Cavity Ring-Down Spectroscopy for Gaseous Fission Products Trace Measurements in Sodium Fast Reactors

Safety and reactor availability are key issues of the generation IV reactors. Hence, the three radionuclide confinement barriers, including the fuel pin cladding, must stay tight during the reactor operation. While manufacturing cladding failures evolve very slowly from a gaseous failure to an open failure, end of life and accidental failures may open very rapidly, generating a concerning situation. It is then important to detect the failure at the early stage through the gaseous fission products xenon and krypton, and evaluate the burning rate of the failed assembly. Therefore the ratio of stable over radioactive fission products needs to be measured. In the frame of the French ASTRID project, an optical spectroscopy technique-Cavity Ring-Down Spectroscopy (CRDS) - is developed to measure the gaseous fission product xenon, either stable or radioactive. A dedicated CRDS set-up, coupled with a glow discharge, is built to detect xenon with a commercial laser. A 6 ·10^-10 molar fraction of stable xenon over argon was measured by CRDS. With the present set-up, the detection limits are estimated from the baseline noise, to approximately 2 ·10^-11 for each even isotope, 6 ·10^-11 for the ¹³¹Xe and 5.5 ·10^-11 for the ¹²⁹Xe. This sensitivity matches the specifications required for gaseous failure measurement. The odd isotopes are selectively measured, whereas the even isotopes overlap, a spectroscopic feature that applies for stable and radioactive xenon isotopes.