ML20086F579

AO 50-261/75-9:on 750501,reactor Coolant Pump Seal Failed, Resulting in Discharge of Reactor Coolant Fluid to Containment Floor.New Seal & Associated Parts Being Installed.Parts Shall Be Certified

ML20086F579
Person / Time
Site:	Robinson
Issue date:	05/12/1975
From:	Utley E CAROLINA POWER & LIGHT CO.
To:	Moseley N NRC OFFICE OF INSPECTION & ENFORCEMENT (IE REGION II)
Shared Package
ML20086F569	List: ML20086F579 NG-75-1139, Summarizes Actions Taken Following AO 50-261/75-9 on 750501. Summaries of Repairs & Insps Performed During Forced Outage Given
References
AO-50-261-75-9, NG-75-701, NUDOCS 8401030416
Download: ML20086F579 (8)

Text

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' ~/~ o GP&L 0 Carolina Power & Light Company oM liay 12, 1975 Files NG-3513 (R) , Serial: NO-75-701 fir. Noru:an C. Moseley, Director Directorate of Regulatory Operations U. S. Nuclear Regulatory Cou:=ission )

Region II, Suite 818 230 Peachtree Strcot, N. W.

Atlanta, Georgia 30303

Dear Mr. Moseley:

H. B. ROBINSON UNIT NO. 2 LICENSE NO. DPR-23 FAILURE OP "C" REACTOR COOLANT PU'IP STAL SYSTEM In accordance with 6.6.2.a of the Technical Specifications for H. B. Robinson Unit No. 2, the attached Abnormal Occurrence Report is sub-mitted for your infor=ation. This report fulfills the requirement for a written report within ten days of an Abnormal Occurrence and is in accordanca with the format set forth in Regulatory Guideline 1.16.

Yours vary truly, p/: 'gg E E. Utley ' {

Vice-President Bulk Power Supply DBW jwk Attachment cet Messrs. N. B. Bessac P. W. Howe R. E. Jone D. Knuth -

J. B. McGirt D. B. Waters b%GA bg, i ,

840103041605000261 gDRADOCK 751030 PDR

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O O Abnormal occurrence Report L

1. Report Number 50-261/75-9 i

! 2a. Report Date May 9, 1975 2b. Occurrence Date May 1, 1975

3. Facility H. B. Robinson Unit No. 2 Hartsville, South Carolina 29550

4. Identification of Occurrence Failure of reactor coolant pump (RCP) seal resulting in discharge of reactor coolant fluid to containment floor.

5. Conditions Prior to Occurrence t

The reactor was operating at full power. All systems were normal.

Dilution of the primary coolant boron concentration was in progress l

to compensate for the buildup of Xenon. The plant had just reached full power operation early in the morning on May 1, 1975 following a maintenance outage.

! 6. Description of Occurrence I During dilution of primary coolant boron concentration, No. 1 seal leakoff from "C" pump flow responded sensitively to all additions.

This sensitivity had existed since its replacement earlier during the week. Since the leakoff was within the prescribed limits and the variations in flow were gradual instead of " spiking", it was considered satisfactory to operate with this seal.

The dilution was still in process when the following events transpired:

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May 1, 1975:

"C" Reactor Coolant Pump leakoff " spiked" several times. The 1

1750 pump was monitored for vibrations and found to be normal.

1811 "C" RCP seal leakoff oscillated full range several times and then stabilized high with a seal flow greater than 6 gpm. A l

load reduction was commenced at a rate of 10% per minute so that "C" pump could be idled as per plant operating procedures.

1817 - Load reduction was stopped at 36% power.

1818 "C" RCP was deenergized.

1819 - Reactor trip occurred due to turbine trip on high level in "B" steam generator. Level instabilities created from the rapid load reduction'and initiation of Automatic Steam Dump induced

the high level.

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l 1832 - Component cooling water valve 626 closed due to high flow from 1

the reactor coolant pump thermal barrier. This was caused by 4

sudden steam formation in the thermal barrier created when the .

