Buoyant round vertical jet had been investigated using Large Eddy Simulations at low Mach number. For the purpose of comparison with in-house experimental data, in the present work helium has been used as a substitute for hydrogen. The influence of the transient concentration fields on the volume of gas with concentration within flammability limits has been investigated, and their evolution and relation with average fields ad been characterized. Transient concentration fields created during initial jet development had been considered. Numerical results have been compared with in-house experiments and data published in the literature.