Introduction
Combined-cycle power plants are one of the most important and efficient types of power plants worldwide, where exhaust gases from gas turbines are utilized for generating steam and additional energy. However, the presence of chemicals such as chlorine and flammable gases like hydrogen in various processes of these plants poses specific risks. Chlorination units, which are used to prevent scaling and microbial growth in cooling systems, are critical safety points due to the production of hydrogen gas, which is highly flammable and explosive. This study aims to identify and analyze the risks of hydrogen gas leakage from the chlorination unit of a combined-cycle power plant with once-through cooling system in Iran using the bow-tie risk assessment method.
This method was selected due to its ability to simulate causes and consequences of industrial accidents, especially in complex processes like chlorination, where multiple factors interact with each other. It was used in this study to identify root causes, analyze the probability of hazardous events, and predict potential outcomes.

Methods
In this research, the scenario of hydrogen gas leakage from the chlorination unit was first selected. To simulate and analyze the probable cause-effect relationships of this scenario, the bow-tie technique was employed. This technique, which combines fault tree analysis (FTA) and event tree analysis (ETA), provides a graphical representation of causes and consequences of incidents. Initially, the causes and intermediate events leading to hydrogen leakage were identified, and then effective safety barriers to prevent the incident were simulated. Finally, the potential consequences of the scenario were examined.

Results
The results showed that hydrogen leakage from the chlorination unit occurs due to failures in three main systems: The chlorine storage tank, rectifier, and electrolyzers. A total of 44 root causes and 20 intermediate causes were identified that could lead to hydrogen leakage. Also, three effective safety barriers were identified: Congestion, immediate ignition, and delayed ignition. These safety barriers play a crucial role in preventing the leakage, and their success or failure can lead to different consequences such as flash fires, jet fires, pool fire, fire ball, explosion, or safe release of hydrogen gas. Quantitative analysis of this scenario indicated that the probability of hydrogen leakage from the chlorination unit was 1.51×10-1%, and the most likely consequence, “explosion/flash fire/jet fire,” was identified with a probability of 4.89×10-2%.

Conclusion
The analysis using the Bow-tie method showed that the probability of hydrogen leakage from the chlorination unit increases significantly if safety systems fail. Furthermore, since the failure of electrolyzers was identified as the primary cause of this incident, it seems that this part of the system requires greater attention and improvements in safety and monitoring. Strengthening safety barriers can significantly reduce the risks associated with hydrogen gas leakage. The findings also indicate that the likelihood of consequences such as explosions and flash fires is very high if safety barriers do not function correctly. Therefore, measures to strengthen these barriers and reduce the likelihood of incidents are essential.
This study demonstrates that the bow-tie technique can be an effective tool for analyzing and assessing risks in complex systems such as the chlorination unit of combined-cycle power plants. The model is particularly useful in cases where factors such as sensitive equipment and environmental hazards interact with each other. This research can help power plants and similar industries and be a basis for future studies in industrial accident risk management of power plants.

Ethical Considerations 
Compliance with ethical guidelines
This research was conducted in compliance with all ethical principles. Since there was no experiment on human or animal samples, the need for an ethical code was waived.

Funding
This research did not receive any grant from funding agencies in the public, commercial, or non-profit sectors.

Authors' contributions
All authors contributed equally to the conception and design of the study, data collection and analysis, interpretation of the results, and drafting of the manuscript. Each author approved the final version of the manuscript for submission.

Conflicts of interest
The authors declared no conflict of interest.


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