ML20086R198

USRE-S2-74-01:on 740214,alarm Received Indicating Low Discharge Flow in Containment Vacuum Sys.Caused by Containment Vacuum Pumps Discharging Air.Pumps Removed & Spare Pump Installed

ML20086R198
Person / Time
Site:	Surry
Issue date:	04/10/1975
From:	Stallings C VIRGINIA POWER (VIRGINIA ELECTRIC & POWER CO.)
To:	Moseley N NRC OFFICE OF INSPECTION & ENFORCEMENT (IE REGION II)
References
USRE-S2-74-01, USRE-S2-74-1, NUDOCS 8402280733
Download: ML20086R198 (9)

Text

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Mr. Norman C. Moseley, Director Serial No. 359 l

Office of Inspection and Enforcement P0&M/JTB:clw l

United States Nuclear Regulatory Commission l Docket No. 50-281 Region II - Suite 818 230 Peachtree Street, Northwest License No. DPR-37 Atlanta, Georgia 30303

Dear Mr. Moseley:

Pursuant to Surry Power Station Technical Specification 6.6.B.2, the Virginia Electric and Power Company hereby submits forty (40) copies of Unusual Safety Related Event Report No. USRE-S2-74-01.

This report is not being submitted within the time interval stipulated by the specification because there has been some question concerning the classification of the event reported and its reportability. However, it has been determined to be an unusual safety related event and the report is contained herewith.

The substance of this report has been reviewed by the Station Nuclear Safety and Operating Committee and will be placed on the agenda for the next meeting of the System Nucicar Safety and Operating Committee.

Very truly yours,

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C. M. Stallings Vice President-Power Supply and Production Operations Enclosures .

40 copies of USRE-S2-74-01 cc: Mr. K. R. Goller

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UNUSUAL SAFETY RELATED EVENT REPORT NO. USRE-S2-74-01 CONTAINMENT VACUUM SYSTEM DOCKET NO. 50-281 LICENSE NO. DPR-37 APRIL 8, 1975 SURRY POWER STATION j

VIRGINIA ELECTRIC AND F0WER COMPANY

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1. INTRODUCTION l

In accordance with Technical Specification 6.6.B.2 for Surry Power Station, Operating License Number DPR-37, this report describes an unusual safety related event which occurred on February 14, 1974.

The Directorate of Regulatogy Operations, Region II, was notified on February 14, 1974.

The condition discovered involved containment vacuum (CV) pump capacity being less than that specified in the Surry Power Station t

Final Safety Analysis Report. This event is classified as an .anusual safety related event pursuant to Technical Specification 1.0.J.2 which states that: "Any substantial variance, in an unsafe or less conservative direction, from performance specifications contained in the Technical Specifications or from performance specifications, relevant to safety

! related equipment, contained in the Final Safety Analysis Report."

j II.

SUMMARY

OF OCCURRENUE On February 14, 1974, Unit No. I was in a cold shutdown condition and Unit No. 2 was operating at 91 per cent reactor power and 730 MWe generator output. Unit.No. 2 containment air partial pressure was 9.6 psia. At approximately 1100 hours0.0127 days <br />0.306 hours <br />0.00182 weeks <br />4.1855e-4 months <br />, an alarm was received at Unit No. 2 computer printout, indicating a low discharge flow in the containment vacuum (CV) system. It appeared that both Unit No. 2 containment l vacuum pumps (2-CV-P-1A, IB) were not pumping air properly from the con-tainment. Therefore, a reduction of load on Unit No. 2 was commenced l

l immediately.

.Following an investigation it'was determined that the containment vacuum pumps were,'in fact, discharging air, but at a lower-rate than a

0 0 normal. The reduction in load on Unit No. 2 was halted and the unit brought back to the original reactor power level.

III. ANALYSIS OF OCCURRENCE The containment vacuum system consists of a steam ejector (used to create the initial vacuum prior to operation) and two (2) mechanical vacuum pumps with related piping, valves and instruments. Each mechanical pump is located within its own leak-tight tank. A pipe, running from the containment to the tank, transports air to the pump, with the pump suction port open to the tank interior. The pumps discharge through the charcoal filters of the gaseous waste disposal system to the process vents.

Two (2) redundant control channels are provided to operate the-containment vacuum pumps. Each vacuum pump is operated through a three-

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position hand-off-auto switch under administrative control to permit either manual or automatic vacuum pump start. Either redundant control channel can actuate either containment vacuum pump. Normally one (1) pump switch is in the "0FF" position and the other in "AUT0".

The actual partial pressure of air in the containment is not measured, but is obtained for each channel by subtracting the partial water vapor-pressure signal from the containment total pressure signal. The value of partial air pressure desired in the containment is set on an instrument common to both channels which contains an adjustable setpoint mechanism.

1 that transmits a signal proportional to the desired partial pressure of air. The desired partial air pressure signal from the common instrument.

and the actual partial air pressure.of each channel are compared.: JTo eliminate the possibility of two different setpoints, a common instrument to set the desired partial air pressure in both channels is used.

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When the actual containment air partial pressure increases 0.1 psi above the desired value, either channel of the control system energizes an electrical circuit which either sounds an alarm to signal the operator in the main control room to manually start and operate a mechanical vacuum pump, or sounds the alarm and initiates starting and operating of one mechanical vacuum pump directly, depending on the position of the adminis-tratively controlled three position switch in the control room. When the actual containment air partial pressure falls 0.1 psi below the desired con-tainment air partial pressure, the mechanical vacuum pump stops and an alarm sounds.

