A fully coupled formulation combining reactive transport and an existing thermo- hydro-mechanical (THM) code is briefly described. Special attention has been given to phenomena likely to be encountered in clay barriers used as part of containment systems of nuclear waste. The types of processes considered in the chemical formulation include hydrolysis, complex formation, oxidation/reduction reactions, acid/base reactions, precipitation/dissolution of minerals and cation exchange. Both kinetically controlled and equilibrium-controlled reactions have been incorporated. The formulation has been implemented in the numerical code CODE_BRIGHT. An application is presented concerning the performance of a large scale in situ heating test simulating high-level nuclear waste repository conditions.