ML20137A393
| ML20137A393 | |
| Person / Time | |
|---|---|
| Site: | Quad Cities  |
| Issue date: | 01/07/1986 |
| From: | Office of Nuclear Reactor Regulation |
| To: | |
| Shared Package | |
| ML20137A391 | List: |
| References | |
| GL-84-11, NUDOCS 8601140342 | |
| Download: ML20137A393 (6) | |
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UNITED STATES
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8 NUCLEAR REGULATORY COMMISSION o
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WASHINGTON, D. C. 20555 e
SAFETY EVALUATION BY THE OFFICE OF NUCLEAR REACTOR REGULATION INSPECTION AND REPAIR OF REACTOR COOLANT SYSTEM PIPING AT QUAD CITIES, UNIT 2
1.0 INTRODUCTION
During the Quad Cities, Unit 21985 refueling outage, a total of 74 stainless steel piping welds were ultrasonically inspected for intergranular stress corrosion cracking (IGSCC). Of these, 34 welds were in the recirculation system, 17 welds were in the residual heat removal (RHR) system, five welds were in the core spray (CS) system, three welds were in the reactor water cleanup (RWCU) system, two welds were in the control rod drive (CRD) return system, three welds were in the heat vent system and 10 welds were in the jet pump instrumentation nozzle penetration assemblies.
All 12 unrepaired welds found defective during the previous outage were inspected during this outage.
Ultrasonic Inspection Qualified personnel from Lambert MacGill and Thomas (LMT) performed the ultrasonic (UT) inspection for the licensee (Commonwealth Edison Company, CECO). The level II and III UT personnel performing detection and sizing of crack indications were qualified at the Electric Power Research Institute (EPRI) nondestructive examination (NDE) Center by successfully passing the respective training courses.
The level I and II personnel performing scanning duties were trained by LMT on site. The LMT inspection results were reviewed and evaluated by qualified CECO UT personnel.
NRC staff in Region III, based on a review of its UT procedures, calibration standards and equipment, and an observation of the field inspection of a few selected welds, has determined that qualified procedures and techniques were used in the UT examination. A total of 14 welds were reported to have crack-like indications. Of these, two welds (#02E-F6A and #02M-F7) were found cracked during this outage and 12 were reported to be defective in the previous outage. Welds #02E-F6A and #02M-F7 were inspected in 1983 outage and were evaluated to be not cracked. UT indications were reported in weld #02M-F7 during 1983 inspection but were determined to have resulted from lack of fusion. Of the 12 defective welds not repaired in the previous outage, only weld #02M-S3 was reported to show some significant crack growth. The licensee originally planned to overlay repair only three welds
(#02E-F6A, #02M-F7 and #02M-S3), however, as a result of NRC's concern regarding the uncertainties in UT examinations, two previously unrepaired welds (#020-F6 and #02B-F7), each containing a crack longer than one-third of the circumference, were overlay reptired during this outage.
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The licensee reported that, because of surface roughness, meaningful UT examinaticn could not be performed on the nine welds overlay-repaired in the previous outage.
One overlay-repaired weld was UT examined on a best effort basis.
In lieu of UT, three other overlay-repaired welds were radiographically examined using the MINAC system, and no crack-like indications were reported.
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] In addition, each of the nine previously repaired welds was visually examined without insulation during hydrostatic testing, and no sign of leakage was observed.
Crack Growth Evaluation NUTECH performed crack growth evaluations for the nine unrepaired welds with j
the use of its NUTCRAC corrputer program. The nine unrepaired welds were all i
treated with induction heating stress improvement (IHSI). NUTECH's crack growth calculations used the crack growth law recommended in NUREG-1061 Vol.
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1, and considered the primary loads from dead weight and internal pressure as well as the secondary stresses from thermal expansion, overlay shrinkages and IHSI. The IHSI through-wall residual stresses were derived from that reported in the EPRI NP-2662-LD, dated December 1982, and the overlay i
shrinkage stresses were calculated by using the measured axial shrinkages resulting from the weld overlay repairs and the PISTAR computer program.
