A contribution of thermal radiation to heat transfer in fires is determined by the divergence of the integral radiative flux only. This makes it possible to use a simple approach based on the P1 approximation for spectral radiative transfer in CFD modelling of fires. This first step of solution is sufficient to determine the flame parameters. The second step, which is necessary to obtain the radiation field outside the flame, can be made independently using the ray-tracing procedure and the source function determined at the first step. Such a splitting of the complete problem results in sufficiently accurate but much simpler algorithm than those used traditionally. An axisymmetric pool fire at the initial stage of fire suppression by a water spray is considered as the case problem. It is demonstrated that the computational time decreases in about two orders of magnitude as compared with direct radiative transfer calculations.