ML20155A247
| ML20155A247 | |
| Person / Time | |
|---|---|
| Site: | Cooper  |
| Issue date: | 10/26/1998 |
| From: | Swailes J NEBRASKA PUBLIC POWER DISTRICT |
| To: | NRC OFFICE OF INFORMATION RESOURCES MANAGEMENT (IRM) |
| References | |
| NLS980178, NUDOCS 9810280224 | |
| Download: ML20155A247 (11) | |
Text
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N Nebraska Public Power District Nebraska's Energy Leader NLS980178 October 26,1998 U.S. Nuclear Regulatory Commission Attention: Document Control Desk Washington, D.C. 20555-0001 Gentlemen:
Subject:
Main Steam Nozzle Weld Indication Relief Request Cooper Nuclear Station, NRC Docket 50-298, DPR-46
Reference:
- 1. Letter (NLS950240) to USNRC from J. H. Mueller (NPPD), dated December 16,1995, " Report of Feedwater Nozzle Examination Results and Relief Request" 1
- 2. Letter to G. R. Horn (NPPD) from William D. Beckner (USNRC), dated June 7,1996," Relief Request from the ASME Code Successive Examination Ryuirements for Feedwater Nozzle to Shell Weld (TAC No. M94260)"
Gentlemen:
The Nebraska Public Power District (District) hereby submits Relief Request, RI-27, for Nuclear Regulatory Commission (NRC) review and approval. The relief request, contained in, is in response to findings related to the ultrasonic examinations of the Main Steam nozzle to vessel weld, N3A at Cooper Nuclear Station (CNS). These examinations have 4
identified an indication that exceeds the acceptance standards ofIWB-3512 of ASME XI,1989 Edition (Code). The indication is being evaluated and is expected to be acceptable for continued service in accordance with Subarticle IWB-3600. A copy of the fracture mechanics evaluation will be submitted under a separate cover letter no later than October 30,1998.
The District has reasonable assurance that the indication in nozzle N3 A is a welding discontinuity that has been present since original construction based on the following:
Ultrasonic Testing (UT) has identified this indication since 1976.
(f UT signal has the characteristics of slag inclusion from original welding. This condition would not appear on the radiographs but has sufficient interface to reflect ultrasonic signals.
The location of the discontinuity is not characteristic of an inservice flaw initiation mect'anism.
9810280224 981026 k
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PDR ADOCK 03000298 P
PDR I
Cooper Nudear Station P.O. Bon 98 / Brownvdle. NE C8171-0098 Telephone: (402) 8253811/ Fax: (402) 825 S211 Ntp //www nppd com
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NLS980178 October 26,1998 Page 2 of 2 -
Th nhture of this indication is similar to the indications previously identified in Feedwater nozzle to vessel welds N4A, N4C, and N4D which were determined to be construction artifacts (References 1 and 2). A description of the indication and an evaluation of the examination data from the previous two intervals is provided as Attachment 2.
Based on the information provided, the District requests approval of th'e relief request prior to startup from Refueling Outage 18 (RFO-18). The District requests a response to this letter by November 11,1998 in order to support the current outage schedule.
Should you have any questions concerning this matter, please contact me.
Sincerely,
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/kbt/dnm Attachment cc: Regional Administrator USNRC - Region IV i
Senior Project Manager USNRC - NRR Project Directorate IV-1 1
Senior Resident inspector USNRC NPG Distribution l
Attachment I to Cooper Nuclear Station NLS980178 Inservice Inspection Program Page1of2 INSERVICE INSPECTION RELIEF REQUESTS RELIEF REQUEST NUMHER: RI-27, REVISION 0 COMPONENT IDENTIFICATION Code Class:
1
References:
IWB-2430(a), ASME XI,1989 Edition Examination Category:
B-D Item Numbers:
B3.10
==
Description:==
Additional Examinations Component Numbers:
Main Steam Nozzle to Vessel Weld NVE-BD-N3A
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CODE REOUIREMENT IWB-2430(a) states that when examinations performed in accordance with Table IWB-2500-1 reveal indications exceeding the acceptance standards of Table IWB-3410-1, additional examinations shall be performed during the same outage. The additional examinations shall include the remaining welds, areas, or parts included in the inspection item listing and scheduled for this and the subsequent period.
