ML20213G652
| ML20213G652 | |
| Person / Time | |
|---|---|
| Issue date: | 05/04/1987 |
| From: | Stello V NRC OFFICE OF THE EXECUTIVE DIRECTOR FOR OPERATIONS (EDO) |
| To: | Mcdonald R ALABAMA POWER CO. |
| Shared Package | |
| ML20213G127 | List:
|
| References | |
| FOIA-87-229 NUDOCS 8705180387 | |
| Download: ML20213G652 (4) | |
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l Mr. R. P. Mcdonald Chaiman, Executive Group Utility Steering Comittee Steam Generator Reliability Project Alabama Power Company 600 North 18th Street Post Office Box 2641 Birmingham, Alabama 35291-0400'
Dear Mr. Mcdonald:
Thank you for your letter of March 16, 1987 which discusses industry activities with respect to steam generator inspection strategies and diagr.ostic capabil-ities, and transmits through its attachments a sumary of NDE activities, a summary of the recommendations from the "PWR Steam Generator Inspection Guidelines (July 1985, Revision 1)" prepared by the EPRI NDE Center for EPRI and for the Steam Generator Owners Group, and the Guidelines document itself.
It is evident from the PWR Steam Generator Inspection Guidelines that the nuclear power industry has recognized the need to improve in-service inspection reliability and diagnostic procedures. The guidelines make use of extensive historical experience and recent research, and recommend inspections in addition to the " Tech Spec" requirements. In our opinion the recomendations given in the guidelines represent an improvement in the basis for developing an in-service inspection (ISI) sampling plan and could improve the results from ISI. However, the guidelines are nonmandatory, and we are aware that they are not consistently and uniformly used and several important recomendations are not followed by many utilities, as for example, " Inspect all steam generators during every scheduled steam generator outage" and " Conduct an independent review of all eddy current data acquired during an outage".
The NRC also has recognized that present Regulatory Guides and Tech Specs ;
governing inspection of steam generators and criteria for plugging defective tubes may need improvement. The technical basis for these was developed a number of years ago when many of the present fonns of steam generator degrada-tion were unknown. In'1976, the NRC embarked on a comprehensive program to develop models for predicting remaining integrity of degraded tubing and to detennine the reliability of multifrequency eddy current (EC) inspection to detect, characterize and size steam generator tube defects. As the project evolved, advanced or alternate inspection equipment and procedures were included for evaluation, and the types of defects used progressed from machined defects to chemically induced flaws including intergranular stress corrosion 1 cracks (IGSCC). The Surry generator project is the final phase of the NRC program to acquire needed validated data for updating the Regulatory Guides on :
in-service inspection and tube plugging.. Besides the NRC, this international j program has participation from EPRI and consortia from France Italy and Japan. l f l g51 7 870512 I KNAPIK87-229 PDR / :
- s. ' ,s R. P. Mcdonald 2 4 The removed-from-service Surry steam generator was chosen as the test bed, i
since it provided a realistic vehicle for testing inspection reliability under i
simulated field conditions. The project is presently correlating metallo-i graphic and visual validation data on tube sections removed from the generator with NDE information obtained in situ from four round robins involving 27
- inspection teams. These rourd robins included inspections using typical multifrequency EC systems and personnel used for ISIS, and advanced and alter-nate systems under development from the U.S., Europe and Japan. The program will be completed by the end of 1987 so our analysis of the results is not yet j complete. However, enough insights have been gained from the research that the '
j following preliminary connents can be made on the industry activities and PWR Inspection Guidelines.
The Guidelines and other ongoing activities are steps in the right direction, but in many cases appropriate analytical and experimental evidence to support a articular recommendation or action is not given. For example, a 17-25% strain p(measured by profilometry) criterion has been used as the plugging threshold
- for dented tubes. No technical basis was given for selecting this particular strain range. Research results from the Surry project have shown the presence of deep IGSCC at strain levels as low as 11% (detennined from profilometry),
suggesting the 17% threshold may be too high. In another instance, multi-frequency EC is noted as offering improved ability to reduce the impact of interfering signals on inspection perfonnance. Our observations suggest 4
that the multifrequency techniques practiced may not always be adequate for filtering out the affects of surface deposits, support plate corrosion, tube denting, and sludge pile accumulations. Many false calls, usually categorized t as "less than 20% wall depth" were caused by copper deposits on the tube OD.
t One advanced technique team using special frequency mixes designed to suppress dent and copper signals reported several 50% wall-loss ID indications, all of wnich turned out to be false calls. A high false call rate may result in excessive and unnecessary levels of inspections and repairs with no gain in
. reliability or safety but with an accompanying increase in radiation exposure i
and cost. We recommend that industry develop or improve inspection techniques that will reduce the number of false calls.
