ML20024J192
| ML20024J192 | |
| Person / Time | |
|---|---|
| Site: | Zion File:ZionSolutions icon.png |
| Issue date: | 10/06/1994 |
| From: | Office of Nuclear Reactor Regulation |
| To: | |
| Shared Package | |
| ML20024J191 | List: |
| References | |
| NUDOCS 9410110267 | |
| Download: ML20024J192 (5) | |
Text
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j NUCLEAR REGULATORY COMMISSION ff WASHINGTON, D.C. SesE4001
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SAFETY EVALUATION BY THE OFFICE OF NUCLEAR REACTOR REGULATION INSPECTION RESULTS FOR THE STEAM GENERATOR GIRTH WELD COMMONWEALTH EDISON COMPANY ZION NUCLEAR POWER STATION. UNITS 1 ANJ_l DOCKET N05. 50-295 AND 50-304
1.0 INTRODUCTION
In a letter dated March 16, 1994, Commonwealth Edison Company (Comed or the licensee) submitted results of the 1993 inspection of girth welds, which are located between the upper shell and transition cone, of the Zion Nuclear Power Station, Units 1 and 2, steam generators. The submittal also included a brief discussion of previous inspection results and disposition of indications found during the inspection.
In letters dated May 26, 1994, and August 2, 1994, the licensee submitted additional information in response to the staff's questions about the flaw evaluation.
The objective of the staff's evaluation was to review the licensee's inspection results and disposition of flaws to determine the acceptability of continued operation of Zicn Nuclear Power Station, Units 1 and 2.
2.0 DISCUSSION Each of the Zion nuclear units has four Westinghouse steam gener& tors, model 51D. The steam generator shell was fabricated using SA 533 Grade A Class I carbon steel.
The thicknesses for the upper shell and transition cone regions are 3.62 inches and 3.68 inches, respectively.
The licensee used 3.62 inches in its girth weld flaw evaluation.
During the Unit I refueling outage in fall 1989 and the Unit 2 refueling outage in spring 1990, indications were found in the girth welds of each steam generator at t'oth units.
All surface indications were removed by grinding and boat samples were taken from the welds for metallurgical evaluation. All subsurface indications were either within the ASME Code,Section XI, acceptance standards or were dispositioned by analysis.
During the Unit I spring 1992 outage inspection using a magnetic particle (MT) technique, the girth welds in three steam generators were found to have clusters of indications. All indications were removed by grinding.
During the fall 1993 outage, Unit 1 steam generators were inspected using the P-scan automated ultrasonic (UT) system and Unit 2 steam generators were inspected using manual UT techniques.
Unit 2 steam generators were not required to be inspected until the scheduled spring 1995 outage but were 9410110267 941006 PDR ADOCK 05000295 P
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inspected during the 1993 outage because the licensee wanted to gain further-understanding of the condition of the Unit 2 steam generator girth welds.
i Section XI of the ASME Code,1980 Edition (including Winter 1981 Addenda),
requires that indications greater than 50% of Dista..ce Amplitude Curve (DAC) be recorded.
However, to be conservative, the licensee recorded indications greater than or equal to the 20% DAC lesel. This procedure results in the recording and evaluation of some indications which would otherwise have gone unrecorded. The following are the results of the 1993 inspections:
Unit 1 Inspection Results 3A Steam Generator There were eight indications found in IA Steam Generator, three of which were within Section XI acceptance criteria, the remaining indications were accepted by fracture mechanics.
All the indications were classified as surface. The longest indication was also the deepest and was found to be 3 inches long and l
0.27 inches deep.
All the above indications were previously found in 1989 using UT but could not be confirmed as surface indications using magnetic particle testing (MT) in 1989 and 1992. No cracking was found in previously r
repaired areas.
JB steam Generator had one spot indication that was non-recordable.
3C Steam Generator had no indications that were recorded at the 20% DAC level.
3D Steam Generator There were six indications found in ID Steam Generator, one of which was within Section XI acceptance criteria, the remaining indications were accepted by fracture mechanics.
All of the indications were classified as surface and were previously detected using UT in 1989 but were not confirmed as surface during the MT examinations in 1989 and 1992.
No cracking was found in previously ground areas.
The longest indication was also the deepest and was found to be 3 inches long and 0.19 inches deep.
Unit 2 Inspection Results 2A Steam Generator l
There were 48 indications found in 2A Steam Generator, 33 of which were within Section XI acceptance criteria, the remaining indications were accepted by fracture mechanics.
Five indications were classified as surface and the i
remaining indications were classified as subsurface.
The longest surface i
indication was 2.05 inches long.and 0.14 inches deep. The deepest surface indication was 0.23 inches deep and 0.85 inches long. The average surface indication was about 0.8 inches long and 0.15 inches deep.
i l
s 2B Steam Generator i
There were 18 indications found in 2B Steam Generator, eight of which were
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within Section XI acceptance criteria, the remaining indications were accepted by fracture mechanics.
Eight indications were classified as surface and the remaining indications were cl&ssified as subsurface. The longest surface indication was also the deepest and was found to be 1.3 inches long and 0.32 inches deep.
2C Steam Generator There were 47 indications found in 2C Steam Generator, 24 of which were within Section XI acceptance criteria, the remaining indications were accepted by fracture mechanics.
