ML20236H059
| ML20236H059 | |
| Person / Time | |
|---|---|
| Site: | Sequoyah  |
| Issue date: | 11/02/1987 |
| From: | NRC OFFICE OF SPECIAL PROJECTS |
| To: | |
| Shared Package | |
| ML20236H040 | List: |
| References | |
| NUDOCS 8711030411 | |
| Download: ML20236H059 (3) | |
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%g UNITED STATES NUCLEAR REGULATORY COMMISSION j
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WASHINGTON, D C. 20555 k.....
SAFETY EVALUATION BY THE OFFICE OF SPECIAL PROJECTS IN REGARD TO j
AUXILIARY FEEDWATER PUMP OPERABILITY
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TENNESSEE VALLEY AUTHORITY SEQUOYAH NUCLEAR PLANT, UNITS 1 AND 2 DOCKET NOS. 50-327 AND 50-328 1
1.0 INTRODUCTION
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The Tennessee Valley Authority (TVA) replaced the pressure control valves in the auxiliary feedwater (AFW) pump discharge lines with cavitating venturis to improve system performance. These modifications resulted in a changing of the system resistance of the auxiliary feedwater flow path.
Subsequently, these modifications also raised a concern regarding the adequacy of the existing Technical Specifications (TS) surveillance requirement to demonstrate full flow capability. At NRC's request, TVA submitted its methodology, calculations and procedures for verification testing.
By letter dated December 23, 1986, supplemented on June 25, 1987 and i
September 24, 1987, the TVA submitted its calculations and justification that the motor-driven AFW pumps are operable and requested that verification testing of the operability of these pumps be done in Mode 3.
2.0 EVALUATION l
TVA has replaced the pressure control valves in the AFW pump discharge lines l
with cavitating venturis.
These impose a greater system resistance (for the same flow) than the pressure control valves. Hence, the 1397 psid recirculation flow operability requirement is in question for demonstrating full-flow capability. Startup testing in Mode 3 at required flows is necessary to determine the actual system resistance with the cavitating venturis. Once this headloss is determined, the system head on the discharge of the pump for the required flow can be determined.
This allows determination of maximum allowable pump degradation and the lowest allowable pressure differential at recirculation flow.
In its review of the methodology, calculations and procedures, there are two safety concerns regarding the AFW operability which were addressed by the staff:
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Is there assurance that the pumps are adequate (with margin) to meet the l
steam generator flow and pressure requirements under worst case i
conditions?
l 8711030411 871102 PDR ADOCK 05000327 P
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J 2.
Will the pump test interval be significantly lengthened by the proposed j
action?
In regard to the concern about meeting steam generator flow and pressure j
requirements, the licensee provided calculations of system resistance across the piping and cavitating venturis for the required flow of 465 gpm (letter of l
i R. Gridley to USNRC and telephone conversation of October 1, June 25, 1987,
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1987, T. Rotella, R. Pierson and R. Wescott of NRC to S. Long and R. Meadors of TVA). This flow of 465 gpm consists of 440 gpm to the steam generators under design conditions of 1100 psi in the steam generators plus 25 gpm for l
pump recirculation flow. The 440 gpm to the steam generators is the minimum
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flow allowed by the design basis accident for the pumps to be considered l
During post modification testing in 1985 TVA discovered that pump 2A-A was not performing in accordance with the manufacturer's curve and that the calculated system resistance with the cavitating venturi may be too high for acceptable j
performance under design conditions. The licensee has subsequently modified t
the cavitating venturi and revised its resistance calculations across the venturi (from 88 psid to 48 psid - October 1, 1987 telephone conversation) and 1
l is rebuilding pump 2A-A.
1 Using the new system resistance data the staff estimated a total expected system head requirement of 2666.5 ft. This will allow a degradation of 233.5 ft from the manufacturer's curve for the cavitating venturis at a flow of 465 gpm before the pump cannot deliver 465 gpm. The actual allowable degra-dation to be reflected in the TS should be somewhat lower because of the j
allowances for instrument accuracy.
TVA will generate a pump performance curve from test data, in Mode 5.
TVA has also committed to revise the surveillance instructions to require full flow testing for the pumps every refueling outage.
The licensee will use the Mode 5 l
test data to calculate that an allowable degradation exists and, allowing for instrument accuracy, will then determine a surveillance instruction acceptance criteria for AFW operability. This surveillance instruction will be performed prior to entering Mode 3.
Mode 3 testing will then be used to confirm or revise the estimated system resistance and the resulting value will be used as the surveillance instruction acceptance criteria. The staff considers the estimated difference between the system resistance and the pump curve for pump 2A-A to be sufficient (233.5 ft) for the licensee to enter Mode 3 in accordance with the above procedure.
In regard to the second concern about the pump test interval, the licensee will have completed a pump flow test which validates the pump curve (including recirculation flow) prior to leaving Mode 5.
Once Mode 3 testing to establish i
the entire system resistance curve has been completed, TVA shall submit a proposed TS revision within 60 days should any changes be warranted.
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The present TS state that the frequency of inspection is to be in accordance with Chapter XI of the ASME Boiler and Pressure Vessel Code, which in the case of pumps requires testing at 3 month intervals. Hence, any TVA submittal
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'. for a proposed TS change, for AFW pump surveillance within 60 days of verification testing should assure sufficient time for NRC to process the proposed TS change.
3.0 CONCLUSION
The staff has evaluated TVA's justification for the calculated motor-driven AFW pump operability. The staff has concluded that the justification is acceptable and the procedures proposed by the TVA are sufficient for Mode 3 without any decrease in plant safety.
i Principal Contributor:
R. Wescott, T. Rotella l
Dated: November 2, 1987 l
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