Study On The Transport Characteristics Of Radioactive Material With Fluid In Pipe Network

Steam generator is an important component of the reactor’s first and second loop system pressure boundary.Due to the large temperature difference and pressure difference on both sides of the steam generator heat exchange tube,it is easy to produce large mechanical stress and thermal stress.Coupled with the vibration and corrosion of the fluid in the first and second circuits,the steam generator heat exchange tube may break under extreme conditions,which leads to the leakage of radioactive materials from the primary circuit to the second circuit.Radioactive materials leaked into the second circuit will gradually spread into the environment over time,causing permanent damage to the natural environment and plant workers.The calculation of the transport and migration of radioactive substances in the pipe network of the second loop is helpful to analyze the process of the steam generator heat transfer tube rupture accident and evaluate the radioactive consequences,so as to provide a reference for the formulation of emergency response plan and accident mitigation measures after the accident.At present,the domestic and foreign radioactive material transport models mainly focus on the transport and migration process of liquid corrosion products in the primary circuit,and did a lot of simplification on the gas-liquid phase transformation and node fine division,which is not suitable for the calculation of radioactive material transport in the second circuit pipe network.In this paper,based on the transport and migration of the typical elements and their upstream and downstream products in the decay chain in the pipeline network model,the phase transition,decay and deposition characteristics of radionuclides in the pipeline network were studied.Based on Gibbs free energy minimization principle and ion chemical equilibrium principle,the calculation method of cobalt ion solubility suitable for the second-loop system is obtained.Based on the analysis of time and accuracy,SOR iterative method is determined to be the better method to solve the transport model.In this paper,the influence of nuclides upstream and downstream of the decay chain of radioactive substances is fully considered,and the program for transporting radioactive substances in pipe network is developed by means of self-programming with Simexec simulation program as the development platform.The comparison of steady-state and transient simulation results between the program and JTOPMERET radioactive computing module shows that the maximum deviation of steady-state simulation results is less than 0.03%,and the trend of transient results is consistent,which proves the applicability of the program model to calculate the real-time migration and distribution of radioactive materials in the second loop pipe network.With the application of the developed program for the transport of radioactive substances in the pipeline network,the calculation and analysis of the transport,decay and leakage characteristics of radioactive substances in the pipe network of the secondary loop after the shear fracture accident of the heat transfer tube of a single steam generator in a small reactor were completed,and the main types of radioactive substances leaked after the accident were determined.