Numerical Simulation Research on Critical Heat Flux of Annular Fuel

Annular fuel features dual cooling channels, inner and outer, which can significantly reduce core temperature, enhancing reactor core safety and economic efficiency. Countries including the United States, South Korea, Canada, Singapore, Iran, and Egypt have undertaken relevant research. Research in the United States and South Korea has been relatively advanced. Following the Fukushima nuclear accident, both countries suspend their research on annular fuel. Meanwhile, China Institute of Atomic Energy has continued to advance its research on annular fuel. The annular fuel assembly adopts a circular ring configuration, loaded between the inner and outer claddings. Heat transfer occurs simultaneously to both inner and outer channels, exhibiting a distinct thermal behaviour compared to rod-shaped fuel. To ensure core thermal safety, critical heat flux (CHF) prediction methodology research constitutes a vital component of novel fuel element development. CHF prediction primarily employs experimental and theoretical analysis methods. Experimental approaches can yield highly accurate empirical formulas but necessitate dedicated experiment rigs, substantial capital investment, and considerable time commitments, typically spanning several years. Theoretical analysis offers significant advantages over experimental methods in terms of both time and financial costs. Numerous scholars have investigated the thermal-hydraulic characteristics of annular fuel using specialized thermal analysis programs, including flow distribution characteristics, resistance characteristics, flow-induced vibration characteristics, and temperature distribution. However, these studies have primarily focused on single-phase flow heat transfer characteristics. To date, no publicly available literature reports on CHF density prediction for annular fuel. In recent years, China’s supercomputing technology has advanced rapidly, providing robust computational power support for conducting phase-change heat transfer calculations based on computational fluid dynamics (CFD) methods. This paper first selected six operating conditions based on the 2006 CHF lookup table. By comparing the calculated CHF values with experimental values for these six operating conditions, the results show that the relative deviation between calculated and experimental values is less than 15.11%, verifying the accuracy of the numerical calculation model. Then, based on the single rod CFD fluid-solid coupling model, numerical simulation research on the CHF of annular fuel was conducted. A CHF point identification method for annular fuel based on the boiling curve was established. The boiling critical sequence of inner and outer channels of annular fuel was calculated. The results show that the boiling curve method can be well applied to the prediction of CHF of annular fuel. When the flow distribution ratio is 1∶1, the boiling critical occurs first in the outer channel of the annular fuel, and the boiling critical position is approximately at 85% of the height. The results of this paper can provide support for the engineering application of annular fuel assemblies.