The work presented in this article relates to the chemical speciation of carbon-14 in the primary circuit of pressurized-water reactors (PWR). The thermodynamic diagrams for carbon under the operating conditions of the primary circuit of PWR were plotted using the CHNOSZ thermochemical code which uses the Hegelson–Kikham–Flowers (HKF) thermodynamic model. The obtained results were compared to the published data. In an operating PWR with the redox potential imposed in the primary fluid by the pair H2/H2O, methane appears to be the predominant form that should be present in the primary circuit. When the reactor is shut down (low temperature, oxidizing medium), the inorganic forms of carbon (carbonate, hydrogen carbonate) predominate.