Limited to the BWR Base Case systems, this section summarizes the worldwide service experience with Code Class 1 piping. The results of this review are input to the pipe failure rate estimation.

D.3.2.1 RR Piping Service Experience — The original piping material in BWR plants commissioned prior to mid-1980 is austenitic stainless steels that contain more than 0.03% carbon. During welding these steels are susceptible to sensitization that results in a loss of corrosion resistance. Intergranular stress corrosion cracking (IGSCC) occurs when the sensitized steel is subjected to stresses and corrosive environment. Sensitization can be avoided by controlling the carbon content to below 0.03%. Another approach to controlling sensitization is to add strong carbide formers such as titanium or niobium to the steel. Stainless steels with additions of titanium or niobium are called “stabilized.” It is noted that low-carbon content unstabilized stainless steel or stabilized stainless steels are not completely immune to IGSCC, however [D.18].

For Plant B, IGSCC is the predominant degradation mechanism acting on the RR piping welds, including heat-affected zones. During early plant life some weld reinforcements were performed where the inservice inspection revealed presence of surface penetrating, and subsurface cracking due to IGSCC. Since the analysis of LOCA frequency distributions is based on a degradation mechanism evaluation, the PIPExp database is queried for service data including IGSCC. The database queries are summarized in a set of charts and tables below. The database currently includes a total of about 1000 records on IGSCC in BWR piping. Figure D. 1 shows the number of weld failures due to IGSCC by calendar year and Figure D. 2 shows the number of weld failures by year of operation. Here a weld heat affected zone with a/t > 10% is characterized as a “weld failure.”

Figure D.1 Weld Failures Due to IGSCC in Code Class 1 & 2 Piping (1970-2002)

Figure D.2 IGSCC Experience by Year(s) of Operation

In Figure D. 3 the IGSCC data is organized by mode of failure (crack — pinhole leak — leak) and pipe size. Figure D. 4 shows the IGSCC data by size and material type.

100.00%

90.00%

80.00%

70.00%

60.00%

50.00%

40.00%

30.00%

20.00%

10.00%

0.00%

NPS12 NPS22 NPS28 All

Figure D.3 IGSCC Data by Failure Mode

300

In Figure D.5, the combination of a/t = 100% and L/C = 100% would indicate a case of DEGB where the pipe ends are separated from each other. As a rule-of-thumb, a through-wall crack (a/t = 100%) with L/C > 40% is unstable and may exhibit unstable crack growth if it were to be left in place.[3]

As seen from the above, there have been a limited number of cases of leaks in large-diameter Reactor Recirculation piping. Only a small fraction of the total number of through-wall flaws have been active leaks; i. e., leaks that have developed during routine power operation. The majority of the through-wall flaws have been “non-active leaks.” That is, leaks that have developed while shutting down for drywell inspection, during performance of weld crown grinding in preparation for ultrasonic examination (“ISI-leaks”), or during the performance of induction heat stress improvement (IHSI — “IHSI-leaks”). There are also some cases where leaks have been discovered during hydrostatic pressure testing to verify the integrity of weld repairs.

Like Figure D.5, Figure D. 8 includes data on all IGSCC-susceptible, Code Class 1 and 2 piping systems in BWR plants. While Figure D.5 includes approximately 300 data points, Figure D.8 includes on the order of 500 data points. This difference in the number of reports represented in respective chart is due to the fact that not all reports on IGSCC include complete details on the crack morphology (dimensions, orientation).

Where through-wall flaws have been observed leak rates have been small. In terms of leak rate and operational impact, so far the two most significant instances of IGSCC occurred at Duane Arnold in 1978 and at the Spanish plant Santa Maria de Garona in 1980. In the former case the leak rate was about 11 lpm (3 gpm) with L/C = 22%. In the latter case the observed leak rate was about 3.0 lpm (0.8 gpm) with L/C = 4.5%.

D.3.2.2 FW Piping Service Experience — Figures D.9 and D.10 summarize the service experience with FW piping. With respect to plant designed by General Electric, the Code Class 1 portion of BWR carbon steel feedwater piping has performed well in the field. There are no reported leaks in medium-or large — diameter RCPB piping. Foreign plants have experienced (and in some cases, continue to experience) thermal fatigue damage due to thermal mixing and stratification. In fact, 80% of the degradation of the RCPB portions of FW piping has occurred in foreign plants with a piping system design that differs from that of U. S. BWR plants.

The U. S. service experience includes a few instances of non-through wall cracking of FW nozzle-to-safe-end (bimetallic) welds. The root cause of the cracking is attributed to weld defects from original construction. As documented in Information Notice 92-35 [D.19], Susquehanna Unit 1 has experienced flow-accelerated corrosion damage about 250 mm (10 inches) from a weld connecting NPS12 piping to a 20-inch by 12-inch reducing tee. There

Figure D.10 Service Experience with FW Piping (ii)