I pump seal failed open. The open seal allowed hot (540'F) primary j coolant to flow upward through the thermal barrier.

i Stopped "A" and "B" Reactor Coolant Pumps. It was the. Shift j Foreman's decision to stop "A" and "B" RCP's when flashing of

! primary coolant in the seal water return line threatened to

cause loss of seal flow due to the pressure surges. The flashing of primary coolant was created by the high flow rate i of coolant through the No. 1 seal of "C" RCP, Valve 303C, leak-I off isolation from "C" RCP, was not closed prior to stopping j RCP's "A" and "B".

1841 - Received automatic letdown isolation due to low level in the-

pressurizer. Supply to charging pump suction changed from '

volume control tank (VCT) to refueling water storage tank l (RWST).

l l 1854 - Pressurizer level returned above letdown isolation setpoint. -

Returned charging pump suction to VCT. ,' f I

1915 - Closed valve 303C to decrease pressure surges in letdown 11ne. '

! Seal flow was lost on "A" RCP. N i 1928 - Seal flow was lost *on "B" RCP. Opened valve WD-1708 to relieve l pressure in Reactor Coolant Drain Tank (RCDT). Pressure build- [

up was due to leakage through No. 2 seal of:"C" RCP. WD-1708  ;

allows drainage of RCDT to containment sump.

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i 1942 - First entry following seal failure was made into containment..

Purpose of the entry was to observe "C" RCP and.to close valve

, CCW-728C, "C" pump thermal barrier outlet manual isolation valve. The control panel-operated valve CCW-303C had not fully

isolated the leakoff and was still causing steam formation in j the thermal barrier. It was not possible to enter "C" RCP bay due to steam escaping from No. 3 seal.' It was previously thought that the thermal barrier on "C" RCP had failed, but

, with no increase on R-17, Component Cooling Water Radiation' Monitor, and the high flow through No.1 seal it was determined -

that steam formation in the thermal _ barrier had caused surges'

in the-outlet line.- The surges in flow ~ caused automatic closure of CCW-626, which isolated all three RCP thermal' barriers.' . ,

1945 ' Valve CCW-626 was manually blocked-open. 'ThisLallowed cooling water to flow through the thermal barriers thus reducing tempera-tures below the boiling point in "C RCP. ~After temperatures were reduced CCW-626 was unblocked and returned to normal operation 4 l in the open position.. >

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O O 1950 - Containment inspection complete. No breaches in primary system other than steam from "C" pump observed. All personnel exited containment and entrance locked.

2000 - Breakers were pulled on containment sump pumps to prevent overfilling of waste holdup tank. Water was originating from reactor coolant drain tank and floor drains in "C" RCP bay.

2007 - Personnel entered Containment to take air samples and noted no ,

steam or leakage from RCP'c. Primary system apparently tight.

2013 - Started "B" RCP bearing oil lif t pump in an attempt to lower shaft, provide more seal clearance and re-establish seal flow.

2015 - Stopped "B" RCP bearing oil lift pump. Failed to establish seal flow through No. 1 seal.

2026 - Personnel exited Containment.

2110 - An entry was made into containment to manually rotate "A" and "B" RCP's in an effort to re-establish seal flow. No inspections of "C" pump was performed during this entry.

2206 - Personnel exited-containment. Failed to establish seal flow through No. 1 seEl's.'

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2215 - The Operating Supervisor called Westinghouse project engineer and notified him that seal flows on "A" and "B" Reactor Coolant Pumps were lost, and that "C" pump's No. 1 seal had opened. It was decided that coola6t pump operation was desirable for proper

,' i mixing of boron in pr'epiaration for cooldown. Also a more con-

' - trolled cooldiwn would be possible with pump operation. Westing-house addsed that "C" pump could be operated with No.1 seal

• leakoff isolgted as long as No. 2 seal remained intact.

N 2242 '- Started "E' icP.

2257 - Prepared foe cooldown by use of main condenser.

e e, 2308 - Blocked valve CCW-626 open to prevent possible loss of thermal barrier cColing, water. _

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2310 - Increased ~ seal whter injection flow to RCP "C" by a factor of 5 l to prevent possible overhcating of pump bearing.

