ML20205M946
| ML20205M946 | |
| Person / Time | |
|---|---|
| Site: | Hatch  |
| Issue date: | 04/24/1986 |
| From: | Office of Nuclear Reactor Regulation |
| To: | |
| Shared Package | |
| ML20205M943 | List: |
| References | |
| TAC-56540, TAC-60943, NUDOCS 8605010049 | |
| Download: ML20205M946 (5) | |
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UNITED STATES
[
g NUCLEAR REGULATORY COMMISSION g'
- j WASHINGTON, D. C. 20555
..... #o SAFETY EVALUATION BY THE OFFICE OF NUCLEAR REACTOR REGULATION RELATING TO INSPECTION AND REPAIR OF REACTOR COOLANT PIPING SYSTEMS AND CORE SPRAY SPARGERS GEORGIA POWER COMPANY EDWIN I_. HATCH NUCLEAR PLANT UNIT 1 DOCKET N0.: 50-321
1.0 INTRODUCTION
During the Hatch Unit 1 1985 refueling outage, a total of 143 welds susceptible to Intergranular Stress Corrosion Cracking (IGSCC) were ultrasonically inspected; 125 in the recirculation system, 12 in the Residual Heat Removal (RHR) system and 6 in the reactor water cleanup (RWCU) system.
The inspected population included 23 overlay repaired welds and 4 unrepaired recirculation piping welds from the previous outages.
Induction Heating Stress Improvement (IHSI) was applied to a total of 107 welds including 12 unrepaired welds to improve its resistance to IGSCC.
In addition, the repaired core spray spargers and associated piping were visually inspected in accordance with I&E Bulletin 80-13.
1.1 Ultrasonic Examination Ultrasonic examinations for IGSCC sere performed by qualified personnel from Southern Company Services (SCS) for the licensee, Georgia Power Company (GPC).
All ultrasonic examination persar.nel performing IGSCC detection were requalified at EPRI NDE Center. Additionally, Level I personnel were given on-site procedural training on EPRI provided IGSCC cracked pipe samples, The UT personnel performing ultrasonic examinations of weld overlays had received training and demonstrated their ability in finding the cracks in the overlay and outer 1/4 T (thickness) of the base materials at EPRI NDE Center. To improve inspectability, the overlay surfaces were mechanically prepared in accordance with the EPRI recommendations.
For those welds that were overlay repaired in the previous outages, the overlay surfaces were also similarly conditioned.
1.2 Inspection Results The licensee indicated that all the accessible piping welds, including the overlay repaired welds as well as the nozzle to safe-end welds in the Recirculation, RHR and RWCU systems, were ultrasonically ir.rpected during this outage.
In addition, each inspected weld was either treated with IHSI or reinforced by weld overlay using IGSCC_ resistant materials. trdTECH performed the IHSI treatment and Welding Services, Incorporated (WSI) applied the weld l
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. overlay repairs for the licensee. About 40% of the welds (56 welds) were UT inspected prior to and following IHSI treatment; every IHSI weld was UT inspected following treatment.
The inspection results indicated that 20 additional welds were found to show crack-like indications (4-6 inch RWCU welds, 9-12 inch recirculation welds, 5-28 inch recirculation welds, 1-12 inch recirculation nozzle to safe-end weld and 1-20 inch RHR weld) during this outage.
Both circumferential and axial cracks were observed in the cracked welds. The worst circumferential cracking was reported in a RWCU weld (RWCV-6-0VT-50) with a maximum crack depth about 70% of the wall thickness. For the cracked welds in the recirculation system, the deepest circumferential crack (50% of the wall thickness) was reported in riser weld (12BR-B-3)gh-wall axial cracks were observed in a recirculation weld 12AR-G-3. Throu after IHSI treatment. The flaw in the recirculation nozzle to safe-end weld (12BR-C-5) was reported to be a small subsurface lamination (0.4 inch long and 0.025 inch deep) in the base metal of the safe end away from the heat affected zone (HAZ) and was about 0.77 inch from the safe-end outside diameter surface.
However, this laminar flaw was not detected after IHSI treatment.
The four unrepaired recirculation welds from 1984 outage were reinspected before and after IHSI treatment during this outage. The inspection results indicated that: welds 22AM1-BC-1 and 22BM1-BC-1 were not cracked because the same indications detected in this outage were determined to be geometrical type reflectors caused by weld geometry; weld 1831-1RC-28A-6 did not show any significant crack growth; and the crack growth in weld 28-B16 was greater than predicted by the previous analysis and, therefore, overlay repair was necessary for structural reinforcement of the weld.
