ML20043F630

LER 90-016-00:on 900511,accident Scenarios Identified by Which Auxiliary Feedwater Piping from Discharge of Turbine Driven Auxiliary Feedwater Pump FW-10 Can Be Overpressurized.Caused by Design deficiency.W/900611 Ltr

ML20043F630
Person / Time
Site:	Fort Calhoun
Issue date:	06/11/1990
From:	Gates W, Hollingsed M OMAHA PUBLIC POWER DISTRICT
To:	NRC OFFICE OF INFORMATION RESOURCES MANAGEMENT (IRM)
References
LER-90-016, LIC-90-0472, NUDOCS 9006150173
Download: ML20043F630 (6)

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-June 11, 1990 LIC-90-0472 s

I i lU. S. Nuclear Regulatory Commission

'4 Attn: Document Control Desk MaiT Station P1-137 Washington,.DC 20555

Reference:

Docket No. 50-285 Gentlemen:-

Subject:

Licensee Event Report 90-16 for the Fort Calhoun Station Please find attached Licensee Event Report 90-16 dated June 11, 1990.

This report is being submitted pursuant to requirements of 10 CFR-450.73(a)(2)(ii)(B).

If.you shoul'd have any questions, please contact me.

Sincerely, A&: 5 W. G. Gates Division Manager

, Nuclear Operations WGG/djm Attachment

.c: .R. D. Martin, NRC Regional Administrator A~. Bournia, NRC Project Manager P. H. Harrell, NRC Senior Resident Inspector INP0 Records Center American Nuclear Insurers 9006150173 900611 PDR ADOCK 05000285 S FDC y

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Auxiliary Feedwater pump FW-10 i; one of two redundant auxiliary feedwater pumas at Fort Calhoun Station. FW-10 is a steam turbine driven pump. designed 1 g" to )e independent of AC power requirements, and is designed to deliver a l minimum flow of 260 gallons per minute to the steam generators against a steam generator pressure of 1000 psia. The pump automatically starts on low steam l generator level or actuation of the Engineered Safeguards Features-(ESF) I system. l l

The speed (and resultant discharge pressure) of FW-10.is governed by a i pneumatic-hydraulic speed control loop which maintains the feed pump discharge.

-pressure at a fixed differential greater-than the steam generator pressure.

This control system is supplied from the Instrument Air system.-. Upon total loss of' Instrument Air, the failure mode of the control system will cause the pump speed to increase to the setting of the mechanical s)eed limiter on the main governor. There is no overspeed trip associated witi FW-10.

The start of FW-10 from the ESF system places the pump in the recirculation mode in preparation for accident mitigation. In the recirculation mode, the pump takes a suction from the Emergency Feedwater Storage Tank (EFWST) and-

-pumps back to the EFWST. The Auxi!iary Feedwater Actuation System (AFAS) then +

determines whether flow is required, and to which steam generator, depending  ;

on the levels and pressures in the steam generators. If pressure in a steam l generator is greater than 500 psia.and level is below the low level setpoint, 1 auxiliary feedwater is initiated to that steam generator. If coincident with l low level the pressure in a steam generator is less than 500 psia, but is more l than 75 psi higher than the other steam generator, then auxiliary feedwater is I initiated to that steam generator. Auxiliary feedwater is not automatically l initiated to the steam generators under any other conditions. l 1

Initiation of auxiliary feedwater automatically opens isolation valves l

'HCV-1107A and HCV-1107B for steam generator RC-2A, and HCV-1108A and HCV-1108B l for steam generator RC-28. These valves are air-operated, and fail open upon I loss of air. Air accumulators were installed on all four valves during the '

1990 Refueling and Maintenance Outage to maintain them closed following a loss of. instrument air to minimize operator actions outside the Control Room.

l As discussed in LER 89-016, in June of 1989 it was discovered that the speed I control loop on FW-10 was functioning improperly and preventing the pump from I reaching an adequate speed for injection of auxiliary feedwater into the steam j generators. The temporary solution was to isolate the instrument air feeding the loop and therefore allow the pump to run at the maximum speed allowed by the speed limiter, which L.s determined to be adequate for auxiliary feedwater injection. After evaluating the options for permanent solutions, it was i decided to repair the speed control loop during the 1990 Refueling Outage, l l

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Subsequently, the potential for a single failure of the speed limiter resulting in an FW-10 overspeed condition was identified.- An engineering analysis was initiated to determine whether a credible single active failure of the speed limiter existed, the maximum pump speed before failure, maximum discharge pressure, and effect on the auxiliary feedwater system. The analysis identified a scenario in which the maximum ~ discharge pressure of-FW-10, 2235 psig, would exceed the design. pressure of the auxiliary feedwater l piping downstream of FW-10, 1660 psig, in the event of a single active failure '

1 of the mechanical speed limiter. The scenario consists of the following: l l

L 1). ALossofCoolantAccident(LOCA),-MainSteamLineBreak(MSLB),  ;

l orMainFeedwaterLineBreakoccurs(MFWLB). l l

2) A coincident seismic event or loss of offsite power disables the instrument air system, causing an air pressure loss. .

