Master's thesis and Internship : A methodology for evaluating the operating margin of safety-significant air-operating valves in nuclear power plants

Abstract

This thesis presents the development of a methodology for evaluating the operating margin of AirOperated Valves (AOV) within the framework of the Long-Term Operation (LTO) program for Belgian nuclear power plants. The project originated from the implementation of the ASME (American Society of Mechanical Engineers) OM (Operation and Maintenance of nuclear power plants) 2020 standard, which introduces mandatory preservice and in-service testing requirements for pneumatically actuated valves classified as High Safety Significance (HSS). These regulatory requirements highlighted the need to demonstrate that such valves maintain sufficient operating margins under design-basis conditions. The core objective of this work is therefore to establish a structured and physically based approach to calculate the operability margins of safety-significant AOV (Air-Operated Valves). To support the practical application of this methodology and facilitate its use in an industrial context, the developed approach is subsequently implemented in a dedicated and user-friendly software tool, named MAP4AOV (Monitoring and Analysis Program For Air-Operated Valves). The proposed methodology combines a risk-informed selection of valves based on a Probabilistic Safety Assessment (PSA) with a physics-based modeling of all forces acting on the valve, including spring forces, fluid-induced forces, friction forces, and inertial effects. The initial scope consisted of 231 air-operated valves, which was progressively reduced to 39 safety-significant valves through the PSA-based screening process. Particular attention is given to friction force modeling, identified as one of the most uncertain yet dominant contributors to the final operating margin. To address this uncertainty, two complementary friction models are developed and implemented : a Karnopp-based model and a first-principles model. The applicability of the proposed methodology is demonstrated through a case study focusing on the EAA-Vx (auxiliary feedwater circuit) air-operated valves. This group comprises eight valves that are geometrically identical but installed at different locations within the circuit, leading to variations in operating conditions. The methodology is applied using two complementary friction modeling approaches under the most restrictive operating conditions, namely with no water in the circuit and therefore a zero differential pressure (∆Pwater = 0 bar). An operating margin of 55.2% is obtained using the Karnopp friction model, assuming a valve idle period of fourteen days, while an operating margin of 58% is obtained using the first-principles friction model. Overall, this work provides a structured and practical approach for evaluating AOV operating margins and delivers a software-based tool that can support future analyses, contributing to improved safety demonstration and maintenance efficiency in nuclear power plants.