Start-up characteristics for the test facility of a prototype sodium heated OTSG

•The modiified RELAP5 is used to analyze the test facility of sodium heated OTSG.•Step change and linear change mode is used to reach steady state.•Step change mode reduce time to steady state and linear change reduce temperature raise rate. The test facility of the prototype sodium heated once-through steam generator (OTSG) is used to verify the accuracy and reliability of the design for the sodium heated OTSG. Besides, according to the test data, the structure and parameters of steam generator can be improved. In this paper, the start-up mode of the test facility of the prototype sodium heated OTSG was pre-analyzed by using the modified RELAP5 MOD4.0. Two kinds of modes to reach the steady state, one is step change mode the other is linear change mode. These two modes are adopted respectively to analyze the thermal hydraulic characteristics of prototype sodium heated OTSG under different conditions. The results show that the step change mode can be used to reduce the time of reaching steady state in low power starting. When the loop is running under high power condition, step change mode can reduce the temperature raise rate and increase the safety of the system.