ML20215K515

Forwards Plans for Inspecting Core Spray safe-end Weld Overlays During 1987 Refueling Outage.Question Re Effect That Presence of Core Spray Nozzle Thermal Sleeve Would Have on Cooling Rate During Weld Overlay Installation Addressed

ML20215K515
Person / Time
Site:	Vermont Yankee
Issue date:	05/07/1987
From:	Capstick R VERMONT YANKEE NUCLEAR POWER CORP.
To:	Rooney V Office of Nuclear Reactor Regulation
References
FVY-87-50, NUDOCS 8705110233
Download: ML20215K515 (9)

Text

VERMONT YANKEE.

NUCLEAR POwliR CORPORATION

. RD 5. Box 169, Ferry Road. Brattleboro, VT 05301 ,,,ty,o y ENGINEERING OFFICE 1671 WORCESTER ROAD

+ FRAMINGHAM, MASSACHUSETTS 01701 e TELEPHONE 6t7 672-6100 May 7, 1987 FVY 87-50 United States Nuclear Regulatory Commission Washington, DC 20555 Attention: Office of Nuclear Reactor Regulation Mr. V. L. Rooney, Senior Project Manager BWR Project Directorate No. 2 Division of Licensing

References:

(a) License No. DPR-28 (Docket No. 50-271)

(b) Letter, VYNPC to USNRC, FVY 86-36, dated May 5, 1986 (c) Letter, VYNPC to USNRC, FVY 86-49, dated June 2, 1986 (d) Letter, USNRC to VYNPC, NVY 86-113, dated June 16, 1986 (e) Letter, VYNPC to USNRC, FVY 87-07, dated January 12, 1987

Subject:

Core Spray Safe-End Weld Overlay Inspection Plan

Dear Sir:

By letter, dated June 16, 1986 [ Reference (d)], the NRC approved Vermont Yankee's 1986 outage weld overlay design and repair methodology for the two core spray nozzle-to-safe-end welds. Subsequently, Vermont Yankee notified you by letter dated January 12, 1987 [ Reference (e)] of our future plans associated with this matter, wherein we determined that replacement of both core spray safe-ends during the 1987 refueling outage was not warranted. The basis for that determination was provided in an enclosure to that letter which demonstrated that acceptable overlay service can be expected beyond the next operating cycle, even when factoring in the conservative assumptions contained in the draft NUREG-0313, Revision 2.

In accordance with our commitment of June 2,1986 (Reference (c)],

Vermont Yankee herein provides, as Enclosure 1 to this letter, the plan for inspecting the core spray safe-end weld overlays during the 1987 refueling outage.

In addition, the following information is provided in response to one of the questions raised by members of your staff during a telephone conversation held on April 16, 1987. This question concerned what effect, if any, the presence of the core spray nozzle thermal sleeve would have on the cooling rate during weld overlay installation, i.e., would its presence have impeded cooling if water backing was used during the process. Previous safe-end overlay procedures developed by other utilities used water-backed welding _ to 0\

8705110233 DR 870507- '

ADOCK 05000271 PDR

United States Nuclear Regulatory Commission May 7, 1987 Attention: Mr. V. L. Rooney Page 2 l .

i' preclude possible weld-induced sensitization of austenitic stainless. steel safe ends. This concern does not exist at Vermont Yankee because the safe end is Inconel 600. The water backing also provides a. heat sink. A-study was

!~ performed to determine the effect of the water backing on control offthrough l wall temperature gradients and...thus,-through wall stress patterns. _The l results of that study.(Enclosure 2, Figures 1, 2, and 3) show that the thermal gradients with and without. water backing are essentially the.same. Therefore,

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the residual stress pattern would be the same with or without water backing.

For this reason it can.be concluded that the presence of the thermal sleeve would have had no effect on the final residual stress pattern.

We trust that our plans with respect to this matter are acceptable; l however, should you have any questions or require additional information, please contact us.

l Very truly yours, VERMONT YANKEE NUCLEAR POWER CORPORATION Y f l R. W. Caps ck Licensing Engineer RWC/sj Enclosures

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l ENCLOSURE 1 Vermont Yankee Core Spray Safe End Overlay Inspection Plan In support of Vermont Yankee's' plans to inspect the weld overlay repairs to both N-5 nozzle to safe-end welds during the 1987 refueling outage an evaluation of the ultrasonic examination techniques available for use has been performed.

Following the decision to weld overlay the Vermont Yankee N-5 nozzle to safe-end weld with Inconel weld metal, Vermont Yankee developed ultrasonic examination procedures for baseline examination of the weld overlay.

Examinations .were conducted to ensure overlay bond, overlay integrity, and to -

provido assurance that flaws present under the overlay did.not propagate into the overlay. The techniques used were developed consistent with EPRI NDE Center techniques for weld overlays. Personnel qualified through EPRI were utilized in technique development and examination performance.

Overall inspectability was enhanced by machining of the weld overlay surface.

Technique development was enhanced through utilization of a mockup of the as-built installation including all materials and interfaces.

Vermont Yankee has reviewed the examinations performed in 1986 and conducted additional investigations intent on optimizing examinations. The appendix to this attachment documents the criteria and results of Vermont Yankee's investigation and is the basis for the planned examination program.

Based on the evaluation described above, Vermont Yankee will implement the following program for inspecting the weld overlay repairs to the N-5 nozzle to safe-end welds during the 1987 refueling outage.

o Personnel Qualification Examination personnel performing and evaluating ultrasonic examination will be a minimum of Level II in ultrasonics and will be currently qualified in accordance with the EPRI program for examination of weld overlays.

