The paper describes the computer code REACFLOW, developed at the JRC for the numerical simulation of compressible gas flows with chemical reactions, with the objective of studying large-scale chemical explosions. The code employs a finite-volume scheme on an unstructured 2-D or 3-D grid, coupled with turbulence models, and modules for either finite-rate chemistry source terms or a model for the interaction between turbulence and combustion. To overcome the problems which are related to different scales, an adaptive fully reversible CFD method is used. New grid points are temporarily inserted into the mesh to track strong disturbances in,the flow (such as shocks or flame fronts) and removed when not required. The code has been tested on a number of different 2-D and 3-D problems, including fast turbulent deflagrations and detonations on medium and large scale problems.