ML20031F142
| ML20031F142 | |
| Person / Time | |
|---|---|
| Issue date: | 06/29/1977 |
| From: | Shao L Office of Nuclear Reactor Regulation |
| To: | Eisenhut D Office of Nuclear Reactor Regulation |
| Shared Package | |
| ML13319A640 | List:
|
| References | |
| FOIA-81-313 NUDOCS 8110190246 | |
| Download: ML20031F142 (7) | |
Text
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': t UNITED STATES a
NUCLEAR REGULATORY CCMMisSION g
h WASHINGTON, D. C. '.0555 e
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u MEMORANDUM FOR:
D. G.,Eisenhut, Assistant Director for Operational Technology, Division of Operating Reactors FROM:
L. C. Shao, Chief. Engineering Branch Division of Operating Reactors
SUBJECT:
STEAM GENERATOR TUBE INTEGRITY - REPORT TO CONGR In response to your request on June 24, 1977 Division of Operating Reactors, has prepared a brief sumary on thethe Eng subject matter in accordance with the format provided by J. C. Guibert.
A copy of the sumary is attached.
.J M
L. C. Shao, Chief Engineering Branch Division of Operating Reactors
Attachment:
As stated cc:
V. Stello, Jr.
R. E. Heineman J. P. Knight R. Bosnak LSlugrt J. Guibert R. Cudlin F. Almeter J. Strosnider R. LaGrange B. D. Liaw l
(8110190246 010827 li i k' PDR FOIA UDELL81-313 PDR 1
INPUT TO QUARTERLY REPORT TO CONGRESS STEAM GENERATOR TUBE INTEGRITY
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Date and Place - During the first half of 1977, significant developments in the area related to steam generator tube integrity were the following:
The discovery of several tube leaks at Duke Power Company's Oconee a.
Units 1, 2, & 3, Babcock & Wilcox manufactured pressurized water reactors located in Seneca. Oconee County, South Carolina, b.
The continued deterioration of steam generator tubes due to a con-dition known as " denting", as reported in the previous quarter, in threc of the six Westinghnuse manufactured pressurized water reactors.
These three units are Virginia Electric and Power Company's Surry Units 1 & 2 in Surry County, Virginia and Florida Power and Light Company's Turkey Point Unit 4 in Dade County, Florida.
The discovery of the tube " denting" phenomenon in two of the pressurized c.
water reactors, manufactured by Combustion Engineering, Inc., which have exclusively used all volatile treatment (AVT) of the secondary coolant since initial start-up.
In May 1977, tube denting was found during inservice inspections at Maine Yankee Atomic Power Company's Maine Yankee unit in Lincoln County, Maine, and at Millstone Unit 2 in New London County, Connecticut.
The condition of steam generator tubes has deteriorated to a stage at Surry Units 1 & 2 and Turkey Point Unit 4 that replacements of their steam generators have been under serious consideratim
'n the interim periods, these three units will probably be operating,,,
swer power levels in the future, depending on conJitions of their steam generators in terms of percent of tubes already or to be plugged.
Nature and Probable Consequences - Steam generator tubes in a pressurized water reactor are an integral part of the reactor coolant pressure boundary of the closed primary system and are designed to serve as the heat exchangers that pass the heat generated in the reactor core to the secondary system
2-where steam is produced and then piped to turbine generators for producing
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sc electricity. The loss of integrity of steam generator tubes is a breach of the primary-to-secondary system boundary.
Safety margins are ensured by conservative design and by administrative contols during operation such that if a steam generator tube leaks, the leakage can be detected rapidly and the reactor safely shutdown. Periodic inspections at a frequency determined by previous service experience are also required to assure that degraded steam generator tubes are removed from service.
During normal operation, tube leaks at a rate observed to date in the above mentioned six units are generally not of safety concern.
However, frequent leaks are normally indicative of a servere tube degrada-tion, which could result in adverse safety consequences should they fail in conjunction with certain postulated accidents. Therefore, leaking or defective tubes are removed from service by plugging both ends of the tube.
Oconee Tube Leak Incident The first tube leak incident at Oconee Station in July,1976 was first thought to be an isolated case caused hy the propagation of a manufacture defect. Subsequently in the last quarter in 1976 and the first quarter in 1977, there have been a total of seven plant shutdowns to plug leaking tubes in all three units at Oconee Station. The majority of these seven leaking tubes are located adjacent to the missing tube lane.
Laboratory examinations of removed defective tubes indicated that the tube failures were caused by the propagation of small cracks in the circumferential direction of the tube, of unknown origin, hy flow induced vibration.
Tube Denting - Westinghouse The phenomenon known as " denting" of steam generator tubes was first found in several Westinghouse manufactured plants, including Surry Units 1 & 2 and Turkey Point Units 3 & 4.
On September 15, 1976 during nornal power operation at Surry Unit 2, a leak of about 80 gallons per minute (GPM) rapidly developed in the U-bend section of a steam generator tube. Subsequent i
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investigation disclosed an axial crack near the top of the U-bend of p:-
approximately 4-1/2 inches in length.
Laboratory analysis determined
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that the. tube failed from intergrarular stress corrosion cracking that had initiated on the inside (primary coolant side) of the tube surface.
The tube leak was quickly detected and the reactor was promptly shut-down. Offsite releases of radioactivity during the event were within
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, regulatory limits.
