Keeping watch on steam generators30 March 2001
A database tracking steam generator operating performance reveals trends in degradation mechanisms. The steam generator degradation database developed by the Electric Power Research Institute (EPRI) contains data from 238 operating plants.
EPRI’s steam generator management project (SGMP) has published their latest annual report on steam generator performance. The report details the major issues and degradation mechanisms contained within the steam generator degradation database.
Repairs for significantly degraded PWR steam generator tubes (see Table, bottom p23) include tube plugging, tube sleeving and rerolling a non-degraded portion of the tube into the tubesheet. In 1999, for the 238 operating plants in the database, a total of 8546 tubes were repaired. The vast majority of tubes repaired have been Alloy 600 mill annealed (I-600 MA) tubing. The I-600 MA category also includes Alloy 600 high temperature mill annealed (I-600 HTMA) tubing used in the combustion engineering steam generators, and Alloy 600 stress relieved (I-600 SR) tubing used in the Babcock & Wilcox OTSG steam generators.
The plant having the longest operating time – the lead plant – with Alloy 600 thermally treated (I-600 TT) tubing is Surry 2, whose replacement steam generators began operation in September 1980. Cook 2, whose replacement generators began operation in March 1989, is the lead plant with Alloy 690 TT (I-690 TT) tubing. Only a few plants, having unique circumstances, have reported corrosion degradation on I-600 TT tubing and, to date, no plants have reported corrosion degradation on I-690 TT tubing.
When the EPRI steam generator owners group (SGOG) was formed in early 1977, denting was responsible for over 80% of tube repairs. By the end of 1982, this figure was less than 4%. During the last five years the degradation mechanism most responsible for tube repair is outside diameter stress corrosion cracking (ODSCC). This damage mechanism has been responsible for approximately 45% of the tubes plugged during this time period.
Steam generator tube pulls have been used to verify the cause of indications identified by non-destructive examination (NDE) and to confirm the type of degradation damage occurring at a specific tube location. Pulled tube eddy current analysis, in-plant eddy current analysis, and the destructive metallography results provide a valuable insight into the condition of the steam generator. Knowledge derived from comparing destructive metallography results with the original eddy current analysis provide direct feedback on the performance of data acquisition and data analysis techniques. Unfortunately, tube pulls are time consuming and the metallography examination quite expensive. In order to minimise the number of tube pulls required, the sharing and dissemination of this information is important. The steam generator degradation database provides information on tubes that have been removed from steam generators throughout the industry. There are over 1600 pulled tubes currently identified in the database.
In situ pressure tests
Degradation of steam generator tubing can lead to decreases in load bearing capacity that may compromise pressure boundary leak tightness. When degradation that may challenge tube structural integrity is found, evaluations are carried out to ensure that required structural margins are maintained and that leak rates, should leakage occur, remain within allowable limits. Structural integrity and leak rate evaluations are based on one or more of the following elements:
•NDE results, such as eddy current, plus analytical or empirical calculations of burst pressures and leak rates.
•Laboratory burst and leak tests of pulled tubes with service degradation.
•In situ leak or pressure testing of sections of tubing with eddy current indications of degradation.
In the past, some combination of the first two items, inspection plus analysis and pulled tube examinations, generally formed the basis for structural integrity and leak rate evaluations. Since 1993, in situ pressure testing has been used to support structural integrity and leak rate evaluations. In situ pressure testing is the hydrostatic pressure testing in the field performed on installed steam generator tubing. The purpose of these tests is to demonstrate that the selected tubes satisfy specified structural and accident-induced leak rate performance criteria. EPRI has developed steam generator in situ pressure test guidelines that provide a standard approach to the performance of in situ pressurisation tests and the application of the test data. The steam generator degradation database has been expanded to collect and disseminate data for those in situ pressure test parameters identified in the guideline document. There is currently in situ pressure test data for over 600 tubes in the database.
One method of removing harmful secondary side deposits that cause tube degradation is by chemical cleaning. As of July 2000 the total number of steam generator cleanings carried out was 77. Of these, 31 cleanings used the EPRI steam generator owners group (SGOG) process, or a modification of this process.
The chemical cleanings have been especially effective at removing deposits from steam generator tubes and in cleaning tube support plate broached holes. Several of the most quantifiable successes include cleaning of four once-through-steam-generator (OTSG) units in order to return to full power operation, cleaning of the broached holes at Bruce 4, Surry 1 and 2 to correct water level oscillation problems, and cleaning four Paks units in Hungary to recover heat transfer capability.
The cleaning of packed tube support plate crevices and hard tubesheet sludge piles has been only partially successful. Utilities using a modified EPRI SGOG process using either higher temperatures or combined with pressure pulsing during cleaning have reported improved effectiveness in crevice cleaning. Tubesheet cleaning can be improved by an effective water lancing programme before and after the cleaning.
Steam generator replacement
Steam generator replacements were completed at Beznau 2, Joseph M Farley 1, South Texas 1, Krsko and Cook 1 in the first six months of 2000. Fifty-two plants had replaced steam generators, as of 31 December 1999. Including the five replacements in the first six months of 2000, the total number of steam generator replacements is 57. All the replacement generators have replaced steam generators which had Alloy 600 milled annealed tubing as the original tubing. Alloy 600 milled annealed includes I-600 MA, I-600 HTMA, and I-600 SR tubing. At least 21 of the remaining 76 plants with Alloy 600 milled tubing have announced plans to replace the steam generators (see Table above). This indicates that steam generator replacement activity will continue, although at a slightly reduced level, for at least the next five years.
TablesPlanned steam generator replacements Worldwide causes of steam generator tube repair