Regarding capability of advanced and alternate inspection techniques; substan-tial inaccuracies were observed in the Surry program for sizing of indications ,
at the hot leg top of tube sheet by multifrequency EC. The predominant defect types observed in this region of the generator consisted of pitting and i wastage. In general, these flaws were undersized by as much as 30% of the 1 thru-wall depth. Round robin inspections of laboratory produced stress j corrosion cracked tubes using currently practiced and advanced techniques under i laboratory conditions produced similarly poor results for both detection and sizing. It is our opinion that considerable improvements are still needed in ISI techniques for detection, sizing and characterization of flaws in steam 4
generator tubes, especially if one wants to take advantage of the relatively 4
high burst strength of flawed steam generator tubes when making tube plugging I
decisions based on ISI findings. We also recognize that a single inspection l
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R. P. Mcdonald 3 technique is not capable of detecting and accurately sizing all relevant forms of degradation. Thus, we agree that proper selection of probes, instrument j
settings and signal interpretation methods should improve inspection perfor-
- mance. Unfortunately, prediction of steam generator degradation can be i uncertain and it may not be possible, prior to an inspection, to accurately j forecast all expected damage foms. Without this knowledge, inspection equip-
- inent and procedures may not be optimized and defects may be missed. Neverthe-less we believe that one objective of ISI should be to detect new, unexpected
- forms of degradation before they become part of an undesired operational c
, experience base. Therefore, we recommend improvements be made in the general j survey techniques used in 151. -
4 We concur with the Guidelines' reconnendation that an independent review of l eddy current data is needed since our research results indicate that the
- analysis of complex eddy current signals is the largest source of inspection i variance. Further, we support industry's activities to provide training for Data Analysts. We reconnend and strongly urge the industry to develop a femal i program for the training of personnel and for qualification of the ISI process
- for inspection of steam generator tubing. The qualification should cover the
! entire "NDE-ISI system", i.e., procedure, personnel and equipment and should f include a rigorous performance demonstration using realistic conditions of j samples, geometries, types and sizes of flaws and ISI conditions expected in the field.
i As stated in your cover letter, other elements of industry's activities for improving steam generator perfomance include water chemistry control, removal of corrosion product deposits, maintenance and repairs, and behavioral charac-l teristics of certain materials and designs. We support these activities and recommend their continuation. The dominant cause of steam generator tube
! degradation is corrosion due to local environmental conditions occurring in crevices. Secondary water chemistry guidelines developed thus far have i
resulted in reduced corrosion, particularly denting. Further work is needed to 2 address other foms of corrosion such as IGA, wastage and pitting in different locations of steam generators. Work should be initiated to gain an understand-
! ing and a predictive capability of the kinetics and steady state conditions with respect to chemistry and electrochemistry in localized regions such as at crevices, crack tips, and sludge piles for full scale steam generators as a l
function of operating conditions, bulk water chemistry, intrusions, additions,
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etc. This information would help in evaluating the potential for corrosion and 2
in optimizing the material selection, design, and bulk water chemistry control i for minimizing corrosion in operating steam generators. Further, data on damage progression as a function of material, stress and environmental
, conditions are needed to properly evaluate NDE indications and arrive at correct plugging decisions. Although not specifically mentioned in your letter, the industry is carrying out programs related to aging and life extension. We believe that these industry programs should also conduct i 4 appropriate analyses and research to identify and resolve issues related to j aging and life extension for steam generators.
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R. P. Mcdonald 4 Finally, we note and support the industry's activities to issue in 1987 Revision 2 of the Inspection Guidelines which will reflect recent inspection experience, provide updated infonnation on new and evolving technologies, and fector in the results of NRC sponsored work on the retired Surry steam generator. We look forward to the results of this activity.
Sincerely.
Original Signed By; jam.es th. Taylor Victor Stello, Jr.
Executive Director for Operations (EDO 002661)
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