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/m³ and 40 GBq/m³ of ¹³³Xe in the argon cover gas, i.e. 14 parts per trillion (ppt) to 2800 ppt of ¹³³Xe, 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.