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

D.3.3.1 RC & HPI/NMU Piping Service Experience — There have only been a limited number of events involving through-wall cracks in the large-diameter RC piping and the Class 1 portion of SI/CV piping. Evidence of axial primary water stress corrosion cracking (PWSCC) in the bimetallic safe-end to RPV nozzle welds of the RC-HL piping has been reported at Ringhals [D.20] and V. C. Summer [D.21].

During an eight-year period, the now decommissioned Trojan nuclear power plant experienced pressurizer surge line movement, which was attributed to thermal stratification [D.22]. In response, the NRC issued Bulletin 88-11 in December of 1988 [D.23] requesting that licensees perform visual inspections of the pressurizer surge line at the first available cold shutdown. Purpose of the inspections was to determine presence of any “gross discernible distress or structural damage in the entire pressurizer surge line, including piping, pipe supports, pipe whip restraints, and anchor bolts.”

The current version (June 2004) of the PIPExp database includes four records associated with degradation of pressurizer surge lines:

• Record # 19849; during the Three Mile Island-1 2003 Refueling Outage (18-Oct-2003 to 3-Dec — 2003), a UT examination found an axial flaw about 13 mm (0.5-inch) deep in the surge line nozzle — to-safe end interface in dissimilar metal weld No. SR0010BM. This weld connects a 10-inch Schedule 140, carbon steel nozzle to stainless steel safe end.

• Record # 19736; in November 2002 during UT examination of RC piping in the Belgian plant Tihange-2 (a 900 MWe series plant designed by Framatome), code rejectable indications were

discovered in the 14-inch Inconel safe-end to nozzle weld. The flaw is believed to be an original construction defect.

• Record # 1119; while in hot shutdown condition, a non-isolable weld leak developed in a 1 — inch drain line off the pressurizer surge line of Oconee-1 (LER 50-269/1998-002-01). The through-wall crack had initiated by TGSCC and propagated through-wall by vibratory fatigue. Small-diameter piping connecting to a pressurizer surge line is not part of the PWR-2 Base Case definition.

• Record # 420; during the 1988 annual refueling outage a pinhole leak was discovered in a 10-inch pressurizer surge line bi-metallic weld of Loviisa-1 (a Soviet designed WWER-440/213 plant located in Finland). The weld degradation was attributed to poor weld penetration and high residual stresses. This event was screened out from the data analysis.

Figures D.11 and D.12 summarize relevant service experience with medium — and large-diameter RC and safety injection (SI) and normal makeup (CV) piping. For comparison, Figure D.13 shows the service experience with small-diameter RC and SI/CV piping (< NPS2). Figure D.14 is a summary of the worldwide, PWR-specific data pipe failures that are attributed to thermal fatigue. In addition to RC-, SI — and CV-piping this figure includes failures in FW — and RHR-piping.

Figure D.11 Weld Failures in PWR RC-, CV — and Si-Piping (1970-2002)

N <ъ <o л o> NN <ь Ф <v ч<ь ф <p <p $ Ф>

Year(s) of Operation

Figure D.14 Pipe Failures Attributed to Thermal Fatigue in PWRs Worldwide

Figure D.14 includes four significant (v > 38 lpm [10 gpm]) events, three in foreign plants (Civaux-1 in France, Tsuruga-2 in Japan and Biblis-B in Germany) and one in a domestic plant (Oconee-2). The latter event involved a failure of a weld between the HPI/NMU and the RCS cold leg (= PWR Base Case Plant A. b). The plant operators correctly diagnosed the leak and brought the plant to safe shutdown. Subsequent to the weld failure in Oconee-2, limited to small-diameter piping the Electric Power Research Institute issued the “Interim Thermal Fatigue Guideline” [D.9] for evaluating and inspecting regions where there might be high potential for thermal fatigue cracking. Additional perspectives on thermal fatigue mitigation are included in an OECD-NEA report [D.24]. The Babcock &Wilcox-designed plants now include a new design thermal sleeve to mitigate or prevent thermal fatigue cracking of welds.

Prior to these ‘four significant events’, thermal fatigue damage occurred at Farley-2 and Tihange-1 (a Belgian plant) during 1988. At these plants, thermal fatigue initiated from cold water leaking through closed check or globe valves in safety injection lines. At Farley-2, the damage occurred in piping connected to the RCS cold leg, and at Tihange-1 in piping connected to the RCS hot leg. In these events the leak rates were 2.6 lpm (0.7 gpm) and 23 lpm (6 gpm), respectively. The U. S. NRC issued Bulletin 88-08 in response to these events.

D.4 Data Processing and Data Reduction

The objective of data processing is to extract from a pipe failure data collection relevant case histories that reflect specific combinations of reliability attributes and influence factors. Next, the data reduction prepares the input to the statistical parameter estimation in the form of event counts and exposure terms to develop Bayesian prior and posterior distributions.