Except for recirculation weld 288-4, all 23 welds overlay repaired during the previcus outages were observed to be free of reportable indications in the overlays. Weld 288-4 was observed to have small lack of fusion indications in the overlav. Similar indications were also observed in weld 28A-12 which was overlay re' paired during this outage. All the lack of fusion indications were reported to "a located in the first two layers of the overlay.
1.3 Core Spray Spargers Inspection Visual inspection was performed on the core spray spargers, associated piping and the installed clamping devices using a remote underwater television camera with resolution capable of viewing in situ a 0.001 inch diameter fine wire.
The inspection results did not reveal any new cracking or evidence of significant degradation of the clamping devices. Because of the visual obstruction by the installed clamping devices, only a small portion of the existing cracks (25-30 degrees of the pipe circumference) could be reinspected. Observation of the visible portion of the existing cracks did not reveal any additional crack growth or increase in crack width.
1.4 Flaw Evaluation and Weld Overlay Design Structural Integrity Associates (SIA) performed flaw evaluation and weld overlay designs for the licensee. Of the 21 welds showing crack like
. indications,12 welds were reinforced by weld overlay repairs, and the others were treated with IHSI.
In the fracture mechanics evaluation, SIA performed bounding crack growth calculations for the 9 unrepaired welds (3-12 inch recirculation welds, 5-28 inch recirculation welds and 1-20 inch RHR weld) to justify continued operation of those welds without overlay repairs. The sustained stresses used in the crack growth calculations included those from thermal expansion and weld overlay shrinkages in the piping system as well as the primary stresses from pressure and dead weight. The results of the calculations indicated that r
the cracks would not grow during the next fuel cycle because the residual compressive stress field that resulted from IHSI treatment mitigated the sustained stresses discussed above. SIA's evaluation also concluded that the existing flaws in the welds discussed above were significantly smaller than 2/3 of the allowable flaw depth for flux welds from IWB-3641-5 of Code Section XI. SIA also evaluated three welds that contained non-IGSCC flaws; small sub-surface lamination in a nozzle to safe-end weld (12BR-C-b) and lack of fusion in the first two layers of the overlay in welds 28A-12 and 28-B-4.
The j
flaws were shown to be acceptable without repair by IWB-3514-3 of Code Section XI.
i SIA designed a full structural overlay for each of the 12 cracked welds that required repair.
In addition, an Inconel cverlay was applied to an unflawed weld (24B-R-12) to improve the inspectability of this weld. The overlay design was based on an assumed 360 throughwall crack in the original pipe.
The PC-Crack program was used to size the weld overlay using Code Section XI IWB-3640 criteria. The overlay design did not take credit for the first layer of weld material that cleared the liquid penetrant test and ferrite measurements (>7.5 ferrite number). The overlay was applied by automatic welding using 308L stainless steel.
With the exception of two RHR welds (24B-R-13 and 24A-R-13), all weld overlays designed in previotss outages were reevaluated to assure that each overlay would qualify as a full structural overlay. As a result of this reevaluation, the overlays of four recirculation end-cap welds were upgraded to full structural overlays. The two repaired RHR welds (24B-R-13 and 24A-R-13) contained only axial cracks and in this case, the overlay served only as a leak barrier. Additional weld metal was added to the overlay in those welds where the surface was improved to facilitate UT to ensure that each overlay would be a full structural overlay after mechanical grinding of the overlay surface.
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1.5 Region II Input i
Region II inspectors reviewed the following: the licensee's examination and repair procedures, inspection records, qualification programs pertaining to the UT inspection, and IHSI and weld overlay repairs performed during this outage. The inspectors concluded that the licensee's UT inspection, IHSI I
application, weld overlay repairs and core spray sparger inspection were performed by qualified personnel using qualified procedures, and that the required Code and Standards, and NRC requirements were met for the activities.
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, 2.0 EVALUATION We have reviewed the licensee's submittals, including the inspection results and SIA's flaw evaluation and weld overlay designs, to support the continued operation of Hatch Unit 1 for one more fuel cycle (18 months) in its present configuration.
2.1 Scope of Inspection During this outage, the licensee has inspected all accessible welds that are susceptible to IGSCC in the Recirculation, RHR and RWCU systems.
