The Monte-Carlo method is often presented as a reference method for radiative transfer simulation when dealing with participating, inhomogeneous media. The reason is that numerical uncertainties are only of a statistical nature and are accurately evaluated by measuring the standard deviation of the Monte Carlo weight. But classical Monte-Carlo algorithms first sample optical thicknesses and then determine absorption or scattering locations by inverting the formal integral definition of optical thickness as an increasing function of path length. This function is only seldom analytically invertible and numerical inversion procedures are required. Most commonly, a volumic grid is introduced and optical properties within each cell are replaced by approximate homogeneous or linear fields. Simulation results are then sensitive to the grid and can no longer be considered as references. We propose a new algorithmic formulation based on the use of null-collisions that eliminate the need for numerical inversion: no volumic grid is required. Benchmark configurations are first considered in order to evaluate the effect of two free parameters: the amount of null-collisions, and the criterion used to decide at which stage a Russian Roulette is used to exit the path tracking process. Then the corresponding algorithm is implemented using a development environment allowing to deal with complex geometries (thanks to computer graphics techniques), leading to a Monte Carlo code that can be easily used for validation of fast radiative transfer solvers embedded in combustion simulators. ``Easily'' means here that the way the Monte Carlo algorithm deals with both the geometry and the temperature/pressure/concentration fields is independent of the choices made inside the combustion solver: there is no need for the design of a new path-tracking procedure adapted to each new CFD grid. The Monte Carlo simulator is ready for use as soon as combustion specialists provide a localization/interpolation tool defining what they consider as the continuous input fields best suiting their numerical assumptions. The radiation validation tool introduces no grid in itself.