Experimental benchmark of a computer code for assessment of baffle gap crossflow effects

The application of core thermal-hydraulic rod bundle crossflow codes (such as COBRA3C, COBRAIV-1, or in this instance Babcock and Wilcox's modified version of COBRAIV-1, called LYNXT) has been extended in order to analyze the flow between the core and the core bypass region (region between core baffle plates and core barrel), called baffle gap crossflows. Baffle gap crossflow geometries differ from typical rod bundle flows in that the baffle region is a relatively open flow region with discrete pressure losses representing the baffle support plates, whereas in typical rod bundle analyses the flow is confined by the rods, which have continuous pressure losses. This difference required modifications to the crossflow resistance and mixing length terms in the rod bundle crossflow codes. To verify the ability of the revised crossflow code, called LYNXTBG, to properly predict the baffle gap crossflow velocities, an extensive test program was conducted on a full scale/partial length fuel assembly/baffle region mockup. The baffle gap crossflow velocities thus obtained were used to benchmark LYNXTBG. The resulting code underpredicted the span weighted average ''inflow'' velocities by approx. =15% and overpredicted the span weighted average ''outflow'' velocities by approx. =10%. These code to experiment variations when combined with statistical uncertainty factors were then used in conjunction with LYNXTBG to calculate in-reactor baffle gap crossflows