L 2316 - Receiv'ed control' panel indication of 0.5 ft. ofLwater in con-i tainment cump not later than 2316 hours0.0268 days <br />0.643 hours <br />0.00383 weeks <br />8.81238e-4 months <br />.

2330 -Reactor Coolant System boron concentration was 1038 ppm which is-approximately cold shutdown concentration.

' 0015 - High Standpipe Alarm on "C" pump was received. Stopped "C"

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reactor coolant pump. . Pressurizer level started falling rapidly

(- , due to failure of No. 2.and No. 3 seals of "C" RCP,.'

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i 0016 - Started "A" safety injection (SI) pump to supplement charging pump. Deliberately opened valves SI-866A and SI-866B, hot leg safety inj ection to loops "B" and "C". Lowest level reached in pressurizer was 6% on level indicator LI-462 (cold calibration) .

Strip chart indication reached zero and remained there for about 10 minutes.

0018 - Started "B" and "C" safety injection pumps. Pressurizer level decrease stopped.

0035 - Opened valve CVC-307, seal water bypass, in an attempt to reduce a source of the loss of primary coolant to containment.

0036 - Diverted charging flow from the cold leg of loop "B" to auxiliary pressurizer spray (opened valve CVC-311) to reduce Reactor Coolant System pressure, (1150 psig at this time).

The pressurizer steam void was rapidly collapsed by the auxiliary spray and pressure dropped accordingly. It was necessary to use auxiliary spray since normal spray was lost when the coolant pumps were stopped.

0039 - Stopped "C" S.I. pump due to rising pressurizer level. The safety injection pumps and valve CVC-311 were used during the remainder of the cooldown to control pressurizer level and

, pressure.

0043 - Started HVH Unit No. 4, (containment recirculation fan and cooler) to r' educe " containment pressure and temperature. Prior to startup of the fourth unit, three inRI units were in operation.

0048 - The S.I. accumulators partially discharged their volumes into the primary system. The discharge isolation valves were closed at this time to terminate this injection phase.

0100 - Reactor coolant system boron concentration at 1521 ppm.

Containment internal pressure reached a maximum of'3 psig.

0145 - Opened condenser vacuum breaker in preparation to terminate cooldown through the main condenser.

l' 0151 - Started "D" Service Water Pump and "B" Service Water Booster Pump (cooling water to HVH units) to aid in the reduction of containment pressure and temperature.

0215 - Shut main steam isolation valves. Cooldown through the main condenser was terminated.

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l 0223 - Started "A" component cooling water pump _in preparation to go on residual heat removal system. (RHR).

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l 0230 - Started' operating RHR pumps one at a time and opened valves 744A l and 744B (discharge valves) and 750 and 751 (suction valves) to

, warmup RHR system. Pressure in the primary system was approxi-

'i mately 400 psig. Proceeded with plant cooldown as per General Operating Procedure GT-lD.

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.' O O 0330 - Reactor coolant system boron concentration at 1861 ppm.

0341 - Residual Heat Removal System in service. Cooldown proceeding as per GP-lD.

0440 - Stopped using S.I. system to assist maintaining pressurizer level.

0448 - Reactor coolant system at cold shutdown, (less than 200'F).

0517 - Commenced decreasing primary system pressure from 100 psig to O psig.

0629 - Entry into containment was made for purpose of inspection.

Contained air supply packs, (Scott Air Packs), were used during entry. The bottom floor of containment was flooded to a height of 8 to 10 inches. Recovery phase initiated at this time.

1000 - Reactor Coolant System drained below +50" with respect to the vessel flange at which point the leak stopped. The containment floor was flooded to a height of 12.5 inches. The containment temperature vas 100*F and 0 psig with 132,500 gallons of water in the containment. The reactor coolant system boron concen-tration was 1694 ppm.

7. Designation of Apparent Cause of Occurrence The failure of the reactor coolant pump seal system created the occurrence.

