Thermal-hydraulic Characteristics Analysis for Lead-bismuth Fast Reactor Fuel Simulation Assembly

The lead-bismuth fast reactor is an important reactor representative of the fourth-generation nuclear energy system, the fuel assembly is the key equipment of the lead-bismuth fast reactor core, and the complex thermal-hydraulic phenomena inside it which are related to the reactor safety. In this paper, the thermal-hydraulic characteristics of the fuel simulation assembly were studied using CFD software for lead-bismuth fast reactor, and the CFD simulation results were compared with the test results to verified the reliability of the used numerical models and methods. Meanwhile, the detailed thermal-hydraulic characteristics analysis is conducted on the steady-state normal operating condition and high-temperature operating condition of the fuel simulation assembly under the rated power, which could support the thermal-hydraulic designs of the lead-bismuth fast reactor fuel assembly. The analysis results show that a certain degree of mass flow non-uniformity between the inner and outer rod bundles of the fuel assembly is existed on the same height cross-section, the flow velocity in the inner channel is relatively high, while that in the outer channel is relatively low. The mass flow non-uniformity leads to a relatively higher average temperature of the lead-bismuth coolant on inner channel rod bundles and a relatively lower average temperature of the lead-bismuth coolant on outer channel rod bundles. The temperature of the lead-bismuth coolant is negatively correlated with its mass flow. As the height increases, the mass flow of the outer channel gradually increases which approaches that of the inner channel, the mass flow non-uniformity between the inner and outer channels gradually decreases. Accordingly, the resulting temperature non-uniformity also decreases. The certain fluctuations in the temperature of the cladding walls at different positions within the fuel simulation assembly are existed. The temperature is less variable on the cladding wall at the bottom of the heating rod bundles, while the temperature of the cladding wall at the top of the heating section reaches the maximum, the maximum temperature of the cladding wall is within the acceptable design ranges. In addition, the maximum temperature of the fuel simulation assembly under normal operating conditions is around 450 Celsius degree, which can meet the requirements for fuel assembly long-term stable operation, the maximum temperature under high-temperature operating conditions is less than 550 Celsius degree, which can meet the requirements for fuel assembly short-term operation.