Examiners will additionally be qualified in examination techniques for planar flaw sizing through the EPRI NDE Center.

o Procedures Liquid penetrant examination of the weld overlay surface and adjacent base material will be conducted.

Ultrasonic examination of the weld overlay and the base materials underlying the overlay will be performed. Examination will be conducted using the procedures used during the 1986 inspection of these overlays.

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Enclosure 1 Continued These procedures will again be available for NRC review at the plant site.

Additionally, they will-be supplemented to include an additional examination using a dual element 450 refracted longitudinal wave t'ransducer. The net effect of the additional beam angle will provide refracted longtiudinal 450, i 600 ,;70 0and O.D. creeping wave examinations, both parallel and normal to the weld axis as well as a 00 scan of the full volume. Transducer selection will be optimized to focus:over the section thickness as necessary to detect-propogation of cracking in the butter area as well as any defects which may develop at interfaces of the materials, i.e., SA508 to Inconel. overlay. weld-metal.

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Appendix to Vermont Yankee Core Spray Safe End Overlay Inspection Plan INTRODUCTION This appendix documents the criteria and results of Vermont Yankee's investigation of the ultrasonic inspectability of the Inconel weld metal overlay as. applied to the Vermont Yankee core spray nozzle to safe-end weld in 1986. This investigation is based upon a comparison of the inspectability of Inconel overlays to the inspectability of stainless steel overlays. Stainless steel overlay inspection techniques have been developed and demonstrated at the EPRI NDE Center.

This investigation addresses the theoretical differences between the two inspections and provides empirical information in support of the conclusions drawn.

INVESTIGATION PARAMETERS Four items are considered to be important to the overall compatibility of inspection techniques. The first item considered is the overall coupling efficiency necessary to achieve examination. The second consideration is the efficiency with which ultrasound penetrates the Inconel weld metal of the overlay and the ability to receive return signals from known reflectors. The third consideration is the degree to which the Inconel overlay exhibits anisotropic properties and how these properties affect examination. The fourth consideration is how the numerous metallurgical interfaces affect examination.

The EPRI techniques are based upon detection of the upper extremities of ICSC cracks through specular-reflected or tip-diffracted sound from refracted longitudinal beams. Techniques based tipon side-drilled holes (SDH) are used by EPRI. EPRI also uses the SDH as the reference reflector for calibration.

Examination techniques for the Vermont Yankee overlays are based upon detections in the same manner and as such will utilize the side-drilled hole for reference, as well as the reflector upon which this report bases its comparisons.

INVESTIGATION The surface condition of the Vermont Yankee overlays exceeds the EPRI criteria. Surface flatness was specified at installation to be flat to 1/64 of an inch in any 1-inch area and roughness not to exceed 250 RMS. As installed, surfaces are flat to within 1/64 of an inch ~over the entire length of the overlay and the surface finish is at approximately 125 RMS. Coupling efficiency is further enhanced through contouring transducer contact surfaces to match the O.D. contour of the examination surface.

The relative overall efficiency of the penetration of ultrasound was determined by comparison of responses from side-drilled holes in an EPRI stainless steel standard and the Vermont Yankee mockups. Utilizing the EPRI

Appendix to Vermont Yankee Core Spray Safe End Overlay Inspection Plan (Continued) standard for basic calibration, the overall response from any side-drilled hole at any location in the mockup through section was a minimum of 2 dB greater than the stainless steel response without accounting for distance amplitude correction. Of the transducers used, as much as 14 dB greater response was noted and the relative efficiency of Inconel overlay weld metal to stainless steel overlay weld metal, allowing for distance amplitude correction, was on the order of 6 dB. From this data it can be concluded that the combined effects of enhanced surface preparation and the apparently less attenuative Inconel weld metal accounts for an examination of Inconel overlays which is effectively two times more efficient.

In considering the efficiency it was also necessary to examine the Signal to Noise Ratios (SNR) observed. The stainless steel overlay SNR was generally observed at 3:1. In the Vermont Yankee mockups the SNR varied from a low of 4:1 to a high of 8:1 and averaged 6:1. The variations appear to be a function of transducer focal length.

When utilizing refracted longitudinal probes, neither the Inconel nor the stainless steel weld metal were demonstrated to exhibit any apparent significant directional properties affecting either the efficiency of sound transmission or the refracted angles of the sound beam. Similarly, no apparent effects were noted through the overlay to safe-end, overlay to weld nugget, or overlay to weld butter interfaces. The interface of the overlay to the carbon steel nozzle does demonstrate some apparent directional effects as exhibited by a slightly reduced amplitude and an increased refracted angle when a single side-drilled hole was insonified in opposite directions. This may be a result of some anisotropic behavior or may be more simply explained as resulting from the overlay clad weld bead interfaces. Neither the amplitude nor beam angle changes are considered significant with respect to the ability to detect or discriminate the reflectors of concern.

The effects of the various weld metal and base metal interfaces has been investigated through the use of the Vermont Yankee overlay mockups. These mockups contain side-drilled holes which require that sound transit every combination of interfaces. With the probes utilized in this investigation, interface signals and interfaces were not apparent above the general noise level.

CONCLUSION Based upon this investigation, Vermont Yankee considers the principles for detection of Inconel overlay flaws to be the same as those utilized for stainless steel overlays. Further, the EPRI program for examination of weld overlays provides the necessary training and techniques to detect and discriminate both the progress of IGSCC and the development of other flaws of concern in the weld overlay area.

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