ML20249A020

Forwards Response to 980508 RAI Re Operations & Control of Containment Cooling Water Systems,Per GL 96-06

ML20249A020
Person / Time
Site:	North Anna
Issue date:	06/11/1998
From:	Ohanlon J VIRGINIA POWER (VIRGINIA ELECTRIC & POWER CO.)
To:	NRC OFFICE OF INFORMATION RESOURCES MANAGEMENT (IRM)
References
98-301, GL-96-06, GL-96-6, NUDOCS 9806150324
Download: ML20249A020 (6)

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Vim;iM A Ei.EcTRic ANI) P(MVE R CONWA N Y Ricusuisis, Vim;INI A 232(al June 11, 1998 Document Control Desk Serial No.98-301 United States Nuclear Regulatory Commission NAPS /JHL R2 Washington, D. C. 20555 Docket Nos. 50-338/50-339 License Nos. NPF-4/NPF-7 Gentlemen:

VIRGINIA ELECTRIC AND POWER COMPANY NORTH ANNA POWER STATION UNITS 1 AND 2 GENERIC LETTER 96-06 ASSURANCE OF EQUIPMENT OPERABILITY AND CONTAINMENT INTEGRITY DURING DESIGN-BASIS ACCIDENT CONDITIONS REQUEST FOR ADDITIONAL INFORMATION In a January 28, 1997 letter Virginia Electric and Power Company provided our response to Generic Letter 96-06. The response included our assessment of the potential for waterhammer and two phase flow in containment cooling water systems as well as our plans to further evaluate containment penetration integrity under the q conditions postulated in the generic letter. In a May 8,1998 letter the staff requested i additional information regarding the operations and control of the containment cooling water systems to complete their review. The attachment to this letter provides the ,

requested information.  ;

If you have any questions regarding these commitments, please contact us.

Very truly yours, O l 4%

ames P. O'Hanlon /

Senior Vice President - Nuclear Attachment 7_7 o

. ., v 'v v 8 9906150324 990611 PDR ADOCK 05000338 P PDR

Commitment made by this letter:

1. To further ensure that 1) the Containment Air Recirculation System is re-established in a controlled manner to avoid a postulated waterhammer scenario and 2) the appropriate containment conditions are satisfied prior to operation, Virginia Power will revise the applicable procedures (e.g., EOPs/APs/ ops) to provide more detailed limitations and precautions. These procedural revisions l will be completed by March 31,1999.

' cc: U.S. Nuclear Regulatory Commission l- Region ll 1 l Atlanta Federal Center 61 Forsyth Street, SW l

Suite 23T85 Atlanta, Georgia 30303 Mr. M. J. Morgan NRC Senior Resident inspector North Anna Power Station i

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RESPONSE TO REQUEST FOR ADDITIONAL INFORMATION ATTACHMENT

1. Describe measures that have been taken to assure that plant operators will not use the containment air recirculation fan coolers and associated cooling water system as an. option during or following a plant accident. Also, confirm that plant Emergency Operating Procedures (EOPs) do not allow plant operators to use the containment air recirculation coolers as an option following a plant accident.

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Response

As stated previously, the Containment Air Recirculation System, which is used to cool the containment atmosphere during normal plant operation, has no Engineered Safety Feature (ESF) function during a Design Basis Accident (DBA). The cooling water supply to the containment air recirculation fans,1(2)-HV-F-1A,1B & 1C, is isolated in response to a DBA and the fans are tripped and deenergized. However, the containment air recirculation fans and associated cooling coils,1(2)-HV-E-2A, 2B & 2C, are noted in the EOPs to provide long-term post-accident cooling of containment atmose here if the equipment is available and when containment

. conditions warrant. It should be noted that the procedure used to start the containment air recirculation fans requires, as an initial condition, that containment pressure must be less than 10 psig. This is based on the concern that the fan will trip if operated with a pressure exceeding 10 psig.

2. Implementing measures to assure that waterhammer will not occur, such as ,

prohibiting post accident operation of the affected system, is an acceptable l approach for addressing the waterhammer concern. . However, all scenarios must be considered to assure that the vulnerability to waterhammer has been eliminated.

