Preparation And Chemical Durability Of Ce_0.5Pr_0.5PO₄ Ceramic Waste-forms Under THMC

Lanthanide praseodymium（Pr3+） was used to simulate trivalent minor actinide amer icium（ Am3 +）, Ce_0.5Pr_0.5PO₄ mo nazite solid so lutions were prepared by the solid state reaction method, and normal pressure sintering and hot-pressing of Ce_0.5Pr_0.5PO₄ monazite ceramic waste-forms were performed using Pr6O11, NH4H2PO4 and Ce2（C2O4）3·10H2O as the starting materials. The law of immobilization of simulated minor actinide praseodymium in mo nazite-（Ce） and the normal pressure sintering and hot-pressing technology were studied by means of XRD, FESEM, EDS and so on. The preparation technological parameters of Ce_0.5Pr_0.5PO₄ ceramic waste-forms were optimized. The phase composition, density, microstructure and morphology of them were investigated as well. The results showed that the optimal preparation parameter of Ce_0.5Pr_0.5PO₄ solid solutions prepared by normal pressure was 1500 oC for 4 h. The Ce_0.5Pr_0.5PO₄ ceramic sample exhibited the round-shaped grains and had few pores, resulting in a relative density of 94.05%, the average of grain size was about 2μ m. The optimal preparation parameter of Ce_0.5Pr_0.5PO₄ solid solutions prepared by hot-pressing was 1150 oC for 2 h. The Ce_0.5Pr_0.5PO₄ ceramic waste-form was well densified with high relative densities of 99.67% and the grains of all the Ce_0.5Pr_0.5PO₄ ceramics were in tight compaction. The chemical durability of Ce and Pr in Ce_0.5Pr_0.5PO₄ monazite ceramic waste-forms sintered by normal pressure and hot-pressing was researched by MCC static leaching using the hydrothermal reaction as the leaching device for simulating the environment of geologic disposal. The effects of p H values and temperature on the chemical durability of the ceramics were investigated. In addition, the effect of sintering process on the chemical durability of the ceramics was also studied. These results demonstrated that the chemical durability of hot-pressing ceramic was slightly better than the normal pressure sintering ceramic. After 42 days, the LRPr and LRCe of the hot-pressing sample remain below 3.6 ×10-7 g·m-2·d-1 and 7.6×10-7 g·m-2·d-1 and the LRPr and LRCe of the normal pressure sintering sample remain below 4.1 ×10-6 g·m-2·d-1and 4.2×10-6 g·m-2·d-1, respectively. After 42 days, for p H=5-11, the Ce_0.5Pr_0.5PO₄ ceramics waste-form prepared by hot-pressing process both show excell chemical durability, the LRCe range from 7.6×10-7¹.5×10-5g·m-2·d-1, LRPr range from 3.6×10-7³.8×10-5g·m-2·d-1. The change of p H value had significant effects on chemical durability of sample. The LRPr and LRCe of the Ce_0.5Pr_0.5PO₄ ceramics decrease with increasing p H at acid conditions, reach a minimum in p H = 7 leachate and increase slightly with increasing p H at alkaline conditions. After 42 days, the LRPr and LRCe reached the highest values（10-3 g·m-2·d-1） when p H = 3, while the LRPr and LRCe had the lowest values（10-7 g·m-2·d-1） when p H = 7. The LRPr and LRCe for p H = 9-11 were lower than 10-6 g·m-2·d-1. The LRPr and LRCe of those samples had no obviously increase when the leaching temperature increases from 90 to 200 oC.Hardness of the ceramics samples were measured using a Vickers hardness testing machine. The results showed that hardness of the HPS ceramics were higher than that of the NPS samples. The hardness reach the highest values（7.5 GPa） for HPS sample sintered at 1150 oC for 2 h, while the hardness have the maximum value（6.2 GPa） for NPS sample sintered at 1500 oC for 4 h. The results showed that the hardness of samples was closely consistent with the relative density, phase composition. The hardness of HPS sample leached in deionised water was slightly improved, while the hardness of NPS sample was significantly decreased.