Preparation And Chemical Durability Of Ce_1-xEu_xPO₄ Monazite Ceramic Waste-forms

Lanthanide europium( Eu3 +) was used to simulate trivalent minor actinide americium( Am3 +), Ce1-xEuxPO4(x=0-1) monazite solid solutions were prepared by the solid state reaction method, and hot-pressing of Ce0.5Eu0.5PO4 monazite ceramic waste-forms were performed using Ce2(C2O4)3·10H2O, Eu2O3 and NH4H2PO4 as the starting materials. The law of immobilization of simulated minor actinide europium in monazite-(Ce) and the hot-pressing technology were studied by means of XRD, FESEM, EDS and so on. The preparation technological parameters of Ce1-xEuxPO4(x=0-1) solid solutions and ceramic waste-forms were optimized. the phase composition, density,microstructure and morphology of them were investigated as well. The results show that at normal pressure,the monazite-type Eu PO4 and Ce PO4 phases coexist between 900 and 1250 oC. The optimal preparation parameter of Ce1-xEuxPO4(x=0-1) solid solutions prepared by normal pressure is 1300 oC for 4 h. The synthesis temperature of Ce1-xEuxPO4(x=0-1) solid solutions at the pressure of 30 MPa by hot-pressing is 1000 oC, which is lower about 300 oC than that of normal pressure. With the increase of Eu content, the unit cell parameters a, b, c and volume V of Ce1-xEuxPO4(x=0-1) solid solutions almost show a linear decrease, except the linear increase is observed in the ? angle. The optimal preparation parameter of Ce0.5Eu0.5PO4 ceramic waste-form prepared byd hot-pressing is 1150 oC for 2 h. The Ce0.5Eu0.5PO4 ceramic waste-form is well densified with high relative densities of 98% and the grains of all the Ce0.5Eu0.5PO4 ceramics are in tight compaction.The chemical durability of Ce and Eu in Ce0.5Eu0.5PO4 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 Eu in the Ce0.5Eu0.5PO4 monazite ceramic waste-form prepared by hot-pressing process both show excellent chemical durability and low normalized elemental leach rate. The chemical durability of Ce is slightly better than that of Eu. After 42 days, the normalized elemental leach rate of Ce and Eu were fairly constant in a lower value 3.91×10-5g·m-2·d-1 and 6.21×10-4g·m-2·d-1, respectively. The p H values have influence on the immobilization for Ce and Eu in the Ce0.5Eu0.5PO4 monazite ceramic waste-forms and the influence of p H values to Eu is relatively obvious. After 42 days, for p H=5-9, the normalized elemental leach rate of Ce ranged from 1.32×10-5 to 3.97×10-5g·m-2·d-1 and Eu ranged from 7.27×10-5 to 6.21×10-4 g·m-2·d-1. The chemical stability in weak alkaline solution is better than in weak acid aqueous solution.