ML13308A822
| ML13308A822 | |
| Person / Time | |
|---|---|
| Site: | San Onofre  |
| Issue date: | 03/07/1983 |
| From: | Palladino N NRC COMMISSION (OCM) |
| To: | Patterson J HOUSE OF REP. |
| Shared Package | |
| ML13308A823 | List: |
| References | |
| NUDOCS 8303290232 | |
| Download: ML13308A822 (3) | |
Text
0 UNITED STATES NUCLEAR, REGULATORY COMMISSION o
WASKINGTON, D. C. 20555 CHAIRMAN March 7, 1983 The Honorable Jerry M. Patterson United States House of Representatives Washington, D.C. 20515
Dear Congressman Patterson:
This is in response to the questions in your January 19' 1983 letter relative to the steam generator leaks at the San Onofre Unit 1 Power plant.
The basic cause of steam generator tube degradation was earlier operation of the unit by the licensees, Southern California Edison Company and San Diego Gas and Electric Company, with a type of secondary water treatment known as "phosphate chemistry" combined with some leakage of sea water into the secondary system through the main condenser. Phosphate chemistry is similar to the water, treatment used in conventional steam plants. A result of its use, however, was the formation of sludge deposits which concentrated at the tube sheet and in the small crevice between the steam generator tubes and tube sheet. During the extended period that the alkaline sludge was present, the tubes were sub jected to a phenomenon identified as caustic-induced intergranular attack.
Continued attack led to localized minute cracks that grew and opened up resulting in leakage.
When tubes are severely degraded, often large numbers of them must be removed from service by plugging to assure safe operation of the plant. Plugging steam generator tubes results in a loss of heat transfer surface and can eventually necessitate a reduction in power. If tubes then continue to de grade and are plugged, the steam generators may have to be replaced. Some utilities have elected to replace their steam generators. Such replacements requirea long outage, involve considerable cost, and entail significant occu pational exposures. To prolong life of severely degraded steam generator tubes, repairs may be done by sleeving. Sleeving leaves the repaired tubes functionall.
Sleeving was developed through laboratory testing and has been successfully used in commercial nuclear plants.
Sleeving provides a sound technical method for restoring the integrity of de graded or defective steam generator tubing as a primary pressure boundary. How ever, there is no absolute assurance.that every sleeved or unsleeved tube will serve throughout the life of the plant without degradation and the corresponding need for corrective action. The aspects of the tube sleeving process and secon dary water chemistry control discussed below, in conjunction with the technical specification leakage limits and surveillance requirements, provide reasonable assurance that degradation will be minimized and that tube integrity will be monitored and maintained within allowable limits adequate for protecting the health and safety of the public. It should be noted that the tubes and sleeves are periodically examined for degradation.
1330337 PDR CO ER coR~.X
-2 The sleeves used at San Onofre were fabricated using the same material as the original tubes in order to provide the same strength but were thermally treat ed to increase their corrosion resistance. The sleeves were inserted inside the existing tube, joined at the upper end to the inner surface of the tube above the tube sheet elevation and rolled into the tube and tube sheet at the lower end.
Structural analyses of the sleeved tube assembly have been performed by the licensee in accordance with American Society of Mechanical Engineers Boiler and Pressure Vessel Code,Section III.
Results of these analyses indicate acceptable fatigue performance and adequate structural margins for the full range of normal operating, transient and accident condition loadings. These analyses considered both the case where the outer tube is intact and the case where the outer tube is completely severed because of corrosion.
Analyses have been performed to establish the minimum wall requirements for the degraded sleeves during normal operation and for postulated accident con ditions. Structural analyses of the sleeved tube assemblies have been supple mented by extensive mechanical and metallurgical testing to verify acceptable structural strength, fatigue performance, and leak-tight integrity of the upper and lower sleeve joints.
In addition to the repairs of the steam generators, the plant is operated with a tight control on condenser leakage and the secondary water treatment program has been modified. Both of these measures are expected to remove the environ ment under which the intergranular attack occurred.
The steam generators in the San Onofre Units 2 and 3 have included a number of improvements which should avoid some of the problems experienced with Unit 1.
Residual tube stress is minimized by having a large bending radius for the inner tube rows and using improved bending techniques which introduce much lower residual tube stress. An explosive technique for placing the tubes in contact with the tube sheet for the full tube sheet thickness has been used.
This eliminates the tube-to-tube sheet crevice which has been the site of corro sion.problems such as stress cracking and intergranular attack.
San Onofre 2 and 3 have employed a chemistry control program to assure that secondary water chemistry is maintained within appropriate control bounds during operation and that timely corrective actions are taken in the event abnormal chemistry occurs. An all volatile treatment water chemistry program is utilized for the secondary system. This method of control reduces the tube corrosion and related problems due to the chemical additives, and it minimizes the amount of sludge deposited within the steam generator. By the above chem istry control program, chemistry related challenges to the integrity of the steam generator tubes are minimized.
-3 As you have requested we will keep you informed of other major issues involving the San Onofre facilities. If you have any further questions, please let us know.
Sincerely, Nunzio J. Palladino