This is classified as an abnormal occurrence under. Technical Specification-1.8.e, abnormal degradation of one of the several boundaries designed to contain the radioactive materials resulting from the fission process.

This seal system consists of three seal surfaces - No. 1 seal is the point of primary pressure drop, No. 2 seal maintains back pressure on No. 1 and diverts seal leakoff from No. 1 seal to the volume control-tank, and No. 3 seal helps maintain standpipe backpressure on No. 2 seal. Both No. 1 and No. 2 seals are designed to maintain system pressure.

8. Analysis of Occurrence The occurrence resulted in no offsite releases or exposures. At no time was there any danger to the personnel involved. Due to the seal failure, overheating of the pump shaf t _and seal housing occurred._ This will necessitate the replacement of the. seals and housing, pump shaft,-and associated equipment.

During the' transient, there was no reversal in coolant flow and the core remained covered. Maximum temperature reached at the core exit was approximately 550*F shortly after the trip. About the time that cold shutdown conditions were reached,- it was determined that another steam bubble existed in the RCS. To reduce the leak rate it was desirable ~to reduce system pressure. This was attempted by cpening valve 311 (auxiliary spray) to collapse the pressurizer bubble. When valve 311

. was opened the RCS pressure did not drop noticeably, but _the pressurizer

. level increased rapidly (more rapidly than charging 'and SI would: raise ?it).

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v l When valve 311 was shut, pressurizer level decreased rapidly as the bubble reformed in the pressurizer. Core thermocouple temperatures showed the reactor temperature to be stable and later when the reactor head vent was opened, little gas or steam escaped. The second bubble probably existed in the steam generator tubes. Due to loss of RCS circulation and the limited steam that had been drawn during the cool-down, none after placing RHR in service, the steam generators remained relatively hot. When pressure in the system was reduced from 400 psig to 100 psig, a bubble could have formed in one or more steam generators.

Proper operation of safeguards equipment and actions taken by the control ,

operators prevented more extensive damage after the seal failure. All systems functioned as designed during the transient and the expected results were achieved.

Apparently pressure surges and high seal leakoff temperatures damaged the No. 2 seal of "C" RCP.

As a result of this incident several other abnormal occurrences took place which were a direct result of the original problem. The primary leakage exceeded those limits given in Section 3.1.5 of the Technical Specifications. This is classified as an Abnormal Occurrence under specification 1.8.b, violation of a limiting condition for operation established in the Technical Specifications. As the leak increased in magnitude the containment pressure, (3 psig at maximum) exceeded 2 psig internal pressure, which is a violation of specification 3.6.2.

This is also classified as an abnormal occurrence under specification 1.8.b. As a result of the subsequent forced cooldown, excessive cool-down rates were experienced, resulting in a violation of Sec. 3.1.2 of the Technical Specifications.

9. Corrective Action Plant cooldown was initiated as soon as conditions permitted. When No. 2 and No. 3 seals failed, operations personnel started depressurizing as fast as conditions would allow in order to reduce the coolant loss through the seal. Safeguards equipment was used as needed in order to maintain the reactor in a safe condition. Proper operating and emer-gency procedures were used to reduce pressure and temperature to cold shutdown conditions.

To prevent reoccurrence of this situation, new seal and associated parts are being installed under the direction of pump vendor representatives and CP&L job coordinators. All parts being installed shall be certified and necessary quality assurance documentation and control maintained.

Since this was the first major incident involving a sizable loss of coolant at H. B. Robinson, the special procedures used to gain control over the transient will be reviewed to determine if any areas warrant revision.

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l 10. Failure Data i . No previous seal failures that resulted in loss of primary coolant to l containment free space have occurred at H. B. Robinson. The following

{ data concerns the affected reactor coolant pump and aeal:

l Controlled Leakage Seal Reactor Coolant Pump - Model V11001-B1 4

No. 1 seal assembly --Identification No. 856C877G01 No. 2 seal ring - Identification No. 856C904G01 No. 3 seal ring - Identification No. 910D670G01 1

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