immediate corrective action to comply with Limiting Conditions for Operation of. Unit No. 4 was to commence reactor cooldown to place No. 4 reactor in the cold shutdown condition.

The immediate corrective action was to replace the grounding transformer and to repair the insulation and shielding of the cable. The repaired insulation was leak tested with j DC test equipment to 30,000 volts and was found to be l satisfactory. Perm &nent repairs will be completed by replacing the repaired section of cable with a complete run of new cable.

Administrative Procedure No. 0103.2 and Operating Procedure No. 0204.2 provide for the review, analysis and evaluation of plant conditions, including the operablility of required engineered safeguards equipment, components or systems.to l

ensure that the requirements specified in the Limiting Conditions for Operation of Unit Nos. 3 and 4 are met.

Actions to be taken if these requirements are not met are specified in these procedures.

Strict adherence to Administrative Procedure No. 0103.2 l

and Operating Procedure No. 0204.2 will prevent recurrence

! of this and similar incidents.

1 Mr. John F. g ,cary, [, July 20, 1973

_. Dircctor W

V. ANALYSIS AND EVALUATION OF SAFETY IMPLICATIONS OF THE OCCURRENCE Unit Nos. 3 and 4 are designed to provide for the continuing availability of engineered safety features under all conditions of operation including the loss of one of the two 4160 volt buses. The incidents analyzed in FSAR, Section 14, are based on the operation of only one intake cooling water pump during the incident period. During this occurrence, No 4A Intake Cooling Water Pump was in continuous operation, which meets the above criteria.

No 4B and 4C Intake Cooling Water T.ups would have been immediately operable upon restoration of power supply to No. 4B, 4160 volt buses. If an incident had occurred during the period No. 4B, 4160 volt bus was de-energized, which required the operation of No. 4B and 4C intake cooling water pumps, the Nuclear Control Center Operator would have manually started No. B Emergency Diesel-generator to energize No. 4B, 4160 volt bus and manually start required engineered safeguard equipment. Complete loss of off-site power to both 4160 volt buses on No. 4 Unit would have automatically started both No. A and B Emergency Diesel-generators, separated the non-vital loads from the vital loads, closed the Emergency Diesel-generator breakers and started one of the three intake cooling water pumps.

Comparison of the actual conditions during the incident described above, with the safety analyses presented in the Turkey Point Final Safety Analysis Report showed that for each incident analysis, the conditions assumed for these analyses are significantly more conservative than the actual conditions which existed during the interval prior to initiating No. 4 reactor cooldown. In addition, operation of the Emergency Diesel-generator would have provided for operation of additional engineered safeguard equipment, if required. Based on the above evaluation, maintaining No. 4 reactor above cold shutdown operation, did not adversely affect the safe operation of No. 4 reactor.

VI. CONCLUSIONS

a. The electrica] ground that resulted in de-energizing No. 4B bus to clear the electrical ground was caused by failure -f the insulation on the cable between No. 4 startup transformer and No. 4B, 4160 volt bus.

b. Maintaining No. 4 reactor above cold shutdown conditions with two intake cooling water pumps out of service, caused by No. 4B, 4160 volt bus being de-energized, did not comply with the Limiting Conditions of Operation for No. 4 reactor.

Udr. sloan tr. @lBigary, -U- .N g Director O O

c. Comparison of the actual operating conditions which existed during this occurrence., show that continuous operation of No. 4A intake cooling water pump meets the conditions assumed for analysic of the incidents presented in the FSAR,.Section 14. In addition, No. 4D and 4C Intake Cooling Water Pumps were immediately operable upon restoration of power supply to No 4B, 4160 volt bus. If loss of off-site power supply had occurred, one of the three intake cooling water pumps would have

'bcen started and operated from power supplied from the Emergency Diesel-generators. Based on the above evaluation, the safe operation of No. 4 reactor was not adversely affected during the period involved in this incident.

d. This abnormal occurrence did not present any danger to the public health or safety.

Very truly yours

$kdshn#

A. D. ScT2midt Director of Power Resources ADS /JHB/AA/VTC:paz cc: Norman C. Moseley, Director Region II, Directorate of Regulatory Operations U. S. Atomic Energy Commission Suite 818, 230 Peachtree Street, N.W.

Atlanta, Georgia Mr. G. A. Olson Edison Electric Institute 90 Park Avenue New York, New York 10016 Dr. James Coughlin J. W. Williams, Jr.

J. R. Bensen J. B. Olmstead W. II . Rogers, Jr. S. G. Brain G. E. Liebler (3) Plant Supervisors J. K. Hays Plant Nuclear Safety Committee C. E. Branning' Documentary Files D. W. Joner i

_ - - _ ___