Eleven indications were classified as surface and the remaining indications were classified as subsurface.
The longest surface indication was 1.6 inches long and had a depth of 0.24 inches.
The deepest surface indication was 0.3 inches deep and 1 inch long.
2D Steam Generator There were 51 indications in 2D Steam Generator, 40 of which were within Section XI acceptance criteria, the remaining indications were accepted by fracture mechanics.
Seven indications were classified as surface indications.
The deepest surface indication was 0.8 inches long and 0.41 inches deep.
The longest surface indication was 1.6 inches long and 0.2 inches deep.
3.0 EVALUATION Insoection Pesults for Unit 1, the licensee compared the results of the 1993 inspection to the results from previous inspections and found no increase in flaw depth for those indications dispositioned by fracture mechanics.
For Unit 2, the licensee concluded that there was no significant increase in flaw depth.
Flaw lengths were longer, but the flaws were accepted by fracture mechanics. A review of the crack depths showed that the cracking is relatively shallow.
The deepest indication found in this outage was 0.41 inches. The licensee indicated that inspection results, boat sample analysis and monitoring showed that crack growth was not rapid.
In addition, boat sample analysis has shown 4
that crack growth rates decreased as (1) the crack grew deeper, and (2) it.
grew from the heat affected zone into the base metal.
1 The licensee attributed the flaws in the girth welds to improper wet layup practices for the steam generators in which the water level was at the girth weld. The licensee found that oxygen causes pitting in the girth weld.
The licensee has changed wet layup practices to reduce dissolved oxygen l
concentration and the pitting inside the steam generator.
Specifically, the steam generators are filled so that the air / water interface at the girth weld will be eliminated. Aluminox, which reacts at lower than operating temperatures, is added as an oxygen scavenger. A blowdown modification was
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6 t
installed to enable favorable chemical mixing inside the steam generator i
during wet layup, t
Flaw Evaluation The licensee evaluated all indications in accordance with Table IWC s510-1 of i
Section XI of the ASME Code,1989 edition.
For those indications that exceed the acceptance criteria of Table IWC-3510-1, the ASME Code specifies that IWB-3600 and Appendix A apply. The licensee used subsurface and surface crack models assuming a circumferential orientation with an elliptical shape.
Each flaw that exceeded criteria in Table IWC-3510-1 Section XI was evaluated and accepted by fracture mechanics.
The applied stresses were calculated using the finite element method and included pressure loading and thermal loading from each of six transients.
i Using both crack models, the licensee calculated fatigue growth of the surface and subsurface cracks according to Appendix A of Section XI of the ASME Code, 1992 edition.
The subsurface crack showed negligible growth over one 18-month fuel cycle.
The surface crack showed a crack growth rate of 0.04 inches in depth per fuel cycle. The licensee judged that the'ASME Code fatigue method underestimates the crack growth rate when compared to actual girth weld inspection data of Zion steam generators. To be conservative, the licensee obtained an upper bound crack growth rate of 0.32 inches in depth per fuel cycle based on a Weibel cumulative distribution function at the 95 percentile of the historical Zion inspection data.
For normal (including upset and test) conditions, IWB-3600 specifies that the allowable stress intensity factor, K, should be less than K //10, where K l
i a
3 is the lower bound of the crack arrest critical K.
From Appendix A of Section XI of the Code, the licensea obtainedaIkmitingK of 200 ksi/in, n
which resulted in an allowable K of 63.3 ksi/in.
The licensee calculated a-i maxirum applied K of 35 ksi/in based on the maximum crack size at the end of t
the next fuel cycle under the worst transient conditions.
The margin between the allowable and applied K demonstrates that the steam generator girth weld g
has enough toughness to resist brittle fracture.
It should be noted that the 1992 edition of the ASME Code that the licensee used for its fatigue growth calculation has not been approved by the NRC.
However, the licensee did not use the fatigue growth rate calculated by the 1992 edition in its conclusions; the calculations were used only as a reference.
Future Inspections For Unit 1, the licensee has committed to inspect lA and ID steam generators as required by Section XI of the ASME Code during next inspection period (Third Interval, First Period), which is tentatively scheduled to end around spring 1997.
Steam generators IB and IC, which had no recordable indications resulting from the 1993 inspection, will be inspected if the results of the 1A and JD steam generators require the inspection as specified by the ASME Code.
'e For Unit 2, the licensee has committed to inspect all four steam generators during the January 1995 refueling outage.
The licensee will also inspect all steam generators as required by Section XI of the ASME Code during the next inspection period (Third Interval, First Period) which is tentatively scheduled to end around spring 1998.
4.0 LONCLUSION The staff has determined that the licensee followed the ASME Code in dispositioning the flaws and was conservative in its practice.
The indications were recorded using a conservative criterion and were all found acceptable by either the acceptance criteria or fracture mechanics analysis of the ASME Code.
Based on its review, the staff concludes that the four steam generators in each of the Zion units are acceptable for continued service because the licensee has satisfactorily dispositioned flaws found in the upper shell to transition cone girth welds in the steam generators.
principal Contributor:
John C. Tsao Date:
0::cber 6, 1994