Plant B is a BWR/4 assumed to have been in commercial operation for at least 25 years. Similar to many other operating BWR/4 plants in the USA, Plant B is also assumed to be operating with a combination of IGSCC Category D and E welds, according to the nomenclature of U. S. NRC Generic Letter 88-01 [D.3, D.4]. In other words, the plant has experienced some IGSCC and the affected welds have been reinforced by weld overlays. It is further assumed that none of the IGSCC susceptible welds have been subjected to any stress improvement (SI) process such as induction heat stress improvement (IHSI) or mechanical stress improvement process (MSIP). It is also assumed that the weld overlay repairs (WOR) were all performed in the 1982-1988 timeframe. Finally, Plant B is assumed to have been operating with normal water chemistry (NWC) at all time.

The system descriptions in this section are extracted from design information supplied by members of the Expert Elicitation Panel. The BWR-specific system information is included in the following documents and drawings:

• Document No. EPRI-156-310: Degradation Mechanisms Evaluation for Class 1 Piping Welds at Plant B [D.5].

• Excel-file entitled “PlantBWelds.” This Excel-file includes weld lists with locations for the RR and FW ASME Section XI Code Class 1 piping. The lists are organized by weld identification numbers (as they appear on the isometric drawings identified below) nominal pipe size and pipe schedule. The Excel file forms the basis for the LOCA frequency model used to derive the LOCA frequency distributions.

• Isometric drawing numbers 6M721-5358-5 (RR System Loop B Ring Header), 6M721-5359-5 (RR Loop B Suction & Discharge Piping), 6M721-2336-1 (FW System Inside Drywell), and 6M721- 3537-5 (FW System Inside Drywell).

D.1.3.1 Reactor Recirculation (RR) System — The RR System evaluated in this study consists of two recirculation pump loops external to the reactor pressure vessel (RPV). These loops provide the piping path for the driving flow of water to the RPV jet pumps. Each loop contains a variable speed recirculation pump and two motor operated isolation valves (one on each side of each pump). The recirculation loops are part of the nuclear system process barrier and are located inside the drywell containment structure. The pipe segments that are subject to evaluation in this study consist of:

Loop A: The Class 1 portion starts at the RPV nozzle N1A and is reconnected to the RPV at nozzles

N2F, N2G, N2H, N2J, and N2K. Class 1 lines for the Residual Heat Removal (RHR) and Reactor Water Cleanup (RWCU) Systems are connected to this loop. These particular Class 1 lines are excluded from the study scope, however. Loop A is excluded from the BWR Base Case.

Loop B: The Class 1 portion starts at RPV nozzle N1B and is reconnected to the RPV at nozzles

N2A, N2B, N2C, N2D, and N2E. Part the original design, a NPS4 bypass line at valve F031B has been removed from the system. Class 1 lines for the RHR and RWCU Systems are connected to this loop. These particular Class 1 lines are excluded from the study scope, however.

D. 1.3.2 Feedwater (FW) System — The FW System provides feedwater to maintain a pre-established water level in the RPV during normal plant operation. The Condensate and the FW Systems take water from the main condenser and deliver it to the RPV after passing it through the feedwater heaters and demineralizer system. The Class 1 portion of the FW System consists of two loops:

Loop A: Loop A starts at valve F076A and a connection to the High Pressure Coolant Injection

(HPCI) discharge line (at valve F006), and connects to the RPV at nozzles N4A, N4B, and N4C. The HPCI discharge line is excluded from the study scope. Loop A is excluded from the BWR Base Case.

Loop B: Loop B starts at valve F076B, connection to the Reactor Core Isolation Cooling (RCIC)

discharge line at valve F013, and a discharge from the RWCU System (at valve F220), and connects to the RPV at nozzles N4D, N4E, and N4F. The RCIC and RWCU discharge lines are excluded from the study scope.