Structure And Chemical Durability Of Ceramic Matrix For The Immobilization Of Radionuclides From The Spent Fuel Reprocessing

Due to the long half-lives and high radiotoxicity, actinides like plutonium, uranium, neptunium, curium and americium, normally existing in the high-level radioactive wastes from past nuclear weapon development and nuclear power generation, need to be immobilized using various waste forms. For actinide immobilization, ceramic waste forms can be envisaged in which actinides are dissolved in the atomic structure of crystalline matrices. Moreover, these waste forms have shown higher density, higher leach resistance, higher thermal stability and superior mechanical durability. Thus, in this article, three types of waste forms (ie. zirconolite-pyrochlore synroc, monazite-xenotime and powellite ceramics) have been fabricated. Among these waste forms, zirconolite-pyrochlore synroc and monazite-xenotime ceramics have been investigated to assess the suitability of the immobilization of actinides from the dismantled nuclear weapon and the used MOX fuel, while powellite waste forms are studied for the simultaneous conditioning of actinides and 99Mo from the UMo spent fuel. In addition, according to isomorphism theory, Ce3+ and Gd3+ ions are used to mimic trivalent actinides (Pu3+, Cm3+, Am3+ and Np3+), while Ce4+ ion is used to simulate tetravalent actinides (Pu4+, Np4+, and U4+).(1) For the zirconolite-pyrochlore synroc with compositions of Ca1-xZr1-(0<≤x≤ 0.4), the partial reduction of Ce4+ in Ce3+ in Ca1-xZr1-ceramics is observed from the Ce 3d XPS spectra. Three different phase fields, namely monoclinic zirconolite, tetragonal perovskite and cubic pyrochlore are observed in this system. The Ca and Zr sites of zirconolite structure are occupied by Ce3+ and Ce4+ ions, respectively. The zirconolite-2Mâ†' zirconolite-4M â†' pyrochlore transformation is based on the extent of Ce3+ and Ce4+ substitution. The zirconolite-4M phase tends to form at slightly higher substitution levels of Ce3+ and Ce4+ ions. At high substitution levels of Ce3+ and Ce4+ ions, cation rearrangement occurs resulting in the formation of pyrochlore structure.7-days normalized mass loss (NLce) of Ce in the Ca1-xZr1-xCe2xTi2O7+δ ceramics is shown in the order of 10-6～10-7 g·m-2.(2) For the zirconolite-pyrochlore synroc based on the general formula of Ca1-xCexZrTi2-xAlxO7 (0.2< x< 0.8), results show that the Ca1-xCexZrTi2-xAlxO7 system can be constituted of three different phases (zirconolite-2M, pyrochlore, and zirconolite-3T). The zirconolite-2M phase can transform into pyrochlore and zirconolite-3T phases depending on the substitution levels of Ce3+ion. The 7-days normalized mass loss (NLCe) of Ce in Ca1-xCexZrTi2-xAlxO7 ceramics is shown in the order of 10-5～10-6 g/m2.(3) For the monazite-xenotime waste forms with general stoichiometry as Gd1-xYbxPO4 (0≤ x≤ 1), the structural boundary between two phases is based on the experimental temperature and Gd content. The pure xenotime-type crystalline phase with Gd0.9Yb0.1PO4 composition is obtained at 1600℃. The monaziteâ†' xenotime transformation is most likely to occur on a (200) monazite plane along the [020] zone. The dissolution-precipitation mechanism governs the Gd and Yb releases. The normalized mass losses of Gd (NLGe) and Yb (NLGe) remain in a state of equilibrium after 7 days leaching.7-days normalized mass losses of Gd and Yb in ceramics increase with increasing the PO4 tetrahedral distortion. For all the surrogates, the 7-days NLGd and NLYb are shown in the order of 10-5～10-6 g·m2.(4) For the waste forms with stoichiometry as Gd--xCexPO4 (0≤ x≤ 1), the optimized temperature for preparation of monazite-type Gdo.4Ceo.6PO4 solid solution is more than 1300℃. At 1400℃, the GdPO4 ceramic is essentially monazite with little metastable xenotime phase. The metastable xenotime (GdPO4) to monazite transformation occurs during the immobilizing phase of Ce element. The change of Ce content has no significant influence on the morphology of the monazite-type Gd1-xCexPO4 compounds. Ce and Gd releases are controlled by the dissolution-precipitation process. Normalized mass losses of Ce (NLCe) and Gd (NLGd) reach a steady state after 7 days leach period. For all the surrogates, the variations in 7-days NLCe and NLCd are related to PO4 tetrahedral distortion (the more tetrahedral distortion, the higher Ce and Gd releases), and the 7-days NLce and NLGd are found to be in the order of 10-4～10-5 g·m2.(5) For powellite-type Ca(1_-x)(LiCe)x/2MoO4 (0<≤x≤ 1) waste forms, the unit parameters and average cation radii in the A site follow different trends. Moreover, with x varying from 0 to 1.0, the average grain sizes are found to increase and then decrease. For Ca(1-xX)(LiGd)x/2MoO4 (0≤ x≤ 1) ceramics, the refined compositions are found to be closes to the nominal compositions. The unit parameters of solid solutions increase with decrease in the concentrations of Gd3+ and Li+ ions. The change of Gd and Li contents has no significant influence on the morphology of the Ca(1-X)(LiGd)x/2MoO4 compounds. For all the powellite-type surrogates, the leaching behaviors of Ce, Gd and Mo are dominated by the dissolution-precipitation process. After 7-days leaching process, normalized mass losses of Ce (NLce) and Gd (NLGd) reach a steady state. The values observed for 7-days NLce, NLGd and NLMo are dependent on the MoO4 tetrahedral distortion in the powellite structure (the more MoO4 tetrahedral distortion, the higher Ce, Gd and Mo leachability).