ML19344E586
| ML19344E586 | |
| Person / Time | |
|---|---|
| Site: | Crane  |
| Issue date: | 08/11/1980 |
| From: | Office of Nuclear Reactor Regulation |
| To: | |
| Shared Package | |
| ML19344E585 | List: |
| References | |
| NUDOCS 8009020308 | |
| Download: ML19344E586 (4) | |
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SAFETY EVALUATION BY THE OFFICE OF NUCLEAR REACTOR REGULATION SUPPORTING-THE ACTION TAKEN ON PIPE CRACKS IN B0 RATED WATER SYSTEMS METROPOLITAN EDISON COMPANY JERSEY CENTRAL POWER AND LIGHT COMPANY PENNSYLVANIA ELECTRIC COMPANY THREE MILE ISLAND NUCLEAR STATION, UNIT NO. 1 DOCKET NO 50-289 Introduction On May 16,1979 Metropclitan Edison Company (Met Ed) informed NRC that evidence of leaks (weepage) was observed in piping of Three Mile Island Unit No. 1 (Tf11-1).
Visual inspection supplemented by liquid penetrant testing revealed a total of five leaks in 8-inch Schedule.40, Type 304 stainless steel piping of the spent fuel pool cooling system.
One leak was also found in a 10-inch Schedule 40 stainless steel pipe in the decay heat removal system.
A metallurgical analysis performed by General Public Utilities Services (GPU) on sections of the 8-inch piping attributed the leaks to intergranular stress corrosion cracking (IGSCC), iniciating on the inside surface of the pipe, at the heat affected zone of welds where the Type 304 stainless steel was sensitized. This piping was exposed to boric acid solutions at temperatures averaging 75 F with a peak tempera-ture of 100 F for a period of 54 months.
By letter dated October 30, 1979 we requested that Met Ed furnish the following information regarding their proposed action related to the pipe crack problems for our review.
1.
Repair Procedures.
2.
Replacement Materials.
3.
Location of the 40 welds with cracks.
4.
UT Procedures.
5.
Technical basis for not removing cracked pipe.
6.
Surveillance program for susceptible pipe systems.
7.
Results of tests and evaluation.
In addition, several meetings and conference calls were held with representatives of Met Ed and GPU to discuss the pipe crack problem at TMI-l and the remedial action taken related to this problem.
Discussion and Evaluation Pipe cracks attributable to IGSCC are due to combination of high residual stress, sensitization of material and a potential corrosive environment. Leaks and cracks in the heat affected zones (i.e., location of high residual stresses) of welds that join austentic stainless steel piping and associated components have been observed 8009020 3cg pu
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at other facilities and the experiences at TMI-l are similar.
Several studies have been performed to evaluate the safety significance of IGSCC in piping systems and to determine the potential for pipe failure due to IGSCC.
The Pipe Crack Study Group (PCSG) Report (NUREG-0531, February 1979), concludes that IGSCC in BWR primary systems, ",..while undesirable, will not be a hazard to the public health and safety."
The recently reestablished PCSG has performed additional evaluation of analysis to determine break and leak potential for cracked pipe in PWR lines.
They concluded that a) cracked lines have a significant safety marain aaainst breaks even though a very large boundary through-wall flaw size and service of ASME Code Level D loading (which includes seismic loads) is assumeo, and td the maximum leak rate expected from a boundary through-wall crack, assuming service of ASME Code Level D loading is a factor of three less than the leak rate that would Sgrade the system function below that assumed in the FSAR.
The PCSG concluded in NUREG-0691 (in preparation) that for those PWR lines where stress corrosion cracking has occurred, the nature of the cracks, the relatively low loads on the systems, and the ability of the system to maintain function lead the PCSG to conclude that these cracking incidents do not represent a substantial safety problem even after considering the application of the minimal surveillance requirements under 10 CFR 50.55a.
Based on our review of the metallurgical analysis performed by GPU and other information regarding crack sizes and boundry conditions, we conclude that the above conclusion of the PCSG would also apply to the failures which are bounded within the failure envelope studies by the PCSG at TMI-1.
In view of the evidence of IGSCC and to assure that other incipient pipe cracks do not exist, Met Ed embarked on an extensive inspection program by ultrasonic means of other Type 304 stainless steel piping containing stagnant or near stagnant boric acid solutions.
This piping ranged in size from 21/2-inch to 24-inch.
Preliminary results using a highly sensitive, non-discriminating procedure resulted in 391 indications of all 1982 accessible welds examined in the spent fuel, decay heat, building spray, ani make up systems.
