Numerical study on the boiling heat transfer and critical heat flux in a simplified fuel assembly with 2x2 helical cruciform rods

The helical cruciform fuel was potential to upgrade the core power density while maintaining the safety margin of a reactor. However, the detailed analysis on the boiling phenomena was inadequate. In this paper, the flow boiling and critical heat flux in a fuel assembly with 2 x 2 helical fuel rods were predicted based on the Eulerian two-fluid framework and auxiliary models for interphase exchanges. The averaged and localized distributions for velocity, temperature, vapor fraction and heat flux were obtained. The four-petalled helical structure introduced significant differences in the flow and heat transfer characteristics compared with traditional cylinder rod bundle. The cross flow intensity was high due to the continuous mixing effects of blades. The azimuthal heat flux distribution was nonuniform, with the maximum to average heat flux ratio larger than 2. The vapor phase crowded at the elbow of the rods, that is, the location of maximum heat flux. The averaged heat flux of helical fuel assembly at critical condition was very close to that of cylinder rod bundle; however, the critical linear heat flux of helical fuel assembly was about 31% higher than cylindrical one, which can upgrade the core power limitation significantly.