Simulation on natural circulation flow characteristics for the FASSIP-03 NSC small loop based on the differences between the heater and cooler positions

Passive safety systems developed increasingly to reduce the risk of an accident in a nuclear reactor, along with active safety system failures triggered by station blackouts. The working fluid has an essential role in passive systems based on natural circulation, such as the FASSIP-03 NSC small loop use nanobubbles as the working fluid. First, the measurement of the size of the nanobubbles fluid inside the pipes of the FASSIP-03 NSC small loop during the experiment conducts using neutron scattering technology. Then, the preliminary investigation is conducting to investigate the flow characteristics of natural circulation in the FASSIP-03 NSC small loop using CFD simulation software before conducting experiments using water or nanobubbles as a working fluid. Then, the purpose of this study was to determine the flow characteristics of the FASSIP-03 NSC small loop equipment based on the different positions of the heater and cooler using fluent simulation with two methods. Method 1 is the position of the heater and cooler on the horizontal pipe, and method 2 is the position of the heater and cooler on the vertical pipe. In this simulation, the working fluid used is still water. The temperature on the heater wall was varied by 70°C, 80°C, and 90°C. At the same time, the temperature on the wall cooler is setting at a fixed setting of 25°C. The simulation results show that the use of a heater and cooler at the vertical positions of the pipe produces the optimal flow. In method 1, the gravitational force of the upper fluid and the force of the buoyancy in the lower fluid do not occur. While in method 2, buoyancy in the heater and gravitational force in the cooler cause the flow to form in the FASSIP-03 NSC small loop. This condition is showing from the velocity vector, which shows that natural circulation flow has occurred. The most significant flow velocity and pressure occurred in the simulation with the temperature on the heater wall at 90°C, respectively 0.0076 m/s and 0.004 Pascal. The difference between the temperature in the heater and the temperature in the cooler is wide, causing the flow velocity to be higher.