# Detection of High-Temperature Gas Leaks in Pipelines Using Schlieren Visualization
## 1 Introduction  
Accidents at nuclear power plants (NPPs) can have significant environmental and human health impacts, cause power outages, and increase plant maintenance costs. To prevent such occurrences, it is crucial to implement proactive measures. We can significantly reduce the risk of major accidents by detecting and addressing elemental component failures through diverse monitoring and diagnostic techniques. Monitoring water and steam pipelines, which constitute the largest portion of the plant, plays a pivotal role in this prevention. Effective monitoring not only helps prevent significant accidents, such as the release of radioactive materials or high vibrations in rotating machinery (e.g., turbine generators and main feedwater pumps), but also enhances the economic and operational efficiency of the plant [1; 2; 3].  
Technologies such as acoustic detection, ultrasonic inspection, and infrared thermography are employed to monitor pipe leaks in NPPs [4; 5; 6; 7; 8]. However, these methods can be diminished in high-noise environments, making precise leak localization difficult. The installation of ultrasonic sensors directly on pipes represents a significant challenge, particularly in the case of insulated pipes, and is also a costly undertaking. Infrared thermography cannot detect leaks in their early stages when only small amounts of fluid escape. To address the limitations of existing technologies, techniques that utilize image processing of pipeline photographs for the visual and optical detection of leaks can be employed.  
The Schlieren flow visualization technique has been the subject of extensive study due to its capacity to discern flows with varying temperatures through visual means. Schlieren visualization represents the refraction of light due to density differences, thereby enabling high-resolution observation of fluid movements with temperature variations [9; 10; 11; 12; 13]. Precise optical system design and alignment enable the detection of gas leaks with temperaturedifferences of just a few degrees. Schlieren technology has been employed in many fields for the visualization of flows, particularly in detecting leaks of flammable or toxic gases such as hydrogen, methane, and ammonia, which are less dense than air [14; 15; 16]. Recent studies have reported the visualization of heat leaks through cracks in building walls or facades using this technique [17].  
The application of Schlieren technology to the monitoring of pipes in NPPs allows for non-contact observation of extensive areas in real time. When integrated with artificial intelligence [18], Schlieren technology has the potential to significantly enhance the precision of leak detection. This advancement in technology instills confidence in the safety of NPPs. This study establishes an experimental configuration for the simulation of pipe leak scenarios in NPPs, with the objective of conducting Schlieren flow visualization experiments. The experiments are designed to encompass two scenarios: leakage of hot air through perforations and gaps in insulated pipes. The temperature of the leaking air is controlled, and Schlieren images are analyzed in accordance with the surrounding temperature disparities. To quantify the leaks, the coefficient of variation in pixel brightness values within the visible flow regions is calculated. An analysis of leak detection based on the coefficient of variation is conducted for both experimental setups. This study is expected to make a significant contribution to the safety management of NPPs by presenting a precise, efficient, and novel approach for pipe leak detection.