CHF Performance of Hybrid Mixing Vane Grid (II)

Numerous studies have shown that the mixing vanes of the spacer grids in a nuclear fuel rod bundle increase the Critical Heat Flux (CHF) significantly. This increase is known to mainly come from the enforced turbulence and lateral flow to homogenize the local enthalpy imbalance in the coolant throughout the downstream of the mixing vanes. These effects can suppress the occurrence of a CHF. However, the amount of the CHF enhancement depends strongly on the design of the mixing vanes such as the vane shape and vane bending angle. Over a couple of decades a split mixing vane has been widely used by most nuclear fuel vendors. As shown in Figure 1 (a), the split mixing vanes are formed as pairs such that each one is bent in an opposite direction. Recently a new mixing vane design was developed for an advanced spacer grid at KAERI. It's called a Hybrid Mixing Vane as shown in Figure 1(b), which comprises two types of vanes; primary vanes and secondary vanes. The primary vanes are to generate a cross flow between subchannels, while the secondary vanes are to create a swirl flow within the subchannels. The main objective of this work is to evaluate the CHF performance of the hybrid vane grid and to compare it with that of a split vane grid. Three kinds of rod bundles were tested for the above objectives: no mixing vane grids, the hybrid mixing vane grids, and the split mixing vane grids. To measure the CHF data, 5x5 rod bundle experiments were conducted in the FTHEL(Freon Thermal Hydraulic Experiment Loop) at KAERI. The test matrix sufficiently covers the PWR operating conditions for water equivalence