The flow field and the intensity field collectively dictate the dose distribution delivered by a UV system. Estimate of the dose distribution represent a key output from the modeling tool developed in this research. As a first component of this modeling work, a modeling tool has been developed for the simulation of the radiation intensity field in the reactor. It is based on the LSI method. It allows the rapid simulation of fluence rate distribution field in different configurations and the analysis of design and operation parameters sensitivity. Uncertainties exist in this model; however, it is not expected to provide an absolute value of the fluence rate field, but rather to provide a sound basis for performance evaluation and comparison. Practically, the model consists in a user-friendly Windows interface developed with Visual Basic. The results are expressed in terms of fluence rate field within a section perpendicular to the lamps. Examples of calculations made for different reactor geometries are presented in this paper, either single lamp or multi-lamp systems. The main advantage of this tool is its capability to simulate the fluence rate distribution in most of the various existing UV reactor lamp configurations. Even though the experimental validation of UV fluence rate could not be performed, the predictions of the complete model combining UV fluence rate simulations with Computational Fluid Dynamics showed a satisfactory agreement with experimental bioassay. The model has been used to design a full-scale prototype. Includes 5 references, figures.