ML20202H263

Safety Evaluation Re Insp & Repairs of Reactor Coolant Piping Sys Welds Susceptible to IGSCC During 1985 Refueling Outage.Insp & Repairs Satisfactory.Facility Can Be Safely Returned to Power & Operated for 18-month Fuel Cycle

ML20202H263
Person / Time
Site:	Millstone
Issue date:	04/03/1986
From:	Office of Nuclear Reactor Regulation
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ML20202H224	List: ML20202H221 ML20202H263
References
NUDOCS 8604150189
Download: ML20202H263 (6)

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'j NUCLEAR REGULATORY COMMISSION WASHINGTON, D. C. 20555 s c g v......f SAFETY EVALUATION BY THE OFFICE OF NUCLEAR REACTOR REGULATION INSPECTION AND REPAIRS OF MILLSTONE UNIT 1 REACTOR COOLANT PIPING SYSTEMS MILLSTONE UNIT NO. 1 DOCKET NO. 50-245

1.0 INTRODUCTION

During the Millstone Unit 1 1985 refueling outage, a total of 115 welds susceptible to intergranular stress corrosion cracking (IGSCC) were ultrasonically inspected; 24 in the recirculation systems, 4 in the shutdown cooling systems, 51 in the low pressure coolant injection (LPCI) system, 8 in the reactor water cleanup (RWCU) system, 4 in the core spray system, 14 in the isolation condenser (IC) system, and 10 in the jet pump instrumentation nozzle assemblies. The inspected population included 7 welds overlay repaired during the previous refueling outage (Reload 9) and one unrepaired weld RCAJ-1 in the 28-inch diameter recirculation piping system.

1.1 Ultrasonic Examination Ultrasonic examinations for IGSCC were performed by qualified personnel from EBASCO, Trutom, Nuclear Energy Services (NES) and Northeast Utilities Service Company (NUSCO) for the licensee, Northeast Nuclear Energy Company, (NNECO). All ultrasonic examination personnel performing IGSCC detection were requalified at EPRI Non-Destructive Examination (NDE) Center.

Additionally, ultrasonic examination technicians were given procedural training, master / slave system training, as well as written and practical examinations, as part of the on-site qualification for IGSCC examination.

Samples containing actual IGSCC were used for the practical portion of the on-site qualification exercise. Ten percent of the welds were examined using the master /sl?ve system. The system was also used for several re-examinations of suspect welds by NUSCO personnel.

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The final disposition of all weld indications was made by qualified NUSCO NDE personnel based on the analysis of the reported data and the additional information obtained during the evaluation, which included the review of radiographs for inside diameter (ID) geometry and the performance of confirmation examinations. The confirmation examinations were performed with the use of ID creepin~g waves and the use of 1.5 Megahertz (MHZ),

4 MHZ, or 55 degrees (longitudinal wave) was also used when the welds were adequately accessible for the equipment. Three suspected welds in the LPCI piping system were examined by the liquid penetrant (PT) technique on the inside diameter surface by disassembling the adjacent valves. The PT test results indicated that those three welds were not cracked.

1.2 Scope of Inspection The licensee indicated that the scope of the piping inspection followed the guidelines in NUREG-1061. Four recirculation safe-end-to-nozzle welds and ten welds in jet pump instrumentation nozzle assemblies were inspected in accordance with I&E Information Notice 84-41. The inspection of five recirculation bypass line welds was based on the recommendations made in our previous safety evaluation report (SER). The scope of inspection was expanded from the original 111 welds to 115 welds after defective welds were reported.

1.3 Inspection Results A total of 7 welds were reported during Reload 10 to show linear indica-tions. Of these, one was a 12-inch diameter isolation condenser weld (code class 2), five were the jet pump instrumentation nozzle assembly welds and one was a 28-inch diameter recirculation weld (RCAJ-1), which was found cracked during the previous refueling outage (Reload 9). All reported crack indications were in circumferential orientation. The worst cracking in those welds was reported in a jet pump instrument nozzle i weld JPBJ-3, which was cracked 360 degrees intennittently with a maximum crack depth about 50% of the wall thickness. All six welds reported to be cracked during this outage were weld overlay repaired. The inspection also indicated no new cracks or apparent crack growth in weld RCAJ-1.

l 1.4 Weld Overlay Repairs The licensee designed the weld overlays by using the computer program PC-CRACK, developed by Structural Integrity Associates. The PC-CRACK program calculated iteratively the required minimu.n overlay thickness based on limit load source equations provided in ASME Section XI. All weld overlays were designed to be full structural overlays, assuming that cracks are fully circumferential and through wall. The designed minimum overlay thickness varied from 0.12 to 0.43 inch. GAPC0 performed the weld overlay repairs for the licensee, using the automatic Gas Tungston ARC Welding (GTAW) process. Low carbon stainless steel (ER308L) weld wires were used for the four stainless steel welds and Inconel 82 (ERNICr-3) weld wires were used for the two bi-metallic welds in the jet pump instrumentation nozzle assemblies. The overlay design took credit for the first layer that passed the liquid penetrant (PT) and the delta ferrite (8 Ferrite Number [FN]) tests, when applicable. In addition to PT tests, ultrasonic examinatiora, using ERPI recommended techniques

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(angled and zero degree longitudinal waves) were applied to the final layer of the weld overlays, to detect flaws in the overlays or the base materials, and the lack of bonding or flaws at the interface between the overlays and the base materials.

