Numerical investigation on effects of fuel rod with different bow deformation ratio X on two-phase boiling and flow performances of coolant in fuel assembly

Abstract

The bow deformation of fuel rod usually occurs in Pressurized Water Reactor (PWR). As the flow channel in fuel assembly changes due to the bowing, coolant in subchannels is redistributed, which influences flow and heat transfer performances of coolant. In this article, based on Eulerian two-phase model and RPI boiling model, a 3 × 3 fuel assembly with bow deformation is studied in the STAR-CCM+, and typical C-shape bow deformation happens to the center fuel rod. The effects of fuel rod with different bowing degrees on coolant two-phase boiling and flow performances are compared. Numerical model is verified by comparing with the experimental data. The results show that significant local boiling occurs in the bowing region and liquid phase temperature increases dramatically in subchannels where cross-sectional area decreases if bow deformation ratio X less than 0.5. The averaged void fraction in bowing region gradually increases with the bowing degree, and it increases dramatically if X less than 0.2. Besides, the axial position of the initial boiling is gradually advanced, and the area of high liquid phase heat transfer coefficient on the wall of bowing rod decreases sharply when X less than 0.4. Meanwhile, the cross flow reduces gradually with the increase of bowing degree, indicating that bow deformation will inhibit the generation of cross flow. The turbulent kinetic energy of coolant in the region with reduced cross-sectional area greatly decreases when X less than 0.2, and the inhomogeneity of distribution brings great challenge to the safe operation of nuclear reactor core. Besides, excessive bow deformation will cause a great pressure drop.