Computational Fluid Dynamics (CFD)-based validation of ultraviolet (UV) reactor performance has enormous potential as a cost effective and versatile tool for facilitating the implementation of UV disinfection for drinking water treatment. Key to using CFD-based validation is obtaining regulatory acceptance. This work provides an assessment of the impact five factors on CFD-based predictions of dose delivery for one commercial UV reactor. Dose delivery by the reactor is influenced by the number of microbes simulated in the model and by the number of simulations that pass fully through the UV reactor and are not "trapped". The Reduction Equivalent Dose (RED) calculated using the CFD-model is dependent on the UV sensitivity of the microbe used in the model. The dependence is greater at lower UV transmittance values. Predictions of RED and velocity profiles with a ninety-degree bend upstream of the reactor did not differ from predictions with a straight pipe inlet. Predictions of RED were also the same with the use of RNG and ke turbulence models. Includes 5 references, tables, figures.