Subchannel analysis of post-dryout flow and heat transfer in a tight-lattice fuel assembly

• The MATRA code is furtherly improved to predict post-dryout heat transfer in tight-lattice fuel assembly. • The improved code can accurately predict post-dryout heat transfer in tight-lattice comparing to experiments. • The improved code is used to study the heat transfer of the 37-rod bundle under high-quality conditions. The tight-lattice fuel assembly is a key research area in the design of the high-performance water reactor, of which the thermal–hydraulic characteristics are significantly different from those used in the conventional pressurized water reactor because of its small pitch-to-diameter ratio. Under loss-of-flow accidents, the dryout and post-dryout heat transfer characteristics in the fuel assembly directly impact its safety. However, the relevant studies are limited, restricting the core design and safety analyzation. In this paper, a subchannel code suitable for post-dryout flow and heat transfer in the tight-lattice fuel assembly is developed based on MATRA program by introducing submodels specialized for various flow and heat transfer regimes. By comparing with experimental data of rod bundles, it is confirmed that the developed subchannel code can accurately predict the dry-out location and the heat transfer of mist flow after dryout in the tight lattice fuel assembly. Subsequently, the subchannel code is utilized to analyze the post-dryout heat transfer behavior in the tight lattice fuel assembly. The results indicate that the hydraulic diameter of subchannel significantly influences the flow field distribution. The corner subchannel with a small hydraulic diameter has the lowest flow rate, and thus the vapor void fraction and quality vary fastest along the axial direction. Though the central subchannel with the largest hydraulic diameter has the highest mass flux, but due to the large heating area, the vapor void fraction and quality still increase relatively high. The transverse flow between different subchannels follows the trend from the corner subchannel to the side subchannel, and then to the central subchannel. Due to the high mass flux and high quality in the corresponding subchannel, the dryout occurs earliest on the center rod, resulting in the largest wall temperature. Meanwhile, the spacer grids show stronger effects on post-dryout heat transfer enhancement in each subchannel. The research conclusions could provide reference to the thermal–hydraulic design of the tight-lattice fuel assembly.