EPRI (Electric Power Research Institute) has been tracking reported steam generator problems since early 1977 and has published an annual status report since 1985. This status report is used to trend progress, identify emerging issues, and assure that current EPRI programmes address the most significant problem areas. The latest report, covering the period through 1996, contains information on 226 operating plants and covers major steam generator issues and the degradation mechanisms encountered.
TUBE PLUGGING
PWR steam generator tubes are subject to a variety of degradation mechanisms. Corrective actions for significantly degraded tubes include removing the tube from service by plugging. Figure 1 shows the number of steam generator tubes plugged on a worldwide basis. Figure 2 shows this same information as a percentage of tubes in service.
During the last five years, approximately 0.32% of the steam generator tubes in service have been plugged each year and this percentage has remained relatively constant. The major causes of steam generator tube plugging are shown in Figure 3. During the last five years the degradation mechanism most responsible for tube plugging is secondary side stress corrosion cracking and intergranular attack (SCC/IGA OD). This damage mechanism has been responsible for almost 40% of the tubes plugged during this time period.
TUBE SLEEVING
Another corrective action for degraded tubes is installing a sleeve to bridge the degraded region of the tube. Successful installation and performance of steam generator tube sleeves requires the integration of a number of factors including material selection, qualified installation procedures, sleeves-to-parent tube joint design, and production controls. Figure 4 shows the number of steam generator tubes sleeved on a world-wide basis. Major sleeving campaigns at Doel 4 in 1994 and Maine Yankee in 1995 account for the large increase in sleeving these two years.
TUBES REPAIRED
Since tube plugging and sleeving are the primary forms of tube repair, combining these two gives an accurate picture of the number of steam generator tubes repaired. Figure 5 shows the percentage of tubes repaired on an annual basis. During the last five years, approximately 0.6% of the steam generator tubes have been repaired.
Figure 5, however, does not show the entire picture concerning steam generator tube repair. Most of the early steam generator tubing (prior to the late 1970s) was of relatively low temperature mill-annealed Alloy 600. After the late 1970s, much of the Alloy 600 tubing was thermally treated. Many of the new and replacement steam generators contain thermally treated Alloy 690 tubing. This tubing has approximately twice as much chromium (30% verses 15%). Figure 6 shows the percentage of tubes repaired by tube material. The vast majority of steam generator tube repairs have been performed on mill-annealed Alloy 600 tubing. This does not mean that the newer steam generator tubing will not experience degradation problems, but the performance of thermally treated Alloy 600 and Alloy 690 to date has been far superior to that of mill-annealed Alloy 600 tubing.
CHEMICAL CLEANING
One method of removing harmful secondary side deposits that cause tube degradation is by chemical cleaning. As of December 1996, 66 steam generator cleanings have been performed. Of the 27 cleanings done in the US and Canada, 26 have used the EPRI Steam Generator Owners Group (EPRI SGOG) process or a modification of this process. Additionally, the EPRI SGOG solvents were applied in Korea and Belgium for a total of 28 applications. Electricité de France (EDF) developed a process for the French utility community and has performed four cleaning applications. Siemens KWU developed its own methodology and as of the end of 1996, has applied this process 34 times. KWU applications made for iron oxide dissolution prior to 1991 used a different chelating agent than later KWU applications.
Tables 1 and 2 list each cleaning performed and the amount of corrosion products removed.
STEAM GENERATOR REPLACEMENT
Steam generators were replaced at Doel 4 (Belgium), Dampiere 3 and Gravelines 2 (France), Mihama 1 and Takahama 1 (Japan), Asco 2 and Almaraz 1 (Spain), and Catawba 1 and Ginna (US) during 1996. This is the largest number of steam generators replaced during any one year. As of end of 1996, 11 utilities in the US, one in Germany, one in France, one in Sweden, one in Belgium, one in Switzerland, two in Japan, and two in Spain have replaced a total of 105 steam generators at 37 nuclear units. Table 3 provides information on these replacements.
Plant operation before steam generator replacement has ranged from 7 to 25 years. However, the effective full power years (EFPY) of operation before steam generator replacement has ranged from 3.6 to 19.6 EFPY. The steam generators replaced in 1996 had an average of 11.3 EFPY. The impact to a utility of having to replace steam generators is still substantial. Table 3 shows that replacement durations have ranged from 38 days up to 365 days. Total doses received by personnel performing the changeout have ranged from 60 to 2141 man rems. Replacement costs have ranged from $37 to $230 million excluding replacement power costs.
The replacements during 1996 required on average only 45% of the time and 6% of the exposure as the first replacement at Surry 2. Replacement steam generators have been ordered for at least 17 additional plants. These orders indicate that steam generator replacement activity will continue to remain high during the next several years.