3) The mechanical speed limiter on FW-10 fails.

The first condition will cause FW-10 to start, but only in the recirculation l

, mode. Following a LOCA, steam generator level will not be immediately reduced l to the AFAS~ low level setpoint; therefore, no auxiliary feedwater flow will

. be initiated to either steam generator. Following a MSLB or.MFWLB, the AFAS L will prevent feeding the disabled steam generator, and the level in the intact steam generator will not be directly affected by the accident; therefore, no auxiliary feedwater flow will be initiated to either steam generator. 'The second condition will disable the speed control loop on FW-10, causing the

, pump speed to increase to the speed limiter setting. With the accumulators on HCV-1107A/B and HCV-1108A/B, these valves will remain closed as instrument air pressure'is lost. A single active failure of the speed limiter would then cause the pump to overspeed, increasing the discharge pressure to approximately 2235 psig until mechanical failure of the pump stops it.

L Inaccordancewith10CFR50.72(b)(2)(i),thispotentialforoverpressuriza-l~ tion was reported to the NRC on May 11, 1990 at 1725 hours0.02 days <br />0.479 hours <br />0.00285 weeks <br />6.563625e-4 months <br /> CDT as being a l condition outside the design basis. The plant was in Mode 5, Refueling Shutdown, for the 1990 Refueling Outage at the time of notification.

When this overpressurization scenario was discovered, repairs were in progress to the speed control loop on FW-10. It was therefore assumed that the loop would be operating properly following the 1990 outage, and that a loss of instrument air would be a necessary component of any FW-10 overspeed scenario. Upon further scrutiny, however, it became apparent that the possibility of an overspeed scenario was inadvertently made less remote in June of 1989 when the instrument air feeding the speed control loop was isolated. This scenario required only an accident which would start FW-10 from the ESF system yet not require immediate auxiliary feedwater flow, and a single failure of the speed limiter.

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a This deficiency appears to have been the result of an inadequate original plant design. Investigation into plant design documents has revealed that an i overspeed trip for FW-10 was originally specified, but was later cancelled and the s)eed limiting governor was substituted for it. No-reason could be found for t11s change. If'the overspeed trip had been installed in conjunction with the speed. limiter, the single-failure scenarios discussed in this report would not exist.

1 Further review of scenarios has identified that inadvertent overpressurization i of the piping has been possible since initial plant startup in 1973. If an l accident signal would have started FW-10 with no requirement for auxiliary 1 feedwater flow to either steam generator, and instrument air was lost, FW-10 l would have been running on the speed limiter in the' recirculation mode. Plant '

procedures would have directed operators to manually close valves HCV-11078 l and HCV-11088 to prevent undesired feeding of'the steam generators. Failure. 1 of the speed limiter at this point would have resulted in the.nec6ssary  !

conditions for FW-10 to overspeed and overpressurize the discharge piping; l therefore, isolating instrument air to the speed control loop'of FW-10-in June of_1989 did not create the potential for overpressurization, but it did increase the probability of the event-by not requiring instrument air to be lost. The system was also determined to be more vulnerable to the overpressurization scenarios during periodic testing of FW-10 in recirculation mode.

An additional factor was that the safety evaluation used to justify isolating instrument air in June of 1989 did not address the potential for a single failure resulting in a condition outside of the system's design basis. The safety evaluation concluded that the consequences of isolating instrument air would be the same as for a loss of instrument air event. The discussion of loss of instrument air stated that the pump would operate at the maximum speed allowed by the speed limiter, and that no new event or unanalyzed condition would be created.

Additional engineering analysis was performed to determine the consequences of a failure of the FW-10 limiter. The turbine key buckets on FW-10 are designed to shear at a s)eed between 10,200 rpm and 10,-500 rpm. The imbalance caused by the bucket slear will then slow down and e .tntually stop the pump due to overheating and bearing failure. At 10,500 r,..a, the discharge pressure of i FW-10 is calculated to be 2235 psig. The auxiliary feedwater piping is class 1600 piping, which has a design pressure of 1660 psig. The engineering .

analysis concluded that a pressure of 2235 psig will not cause failure of the piping or associated components. No immediate corrective actions were determined to be necessary. Based on the analysis of FW-10 limiter failure, Safety Analysis for Operability 90-012 was approved on May 13, 1990 to justify acceptability of plant operation .

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r The following corrective actions will be implemented:

, , ;1.. Lessons learned from this event will_be included in the training i p program for 10 CFR 50.59 safety evaluations. ~This will be t-completed by September 1, 1990. s s 2. A permanent resolution _to the concern for overpressurization of.

the auxiliary feedwater piping from overspeeding of FW-10 will be .l

'; , completed during.the 1991 Refueling and Maintenance Outage.- '

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As discussed earlier, LER 89-016 reported on a condition of FW-10 being outside the design basis. LER's90-003 and 90-009 reported on different

-sections of auxiliary feedwater piping being outside the design basis. LER's87-018, 88-019,88-020, 88-032,88-033, 89-007,-89-010,.89-014,89-015, 89-017,90-005,.and.90-007 reported design' deficiencies in various other systems-at Fort Calhoun Station.

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