ML19273D734
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OLk N 2 3 1980 MEMORANDUM FOR:
F. Schroeder, Acting Director Division of Systems Safety FROM:
D. G. Eisenhut, Acting Director Division of Operating Reactors
SUBJECT:
OPERATING EXPERIENCE MEMORANDUM N0. 25 STEAM GENERATOR TUBE DEGRADATION PROBLEM Steam generator tube degradation has been and continues to be a major problem in operating pressurized water reactors (PWRs). Major forms of degradation include local thinning or wastage, denting, deep crevice corrosion, stress corrosion cracking, U-bend cracking, erosion-corrosion and circumferential fatigue cracking (Attachment 1).
SAFETY SIGNIFICANCE Steam generator tubing comprises over 50% of the reactor coolant pressure bound-ary in pressurized water reactors.
Tube ruptures during normal operation have - ~~ ~ ~
resulted in primary to secondary leak rates as high as 390 gpm. Orderly plant shutdowns have been effected in major tube failure incidents with no adverse affects on public health and safety. However, these incidents do present an undesirable challenge to plant operators and systems.
Response of degraded steam generator tubes during postulated accidents is of major concern.
The steam generator tubing will be subjected.to..the worst _ load-ing conditions under postulated MSLB or LOCA and failure of tubes during such accidents can have serious adverse effects including the potential for steam binding during a LOCA or offsite releases of radiation in excess of allowable limits during a MSLB. Design basis accidents do not include postulat-ed concurrent steam generator tube failures.
It is therefore necessary to ensure that steam generator tube integrity will be adequately maintained during postulated accident conditions.
In addition, plugging of deg.aded tubes has led to derating in power and ex-tended outages for steam generator replacement at some plants. Steam generator tube degradation also results in frequent plant shutdowns, more inservice inspection, and increased personnel exposure.
RECOMMENDED REMEDIAL ACTIONS Operating experience has identified many improvements in mechanical and materials design and operating methods which can improve steam generator inspectibility and eliminate or reduce tube degradation.
These improvements include provisions for greater accessibility and inspectibility of steam generator internals, g.-
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elimination of high tube residual stresses, proper secondary system mechanical design and materials selection, improved blowdown design and flow distribution, provisions for chemical cleaning, and finally, provisions for steam generator replacement.
ACCESSIBILITY AND INSpECTIBILITY For some forms of degradation which have occurred, eddy current testing and tube gauging alone are not sufficient to assess and monitor tube degradation.
Several operating plants have found it necessary to install access and inspec-tion ports in their steam generators.
In order to assess and monitor the condition of secondary side tube supports. These installations result in extended outages and additional personnel radiation exposure when they are performed in the field on a " hot" unit.
Therefore, these ports should be installed in steam generators before they are put into operation. At a mini-mum, ports should be installed just above the upper tube support plate and between the tubesheet and the ?ower support plate.
These ports should be located such that inspection of the upper support plate and inner row "U-bends" and the lower support plate and secondary face of the tubesheet can be made.
The inspection ports should be of adequate size to allow tube specimen removal, when necessary.
Ports at intermediate elevations are also advisable, depending upon the design.
A second requirement which would improve quality assurance associated with inservice inspection is the numbering of tubes on the primary surface of the tubesheet. Many plants have misplugged tubes; therefore, easy tube identifi-cation along with photographic verification will improve the tube plugging procedures.
CREVICE ELIMINATION Mechanical components subject to aggressive environments should be free of crevice geometries which serve as a concentrator for aggressive chemicals and enhance or compound corrosion phenomena.
Corrosion of tube support plates with drilled tube holes has resulted in a wide-spread fom of degradation known as " denting." Denting occurs when support plate corrosion products build up in the tube to tube support plate crevice and exert sufficient forces to dent the tube diametrically.
Alternate designs of support plates without the tight crevice geometry have not resulted in tube denting. These designs include the Combustion Engineering " egg crate" design and the Babcock and Wilcox broached plate design. New steam generators should have similarly designed support platcs which eliminate the tight crevice geometry and minimize the potential for concentration of aggressive chemicals and tube denting.
A second form of degradation known as " deep crevice corrosion" has resulted in intergranular attack and stress corrosion cracking in the tube to tubesheet crevice of some units. The exact mechanism of this form of corrosion is not well understood.
The crevice exist in those units where the tubes were only
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F. Schroeder partially rolled in the tubsheet.
Full depth expansion of the tubes in the tubesheet has been performed in the filed and should be required for all steam generators prior to operation.
ELIMINATION OF HIGH TUBE RESIDUAL STRESSES Cracking of the tight U-bends in Westinghouse designed steam generators has been observed in some foreign and domestic plants.
It is suspected that high residual manufacturing stresses are a main contributor to this problem.
Laboratory tests and operating experience has indicated that high residual stresses can lead to stress corrosion cracking in Inconnel. Therefore, future design should minimize those geometries and manufacturing processes which in-troduce high residual stress, and stress relief annealing should be required when necessary.
Recent experience has demonstrated that, mw 1 tubes in West-inghouse steam generators have high residual stresses and are particularly susceptible to cracking.
Procedures for stress relieving tubes with small bend radii prior to operation should be investigated and required if practicable.
SECONDARY SYSTEM MECHANICAL DESIGN AND MATERIALS SELECTION Operating experience has also shown that steam generator tube degradation is primarily affectedsby secondary system component integrity and materials selec-tion. Contamination of the secondary coolant by inleakage of condenser cooling water is related to the various tube degradation phenomena mentioned above and resin break through from demineralizers has recently caused denting at a West-inghouse unit.
