In a sodium-cooled fast reactor system, a minor damage to the steam generator in the secondary circuit may trigger a chemical reaction between sodium and water, leading to a small leak accident. In this study, we modeled the system changes that may be caused by such a small leak accident by conducting a small sodium-water reaction experiment. During the experiment, we recorded and compared data on temperature, pressure, flow rate, liquid level, and hydrogen ion concentration at different monitoring points.The selected test operating conditions are as follows:operating pressure: sodium loop at 0.2 MPa, water loop at 14 MPa;operating temperature: sodium loop at 250–500 °C, water loop at 320 °C;operating flow rate: sodium loop at 20 m3·h-1, water loop determined by operating conditions. The results showed that the hydrogen ion concentration changed most significantly during the reaction, especially in the early stages of the reaction. By comparing the hydrogen ion concentration at different locations during the reaction, we found that the movement direction of hydrogen ions is the same as the flow direction of sodium, which can be used as an effective signal for detecting leak accidents in small-scale sodium-water reactions. Based on the study of hydrogen ion concentration changes at different locations, we determined a reasonable hydrogen ion concentration alarm threshold, and this threshold is consistent with the previously developed small leak detection subsystem. Finally, we verified the feasibility of the previous settings and the accident protection logic by detecting the change of working conditions in the loop after the alarm trigger was put into the accident protection logic.