An experiment was conducted at Idaho National Laboratory to investigate the bypass flow associated with a Gas Turbine-Modular Helium Reactor in direct support of Computational Fluid Dynamic validation . Velocity fields within a representative quartz model, consisting of an upper plenum, upper block, and lower block, were measured using Particle Image Velocimetry; after which, flow rates were calculated in each section. The present study was carried out to determine flow distribution from the upper plenum to the fuel block assembly. It was found that the flow rates in the lower six coolant channels varied from their average only by 2.4, 4.6, and 2.5% for the low, medium, and high flow cases, respectively. Consequently, it was concluded that the non-uniform inlet velocity condition in the upper plenum had insignificant effect on flow distribution to the coolant channels and interstitial gap.