ML20154N882

Responds to NRC Bulletin 88-008, Thermal Stresses in Piping Connected to Rcs. Low Flow Situations Resulting in Failures Cannot Occur at Plant.No Piping Connected to RCS Sufficiently Pressurized to Cause Leakage Into Loops

ML20154N882
Person / Time
Site:	Yankee Rowe
Issue date:	09/23/1988
From:	Drawbridge B YANKEE ATOMIC ELECTRIC CO.
To:	NRC OFFICE OF ADMINISTRATION & RESOURCES MANAGEMENT (ARM)
References
FYR-88-129, IEB-88-008, IEB-88-8, NUDOCS 8809300031
Download: ML20154N882 (3)

Text

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y TWX 710480-7619 YANKEE ATOMIC ELECTRIC COMPANY Q%.

1671 Worcester Road, Framingham, Massachusetts 01701 September 23, 1988 FYR 88-129 United States Nuclear Regulatory Comission Document Control Desk Washington, DC 20555

References:

(a) License No. DPR-3 (Docket No. 50-29)

(b) USNRC Bulletin 88-08, "Thermal Stresses in Piping Connected to Reactor Coolant Systems," dated June 22, 1988 (c) USNRC Bulletin 88-08, Supplement 1, dated June 24, 1988 (d) USNRC Bulletin 88-08, Supplement 2 dated August 4, 1988

Subject:

Response to NRC Bulletin 88-08

Dear Sir:

NRC Bulletin 88-08 informed licensees of the potential for thermal fatigue in unisolable piping connected to the Reactor Coolant System (RCS) due to thermal stresses not considered in the design basis. This phenomenon, which occurred at Farley 2, can result if the following three conditions are present: the piping connected to the RCS is isolated by a block valve which is litaking; the pressure upstream from the valve is higher than the RCS pressure; and the temperature upstream is significantly cooler than the RCS temperature.

f,ulletin Action 1 requested that a review be made of systems connected to the RCS to determine if such a condition exists. This review has been completed for Yankee Nuclear Power Station (YNPS).

We hcve determined that there are no locations where a leaking valve, during normal operatica, could subject unisolable sections of piping connected to the RCS to stresses from temperature stratification, or temperature oscillations that were not evaluated in the design basis. During normal operation, there is no piping cor.nected to the RCS sufficiently pressurized to cause leakage into the loops. Therefore, the low flow situation, which resulted in the failures described in the bulletin and bulletin supplements, cannot occur at YNPS. The reason for this is presented below.

The safety injection tie-ins to the RCS are the only piping which possess some of the characteristics required for this phenomenon to occur. At YNPS dedicated safety injection pumps provido coolant to the RCS cold leg in the  ;

event of an Emergency Core Cooling System actuation (see attached sketch). A l cross-connect from the normal charging system utilizes these piping  !

connections for loop filling operations during outage periods. Leakage through a normally closed block valve on this cross-connect line could subject f 9300031 999923 (, t o ADoCK 05onoo29 .

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1. United States Nuclear Regulatory Conunission September 23, 1988 Page 2 the safety injection tie-ins to charging line pressure. The line pressure at points E and F would then be equal. However, unlike Farley 2 our normal charging line ties into the RCS hot leg, which is at a lower pressure than the cold leg due to losses through the reactor pressure. vessel. Charging pump discharge pressure (Pf) is greater than hot leg pressure (Pa), resulting in desired flow into the hot leg. Flow calculations conclude that the pump discharge pressure is less than the cold leg pressure (Pb, c, d), therefore, there can be no flow into the cold leg through the check valve located between points D and E.

Following a turbine trip, RCS shrink requires higher charging system flows. For this short period of time (approximately three minutes), the pressure differential is no longer sufficient to ensure that there is no leakage into the loops. However, this event is estimated to occur about twice per year, and therefore, could not lead to the high cycle fatigue failuren described in the bulletin.

Stratified flow in unisolable sections of piping connected to the RCS cannot accur at YNPS due to the piping configuration and plant operational characteristics. The analysis of this piping performed under the Systematic Evaluation Program further confirms the acceptability of the piping described herein.

We trust that you will find this information satisfactory; however, should you have any questions, please contact us.

Very, truly-yours,

,/ )

ANKEE ATOMIC TRIC COMPANY YV B. LA Drawbrid e Vice P?esiden't and Manager of Operations BLD/25.824 Attachment cct USNRC Region I USNRC Resident Inspector, YNPS W. T. Russell - Regional Administrator USNRC Region I COMMONWEALTH OF MASSACHUSETTS) I

)ss MIDDLESEX COUNTY )

Then personally appeared before me, 8. L. Drawbridge, who, being duly sworn, did state that he is Vice President and Manager of Operations of Yankee Atomic Electric Company, that he is duly authorized to execute and file the foregoing document in the name and on the behalf of Yankee Atomic Electric Company and'that the statements therein are true to the best of his knowledge l and belief.

[ Lecn Robert H. Groce Notary Public My Conunission Expires August 29, 1991

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NORMAL HEAT EXCHANGER CHARGING { CHARGINg p3 3,M n raI, STM. '

BLOCK GEN. CROSS-CONNECT V FROM CHARGING VALVE

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