A screening procedure was developed to dis-tinguish between Ul indications of IGSCC and geonetric reflection and other welding rela ted con, itions (grind marks, etc. ).
This screening procedure resulted in reducins, the 391 potential indications of IGSCC to 40.
After removing and destructively examining approximately half of the 40 weldments determined to have UT indications by the screening examinations, Met Ed found that many of the UT indications reported to be IGSCC were, in fact, reflections from weld roots or machine marks.
The remaining weldments are being reevaluated using radiographic and UT techniques to establish nore accurately the cause of the reported indications.
The procedures for detecting IGSCC were transmitted by letter dated January 4,1980 for our review. These procedures are supplemented by investigative techniques, using UT and radiography, that differentiate IGSCC indications from indications of weld root geometry or other inocuous conditions next to the weld root such as machine marks. These areas identified as having IGSCC are examined metallographically to verify the source of the UT indications.
As part of developing the inspection program Met Ed investigated the possibiiIty of monitoring future growth rate of crack identified in the inspection program.
- However, because of the difficulty in accurately measuring the insitu crack sizes in pipe
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systems, Met Ed decided to repair any weldment that shows positive indication of IGSCC regardless of size consideration. We agree with this conclusion, We have reviewed the radiographic and UT techniques being used and conclude that there is reasonable assurance that any IGSCC pipe cracks that may exist in the remaining 20 weldments (approximately) will be detected by these methods.
As for those weldments (1962 approximately) that have been judged to be free of IGSCC, we agree with Met Ed that the screening process and the metallographic results confirming the UT examination gives reasonable assurance that these welds are satisfactory.
Furthermore, these weldments will be reexamined in the future as part of the surveillance program as discussed below giving additional weldment integrity.. Therefore, we find the inspection techniques employed by Met Ed are acceptable, e
Repair procedures to eliminate IGSCC have been qualified by Met Ed and are des-cribed in the March 31, 1980 letter, (TLL 136). The procedures, which conform with the recomendations of NUREG-0313, Revision 1, include the removal of the leaking or cracked weldment, replacement of removed pipe with a corrosion resis-tant grade of stainless steel (Type 304L), and the application of corrosion resis-tant cladding by weld overlay in the ID surface of the remaining original pipe where the final field welds are made. We conclude that the repair procedures pro-posed by Met Ed are acceptable since the latest state of the art methods are being emoloyed which were found successful elsewhere in the industry.
Uet Ed has developed surveillance criteria for monitoring those pipe systems that are susceptible to IGSCC (i.e., spent fuel pool cooling, decay heat removal, building spray.and make up systems). The basis for selecting these systems is that these at e the only safety related systems in which the pipe material is Type 304 stainless steel having a stagnant or near stagnant boric acid solutions as a service environ-ment. Met Ed has connitted to a program of visual examination (walk down) of each system to be done monthly.
UT examination will be applied on a sample basis using the results of the initial inspection for selecting cardidate weldments that show susceptibility to IGSCC.
The frequency of the examination will be based on the initial inspection results and the results of future examinations.
The sampling categories of weldments to be examined will include as a minimum (1) weldments that were originally examined (1982) having UT indication exceeding 30% amplitude response attributed to inside surface and weld root geometry, (2) weldments exhibiting any reflectors regardless of amplitude during the original examination of the 1982 weld population, and (3) any group of welds considered susceptible to IGSCC as a result of correlations and investigations now in progress or in the future. This surveillance criteria has been judged as being more extensive than the requirement imposed by 10 CFR 50.55a and therefore this program is considered to be beyond the requirements imposed under the existing operating license.
Furthermore, we find this proposed surveillance criterion is acceptable after considering that the minimal hazards associated with the potential pipe cracks and that the failure mede (i.e., small leaks or pipe weeping) is such that the proposed surveillance would be adequate to detect such anomalies well before any significant loss of structural capability or large leakage would occur.
-TMI-l,
We conclude that the nondestructive inspections performed, the metallurgical examinations performed, the pipe repairs, the surveillance program proposed, and the inherently good material properties of the Type 304 stainless steel pipe will ensure adequate pipe integrity.- This piping integrity will be maintained to the extent that the systems _will perform their function if called upon and that any additional ~ IGSCC that may occur will be discovered with sufficient time to make appropriate repairs without being hazardous to the public health and safety.
Met Ed has comm.itted to furnishing the NRC a final report covering this program in its entirety.
Dated: August 11, 1980
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