Computational Fluid Dynamics (CFD) was used to model an annular photocatalytic reactor by solving the Radiative Transfer Equation (RTE) using the Discrete Ordinates Model (DOM) and the Six-Flux model (SFM) with isotropic scattering. The RTE boundary condition (BC) at the light entrance wall with the SFM was either the irradiance or the fluence rate, calculated using the LSSE, LSDE or ESDE light emission models. The Total Rate of Photon Absorption (TRPA) calculated with the SFM and fluence rate BC was overestimated by 29 21 % in comparison to the DOM, when the optical thickness varied between 1.8 and 3.2 %, and was underestimated by 3.1-8.8 % when irradiance was the BC. The intrinsic reaction kinetics constants of 2-hydroxybenzoic acid (2HBA) determined using the SFM in experimental reactors operated at very high optical thicknesses were 1 % higher and 18 % lower, than the constants determined with DOM, when irradiance or fluence rate, respectively, was used as BC. Overall, the SFM combined with the irradiance BC provides a more accurate evaluation of the LVRPA and intrinsic reaction kinetics constants, with instantaneous solutions, while the DOM computational time > 20 min. This aspect is highly important in solar photocatalytic reactors with fluctuating irradiance.