The results of these crack growth evaluations showed that the final flaw l
sizes at the end of an 18-month operating cycle in each of the nine i
unrepaired welds were well within two-thirds of the ASME Code Section XI IWB-3640 allowable limits. Therefore, the licensee concluded that the nine unrepaired welds are acceptable without repair for an additional operating j
cycle.
j Weld Overlay Repair j
A total of five flawed welds were overlay repaired during this outage. These were three 12" recirculation pipe welds, one 20" RHR pipe weld and one 28" i
l recirculation pipe weld. NUTECH performed the weld overlay design for the i
licensee by using the same design criteria that were used during the 1983 outage. Credit for the first overlay layer and portions of the uncracked piping were taken in the overlay design to meet the ASME Code Section XI IWB-3640 allowable limits. The weld overlay was fabricated with Type 308L stainless steel, which is considered to be IGSCC resistant. The licensee indicated that enhanced visual examination was performed during the fabrication of the first layer to detect evidence of any steam blow-out.
i The ferrite content of the first layer was also measured. After completion of the weld overlay repair, liquid penetrant examination and ultrasonic i
examination were performed to demonstrate the integrity of the overlay and
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adequate bonding with the base metal.
The licensee reported that the average "as-built" overlay thickness and length varied from 0.156 inch to 0.263 inch and 2.5 inches to 8 inches, 4
l respectively. These exceeded the minimum overlay design requirements in each case. The measured axial shrinkages resulting from weld overlay repairs were reported to vary from 0.038 inch to 0.229 inch.
l 2.0 EVALUATION We reviewed the licensee's submittals, including NUTECH's analys.is of the weld overlay design and the calculation of IGSCC crack growth to support the continuing service of Quad Cities, Unit 2 for a 18-month operating cycle in l
the present configuration.
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. Scope of Inspection Except for the inspection of 69 IHSI treated uncracked welds and nine overlay repaired welds, the percentage of welds inspected in each pipe size of each piping system met the guidelines in Generic Letter 84-11. For the inspection of the IHSI-treated uncracked welds, the licensee followed the guidelines in NUREG-1061 for category B welds and a total of seven such welds were inspected. We consider that the deviation in the licensee's inspection of the IHSI-treated welds in this outage is acceptable for this outage, because all those welds were UT inspected after IHSI treatment during the previous refueling outage.
For the inspection of previously overlay-repaired welds, we consider that the licensee's inspection scheme performed during this outage is marginally acceptable for the next operating cycle. Therefore, to assure that the code-required safety margin is maintained in each overlay-repaired weld, UT inspection should be performed during the next refueling outage by qualified personnel, using the specified procedures developed at the NDE Center.
Since the licensee's weld overlay designs took credit for the uncracked piping ligaments, the UT inspection techniques employed should demonstrate the capability of detecting and sizing the cracks underneath the overlay with an adequate level of assurance.
Unrepaired Welds NUTECH used the favorable residual stress distribution resulting from IHSI treatment in its IGSCC crack growth calculations because each of the nine unrepaired defective welds was mitigated by IHSI. The results of NUTECH's calculations indicated that the cracks were arrested in every IHSI-treated weld, except for weld #10S-FS. Even in weld #10S-FS, the cracks were calculated to propagate only a very short distance in 18 months. The crack arrest and slow crack propagation are caused by the presence of large compressive residual stresses at the front of the crack tip.
NUTECH indicated that the calculated crack depth in each unrepaired defective
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weld is well within two-thirds of the Code IWB-3640 allowable limits.
m The allowable flaw s1zes in Code IWB-3o40 are currently being revised to take account of the low toughness associated with the flux welds. The revised Code IWB-3640 is expected to be published soon. Prior to the publication of the revised Code IWB-3640 to derive the allowable flaw sizes for the flux welds, the key points of the staff interim guidelines are summarized below:
(a) The secondary stresses (mainly thermal stresses) should be included in the calculation of the stress ratio.
(b) The acceptable flaw size limits for the flux welds are' two-thirds of the Code IWB-3640 allowable using the stress ratios as modified by (a).
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. The nine unrepaired welds are all flux welds. We have reviewed NUTECH's crack growth calculations and have determined that their calculated final crack depth of each unrepaired defective weld meets the current staff acceptance criteria.
However, there is concern regarding the effectiveness of the IHSI process when applied to cracked welds. This concern is compounded by the uncertainties associated with the UT determination of the crack sizes. Therefore, at this time we consider IHSI to be an effective repair only for welds with minor cracks (crack depth less than or equal to 30% of wall thickness and crack length less than or equal to 10% of the pipe circumference). Accordingly, only welds #02D-53 and #02M-54 qualify for IHSI credit as the other seven unrepaired welds, each having a total crack length exceeded 10% of the pipe circumference, do not meet the staff criteria.
We performed an independent crack growth calculation by using an operating stress of eight ksi and an initial crack depth of 18% of the wall thickness.
