Preparation And Chemical Durability Of Pr-doped Monazite-(Ce) Ceramic Waste-forms

Lanthanide praseodymium(Pr3+) was used to simulate trivalent minor actinide neptunium(Np3+), Ce1-xPrxPO4(x=0-1) monazite solid solutions were prepared by the solid state reaction method, and hot-pressing of Ce0.5Pr0.5PO4 monazite ceramic waste-forms were performed using Ce2(C2O4)3?10H2O, Pr6O11 and NH4H2PO4 as the starting materials. The law of immobilization of Ce PO4 and Pr PO4 with each other was studied by means of TG-DSC, XRD, FESEM, EDS and so on. The preparation technological parameters of Ce1-xPrxPO4(x=0-1) solid solutions and Ce0.5Pr0.5PO4 ceramic waste-forms were optimized. The phase composition, density, microstructure and morphology of them were investigated as well. The results show that the optimal preparation parameter of Ce1-xPrxPO4(x=0-1) solid solutions is prepared by two grinding/heating cycles after secondary calcining at 1200 oC for 6 h; With the increase of Pr content, the unit cellparameters a, b, c and volume V of Ce1-xPrxPO4(x=0-1) solid solutions almost show a linear decrease, except the linear increase is observed in the ? angle. The optimal sintering temperatureand holding time of Ce0.5Pr0.5PO4 ceramic waste-form prepared by hot-pressing are 1150 oC and 2h, respectively. The Ce0.5Pr0.5PO4 ceramic waste-form is well densified with high relative densities of 99%. The phase is Ce0.5Pr0.5PO4 solid solution. The grains of all the Ce0.5Pr0.5PO4 ceramics are in tight compaction and the grain boundaries are obscure.The chemical durability of Ce0.5Pr0.5PO4 monazite ceramic waste-forms sintered by hot-pressing was researched by MCC-1 static leaching using the hydrothermal reaction as the leaching device for simulating the environment of geologic disposal. The results show that Ce and Pr in the Ce0.5Pr0.5PO4 monazite ceramic waste-form prepared by hot-pressing process both show excellent chemical durability. After 42 days, the normalized elemental leach rate of Ce and Pr were fairly constant in a lower value below 9.93×10-6 g·m-2·d-1 and 7.74×10-5 g·m-2·d-1, respectively. The leach rate of Ce and Pr were in a low value below 1.90×10-12 g·m-1 and 1.48 × 10-11 g·m-1, respectively. The p H values have influence on the immobilization for Ce and Pr in the Ce0.5Pr0.5PO4 monazite ceramic waste-forms, the chemical stability in neutral aqueous solution and weak alkaline solution is better than in weak acid aqueous so lution.