The width of the designed overlays varied from 4 to 4.2 inches. For the two repaired bi-metallic welds (JPAF-2 and JPBF-2), the width of the overlays at the reducer side of the welds were shortened at some locations because the geometry of the eccentric reducer prevented the use of a full

.- overlay design width.

1.5 Region I Input The Region I inspectors have reviewed the ultrasonic examination procedures and data, and held discussions with cognizant licensee personnel regarding the nondestructive examinations performed during this outage. Region I concluded in their report that the nondestructive examinations were performed by qualified personnel using the latest recognized techniques and equipment.

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2.0 EVALUATION We have reviewed the licensee's submittals, including the inspection results and weld overlay designs, to support the continued operation of Millstone Unit I for one fuel cycle (18 months) in its present configuration.

I 2.1 Scope of Inspection Except for the inspection of IHSI treated recirculation welds the percentage of welds inspected in each pipe size of each piping systems met the guidelines in Generic Letter 84-11. For the inspection of the IHSI treated welds, the licensee followed the guidelines in NUREG 1061 for Category B welds; a total of 9 such welds were inspected. We consider that 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.

In the proposed NUREG 0313, Revision 2, such IHSI treated welds are recomended to be 100% re-inspected within three and one-third years subsequent to the post-IHSI inspection. Recently, many IHSI treated '

welds were reported to be cracked in Peach Bottom Unit 3. These welds were reported to be not cracked in the post-IHSI inspections. Therefore, the licensee should consider inspecting more of those IHSI treated welds during the next refueling outage. '

2.2 Unrepaired Weld RCAJ-1 Recirculation weld RCAJ-1 (18 inches) was reported to be cracked during ,

the previous refueling outage. This weld was not repaired because of  !

the reported crack sizes were small (total length of 9 inches and l

maximum depth of 17% of wall thickness) and continued operation of this weld for one fuel cycle was justified by fracture mechanics evaluation, i In addition, IHSI was applied to this weld to slow down further crack growth and to prevent the initiation of new cracks. The current i

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inspection results indicated that there was no apparent crack growth or initiation of new cracks in this weld. Therefore, we conclude that weld RCAJ-1 can be safely operated for one more fuel cycle of 18 months because the Code designed safety margin would continue to be maintained.

2.3 Weld Overlay Designs -

The licensee's overlay designs took credit for the first layer of overlay, which passed the PT and ferrite content tests, when applicable. The staff's earlier position was that this was 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 that 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 cons.ider that the original additional design conservatism for overlay is not needed if full IGSCC resistance of the first layer is demonstrated. The licensee reported that low carbon austenitic stainless steel and INCONEL 82 were used for the weld overlay and the ferrite content of the first layer of each stainless steel overlay was measured to be at least 8 FN.

We conclude that the licensee has demonstrated adequate IGSCC resistance in the first layer, and therefore, the licensee's overlay design j

thickness, which includes the first layer, is acceptable.

Although the licensee's overlay designs were performed by using the limit load source equations provided in American Society of Mechanical Engineers (ASME) Code,Section XI, we noted that the calculated minimum overlay

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thickness for the five cracked jet pump instrumentation nozzle assembly welds did not meet the cut off valve (75% of wall thickness) of the maximum crack depth allowed ir, the Code IWB-3640. The purpose of the cut

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off valve was to provide additional margins because of concerns with the uncertainties associated with the crack growth rate and the UT sized flaws that were used in the fracture mechanics evaluation. However, the licensee reported "as-built" overlay thicknesses did meet the maximum crack depth cut off valve in IWB-3640. Therefore, we conclude that the six overlay repaired welds were adequately reinforced to meet the Code required safety margin for at least the next fuel cycle of operation.

3.0 CONCLUSION

Based on our review of the licensee's submittals and considering Region I input, we conclude that the IGSCC inspection and repairs performed in this outage is satisfactory and' that the Millstone Unit 1 plant can be safely returned to power and operated in its present configuration for an 18-month fuel cycle.

Nevertheless, there remains a residual concern regarding the long term growth of small IGSCC cracks that may be present, but not detected, during this operation. Therefore, plans for inspection and/or modifications of the recirculatidn and other reactor coolant pressure boundary piping systems during the next refueling outage should be submitted for our review at least three months before the start of the next refueling outage, i

Prepared by: William H. Koo *-

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