Experimental investigation and modeling of void drift in modern BWR fuel designs

Modern fuel designs for Boiling Water Reactor (BWR) include complex features, such as part-length rods (PLRs) and spacer grids with mixing vanes that make the flow very heterogeneous and challenging to simulate accurately (e.g. using sub-channel analysis codes or CFD tools). Due to the lack of reliable local experimental data, the codes have thus far not been validated for the simulation of two-phase flow parameters under BWR operating conditions across the resulting heterogeneous fuel lattice. In order to improve the thermal-hydraulic code predictions, local annular two-phase flow parameters (void and velocity) have been recently measured in a test fuel bundle (including PLRs and spacer grids with mixing vanes) at the Westinghouse FRIGG facility under prototypical BWR core operating conditions. The instruments (local optical probes and Pitot tubes), have been positioned both in an open region downstream PLRs and in a neighboring region surrounded by full length rods (FLRs) at the outlet of the test section. The measurements show higher void fraction and velocity in the region downstream PLRs, thus clearly illustrating a steam crossflow phenomena towards open regions, also known as "void drift". The measurements are used to support the development and validation of a new void drift crossflow model in the Westinghouse sub-channel analysis code VIPRE-W/MEFISTO-T, applicable to modern fuel rod bundles with PLRs. The results show significant improvements of the code predictions.