Numerical study on the transportation characteristics of nuclide ¹⁶N in steam generator

The steam generator is a critical component that couples the primary circuit and the secondary circuit of a nuclear reactor. During operation, steam generator tubes are exposed to corrosive environments and high pressures. Since leakage locations are not directly observable and the leakage flow rate from cracks in the heat transfer tubes cannot be measured during operation, leakage incidents are typically diagnosed by sampling emissions from the steam generator exhaust system and analyzing their radioactive activity levels. This study integrates a species transport model into the thermal–hydraulic analysis code STAF (Steam-generator Thermal-hydraulic Analysis code based on Fluent) to investigate the thermal–hydraulic conditions on the secondary side of the steam generator, along with the distribution and concentration evolution of leaked radioactive substance ¹⁶N from the heat transfer tubes, while taking radioactive decay into account. The simulation results reveal the three-dimensional distribution and transient behavior of the leaked substance on the secondary side. Notably, under 50 % power conditions, the equilibrium mass fraction of ¹⁶N exhibits a maximum spatial fluctuation of 38.5 %, indicating that the commonly used assumption of uniform concentration in tube leakage diagnostics may lead to significant errors. The three-dimensional simulation approach developed in this work offers improved accuracy for steam generator tube leakage detection and diagnosis, and provides valuable insights into the transport behavior of radioactive species under realistic operating conditions.