Application of Containment Integrity Margin to Allow for Increased Operational Flexibility of Service Water Systems

Abstract

Recent experience has shown that the cooling capacity of service water systems in Nuclear Power Plants has been degraded by various causes, such as heat exchanger tube fouling, corrosion/erosion, failures, etc. Recognizing this, the NRC has addressed this concern via Generic Letter 89-13 (Reference 1), and has provided guidance toward the resolution of this issue. The purpose of this study is to show that margin to equipment operating limits can be quantified and applied to other areas requiring margin, thereby allowing greater flexibility in efficient plant operation. Following a postulated Loss-Of-Coolant-Accident (LOCA) for a Light Water, Pressurized Water Reactor (PWR), the Containment safeguards systems (Emergency Fan Coolers (EFCs) and Containment Spray (CS)) are relied upon to remove energy from the containment atmosphere in a post-accident environment, in order to ensure that the design limits of the containment structure and equipment within the containment are not exceeded. In a PWR plant, the EFCs transfer energy to the ultimate heat sink via the Service Water (SW) system or the Component Cooling Water (CCW) system. This paper focuses specifically on the contribution of the EFCs in transferring energy from the containment to the Component Cooling Water system, and the utilization of margin to compensate for system degradations. The effectiveness of the EFCs in removing energy from the containment atmosphere is influenced by the capability of the CCW system. The results of the analyses performed are used to: 1. better quantify the post-accident heat removal requirements; 2. define CCW heat exchanger requirements; and, 3. provide the basis for an improved quantification of design limits and margin. This margin may then be allocated for safety evaluations, or used to obtain greater operational flexibility, such as compensating for increased service water temperatures, higher heat exchanger fouling, or degraded flowrates. The analysis of various aspects of emergency containment cooling has yielded important information relative to I better understanding of the significance of EFC operation, by quantifying both the heat removal effectiveness and the potential for margin improvement. This study demonstrates that margin in the calculated contain111nt temperature can be applied to compensate for degradations in the heat removal capability. Improvements in margin obtained may then be utilized to achieve greater operational flexibility.