Carbon steel support plates are particularly susceptible to corrosion, and copper ions in the presence of a chloride environment are related to the denting reaction. The primary source of copper ions is corrosion of con-denser tubes and feedwater heaters made of copper alloys. Therefore, although it is not included in the NRC scope of review, the licensees should be made aware of the significance of the secondary system on steam generator degradation and should be encouraged to use improved designe.
Specifically, condenser design should be such to minimize leaks, support plates should be made from corrosion resistant materials, and cooper alloys should be avoided throughout the secondary system.
Demineralizers can be beneficial and should be encouraged when properly designed and operated; however, resin break through and contaminant ingress from demineralizers are deleterious. Mechanical design should minimize the potential for such incidents and performance of demineralizing systems should be carefully monitored.
IMPROVED BLOWDOWN AND PROVISIONS FOR CHEMICAL CLEANING Corrosion product deposition as sludge is a major cause for concentration of feedwater impurities in operating steam generators.
Design and operational methods for the control and removal of sludge have not received their deserved attention.
Generally the control of concentration of bulk water impurities has been based on mass balance with zero or minimum blowdown.
Based on this principle PWR steam generators nave been designed with inadequate blowdown capaci ty.
Ja 2 31980 F. Schroeder Little can be done to improve the blowdown capacity for plants that are now in operation.
However, in new designs, the posJtioning of blowdown line entries (in areas where flow stagnation is suspected) should be required to facilitate the removal of corrosion product deposition.
To control the mass balance of bulk water in the steam generator, new plants should be required to have maximum blowdown efficiency or be required to operate with a continuous blowdown.
Return lines should be routed back to the condensate for waste clean-up in the fullflow condensate demineralizers of the feedwater system.
As discussed briefly earlier, a large fraction of corrosion products and other insoluble impurities in the feedwater generally stay in the steam generators at zero or low blowdown. To remove these deposits in present operating re-circulating and once-through type PWR steam generators, chemical cleaning will be necessary to minimize existing or potential tube degradation.
If chemical cleaning is not encouraged, to arrest initiating corrosion in PWR plants, the service life of the equipment will be seriously reduced.
Once significant tube degradation has occurred, chemical cleaning may be ineffective.
This is particularly true when dealing with the denting phenomenon.
High pressure steam generation has high potential for component degradation in steam generators and particularly in turbines.
Therefore, regardless of material selection or unit design, the emphasis on system cleanlines is of far greater importance than chemical control of the feedwater and chemical cleaning will need to be extensive and thorough.
Future PWR steam generators should be designed to facilitate preoperational and periodic chemical cleaning during operation to extend the service life for 40 years.
PROVISIONS FOR STEAM GENERATOR REPLACEMENT Tube plugging due to extensive denting has resulted in a prolonged outage at Surry Unit 2 to replace the degraded steam generators.
Plans have also been made for replacement at Surry Unit 1 and possibly Turkey Point Units 3 and 4 because of extensive tube plugging due to denting.
Deep crevice corrosion at Point Beach Unit I has also resulted in extensive tube plugging during the past year and, unless the degradation can be arrested, steam generator replace-ment may be necessary.
Many units with less than half their intended operating life have significant percentages of their tubes plugged.
Therefore, future replacement or repair of steam generators has a finite possibility and provisions should be made for these type of operations. These provisions should include containment designs which allow easy access to the steam generators and equipment hatches which allow movement of steam generators in and out of containment.
Regarding the steam generator itself many designs exist and improvements can be made to facilitate steam generators replacement.
Features which can facili-tate replacement include removable upper shells and assemblies and removable tube bundles and tubesheets.
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JAN 2 31980 F. Schroeder s D0R ACT40NS 00R has expended a great deal of effort in monitoring and evaluating steam generator operation. The main mechanisms for ensuring adequate steam generator tube integrity are inservice inspection, tube plugging, and primary to secon-dary leak rate limits.
For units with excessive tube degradation the frequency of inservice inspection has been increased, more conservative plugging criteria and leak rate limits have been imposed, and more stringent limits have been applied to primary coolant activity levels.
Unfortunately many corrective actions which could improve and prolong steam generator operation are impossible or impracticable in operating reactors.
The result, as mentioned previously,is that some units are facing replacement of steam generators or derating in power levels.
00R is also actively participating in Task Action Plans A-3, A-4, and A-5 regarding steam generator tube integrity.
These task action plans will address steam generator tube degradation problems on a generic basis and recommend im-proved licensing criteria.
RECOMMENDATIONS It is important to recognize that a systems approach, properly integrating the above considerations, is necessary to solve steam generator tube degradation probl ems. No one " super" material, water chemistry, or design alone win produce a trouble free steam generator. The combination of materials, water chemistry, and design should be reviewed in a holistic manner to ensure safety and optimum performance.
We recomend that the above items be considered in the design of new plants and to the extent possible in plants already designed and under construction.
The D0R staff is available to discuss the above subjects in greater length and detail.
Principal 00R Personnel B. D. Liaw, Section B Leader, Engineering Branch W. S. Hazelton, Section A Leader, Engineering Branch J. Strosnider, A-3 and A-5 Task Manager F. Almeter, A-4 Task Manager wit 6 Fat sighe3tif Darreu u. Z,iS0Dh11%
Darrell G. Eisenhut, Acting Director Division of Opera ing Reactors 7
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