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International audience

International audience

International audience

International audience; Under irradiations in nuclear reactors, the microstructure of oxide nuclear fuel changes. To improve the modeling of the UO2 fuel behavior under irradiation, it is necessary to understand the elementary mechanisms of fission products diffusion. Among them, rare gas Xenon and Krypton represent 30% of created elements and fission products such as Iodine and Caesium are corrosive for the clad. Our experimental work consists in the measurement of the fission products release kinetics by Knudsen cell mass spectrometry. For that, 8mm diameter-1 mm height fresh polycrystalline UO2 pellets are implanted with different concentrations in 129Xe, 83Kr, 127I, 133Cs to understand the effect of the fission products density on the diffusion. The release kinetics is studied either during the heating at a given rate from room temperature to about 2050DC (Figure) or during isothermal annealing every 100 DC from 100 to 1600 DC.

Over the last decades, XAS has evolved as a very useful tool to investigate nuclear materials with applications for fuel manufacturing of for the behavior for fission products. Nowadays, the possibility of applying HERFD measurements to radioactive compounds offers even more possibilities. Two recent studies dealing with the behavior of fission gases in UO2 nuclear fuel and investigating both the electronic and structural properties of uranium based mixed oxide fuels will be presented to illustrate the use of HERFD. Furthermore, recent experimental developments allowing to perform in-situ XAS measurements on actinide-based materials at high temperature and under controlled atmosphere, will be illustrated by the first results obtained on UO2±x and (U,La)O2±x samples.

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.

International audience; A new procedure for (U,Pu)O2 nuclear fuel manufacturing based on the oxalic co-precipitation of U(IV) and Pu(III) followed by the thermal conversion of the co-precipitate into oxide is under development. In order to fully investigate the ideality of solid solution with Pu content equal to 50, 30, 15 and 7 at.percent, XRD and XAS characterizations at uranium and plutonium LIII edges have been undertaken. Using XRD, a face centred cubic structure was observed in each case, and the cell parameter deduced obeys satisfactorily the Vegard-s law. However, EXAFS measurements moderate these results, only the (U0.5Pu0.5)O2 sample leads to the same conclusion as XRD. For the lower plutonium concentration, a disordered hyperstoichiometric structure (U1-yPuy)O2+x have been evidenced. In those compounds, cuboctahedral oxygen defects are only located around uranium atoms and not in the Pu environment. Thus, a much complex structure than suggested by the XRD is observed with certainly a non-random distribution of plutonium atoms into the uranium sites of the UO2+x structure

Théorie, ACE