ML20211G608
| ML20211G608 | |
| Person / Time | |
|---|---|
| Site: | Brunswick  |
| Issue date: | 06/12/1986 |
| From: | Office of Nuclear Reactor Regulation |
| To: | |
| Shared Package | |
| ML20211G600 | List: |
| References | |
| GL-84-11, GL-84-LL, IEIN-84-41, NUDOCS 8606200030 | |
| Download: ML20211G608 (4) | |
Text
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SAFETY EVALUATION BY THE OFFICE OF NUCLEAR REACTOR REGULATION RELATIVE TO INSPECTION AND REPAIR OF REACTOR COOLANT PIPING SYSTEMS CAROLINA POWER & LIGHT COMPANY BRUNSWICK STEAM ELECTRIC PLANT, UNIT NO. 2 DOCKET NO.: 50-324
1.0 INTRODUCTION
During(the Brunswick Unit 2 December 1985 (refueling) outage, a total of 107 IGSCC intergranular stress corrosion cracking) susceptible stainless steel welds in the reactor coolant piping systems were ultrasonically inspected; 102 welds in the recirculation system (including 5 welds overlay repaired in 1983 outage) and 5 welds in the residual heat removal (RHR) system.
In addition, 9 Inconel buttered nozzle to safe-end welds (2-28 inch and 7-12 inch) in the recirculation system were inspected in accordance with I&E Notice 84-41.
During this refueling outage 65 recirculation welds were mitigated with IHSI (induction heating stress improvement) and each weld was reinspected after the treatment; and the entire Code Class 1 6-inch reactor water cleanup (RWCU) system piping was replaced with nuclear grade 316 stainless steel piping.
l 2.0 DISCUSSION j
2.1 Ultrasonic Examination Ultraso'nic examinations for IGSCC were performed by qualified ultrasonic testing (UT) personnel from General Electric Company (GE) for the licensee.
l All welds were examined by using the GE " SMART" UT i.n automatic mode where geometrically feasible. The SMART UT system utilized the Ultra Image III computer-driven data acquisition system with the ALARA remote scanning device.
Manual examinations were performed on welds where SMART UT could not be used.
For re-inspecting the weld overlays repaired in previous outage, the techniques recommended by EPRI for examining weld overlays and associated base materials were used.
2.2 Inspection Results And Mitigations A total of 35 welds in the recirculation system (3 4-inch, 21 12-inch, 2 22-inch and 9 28-inch) were reported to be cracked. Eleven welds reported to be cracked in previous outages were re-examined during this outage, and three of those welds (28-A14,28-89 and RHR-20-A2) were determined to be free of IGSCC because the UT indications were attributed to the geometry of the internal surface.
The cracks in the flawed recirculation welds were relatively short and predominately axially oriented. A pin hole leak was visually observed in a recirculation riser weld (12-BR-GI) and throughwall flaws were reported in 8606200030 860612
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- three 4-inch bypass line weldolets. Except for the two recirculation outlet nozzle-to-safe-end welds, all the cracked recirculation welds were repaired by weld overlay. The two unrepaired outlet nozzle welds were evaluated ~ for continued operation by fracture mechanics.
Sixty-five recirculation welds were treated with IHSI during this outage.
IHI, Incorporated of Yokohama, Japan performed the IHSI treatment for the licensee. Seven of those IHSI treated welds were subsequently overlay repaired because cracks were found in those welds during post-IHSI inspection.
During this outage the entire RWCU Code Class 1 suction line was replaced with nuclear grade stainless steel nuclear grade 316 piping and the containment penetration was redesigned to eliminate an inaccessible weld. The licensee reported that, except for two 4-inch decontamintcion port branch connection welds, all IGSCC susceptible stainless steel welce eere either weld overlay repaired or mitigated with IHSI.
2.3 Fracture Mechanics Evaluation General Electric (GE) performed the fracture mechanics evaluation of two cracked recirculation outlet nozzle-to-safe-end welds for the licensee. One small axial crack located in the inside diameter surface of the Inconel 182 weld butter was reported in each of the two cracked nozzle welds. The cracks were sized as being 0.25 inch deep and about 0.25 to 0.3 inch long. The crack growth rate used in the evaluation was based mainly on proprietary test data.
The results of GE's crack growth calculations indicated that the axial crack would grow to a depth of 0.85 inch (41% of the wall thickness) after an operating period of 12000 hours (one fuel cycle). The calculated final crack depth was well within the two-thirds of the Code allowable in Section XI IWB-3640.
Therefore, GE concluded that the two cracked nozzle-to-safe-end welds were
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Justified for continued operation without repair for at least 12000 hours because the Code required design safety margins would be maintained.
2.4 Weld Overlay Designs Nutech designed a total of 39 weld overlays for the licensee including one 4-inch bypass line weld (B-11) which was not cracked and the upgrading of five weld overlays repaired in the November,1983 outage to full structural overlays.
Except for seven 28-inch welds, full structural overlays were designed for all smaller cracked pipe welds (four 4-inch, twenty-six 12-inch and two 22-inch).
i For the seven 28-inch welds containing mainly axial cracks, the overlay design l
consisted of a minimum of two layers since the purpose of the repair was to provide a leakage barrier. Nutech's full structural overlay designs assumed the pipe to be completely cracked through and did not include the first layer that cleared the liquid penetrant and ferrite tests. The required minimum overlay thickness was calculated from the ASME B&PV Code,Section XI, IWB-3640.