Con'irm that all scenarios have been considered, including those where the affected containment penetrations are not isolated (if this is a possibility), such that the measures that have been established are adequate to prevent the occurrence of

, waterhammer during (and following) all postulated accident scenarios.

o i o Response:

The Containment Air ' Cooling system is not used during a DBA to reduce containment air temperature and pressure. However, the system may be used to facilitate long-term post-accident containment cooling. If flow is re-established to the containment recirculating air cooling ' coils under the containment cond!tions permitted by the operating procedure, steam induced waterhammer in the coils will not occur

. due to the reduced containment air temperature.

During the temperature transient associated with a DBA (LOCA or MSLB) the cooling water return from the fan cooling coils is isolated by valves 1(2)-CC-TV-100A, B & C (200A, B & C) and 1(2)-CC-TV-105A, B & C (205A, B & C) which close on a containment isolation signal. The cooling water supply penetration to the fan

- cooling ' coils is isolated by a containment isolation check valve located outside of the

containment in each of the three supply lines to the containment fan cooling coils.

The containment fan cooling coils are located in the lowest elevation in containment

' (floor elevation 216'-11") as shown on the enclosed Figure 1.

With the fan chilled water supply isolated by the containment isolation valves, the postulated heat transfer path to the trapped cooling water is through the exposed containment fan cooling coils. As the bulk temperature of the isolated fluid in the cooling coils and the 6 inch supply and return piping rises, the internal pressure increases. The containment air fan cooling coils are protected by a thermal relief valve,1(2)-CC-RV-104A, B & C (204A. B & C), which is set at 150 psig. The saturation temperature associated with a pressure of 150 psig is 366*F. If the trapped fluid expands enough to raise the pressure to the set pressure of the relief valve, then the relief valve will lift and reset at a lower pressure. If containment conditions continue, elevated heat would agsin be transferred to the exposed cooling coils and the set pressure of the relief valve may be reached again. This scenario would continue until not enough heat could be transferred to the trapped fluid to cause the relief to lift. It should be noted that no steam voids are formed due to the pressure and associated high saturation temperature in the cooling coils.

In the above scenario, if the containment air fan cooling coils thermal relief valve does not reseat, a volume of water would be discharged to the containment sump.

As a result,-the water in the cooling coils supply and return piping will reach an equilibrium level based on pressure. Although this condition may result in steam void formation initially during the accident, by the time the conditions would permit containment fan cooler operation in post-accident recovery, the steam void (s) would have condensed.

3. Confirm that the waterhammer and two phase flow analyses included a complete failure modes and effects analysis (FMEA) for all components (including electrical and pneumatic failures) that could impact performance of the cooling water system and confirm that the FMEA is documented and available for review, or explain why a complete and fully documented FMEA was not performed.

Response

The containment air recirculation fans are tripped by a Containment Depressurization Actuation (CDA) signal. The cooling water supply flow to the containment air cooling coils is isolated via containment isolation valve closure

. during accident transients. The Containment Air Recirculation System does not perform an ESF during a DBA. Therefore, it was concluded that waterhammer and two phase flow is not a concern because the system is isolated and deenergized dunng a DBA.

The failure of the cooling coil relief valve discussed above is the worst case failure considering conditions that could cause a waterhammer. ,

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4. Provide a simplified diagram of the affected systems, showing major components, active components, relative elevations, lengths of piping runs, and the location of any orifices and flow restrictions.

Response

The enclosed Figure 1 provides a simplified diagram of the affected systems as requested.

5. Describe in detail any plant modifications or procedure changes (other than those already described in response to question 1, above) that have been made or are planned to be made to resolve the waterhammer issue.

Response

Virginia Power is not performing any modifications as a result of the GL 96-06 evaluations for waterhammer. Virginia Power believes that it is still beneficial to '

have the flexibility of using the containment air fan and associated cooling coils post-accident to facilitate long-term containment cooling following a DBA. Virginia Power believes that the existing procedural controls for containment air recirculation fan operation are adequate to prevent a postulated waterhammer due to steam void collapse. However, to further ensure that 1) the system is re-established in a controlled manner to avoid a postulated waterhammer scenario and 2) the appropriate containment conditions are satisfied prior to operation, Virginia Power will revise the applicable procedures (e.g., EOPs/APs/ ops) to provide more detailed limitations and precautions. These procedural revisions will be completed by March 31,1999.

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