ML19351F211
| ML19351F211 | |
| Person / Time | |
|---|---|
| Site: | Rancho Seco |
| Issue date: | 12/24/1980 |
| From: | Office of Nuclear Reactor Regulation |
| To: | |
| Shared Package | |
| ML19351F210 | List: |
| References | |
| IEB-79-13, NUDOCS 8101100449 | |
| Download: ML19351F211 (21) | |
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SAFETY EVALUATION BY THE OFFICE OF NUCLEAR REACTOR REGULATION ACTIONS TAKEN IN RESPONSE TO BULLETIN 79-13 "CRACKIf;G IN FEEDUATER SYSTEM PIPING" l
FOR RANCHO SECO, UNIT NO. 1 SACRAfiENTOMUNICIPALUTILIkYDISTRICT(SMUD)
_ DOCKET NO. 50-312 Backoround and Discussion l
Indiana and flichigan Power Company notified the NRC of pipe On May 20, 1979 cracks in two feedwater lines at their D. C. Cook Unit 2 facility.
Pipe cracks were discovered following a shutdown on May 19 to investigate leakage inside containment. Circumferential cracks were identified as leaking in.the l
16-inch feedwater elbows adjacent to two steam generator nozzle to elbow welds.
Subsequent' radiographic examinations revealed cracks in all eight steam genera-t tor feedwater lines at this location on both Units 1 and 2.
On !by 25,1979, a letter was sent to all PWR licensees by the Office of Nuclear l
Reactor Regulation which infomed licensees of the D. C. Cook failures and re-quested specific infomation on feedwater system design, fabrication, inspection To further explore L e generic nature of the cracking and operating histories.
problens, the Office of Inspection and Enforcaent requested licensees of PWR plants in current outages to imediately conduct volumetric examination of l
certain feedwater pipe welds. As a result of these actions several other licensees reported cracking in the steam generator nozzle to feedwater piping weld vicinity. On June 25, 1979, IE Bulletin 79-13 was issued. The Bulletin re-quired inspection of the steam generator nozzle-to-pipe welds and adjacent areas to be conducted within 90 days. The Bulletin also required an expanded inspec-tion program to be conducted during the next refueling outage consisting of the reinspection of all previously inspected welds, main feedwater pipe welds to l
the first support, main feedwater pipe welds to the containment penetration and auxiliary feedwater pipe joining the main fee.1 water pipe.
Instrumentation was installed at severas plants whici, had experienced feedwater pipe cracking. As a result of this instrumentation and other modeling and analysis by a Westinghouse Owners's Group, significant cyclic stresses were discovered to have occurred in the feedwater piping in the vicinity of the steam generator nozzle.
The cyclic stresses were caused by mixing and stratification of cold auxiliary feedwater with hot water from the steam generator in the feedwater piping during low flow conditions. fietallurgical analysis of the cracked feedwater piping has identified the mode of failure as fatigue assisted by corrosion which supports the findings of the' Westinghouse Owner's Group.
8101100
- In an August 24, 1979 meeting with the Babcock & Wilcox (B&W) Owners Group at Bethesda, Maryland, the group identified steam generator design differences between those fabricated by B&W and those fabricated by Westinghouse and Com-bustion Engineering. Furthermore, operating conditions of the B&W plants pre-cludes the occurrences of stratification of cold auxiliary feedwater with hot water from the steam generator. In addition, the B&W Owner's Group reported a
that 95 welds in the main and auxiliary feedwater piping at Crystal River Unit No. 3 and at Davis-Besse 1 had been radiographically examined and no cracks had been detected.
By letters dated June 15, 1979, July 27, 1979 and August 13, 1979, SMUD addressed the issues presented in Bulletins 79-13 and 79-13(2) on the feedwater pipe cracks as they relate to Rancho Seco, a B&W fabricated facility.
In the response SMUD addressed the design, fabrication, preservice/ inservice inspections and operating history of the feedwater system. Furthemore, the presentation given by B&W Owner's Group on August 24, 1979 describing the B&W facility design applies to Rancho Seco.
Evaluation 9ased upon the identified mode of cracking of the feedwater to nozzle weld, the sperating and design characteristics of the B&W steam generator, the satisfactory inspection results on welds in the main and auxiliary feedwater piping on Crystal River Unit No. 3 and on Davis-Besse 1, and the review of SMUD's response, we conclude that feedwater nozzle cracks are unique to the Westinghouse and Combustion Engineering fabricated facilities.
Furthemore, stratification of the cold and hot water is imoossible to occur under all modes of plant operation (including analyzed accident conditions) at the B&W fabricated facilities including Rancho Seco because the cold auxiliary feedwater enters the steam generator thru a separate line, and thus cannot mix with the hot water that would backup from the steam generator into the main feedwater pipe where cracks have been observed. This is the case for the B&W main feedwater system because the licensee has indicated that the main feed-water is always preheated before entering the steam generator.
In addition, the velocities are. high enough to prevent the'high temperature water or steam from On this basis, we backing up from the steam generator into the feedwater lines.
have concludei that feedwater pipe cracks caused by themal cycling similar to those observe at the Westinghouse fabricated facilities will not occur at the For this reason, we have B&W fabricated facilities (including Rancho Seco).
exempted the B&W fabricated facilities from the augmented inspection program under ISE Bulletin 79-13. This decision was conveyed to all licensees by issuing a re-vision to Bulletin 79-13. On this basis, Rancho Seco is exempted from the in-spection requirements of Bulletin 79-13. We conclude that granting the exemption from Bulletin 79-13 will not in any wa endanger public health and safety.
Feedwater nozzle to pipe weld cracking is still under generic review by the staff.
Should the staff determine that actions beyond those stated in IE Bulletin 79-13 are necessary to ensura adequate feedwater line integrity, the licensee will be notified at that time.
Dated: December 24, 1980
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