This calculation bounds all the seven unrepaired welds that are not qualified for IHSI credit. The results of our calculation indicate that the final crack depth at the end of an 18-month period will not exceed 40% of the wall thickness which meets the staff's acceptance criteria. We assume that IHSI will prevent crack growth in the length dimension.
Based on the above considerations and considering the insignificant crack growth during the last operating cycle, we conclude that the continued operation with the nine defective welds for an 18-month operating cycle is acceptable, because the Code-required safety margin is expected to be maintained.
Weld Overlay Design NUTECH's overlay design for the five defective welds was based on assuming the crack depth to be two times the reported maximum crack depth for pipe diameter larger than 12 inches, or completely throughwall for pipe diameter equal to or less than 12 inches. The favorable residual stress distribution resulting from weld overlay repair is depended upon to inhibit further crack growth. The residual stress distribution used in their analyses was calculated using a finite element model.
The required overlay thickness determined this way is much thinner than that determined by assuming the whole pipe is completely cracked. Although there are many important advantages in using thin overlay (lower fabrication time and cost, less radiation exposures, increased UT inspectability of the overlaid weld, and less distortion in the piping system), the thinner overlays do not provide complete Code compliance if the cracks were to continue to grow until they reach the overlay.
The specific residual stress distribution used by NUTECH in this' analysis appears to us to be somewhat more favorable than can be justified at this time; therefore, for cur analysis, we relied on distributions derived from
. later work, and results from closely related work performed under EPRI contracts.
Even with our more conservative approach, we still conclude that all repairs will provide adequate assurance of safe operation during the next operating cycle of 18 months.
The licensee's overlay designs took credit for the first layer of overlay.
The staff's earlier position was that this is not permitted. The purpose for not allowing credit for the thickness of the first layer was to provide additional conservatism in the overlay design. These additional design margins were considered desirable because, at the time the requirement was introduced, there were substantial uncertainties in the UT crack depth sizing, and experimental measurements had not been performed to support the presence of beneficial residual stresses resulting from weld overlay repair.
Since then, the quality of UT crack depth sizing has been greatly improved, and the extent of the beneficial compressive residual stresses from weld overlay repair have been substantiated by both analytical and experimental measurements. Therefore, we consider that the original additional design conservatism for overlay is not needed if full IGSCC resistance of the first layer was demonstrated. The licensee reported that low-carbon stainless steel was used for weld overlay and the ferrite content of the first layer in each repaired weld was measured to be at least 7.5 FN except at one location on weld #10S-F5, where the depth ferrite measurement was reported to be 5 FN. We conclude that the licensee has demonstrated adequate IGSCC resistance in the first layer, and therefore the licensee's overlay design thickness, which includes the first layer, is acceptable.
Region III of the NRC also concluded that the overlay repairs were performed in accordance with the required Codes and Standards, and NRC requirements.
Leak Detection Although the conservative calculations discussed above indicate that the cracks in the unreinforced welds will not progress to the point of leakage during the next operating cycle, and very wide targins are expected to be maintained over crack growth to the extent of compromising safety, some uncertainties in crack sizing and growtn rate still remain.
Further, not all welds were UT examinable, and cracks could be present in welds that were not examined. Because of these uncertainties, it is prudent to tighten the requirements for the monitoring of unidentified leakage.
In its submittals, the licensee did not discuss further actions to be taken to upgrade the leak detection capability in the plant to ensure timely investigation cf unidentified leakage.
Since there are nine cracked welds not being repaired and not all nonconforming stainless steel piping welds were examined or mitigated, the licensee will continue to implement the augmented leakage limits and monitoring in accordance with the guidelines in Attachment 1 to Generic Letter 84-11.
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3.0 CONCLUSION
i Based on our review of the licensee's submittals and a consideration of l
the finding of NRC staff in Region III based on their observations of the inspection and repair program, we conclude that the IGSCC inspection and i
repair performed in accordance with Generic Letter 84-11 is satisfactory and that the Quad Cities Unit 2 plant can be safely returned to power and operated in its present configuration for an 18-month operating cycle.
Nevertheless, there remain residual concerns regarding the long term growth of small IGSCC cracks that may be present but not detected during this inspection. Therefore, plans for inspection and/or modification of the recirculation and other reactor coolant pressure boundary piping systems during the next refueling outage should be submitted for our review at least 3 months before the start of the next refueling outage.
Principal Contributor:
W. Koo Date'd: January 7, 1986, t
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