BASIS FOR RELIEF Specific reliefis requested on the basis that the proposed alternative would provide an acceptable level of quality and safety.
I During the Fall 1998 Refueling outage, the last outage of the first period, an indication was identified in Main Stearn nozzle to vessel weld N3A that exceeds the acceptance criteria ofIWB-3512. The District has reasonable assurance that the indication is a welding discontinuity that has been present since original construction. The location of the indication and the signal characteristics are consistent with the presence of a thin layer of slag. This condition would not appear on the radiographs but has sufficient interface to reflect ultrasonic signals. This condition is being evaluated and is expected to be acceptable for continued operation in accordance with j
The nine nozzle to vessel welds, scheduled for examination in the first period of the Third Ten-year interval, have been performed during the Fall 1998 Refueling outage. Nine nozzle to vessel welds are scheduled to be examined during the second period. Strict applicatt u of the Code would require these nine nozzles to be examined during the current outage ano again during the next period. In order to perform the ultrasonic examinations of these additional welds, the nozzles are first hydrolyzed to reduce the c'ese, and then scaffolding is erected (as needed),
insulation and shield blocks are removed, and the surface is cleaned. These activities, plus the time for the examinations, followed by the reinstallation of shield blocks and insulation, and scaffold removal will add several days to the outage. The dose for these additional weld examinations is estimated to be 12 person-rem.
.. to Cooper Nuclear Station NLS980178 Inservice inspection Program Page 2 of 2 '
INSERVICE INSPECTION RELIEF REQUESTS The only potential service degradation mechanisms that could affect the steam nozzles are fatigue, stress corrosion cracking and erosion corrosion. For all these mechanisms, cracking indications would be expected to occur at the inside surface. Since the indications of current concern are subsurface near the mid-depth of the nozzle, fabrication is the most likely cause of origin.
In the 1991 Addenda, IWB-2430(a) was revised to clarify that the additional examinations shall be selected from welds, areas, or parts of similar material and service. This clarification was provided to ensure that the additional examinations focused on welds, areas, or parts that would be subject to a possible common service degradation mechanism. A common service degradation mechanism does not exist for original weldino related discontinuities. The Main Steam nozzles are unique when considering all aspects or wecice conditions applicable to reactor vessel nozzles. The most notable factor is that the main steam nozzles are in a steam environment during normal plant operation. The thermal cycles that the vessel can experience are not greatly different when comparing the upper vessel where the steam nozzles are located and the mid-vessel where most other nozzles are located. Ifowever the steam environment has significantly different heat transfer properties which make the thermal response of the nozzles in the steam region different from those below the water line.
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In addition to the main steam nozzles, the top head nozzles are also in the steam region, llowever, the top head nozzl are small diameter nozzles welded to the hemispherical top head, where it isjust over three i.i.. : thick. The steam nozzles are 24 inch diameter nozzles welded to the six inch thick cylindrical shell. Therefore, the stress conditions associated with the nozzle-specific geometries are significantly different.
PROPOSED ALTERNATE EXAMINATION Based on the nature of the indication, the anticipated dose, the impact on the outage schedule, and the changes to IWB-2430(a) in later editions of the Code, the District requests that the additional examinations of reactor vessel nozzles be limited to the two Main Steam nozzle to vessel welds scheduled ie' period two, and that the remaining additional nozzle to vessel weld examinations required by IWB-2340(a) be deferred until the next refueling outage. The person-rem exposure for inspecting the additional nozzles this outage and again next period provides no concurrent safety benefit.
Reliefis requested in accordance with 10CFR50.55a(g)(6)(i). The ASME Section XI requirement is impractical due to dose considerations and the proposed alternative grovides an adequate level of quality.
APPLICABLE TIME PERIOD Reliefis requested for the first period of the Third Ten-year interval of the Inservice Inspection Program for CNS.
' to NLS980178 Page 1 of 6 Description ofIndication Preservice ultrasonic examinations were performed in accordance with.n SME Section XI,1971 Edition. No actual data reports were available for review; however the summary report for the preservice examinations shows main steam nozzle N3A as having no recordable indications.
