Noble gas diffusion studies by Thermal Desorption massSpectometry in materials for the nuclear industry.
Résumé
Thermal Desorption coupled with high sensisitivity mass Spectrometry (TDS) is a part of the interdisciplinary platform for noble gas analysis PIAGARA (French acronym for Platforme Interdisciplinaire pour l'Analyse des GAz RAres). TDS is used to study the release of noble gas present or implanted in materials like TDP (beta- Thorium phosphate diphosphate), UO2, Mo, SiC. All these materials are involved in nuclear fuels, nuclear waste long term storage or for new generation fuel. He, Kr and Xe are actively studied in all these samples to predict the evolution and to understand the behaviour of fission gases or actinide alpha decays. Noble gas diffusion can also be studied in geological formations with low permeability such as shale and clay-rich formations. In France, these are considered as potential hosts for underground repositories of nuclear waste. The experimental setup is based on micromass 12 from VG specialised in noble gas analysis. There are 3 principal parts. 1) The extraction part with the possibility to anneal the sample until 1350°C. 2) The purification and splitting zone are for cleaning the released gas and separated the different noble gases. 3) The mass spectrometer gives detection limits around 1E6 atoms for Kr and Xe and 1E8 for He. Experimental process consists to anneal the sample at constant or ramp temperature and to measure release gas at different time. Released curves can be modeled and fundamental parameters as diffusion coefficients or activation energies can be deduced. I will present some examples of noble gas releases that cover the longterm storage (TDP), fuel operating (UO2) and new generation fuel liner (Mo, SiC). For underground deposit, 2 types of boreholes were investigated. Boreholes from surface to -1000m across the storage zone (-500m), are for the study of hydrogeological process and radionuclides transportation. Boreholes at the deposit level, are for the study of the excavation zone. I will present, at the deposit level, noble gas profiles along boreholes that show diffusive process correlated to the damage of the gallery.