An Investigation of Safety Improvements using Additional Safety Class Instrument Air System in Nuclear Power Plant

An additional safety class instrument air (IA) system delivers air to safety class air operated valves (AOVs) such as pressurizer PORVs, the steam generator PORVs, and auxiliary feedwater modulation valves related to natural circulation, when a loss of offsite power (LOOP) occurs. This paper analyzes the risk change before and after installing the new IA system based on the probabilistic safety assessment (PSA) methodology. This approach identifies the cause of the risk reduction through a detailed cutset investigation and measures the importance of the added components and human error. Three main conclusions were elicited: there was a 64.6%∼99.1% reduction in the unavailability of the supporting system related to the safety AOVs, a 19.1% CDF reduction due to the new IA system, and the importance measures of human error were decreased