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. Cladding failure may occur for 3 different reasons: manufacturing defect along the welding, end of life and accidental operation. While manufacturing 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 to know if the failure belongs to the end of life type, and to localize the failed assembly. Therefore the ratio of stable over radioactive fission products needs to be measured. Conventional nuclear measurement can't measure stable isotopes, and gamma spectrometry is suffers from the strong emission from the gaseous activation products. A review shows that a failure releases between 0.2 GBq/m(3) and 40 GBq/m(3) of Xe-133 in the argon cover gas, i.e. 14 parts per trillion (ppt) to 2800 ppt of Xe-133, and about 100 times more stable isotopes in case of an highly burned fuel. In the frame of the French ASTRID project, an optical spectroscopy technique - Cavity RingDown Spectroscopy (CRDS) - is developed to measure the gaseous fission products, either stable or radioactive. A dedicated CRDS set-up is needed to detect the rare gases with a commercial laser. Indeed, the CRDS is coupled to a glow discharge plasma, which generates a population of metastable atoms. The xenon plasma conditions are optimized, to get a 0.8 % production efficiency of metastable Xe. The metastable number density is proportional to the xenon over argon molar fraction. The spectroscopic parameters of the strong 823.16 nm xenon transition are calculated and/or measured in order to make a quantitative measurement of the metastable xenon. The laser intensity inside the cavity is limited by the optical saturation process, resulting from the strong optical pumping of the metastable state. The resulting weak CRDS signal requires a fast, low noise and sensitive photodetector. A 600 ppt xenon molar fraction was measured by CRDS. With the present set-up, the detection limits are estimated from the baseline noise to approximately 20 ppt for each even isotope, 60 ppt for the Xe-131 and 55 ppt for the Xe-129. 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 or radioactive isotopes.