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A versatile reactor system analysis code RESYS for advanced nuclear reactor was developed by using object-oriented language C++. Based on RESYS code, the model of thermionic nuclear reactor TOPAZ-Ⅱ developed in the former Soviet Union was established, and its start-up process was simulated. The established TOPAZ-Ⅱ reactor system model included reactor core thermal model, thermionic conversion system model and the radiator model. The current density model of cesium thermionic converter used model by Rasor. And the 6 groups of delayed neutron point reactor kinetic model was used to consider the influence of various structural components on reactivity. The point reactor equation with strong rigidity was solved using the Gear algorithm in the RESYS code. The reactivity feedback model of TOPAZ-Ⅱ reactor took into account the Doppler reactivity feedback of the fuel, the reactivity feedback of the electrodes, the reactivity feedback of the moderator and reflector, and the introduction of reactivity control drums. The steady state and start-up transient were simulated based on established thermal-hydraulic model and RESYS code. At steady state, the maximum temperature on the inner surface of the central thermionic fuel element is 2 291 K, the maximum temperature on the outer surface is 2 066.55 K, the maximum temperature on the emitter is 1 961 K, the maximum temperature difference between the electrodes is close to 1 200 K, the temperature at the coolant inlet is 743 K, and the temperature at the core outlet is 837 K. The comparison between the steady-state operating conditions calculated by the RESYS code and the TOPAZ-Ⅱ design values was carried out. The calculated steady-state electric power output is consistent with the calculation results of TITAM code. The calculation results verify the correctness of the developed RESYS code and the established TOPAZ-Ⅱ system model. During the startup process, the reactivity feedback of the moderator and reflection layer is positive feedback, which dominates all feedback effects. The overall reactivity feedback of the TOPAZ-Ⅱ reactor core is positive and does not have a self-stable negative feedback effect, which increases the difficulty of reactor control. During the start-up process of TFE, due to changes in the gas thermal conductivity of the electrode gap, the temperature of the fuel pellets and emitter rapidly increases. At this time, some of the thermal energy generated by fission is stored in the fuel pellets, causing a decrease in coolant temperature and the power of the radiation radiator.

The fuel assembly and fuel rod bundle of sodium-cooled fast reactor are arranged in a triangle form. The fuel assembly adopts closed structure deign without mixing of coolant between assemblies, which is cooled by the sodium coolant through the distribution header on the bottom of the core to accomplish the matching of coolant and nuclear power and flattening the outlet temperature of the reactor core. It is of great significance for the core design and flow distribution scheme to carry out the whole core pinbypin refinement power distribution and neutronicthermal coupling calculation. There are many internal components and complex structures in the pool type fast reactor container including the core, the internal shielding structure, the primary main pump, intermediate heat exchanger, independent heat exchanger and loading and unloading elevators which are arranged around the core. The design of neutron extraction channel in the direction of loading and unloading lifting devices leads to the asymmetry of the structure in the reactor container. It causes obvious anisotropic scattering in the process of neutron transport calculation, which is also a typical deep penetration problem, so more reliable and accurate calculation results require threedimensional calculation with the whole range of the reactor container. The above problems pose challenges to modeling and neutron transport calculation. The highperformance simulation system of sodiumcooled fast reactor adopts advanced modeling technology, largescale parallel computing technology, transportburnup coupling function, coreassemblylattice multilevel mapping and conversion technology to realize the full range threedimensional fine modeling and multiphysical coupling calculation, including the shielding calculation of the container of pool structure fast reactor, the deep penetration calculation of neutron transport of detectors outside the reactor, pinbypin refined power distribution calculation, transport burnup coupling calculation and full core neutronic-thermal sub-channel coupling calculation with the parallel computation scale, and reaches thousands of cores. Through CAD preprocessing modeling, mesh generation, highperformance calculation and visualization, the threedimensional calculation results of large pool type fast reactor can be given more comprehensively and finely, and a clearer physical image of the core or reactor container can be obtained. The comparison between the local key position and the existing calculation program verifies that the highperformance simulation system of fast reactor is reasonable and feasible which provides support for further reducing the conservative margin of design and local structure optimization.