In addition, each weld was irispected after IHSI treatment and weld overlay repairs. We conclude that the inspection performed in this outage meets the guidelines in Generic Letter 84-11.
2.2 Unrepaired Welds A total of 24 welds were reported to show linear indications during this outage. Twelve of those welds were overlay repaired and the others were treated with IHSI. Of those 12 welds not overlay repaired, 9 welds contained IGSCC flaws, one weld contained only a small non-IGSCC subsurface lamination and 2 welds were determined to be not cracked.
SIA's fracture mechanics evaluation of these 9 unrepaired welds containing IGSCC flaws took advantage of the beneficial residual stress distribution resulting from the IHSI tra tment. Three welds in the Recirculation System welds (12AR-G-4,- 12BR-E-4 and 288-10) did not meet the staff criteria (Id permit the IHSI credit in flaw
< 30% of wall thickness and total crack length < 10% of circumference) that wou evaluations. Both welds 12AR-G-4 and 12BR-E-4 were reported to have a total circumferential crack length of 14% of the pipe circumference, and weld 288-10 was reported to contain short axial and circumferential cracks with a maximum crack depth of 31% of wall thickness in axial orientation. However, since the deviations are minor, we find ti:em acceptable for one cycle of operation since the maximum crack depths in welds 12AR-G-4 and 12BR-E-4 are shallow (< 20% of wall thickness) and the axial cracks in weld 288-10 would not have a significant impact on structural integrity even if assumed throughwall. However, to assess if the IHSI treatment has bcen effective in limiting crack growth, these three welds are to be reinspected during the next refuel outage. Based on our review as presented above, we conclude that the 12 unrepaired welds can be safely operated for one more fuel cycle of 18-months since the Code design safety margins would continue to be maintained.
2.3 Weld Overlay Repairs-n Based on our review of SIA's weld overlay design report, we have determined that all weld overlays fabricated at Hatch Unit I were designed and inspected in accordance with the guidelines in Generic Letter 84-11. Therefore, we conclude that all 36 overlay repaired welds can be safely operated for one more fuel cycle of 18-months. For the two repaired welds (288-4 and 28A-12) showing lack of fusion in the first two layers of the overlay, we request that the welds be ultrasonically inspected during the next refueling cutage to assure that no growth of these repair defects has occurred.
2.4 Core Spray Spargers Inspection We have reviewed the licensee's inspection results, and conclude that the inspection is satisfactory and the repaired core spray spargers can be safely operated for one more fuel cycle of 18-months. However, we have some concerns -
regarding the long term behavior of the existing cracks since a large portion of the cracks are not visible, and of those pipes located in areas that can-not be fully inspected. To resolve those concerns, we recommend the licensee perform an air bubble test at the next refueling outage to supplement the visual inspection. The air bubble test would be advantageous in identifying small throughwall cracks especially if the cracks are located in nonvisible areas.
Furthermore, significant growth in the length or width of the throughwall cracks could be detected by comparing the re:orded air leakage i
rates.
As indicated in our previous safety evaluation (SE) the NRC approval for continued operation of the repaired core spray spargers is dependent on the inspection results for each of the tollowing refueling outages. The licensee stated that the sequential events of inspecting, reporting and obtaining NRC startup approval impacts outage scheduling and, therefore, requested relief for this item. To provide the licensee some relief in outage scheduling, we will not require staff review and approval of sparger' inspection results unless new cracking, growth of existing cracks in the spargers and associated piping, or evidence of degradation in the clamping devices is identified during the inspection. However, the detailed inspection results should be submitted for NRC information within two weeks after completion of the inspection.
In the event that new cracking, growth of the existing cracks or degradation of the claaping devices is observed, the acceptability of the sparger for continued operation would require staff review and approval.
3.0 CONCLUSION
l We find that the core spray sparger and IGSCC inspections and repairs performed in this outage are satisfactory. Therefore, we conclude the Hatch Unit 1 plant can be safely operated in its present configuration for an 18-month fuel cycle.
Nevertheless, the staff is still concerned with long term growth of small IGSCC cracks that may be present but were not detected during this inspection.
Therefore, plans for inspection and/or modification of the Recirculation and any other service sensitive austentic stainless steel piping systems operating over 200 F should be submitted for our review at least three months before the start of the next refueling outage.
Principal Contributor:
W. Koo Date: April 24, 1986
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