If the containment air partial pressure increases 0.25 psi above the preset partial air pressure setpoint, an alarm indicating increasing pressure is activated. If the pressure is .25 psi above the maximum value allowed by Figure 3.8.1 of the Technical Specifications, the operator ini-tiates an orderly power reduction to cold shutdown.

At the time of the incident, pump 2-CV-P-1B was in the automatic mode and 2-CV-P-1A was secured. The low discharge flow alarm setpoint for the containment vacuum system is 2.0 SCFM. The discharge flow of the pump was 1.8 SCFM.

IV. CORRECTIVE ACTION TO PREVENT RECURRENCE Being concerned that Technical Specification 3,15.2.B may have been violated, the immediate action was the load reduction of Unit No. 2.

However, af ter further investigation it was determined that a violation did not exist (both unit vacuum pumps were operable). The power level of Unit No. 2 was restored to normal.

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v The containment vacuum pumps were removed one at a time f rom the system and spare pumps were installed. Operction of the spare pumps in-dicated a flow discharge equal to the pumps removed. In the shop, the vacuum pumps were disassembled, inspected, cleaned and reassembled; no visual problems were detected. A shop test of the pumps indicated satis-factory operation (no performance data was taken at this time) and both pumps were subsequently returned to the containment vacuum system. With the newly overhauled pumps operating, no apparent increase in flow was obse rved .

A check of the rotometer at the containment vacuum pump discharge indicated an initial discharge flow surge, causing the flow meter's disc to oscillate to the full range of the gage. Af ter abouc one (1) minute of pump operation, the disc set led to a position below the lowest value of the scale on the rotameter (less than 2 SCFM). The flow meter was removed from service, cleaned, recalibrated and returned to service.

Suction lines within the containment were also drained. A small amount of water was removed from the lines but these corrective measures did not improve pump performance.

Another shop test was performed, this time with a newly rebuilt con-teinment vacuum pump, to determine actual pump performance. Data was recorded and a plot of discharge flow vs suction pressure drawn. From the test results it was evident that the pump did not meet its specifi-cation. Test data indicated that at a suction pressure of 8.3 psia, the discharge flow of the pump was approximately 2.2 SCFM at 14.7 psia dis-charge pressure. According to the specification, the pump discharge should be 5 SCFM at suction pressure 8.3 psia and discharge pressure

.15.1 psia.

%) G A special test was developed to test the vacuum pumps and containment vacuum system lines in a simulation of actual system operation. Containment isolation valves were shut during test. Suction pressure was adjusted to desired values by throttling the suction line valve 2-CV-27, downstream of the containment isolation valves. The discharge flow of the containment vacuum system was subjected to a slight negative pressure (approxima tely lig-inches W.G.) at the point where the containment vacuum system joins with the process vent system, and therefore the flow should have increased slightly. Special test results showed a pump discharge flow less than that obtained in the previous shop test. It appeared as though some line block-age existed.

Visual inspection of system discharge piping revealed a " loop seal" formed in the line, with no means avaiable to drain it. The discharge lines were blown out with compressed air and approximately 293 grams of debris were expelled from the line. The special test previously performed was rerun with almost identical results.

Additional investigations indicated the proper corrective action was to purchase new vacuum pumps of an improved design to replace the existing pumps. An order for the new pumps has been placed and they are expected to be delivered June 30, 1975. They will be installed at the next convenient shutdown following receipt of the new pumps.

V. ANALYSIS AND EVALUATION OF SAFETY IMPLICATIONS OF THE OCCURRENCE Operation of the containment vacuum system is not required for several months af ter the Design Basis Accident (DBA, FSAR 14. 5.2) .

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The containment is designed and demonstrated to have a leak rate not exceeding 0.1 per cent of containment volume (containment volume 0

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approximately 1.86 x 10 cu. ft.) per day at accident pressure.

Under normal plant operation and during the post accident period, i when the containment has been returned to subatmospheric pressure, the q

inleakage rate will be assumed less than that at design pressure. However, assuming a rate of 0.1 per cent per day, corresponding to a leakage flow

' of 1.2 SCFM, the containment pressure would increase approximately 0.01 psi per day or i psi in 100 days. Each of the two (2) mechanical vacuum ,

pumps are required to have about 5 SCFM capacity which is approximately four (4) times the rate necessary to remove the assumed leakage.

The bench test of a containment vacuum pump produced the following data:

Suction Pressure Discharge Flow PSIA SCFM 2.4 8.5 9.5 3.0 t

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3.6 10.5

~1 1.5 . . 4.3 i 5.1 12.5 s A discharge capacity of 5 ~SCFM will not be realized until the pump

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suction pressure is 12.5 psia. With' the reactor coolant systam at

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and 350 degrees F, 'the containment partial air pressure ~is required ,

~ ;e ,"k 11 psii+(FNhR 6.3.2.1) .

• 1 to be maintained'11etween19 -

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It is' apparent from the data that the pump capacity is sufficient

~to' keep up with inleakage and a safety hazard does not exist.

3 VI. -CONCLUSIONS 6

The licensee concludes that:

. 1. The containment vacuum pumps, though capable of reducing the-inleakage of the containment, are below requirements.

2. The corrective measures described herein will increase the capacity of the containment vacuum systems (Units ,

No, I and 2).

l 3. This unusual safety related event did not present any I

i hazard to the health and safety of station personnel or to the health and safety of the general public.

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