Optical on line techniques enable non intrusive physical measurements in harsh environments (high temperature, high pressure, radioactivity,…). Optical absorption spectrometries such as UV-Visible or FTIR spectrometries have been successfully used to study reaction kinetics in thermochemical cycles for example. To achieve very low detection limits often required in studies relevant with fission product release in nuclear reactor studies or explosion risks in waste disposals, cavity ring down spectroscopy is a promising on line optical technique.