ML20196H033

Informs NRC of Deletion of 1991 Commitment Contained in Ltr Re Feedwater Nozzle Bypass Leakage Monitoring at Plant

ML20196H033
Person / Time
Site:	Cooper
Issue date:	06/24/1999
From:	Swailes J NEBRASKA PUBLIC POWER DISTRICT
To:	NRC OFFICE OF INFORMATION RESOURCES MANAGEMENT (IRM)
References
RTR-NUREG-0619, RTR-NUREG-619 NLS990060, NUDOCS 9907010323
Download: ML20196H033 (3)

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H Nebraska Public Power District i Nebraska's Energy Leader i

l NLS990060 June 24,1999 U.S. Nuclear Regulatory Commission Attention: Document Control Desk l Washington, D.C. 20555-0001

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l Gentlemen:

Subject:

Revised Commitment: Feedwater Nozzle Bypass Leakage Monitoring

Cooper Nuclear Station, NRC Docket No. 50-298, License No. DPR-46

Reference:

Letter (No. NLS9100042) to USNRC Document Control Desk from G. R. Horn (NPPD) dated January 22,1991, "BWR Feedwater Nozzle Inspections" l The purpose of this letter is to inform the Nuclear Regulatory Commission (NRC) of the deletion of a 1991 commitment contained in the above reference regarding feedwater nozzle bypass leakage monitoring at Cooper Nuclear Station (CNS).

As a result of feedwater nozzle cracking experienced at various Boiling Water Reactor plants in past years, the NRC issued NUREG-0619. In response to NUREG-0619, in 1980, Nebraska i Public Power District (District) performed a design change at CNS which removed the cladding from the feedwater nozzles, installed triple thermal sleeve, double piston ring spargers, and committed to perform dye penetrant (PT) examinations on a periodic basis. Pursuant to the referenced Letter CNS committed to monitor for bypass leakage and perform Automated Ultrasonic Inspections as an alternative to the PT examinations.

The technical basis for monitoring is that feedwater nozzles experience thermal stresses, under normal operating conditions, due to the incoming feedwater being colder than that in the reactor vessel. The magnitude of the thermal stresses is increased during reactor startup (before l feedwater heaters are in service) and during shutdown (after heaters are taken out of service).

Turbulent mixing of the hot water retuming from the steam separators and dryers and the

incoming cold feedwater causes thermal stress cycling of the nozzle bore unless it is thoroughly l protected by the spx 'r thermal sleeve. Bypass leakage past the juncture of the thermal sleeve and nozzle safe end it . . primary source of cold water impinging upon the nozzle bore. This thermal cycling caused by turbulent mixing can lead to fatigue surface crack initiation in the feedwater nozzles. Also, stainless steel cladding on feedwater nozzle surfaces contributes to the fatigue cracking because thermal stresses from the high frequency cycling are higher in the i stainless steel than they would be in the unclad base metal.

9907010323 990624 ii PDR ADOCK 05000298 x

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! P PDR :ooper Nudear Station PO. Box 98/ Browndlie, NE 6832h0098

):4*)V) i Telephone: (402) 825-38n / Fax: (402) 825-52H http//www nppd com

1 ' NLS990660 June 24,1999 Page 2 of 2 After review of the above described basis, the District has determined that the commitment to perform on-line monitoring of the feedwater nozzle bypass leakage is no longer necessary for the following reasons:

1) The removal of the feedwater cladding in 1980 resulted in the elimination of the dissimilar metal that contributed to fatigue. Additionally, the new sparger design, a triple thermal sleeve, double piston ring, also installed in 1980, has minimized bypass flow (no nozzle bypass leakage in excess of 0.3 gallons per minute has been identified).

2) Since the implementation of the design changes, there has been no indication of flaw initiation. This has been verified through the use of advanced ultrasonic techniques to '

monitor for crack initiation in the bore and inner radius regions of the feedwater nozzles.

3) Results of a recent (completed June 7,1999) fracture analysis support the conclusion that should a flaw exist, it will not exceed the critical flaw size during the remaining life of the plant. By this analysis, a postulated .25" flaw in the limiting location of the CNS feedwater nozzles will not reach the critical flaw depth (0.575") before 37 years of rated plant operation from the last inspection.

CNS plant-specific experience and analysis (historic data, ultrasonic testing, and fracture mechanics analysis) has demonstrated that the conunitment to monitor feedwater nozzle bypass

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leakage is no longer technically necessary. j Should you have any questions concerning this commitment change, please contact me.

Sincerely, f ]

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' i e Presi nt fNuclear Energy l

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/dnm cc: . Region Administrator Senior Resident Inspector USNRC - Region IV USNRC Senior Project Manager NPG Distribution USNRC -NRR Project Directorate IV-1

i p4 O 'iTTACHMENT 3 LIST OF NRC COMMITMENTS l

Correspondence Nor NTA990060 The._following table identifies those actions committed to by the District in this document. 'Any other actions discussed in the submittal represent intended or planned actions by the District. They are described to the NRC for the NRC's information and are not regulatory commitments. Please notify the l543 Manager at Cooper Nuclear Station of any questions regarding this

' document or any associated regulatory commitments.

COMMITTED DATE COMMITMENT OR OUTAGE None  !^

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h PROCEDURE NUMBER 0.42 l REVISION NUMBER 6 l PAGE 9 OF 13 l

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