Preparation And Characterization Of The Analog Waste Forms For Actinide Nuclides And Fission Product Strontium

Pyrochlore–type rare earth zirconates(Ln2Zr2O7, Ln=rare earth) are the current ideal base material internationally recognized for curing actinide, because of their ability of simultaneously curing a variety of actinides, large nuclide package capacity, good chemical stability, high radiation resistance. In the present study, Nd3+, Ce4+ and 89Sr2+ were used as rare earth analogues for An3+, An4+ and 90Sr2+ ion.(1) Single and composite rare earth zirconate–based curing materials,(2) multivalent actinide nuclides simulating waste forms, and(3) simulating waste forms for the coexistence of actinide nuclide and fission product 90 Sr were synthesized by using spray pyroysis method. The phase composition, valence state and microstructure of the sintered bodies were investigated by XRD, Raman, XPS, SEM and TEM techniques, the main research contents and results are as follows:The phase composition and crystal structure of tht target powders synthesized from zirconium and rare earth nitrates via spray pyrolysis were studied. The results indicated that compounds of Ln2Zr2O7(Ln=La, Nd, Eu and Gd),(La1–xLnx)2Zr2O7(Ln=Nd and Gd) and(Nd1–xEux)2Zr2O7(x=0–1) synthesized at 1200 oC have a pure pyrochlore structure. Aftering a heat treatment at 1200 oC in air, Ce2Zr2O7 decomposed into two phases of Ce–rich cubic and Zr–rich tetragonal phases. Compounds(Nd1–xCex)2Zr2O7(x=0–1) were in pyrochlore form after annealing under the protection of graphite. The lattice parameter decreased with increasing x value. The Ce was in a single oxidation state of +4 in above compositions. The in–situ reaction product of(1–x)LnO1.5–xSrO–ZrO2 ternary system(Ln=La, Ce, Nd, Eu and Gd, x=0–1) composed of pyrochlore Ln2Zr2O7 and orthorhombic Sr ZrO3(lattice parameter a=0.5817 nm, b=0.8204 nm, and c=0.5797 nm) phases.The phase stability and microstructure of the simulating waste forms sintered at high–temperature in air were studied. After sintering at 1500 oC for 24 h, the densities of(Nd1–xCex)2Zr2O7(x=0–1) bodies were in the range of 5.97–6.30 g/cm3(95%–98% of the theroretical values). The particle sizes were 1 μm~10 μm, and which were decreased with increasing x value. When x=0–0.4, the samples were in pyrochlore form, when x=0.5–0.8, the samples were in fluorite form, when x=0.9, the sample exhibited a small amount of tetragonal phase in addition to fluorite phase. After sintering at 1550 oC for 48 h, the simulating waste form Nd2Zr2O7–SrZrO3 was chemical compatibility and had a relative density higher than 95%.