Thermal-hydraulic analysis of dual-cooled annular fuel element

Compared with the traditional solid-cylindrical pin nuclear fuel, the dual-cooled annular nuclear fuel with both inner and outer coolant channels can greatly increase the plant power density at the same or even higher safety margin condition, because it can improve the cooling capacity and reduce the maximum temperature of fuel pellets. In this study, the thermal hydraulic characteristics of a typical PWR reactor using annular fuel elements are investigated, especially the subcooled boiling characteristics in the inner and outer coolant channel. The Eulerian two-fluid model coupled with inter-phase heat and mass transfer model, inter-phase momentum transfer model and RPI wall-boiling model was applied to predict the 3-D conjugate heat transfer in the annular fuel element clad surface by using the general-purpose computational fluid dynamics (CFD) solver FLUENT. In addition, the coupled model was also used to predict the local flow and heat transfer of the subcooled boiling in both inner and outer coolant channel of annular fuel. The numerical results demonstrate that the mild subcooled boiling occurs both in inner and outer coolant channel of PWR annular fuel element during steady state operation and RPI wall-boiling model can predict the onset of subcooled flow boiling in annular fuel coolant channels. Because of the narrow gap (≈ l.0 mm) between the fuel element pitch, the initial point of subcooled boiling of the outer coolant channel occurs at axial height l.0 m, while that of the inner coolant channel occurs at axial height l.5 m. The maximum average void fraction in the inner and outer coolant channels are 0.0037 and 0.0047 corresponding to the axial height 2.5 m and 3.5 m, respectively. While the maximum void fraction of the coolant channel is 0.0206 at the axial height 3.5 m in the outer coolant channels. The results also show that the peak temperature of annular fuel pellet is 957 K at the axial height 1.9 m. The minimum DNBR of inner and outer coolant channels is 1.69 and 1.55, respectively. It is important to find that annular fuel shows a sufficient margin below the peak temperature 1300 K compared with cylindrical fuel. (authors)