ML19261E812

Forwards Proposed Repair/Replacement Program for Steam Generator Feedwater Nozzle Safe-End-To-Pipe & Pipe-To-Elbow Welds, in Response to 790825 Ltr

ML19261E812
Person / Time
Site:	Millstone
Issue date:	09/28/1979
From:	Counsil W NORTHEAST UTILITIES
To:	Reid R Office of Nuclear Reactor Regulation
Shared Package
ML19261E813	List: ML19261E812 ML19261E814
References
TAC-11793, NUDOCS 7910040319
Download: ML19261E812 (3)

Text

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NOltrHI!AST IFFII.ITII!S P o Box 270 j' ,,

.,,' HARTFORD. CONNECTICUT 06101 (203) 666-6911 k k j ,

September 28, 1979 Docket No. 50-336 Director of Nuclear Reactor Regulation Attn: Mr. R. Reid . Chief Operating Recctors Branch #4 U. S. Nuclear Regulacory Commission Washington, D. C. 20555

References:

(1) R. Reid letter to W. G. Counsil dated August 25, 1979.

(2) W. G. Counsil letter to R. Reid dated August 22, 1979.

(3) W. G. Counsil letter to R. Reid dated August 24, 1979.

(4) W. G. Counsil letter to R. Reid dated August 24, 1979.

Gentlemen:

FEllstone Nuclear Power Station, Unit No. 2 Feedwater System Piping In Reference (1), the NRC Staf f documented its conclusion and the supporting analysis of the acceptability of interim operation of Millstone Unit No. 2 with feedwater line crack indications. The Safety Evaluation enclosed in Reference (1) was based an the documentation provided in References (2) l through (4).

Reference (1) further documented seven commitments which served as the basis for the acceptability of continued operation. Items 1, 2, and 6 of Reference (1) have been accomplished. Items 3 and 7 postulate the occurrence of conditions which have not taken place; as such, they are not applicable at this time. With regard to Item 5, a plant outage is currently scheduled to commence on October 27, 1979. In response to Item 4 of Reference (1), the attached document, entitled " Millstone Unit No. 2, Proposed Repair / Replacement Program fo- the Steam Generator Feedwater Nozzle Safe-End-to-Pipe and Pipe-to-Elbow Welds", is provided for your review. In addition to providing a qualitative description of the two major repair alternatives, a detailed fabrication and installation procedure is included in the attached material to facilitate NRC Staff comprehension of the complications associated with these alternatives. NNECO has developed this document in recognition of the Staff concerns stated in our meeting of August 21, 1979 and in subsequent telephone conversations and, as such, believe it to be fully .esponrive to your concerns. However, should the Staff require supplemental inrormation, ample time exists before the scheduled shutdown date of October 27 to clarify any specific points. Execution of either of the repair options is estimated to require approximately four weeks.

22iL 024 7910040 1/f a

In addition to fulfilling the Item 4 requirement of Reference (1), NNECO hereby provides information relative te the instrumentation installed on the feedwater piping system. A desci .on of the instrumentation, which includes thermocouples, accelerometers, strain gauges, and acoustic leak detectore, was provided in Reference (3). A preliminary evaluation of the data available to date has indicated the following:

(1) The thermal variations (stratification) during hot standby, auxiliary feed, main feed, and power escalation are consistent with data collected at similar locations of the feedwater piping at the plants previously monitored (D.C. Cook, R. E. Ginna , and H. B. Robinson) . The data indicates stratification of temperature during low flow conditions with eventual uniform temperature as feedwater flow increases.

Maximum static stratification of approximately 400*F with tempera-ture increasing from the bottom of the piping towards the top was identified. Additionally, there are transient variations at any specific location on the pipe lasting approximately 40 minutes during which time one full cycle of temperature variation may occur. The most severe variation is found at the top of the piping system where the temperature changed by 360 F.

(2) The strain levels from piping vibration in the piping system are signifi-cantly below levels that would cause crack growth from high cycle fatigue.

Typically, dynamic strains do not exceed 30 micro-strain at any time.

This results in stresses less than 1000 psi.

(3) The accelerometer responses do not indicate significant motion in the piping system from flow induced vibration.

(4) The data collected during startup through 100% power operation provides sufficient bases to establish loading conditions to determine stress levels in the piping from static thermal stratification and trom the transient thermal distribution. There is no evidence from the data collected that would invalidate the results of the model testing being conducted even though the feedwater piping at Millstone Un.t No. 2 is slightly larger than the model test configuration.

An ultrasonic examination of the four previously detected cracks in the feedwater piping system is presently scheduled for October 29, 1979. This examinatien will employ the exmnination te-hnique previously used and results will be plotted and compared with the original results. A succinct statement regarding NNECO's acceptance criterion is that no confirmed crack growth in the through-thickness direction (depth) is permissible. Circumferential crack length will also be quantified during the examination; however, crack length is not a critical parameter, and as such, no acceptance criterion is appropriate. Ultrasonic measurement sensitivity is such that changes in crack depth exceeding 30 mils can be detected reliably. Changes of this magnitude correlate approximately to an increase in gain of 2.5 decibels or a 25 percent increase in signal amplitude.

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Should the examination verify that crack depth has not increased, plant operation without repair will continue. A report describing the findings will be docketed within two weeks of subsequent plant startup. The instru-mentation on the feedwater piping system will be removed with the exception of the acoustic leak detection equipment. This is appropriate in that no substantive information beyond that already acquired would accrue from continuing to collect data from the remaining instruments. The leak detection equipment will remain functional and be monitored without change from the current program until the issue is permanently resolved.

A permanent repair is planned for the 1980 refueling outage. At that time, sufficient data will have been reduced and causative mechanisms identified such that positive solutions can be employed to minimize the potential for further cracking. This approach results in the significant advantages of minimizing personnel exposure and reducing plant outage time. Deferring the replacement, therefore, results in substantial man-rem and economic benefits. Should NNECO's anticipations of no detectable crack growth be realized, the 1980 replacement program is concluded to be technically defensible and appropriate. No other interim ultrasonic examinations before the 1980 refueling outage are required to justify continued safe plant opera-tion and, therefore, none are planned.

We trust you find the above information responsive to the Reference (1) commitments.

Very truly yours, NORTHEAST NUCLEAR ENERGY COMPANY

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