March 30, 2018 Current methods used for the detection of long-lived radioisotopes are mainly laboratory-based techniques, which cannot provide on-site monitoring capabilities. Optical techniques, such as laser spectroscopy, on the other hand, have inherent on-site monitoring capabilities, as it can provide compact, transportable, and reliable instrumentation, while still achieving very high sensitivity. Cavity ring-down spectroscopy is one of the most sensitive laser spectroscopic technique as it uses an absorption path length of several kilometres, thanks to the use of an optical cavity with high-reflectivity mirrors. Its principle is described in Figure 1. The technique is routinely used in environmental and emission monitoring but has never been applied to the characterisation of nuclear waste outgassing. The development of the technique for the detection of radioisotopes of interest will provide a sensitive and compact way of monitoring the waste outgassing in waste repositories and during the decommissioning process. The prototype being developed in MetroDECOM II focuses on the detection of radiocarbon, which is one of the most common source of radioactive gaseous emissions in nuclear facilities. The technique also allows differentiating between the different molecular forms of radiocarbon, such as carbon dioxide and methane. In the future, it is envisioned that the technique will be ideal to provide in-situ continuous monitoring of radiocarbon emissions in nuclear facilities Figure 1: Principle of cavity ring-down spectroscopy. At each wavelength, the ring-down rate, τ, is recorded and by comparing it with the vacuum ring-down time, τ0, the sample absorption spectrum is computed. If the absorption line strength is known, the gas concentration can then be inferred. .