2.5 Region II Input l
The Region II inspections included the following:
review of selected examination procedures and representative records, interview of UT personnel and observation of activities in progress. The inspectors concluded that the l
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licensee's inspection and repairs were perfonned by qualified personnel using qualified procedures, and that the required Code, Standards and NRC requirements for the activities were met.
3.0 EVALUATION We have reviewed the licensee's submittals to support the continued operation of. Brunswick Unit 2 for the upcoming 18-mont!. fuel cycle in.its present configuration.
3.1 Scope of Inspection During the current' refueling' outage, the licensee had inspected all Code Class 1 IGSCC susceptible stainless steel welds ~with pipe diameter /4 inches in the recirculation and RHR systems.
In addition, each weld was re-inspected
.after IHSI treatment, and weld overlay repairs as needed. We conclude that the inspections performed during this outage meet the guidelines in Generic Letter 84-11.
3.2 Unrepaired Welds We reviewed GE's facture mechanics analysis of the two unrepaired recirculation outlet nozzle-to-safe-end welds. GE's flaw evaluation has shown that the final flaw sizes in the two unrepaired welds after operating a period of 12000 hours (one fuel cycle) are well within the Code allowable in Section XI, IWB-3640.
i However, we have some concerns regarding flaw size estimation and evaluation.
Our concerns are:
4 (1) GE's crack growth calculation of a plateau crack growth rate of 5x10-g in Inconel 182 was based on the use inch / hour. The available test data to support this plateau crack growth behavior in Inconel 182 is very. limited.
Furthermore, GE's proprietary test data covered only a small range of stress intensity factor (K) with an upper limit at about 40 ksi/in. Thus, the. crack growth calculation of an axial crack under a combined stress condition of 79 ksi with 9 ksi in pressure and 70 ksi in residual stresses would require an
. extensive extrapolation of crack growth rate from the limited test data.
Therefore, we consider. that GE's assumption of a plateau crack growth behavior in Inconel 182 can not be fully justified at this time.
. (2) Past experience with UT inspections have shown that the detection and sizing of axial cracks are not very accurate or reliable. Many axial, cracks that were either unidentified or undersized by field inspections were i
leaked during overlay repair'. Therefore.-the licensee's reported depth of the axial cracks in the two unrepaired welds may not be accurate and the cracks could be substantially deeper than reported.
(3) The staff is concerned that cracks in the Inconel 182 butter may extend into the reactor vessel nozzle during cortinued plant operation.
Cracking into the nozzle would represent a more significant problem since it would constitute a degradation of the reactor pressure vessel itself and would be~ extremely difficult to repair.
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o In view of our concerns regarding the flaw evaluation as discussed above, we consider it prudent to have a mid-cycle inspection on the two unrepa. ired nozzle welds to ensure that the crack growth does not exceed that estimated by GE, and that the cracks do not extend into the nozzle material. The recommended augmented mid-cycle inspection would provide ~ an additional assurance of safe plant operation and is consistent with NRC's philosophy of defense-in-depth in protecting the public health and safety. Therefore, we recommend that the licensee perform a mid-cycle inspection on the two unrepaired nozzle-to-safe-end welds after operating for a period of about 8 to 10 months. The test results should be reported to NRC for review prior to restart from the mid-cycle inspection outage.
3.3 Weld Overlay Designs We reviewed Nutech's weld overlay designs for the 39 repaired welds; 7 were designed with a minimum of two layers of overlay for the purpose of providing a leak barrier and 32.were designed for full structural reinforcement in accordance with the Code Section XI, Table IWB-3951-1. The full structural overlay designs did not take credit for the first overlay layer and the remaining uncracked ligament. Nutech's overlay designs for welds requiring only a leak barrier did not fully meet the guidelines in the Generic Letter 84-11, because the designs took credit for the first overlay layer. However, the IGSCC resistance of each first overlay layer was fully demonstrated by successfully passing the liquid penetrant and ferrite tests. Therefore, the subject designs are considered acceptable. We conclude that all 39 overlay repaired welds can be safely operated for the upcoming 18-month fuel cycle.
3.4 RWCU Piping Replacement Materials The entire Code Class 1, 6-inch RWCU system suction line piping was replaced with nuclear grade 316 stainless steel piping. The replaced RWCU piping material is acceptable for this application because of its greater resistance to IGSCC.
4.0 CONCLUSION
We find that the IGSCC inspections and repairs performed during this outage are satisfactory. Therefore, we conclude the Brunswick unit 2 plant can be safely operated in its present configuration for the upcoming 18-month fuel cycle. We recommend a mid-cycle inspection of two unrepaired nozzle-to-safe-end welds to ensure that excessive crack growth has not occurred.
Nevertheless, the staff is still concerned with long term growth of small cracks that may be present but were not detected during these inspections. Therefore, plans for inspection and/or modificaiton of the recirculation and any other service sensitive austenitic 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.
Date:
June 12,1986 Principal Contributor:
W. Koo
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