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UNITED STATES y i )yM J j NUCLEAR REGULATORY COMMISSION WASHINGTON. D. C. 20555
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NOV 2W MEMORANDUM FOR:
K. R. Goller, Assistant Director for Operating Reactors FROM:
D. G. Eisenhut, Assistant Director for Operational Technology
SUBJECT:
PLlR STEAM GENERATOR TUBE DENTING No. 2 have demonstrated that steam generator tu n
plants that have exclusively utilized AVT for the secondary coolant ch istry control.
indicates that denting was preceded by condenser tube resulted in contamination of the secondary coolant by the conde water.
Although an exact correlation between the extent of condenser inl all licensees of PWR facilities (excluding B&W de of this new development so that they can take appropriate actions.
dary coclant impurity levels-or the rate of tube den Since con-tacility Technical Specifications do not establish requirements i rent regard, we suggest that a letter should be sent to affected licensees n this recommending that they take appropriate actions.
for this purpose is attached.
A suggested sample letter i
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- 1. G. Eisenhut," Assistant Director for Operational Technology Division o*' Operating Reactors
Enclosure:
As stated-l cc:
V. Stello R. Mattson L. Shao A. Schwencer D. Davis G. Lear
- 2. Reid R. Stuart --(
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TO:
Licensees of PWR Operating Facilities (Except B&W) 2'
SUBJECT:
PWR STEAM GENERATOR TUBE DENTING During the past year and a half, the NRC has been following experience with PWR steam generator tube denting in operating reactors and the associated problems, particularly for the six PWR units which have experienced moderate to extensive tube denting.
During this time period, there have been several meetings between the NRC staff, the licensees of affected units and the NSSS vendors on this subject.
Regarding the initiating mechanism of tube denting, most discussions have centered eround units that had started up with phos-phate treatment and had later converted to AVT or had continued on phosphate treatment for the secondary coolant.
Recent inspections of steam generators at Maine Yankee and Millstone Unit No. 2 have demonstrated that tube denting can occur in plants that have exclusively used AVT for the secondary coolant treatment.
Both our evalua-tions and recent model boiler tests conducted by a PWR vendor have acknow-ledged that phosphates are not a necessary precursor for the formation of magnetite which causes tube denting. Nonetheless~, the recent experience at these two units is significant in that it represents the first times that tube denting has been observed in plants solely using AVT.
Review of the operating histories for these two units indicates that denting was preceded by condenser tube failures which resulted in contamination of the secondary coolant with condenser cooling water.
As you are aware, the NRC staff is currently evaluating a number of methods for minimizing the occurrence of tube denting and the associated 1
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problems. We would like you to be fully aware of the developments in this area since they may affect reactor licensing considerations, and also to assure continued and coordinated progress toward the resolbtion of this concern.
Accordingly, we would like you to consider the following NRC recommendations:
i) To implement a program to monitor and/or control the secondary coolant chemistry; e.g., monitor the impurity levels.
ii) To repair condensers as soon as practicable after tube failure occurs.
iii) To specifically probe for tube denting in your steam generator inspection program. and to develop a program for monitoring the rate of denting, if found.
K. R. Goller, Assistant Director for Operating Reactors Division of Operating Reactors Office of Nuclear Reactor Regt'ation I
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