ML20031F138
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UNITED STATES
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NUCLEAR REGULATORY COMMISSION
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MAY l'J 1977 e
MEMORANDUM FOR:
D. G. Eisenhut, Assistant Director Operational Technology, Division of Operating Reactors THRU:
L. C. Shao, Chief. Engir;aering Branch j
Operational Technology, Division of Operating c
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F. M. Almeter, Section A, Engineering Branch Operational Technology, Division of Operating Reactors
SUBJECT:
MAINE YANKEE STEAM GENERATOR TUBE DENTING On May 18, 1977, the Engineering Branch contacted Yankee Atomic to d hcuss the discovery of tube denting during the scheduled ISI (completed on May 10,1977) of the steam generator tubes at Maine Yankee. Maine Yankee is a 3 loop unit, designed by Combustion Engineering, that has operated with an AVT chemistry for the secondary coolant since initial start up in November 1972. This C. E. steam generator design has a combination of drilled carbon steel plates and carbon steel egg crates for tube supports. Maine Yankee uses sea water for condenser cooling, but does not have condensate demineralizers in the secondary water cycle system.
Three ISI's performed prior to May 1975 indicated no tube degradation or sludge accumulation on the tube sheet and tube supports. The recent ECT inspection of 200 tubes in each steam generator, after 24 months operation, indicated no tube degradation due to wastage or cracking and no sludge accumulation. However, an average 1 to 2 mil dent was indicated in 32 tubes in S.G.-l and 9 tubes in S.G.-2.
The maximum degree of denting was approximately 4 mils. The Maine Yankee steam generator tube size is 0.750" 0.D. and 0.654" I.D., and an 0.540" probe at 400 KHz is used for standard ECT inspections. All tubes with indications of denting did allow passage of the 0.540" probe. The denting occurred at the 7th and 9th level tube support plates even though boroscope inspections indicated no magnetita in the tube / tube support plate annulus. However, the absence of magnetite at the mid-section of the annulus could not be confimed.
Although Combustion Engineering has not completed their evaluation of I
the eddy current tapes to confirm the existance of denting, Maine Yankee postulates that denting is the result of chloride 1.ncursions due to a number of broken condenser tubes during the past 24 months. The condenser tubes at Maine Yankee are 90-10 copper-nickel. Maine Yankee intends to try several methods to reduce the chloride concentration in the steam generators prior to start up May 25, 1977. Although the failed condenser tubes have been plugged, there was no indication to retube the condensars.
CONTACT:
Almeter
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\\._.I Dr. D. Van Rooyen at BNL has informed us that C.E. has reproduced denting in a.model boiler with an AYT bulk water doped with Cu0 and 1.0 ppm Cl-impurities. The C.E. and Westinghouse experiments correlate with Potter and Mann's obserYations of magnetite formation in the crevices of carbon steel in high temperature pure water containing iron or nickcl or copper chloride salts. The C.E. experiments indicate that denting would require the corrosion of copper base condenser tubes and condenser in-leakage of cooling water containing chloride ions. Of course, the type of copper-base tubing would be important in the accumulation of Cu0 in the secondary side of the steam generator (copper-nickel alloys being more corrosion resistant) to initiate rapid magnetite fonnation and tube denting. Obviously, the initiation of denting in AVT water without the presence of phosphates would be slower and less extensive as experienced by Maine Yankee.
In regards to the Cu0 and Cl ion contamination of AVT water causing denting, there is now a need for a closer look at PWR plants with cooling towers which in some cases have chloride treatment to reduce fouling of the cooling towers.
We have heard that Millstone Unit 2 with C.E. designed steam generators, which is also a sea coast plant that started up with an AVT chemistry, has had severe condenser leakage and shutdown was required May 13, 1977 to retube tha condensers.
In view of these events at Maine Yankee and Millstone Unit 2, the Engineering Branch has requested L. Olshan, Project Manager for Maine Yankee, to set up a meeting with Yankee Atomic and Combustion Engineering prior to Maine Yankee's return to power on May 25,1977. We will ask Yankee Atomic to present more details of the ISI at Maine Yankee, and have C.E. give us information on their denting experiments, the ceasequences of the Millstone Unit 2 condenser failures, and the potential for other C.E. plants to develop tube denting, e.g. Calvert Cliffs which has cooli,ng towers and condensate demineralizers.
Y. ?M, Ws<282 F. M. Almeter Engineering Branch Division of Operating Reactors cc:
V. Stello L. Olshan l
R. Heineman M. Fairtile l
J. Knight H. Conrad L. Shao B. Turovlin S. Pawlicki W. Hazelton R. Bosnak R. Stuart J. Guibert A. Liaw R. Cudlin F. Almeter I
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