- 39 Examinations of previous inspection results revealed that six of Westinghouse manufactured uhits had suffered various degrees of tube denting and thus had the potential for occurrences similar to that at Surry Unit 2.
Subsequently, these six plants were voluntarily shutdown to inspect specifically the conditions at U-bend and the degree of tube denting. Analyses of the stress condition at the top of the U-bend and laboratory data showed that enly the inner most row tubes were suscept-ible to the type of failure which occurred at Surry Unit 2 on September 15, 1976. Corrective actions were taken by licensees of these plants by plugging all tubes in the inner most row of the tube bundle.
The continued deteriorations of steam generator tubes have resulted in numerous small tube leaks in later part of 1976 and the first quarter of 1977 at Surry Units 1 & 2 and Turkey Point Unit 4.
Laboratory examin-ations of removed tube and tube support plato samples indicated that these leaks were caused by small longitudinal cracks in sections of tubes masked within support plates. By analyses and tests, Westinghouse has established, I
and the NRC staff has concurred, that tube leaks or cracks of this nature l
will not result in adverse consequences during normal operation or postu-lated accidents. However, continued deterioration and plugging following each inspection will lead to the de-rating of power for these three units, until it becomes economically unfeasible for continued operation. The licensees involved; i.e., Virgini.a Electric and Power Company and Florida Power and Light Company, have undertaken steps for the eventual replacements l
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of these steam generators.
The replacement units have been ordered and ieplacement schemes are being developed by their respective Architect /
Engineers.
Tube Denting - Combustion Engineering In the past the phenomenon of steam generator tube " denting" was observed only in plants that had been using phosphate treatment for the secondary coolant or had made conversion from phosphate treatment to AVT.
During an inservice inspection of the steam generator tubes at Maine Yankee, a Combustion Engineering pressurized water reactor that had utilized AVT exclusively for secondary coolant since its initial start-up, it was found to be experiencing tube denting. Primarily shutdown for retubing the condenser and because of the finding at the Maine Yankee unit, Northcast Utilities Company's Millstone Unit 2 (another Combustion Engineering pressurized water reactor which started up with AVT) steam 9enerators were inspected in May,1977. Results of this inspection at Millstone Unit 2 also indicated the existence of the tube denting phen-omenon in its steam generator tubing. Although the degrees of denting at both Maine Yankee and Millstone Unit 2 are minor to moderate in com-parison to other Westinghouse units, the significance of these findings is that both units have been on AVT since their initial start-up.
Cause or Causes - The tube leaking incidents at Oconee Station Units 1, 2,
& 3 were caused by the propagation of small cracks by flow induced vibra-tion. The initiation mechanism of the small cracks is not fully understood at this time.
Preliminary indications are that the small cracks may have been initiated by the frequent turbine stop and control valve operability tests, which may have caused pressure or flow transients in the steam generators.
The causes of tube failures at Surry Units 1 & 2 and Turkey Point Unit 4 have been identified to be stress corrosion cracking in the primary coolant environment of severely dented or strained tubes, and tubes at U-bend e
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The tube denting is a consequence of the tube support plate
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corrosion which can be attributed to the ingress of chloride, from the ruptured. condenser tubes, to form chlorine salts which concentrated in the annulus between the tubes and tube support plates.
Actions Taken to Prevent Recurrence Licensee (s)/ Vendor (s) - The licensee of uconee Station has actively participated in an investigative program conducted by Babcock & Wilcox to assure continued safe operation of the plants. Specifically, a series of instrumented tests of the turbine stop and control valves were conducted, which appeared to help explain why tube leaking incidents had been limited to Oconee Station among all Babcock & Wilcox manufactured operating units.-
The licensees of the six Westinghouse plants affected by the tube denting have actively engaged, in cooperation with the vendor, in a program to assure continued safe oper tion of the uni'ts. These licensees have shutdown their reactors and nave completed inspections and taken remedial actions by plugging defective and affected tubes.
In addition, the licensees *f the three units that have experienced the most denting and tube leaking, i.e., Surry Units 1 & 2 and Turkey Point Unit 4, have taken steps to order replacement steam generators and to study various replace-ment schemes for actual implementation in the near future.
The licensees of the two Combustion Engineering built reactors have j
completed their inspections and repaired their condensers N prevent further condenser tube failures and tube support plate corrosion.
In cooperation with their vendor, various short term cleaning processes have been studied and the most effective one was implemented before the restart of the units.
Combustion Engineering is also actively seeking a long term solution, such as a chemical cleaning of the affected steam generators.
NRC - Following inspections performed by licensees of their steam generators and the completion of any necessary repair programs, the NRC must approve i
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6-or concur with the restart of each of the affected facilities. To date,
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the six Westinghouse units affected by the tube denting have completed the inspection and repair program and received NRC approval for opera-tion for a limited time period. Reinspections and further analyses of the conditions of tubeshave been made for the three units that have found severe degradation of the tubes and support plates.
In the interim period for these three units, safe operation will be assured by the imposed strict conditions of licensed operation.
The generic implications of.the three issues summarized above are being actively studied by the NRC.
In this respect, NRR has recently formed three task groups to monitor the progress made by each of the three vendors affected, in conjunction with the utilities involved, and is pursuing the final resolution of these problems.
Future reporting will be made as appropriate.
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