EXPERIMENTAL AND THEORETICAL STUDY OF THE PORE STRUCTURE AND DIFFUSION PROPERTIES OF AN EVOLVING HETEROGENEOUS MATERIAL: APPLICATION TO RADIOACTIVE BITUMINIZED WASTE

Placed in a geological repository, radioactive bituminized waste (BW) could be altered in the long term by water, leading to the release of chemical and radioactive elements. The main difficulty, in terms of experimental characterization, comes from the fact that the BW material evolves in time due to the swelling associated to the water osmotic flux. To overcome this difficulty, a new approach is proposed in this work, based on the leaching of BW samples in aqueous solutions where the chemical activity of water is controlled. These specific leaching conditions allow one to control the swelling of the degraded BW matrix. The chemical activity of water being fixed, the pore structure of the leached BW samples was quantitatively studied by ESEM pictures further treated by image analysis. In parallel, diffusion cells using radioactive tracers were used in order to measure mass transfer characteristics in the leached BW. Coupling image analysis with diffusion experiments for each degradation state leads to a diffusion coefficient-porosity relation that is then compared to standard diffusion models in biphasic materials.     

Room-temperature CW laser operation at ~1.55 μm (eye-safe range)of Yb:Er and Yb:Er:Ce:Ca2Al2SiO7crystals

Room-temperature CW laser operation at 1.55 μm of Yb:Er:Ca₂Al₂SiO₇ (CAS) single crystal pumped at 940 nm and 975 nm has been achieved for the first time. Introduction of a third doping ion, Ce³⁺, decreases the Er ³⁺⁴I_11/2 excited-state lifetime and improves the laser properties. For Yb:Er:Ce:CAS single crystal, a maximum of 20 mW output power is produced for 285 mW absorbed power. With this material, a low threshold of 20 mW and a relatively high slope efficiency of ~5.5% are obtained. Preliminary results indicate possible improvement in the near future. Experimental threshold values and laser properties of CAS crystals with various compositions are in good agreement with calculations, performed using the rate-equations modeling. Comparison with a Yb:Er:phosphate glass laser is also presented     

Yb to Er energy transfer and rate-equations formalism in the eye safe laser material Yb:Er:Ca2Al2SiO7

Rate equations formalism is used to predict the population ratio of the Er^{3+ 4}I_13/2 levels involved in the 1.55 μm laser transition in the Yb:Er:CAS laser materials. An effective Yb → Er energy transfer, favourable to the Er³⁺ 1.55 μm laser emission, is demonstrated in this laser host. Indeed, the Yb → Er transfer and the Er → Yb back transfer rates are calculated to be 6 x 10⁻¹⁶ and 0.45 x 10⁻¹⁶ cm³ s⁻¹, respectively. Attempts of codoping the system with Nd³⁺, Eu³⁺ and Ce³⁺ have been realised in order to increase the population of the Er^{3+ 4}I_13/2 laser emitting level. Best results are obtained with Ce³⁺ ion since in the sample containing 6 x 10²⁰ Ce³⁺/cm³, the Er^{3+ 4}I_11/2 level lifetime is divided by a factor of 3 while the Er^{3+ 4}I_13/2 fluorescence lifetime remains unaffected. On the contrary, codoping with Nd³⁺ or Eu³⁺ ions simultaneously decreases the Er^{3+ 4}I_11/2 and ⁴I_13/2 kinetics parameters. The role of the other parameters such as Yb/Er concentrations ratios is also discussed.     

EXPERIMENTAL AND THEORETICAL STUDY OF THE PORE STRUCTURE AND DIFFUSION PROPERTIES OF AN EVOLVING HETEROGENEOUS MATERIAL: APPLICATION TO RADIOACTIVE BITUMINIZED WASTE

Placed in a geological repository, radioactive bituminized waste (BW) could be altered in the long term by water, leading to the release of chemical and radioactive elements. The main difficulty, in terms of experimental characterization, comes from the fact that the BW material evolves in time due to the swelling associated to the water osmotic flux. To overcome this difficulty, a new approach is proposed in this work, based on the leaching of BW samples in aqueous solutions where the chemical activity of water is controlled. These specific leaching conditions allow one to control the swelling of the degraded BW matrix. The chemical activity of water being fixed, the pore structure of the leached BW samples was quantitatively studied by ESEM pictures further treated by image analysis. In parallel, diffusion cells using radioactive tracers were used in order to measure mass transfer characteristics in the leached BW. Coupling image analysis with diffusion experiments for each degradation state leads to a diffusion coefficient-porosity relation that is then compared to standard diffusion models in biphasic materials.