ISO 2 2125-1 standard Part 1 specifies a test method for the determination of tantalum-99 (T-99) isotopes in water quality using the liquid scintillation counting method (LSC). This method has high Sensitivity and Accuracy, and is widely used in water quality monitoring, especially for the detection of low-concentration radioactive substances. The liquid scintillation counting method reacts tantalum-99 in water samples with scintillants to generate optical signals, and then detects the radioactivity intensity through a counter to realize the quantitative analysis of tantalum-99. The standard provides a scientific basis for environment monitoring and water quality security evaluation in nuclear facilities, which helps to effectively control the concentration of radioactive substances in water and ensure the safety of water resources.