During the first refueling outage in 1976, manual ultrasonic examinations were performed on nozzle N3A in accordance with the 1974 Edition of ASME Section XI. During the examination with the 60 shear wave transducer one (1) indication with the following parameters was recorded: Distance to weld centerline = 3.25"; Metal path to reflector = 5.4"; Length to maximum amplitude = 14.5"; and Maximum amplitude = 100% distance amplitude correction (DAC). The evaluation of this indication determined that it was located in the nozzle forging and not in the weld. See attached sketch.
Main Steam nozzle (N3A) was examined again in 1986 with similar results. During the examination with the 60 shear wave transducer one (1) indication with the following parameters was recorded: "W" distance = 1.75"; Metal path to reflector = 5.3"; and Maximum amplitude =
90% DAC. The evaluation of this indication determined that it was located in the nozzle forging and not in the weld.
In 1998 (RFO-18) the examination with the 60 shear wave transducer identified one (1) recordable indication with the following parameters: "W" distance = 1.20"; Metal path to reflector = 5.5"; Length to maximum amplitude = 13"; and Maximum amplitude = 80% DAC.
This indication is located at mid-wall along the nozzle side fusion line between 10.5" and 22.75" (see attached sketch). No flaw indication could be found in the nozzle forging as was found in 1976 and 1986. The 1976 and 1986 indication mistocation were entered into the District's corrective action program. The following comparisons clarifies the size and locational differences.
Comparison The construction drawings show the weld prep to be a 1.5" gap between the nozzle and the vessel wall with a backing ring attached to the outside. After welding the backing ring was removed and a cosmetic weld cap was installed. This cap was machined to a 5.5" radius to provide a smooth transition between the nozzle outside diameter and the vessel wall. This resulted in a weld cap which is approximately 3.5" wide on top of a weld which is 1.5" wide. Due to this 2.0" difference between the weld cap and weld width, the plotted location ofindications could also vary by 2.0" due to differences in the "Wo" reference.
The metal path to the reflector is very consistent in all three examinations which would indicate
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the same reflector. The 14.5" length location in 1976 is well within the length of the 1998 location. The differences in "W" locations may be explained by differences in the reference i
l point. If the 1976 data used the center of the weld crown cap as "Wo" then the 3.25" recorded is essentially the same measurement as the 1.2" location recorded in 1998 which used the nozzle blend radius weld toe as "Wo" reference.
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l j to NLS980178 Page 2 of 6 '
The 1976 and the 1986 examinations were performed to editions of the ASME code which required only a 50% DAC recording level with 0.9" increment measurements. Using this criterion, this indication was recorded as a series of spot indications which were observed all around the weld at this metal path. The 1998 data was recorded at 20% DAC levels which resulted in the 12.25" length. Other indications were observed below the 20% recording levels at the same metal path around the weld.
Conclusions Based on the similar metal path to the reflector and the changes in the "Wo" reference points, this appears to be the same indication. Because of the change in recording criteria, no comparisons can be made to the length of these indications. Due to the improved techniques available for plotting recorded indications this indication is shown to be located in the weld material.
The flaw signal characteristics are typical of those of a slag type flaw with multiple reflectors and the ability to maintain signals over a nide range of skew angles. The flaw signal characteristics are not similar to those expected froni lack of fusion or fatigue cracking. These indicatioris have been recorded since 1976 and the signal characteristics observed. Thus, it is apparent that the indication was the result of the original construction process and not service induced.
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l ATTACHMENT 3 LIST OF NRC COMMITMENTS l
Corresponde'nce No:
NLS980178 The following table identifies those actions committed to oy m.ie 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 NL&S Manager at Cooper Nuclear Station of any questions regarding this document or any associated regulatory commitments.
COMMITTED DATE COMMITMENT OR OUTAGE Examine two additional Main Steam Nozzle to vessel welds Refueling Outage No.18 as required by ASME XI.
Examine remaining Period two Nozzle to vessel welds as Refueling Outage No.19 required next outage.
Submit fracture Mechanics evaluation of MS-A Nozzle to 10/30/98 vessel weld indication l
PROCEDURE NUMBER 0.42 l
REVISION NUMBER 6 l
PAGE 9 OF 13 l
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