Study On Direct Immobilization Of Simulated TRPO Waste By Gadolinium Zirconate Pyrochlore

In order to properly handle the end-of-process waste from the TRPO（Trialkyl Phosphine Oxide）process（namely TRPO waste）,this study conducted immobilization treatment of TRPO waste.TRPO waste consists of residual transuranic elements or fission products from the TRPO process,which are not suitable for further separation and become complicated by various factors before and after the extraction-re-extraction process.Inspired by the diverse and similar characteristics between TRPO waste and high-entropy oxides,the synthesis method of high-entropy oxides was borrowed to conduct fusion experiments on simulated TRPO-ANP waste（TPRO-ANP waste contains 16 waste components,ANP means that the main waste elements are Am,NP,and Pu.）.The results showed that most of the waste components could be fused and further consideration was given to introducing stable sub-lattices to constrain the unfused waste components.Based on previous research and reports,pre-synthesized Gadolinium zirconium pyrochlore（Gd₂Zr₂O₇,GZO）was directly selected as the immobilization matrix,and fusion was conducted again on simulated TRPO-ANP waste.The results showed that a regular and dense waste form could be obtained after the mixture of waste and matrix were sintered at 1500°C for 72 hours（h）,48 h,or 24 h.The characterization results show that under the three selected sintering durations,GZO matrix can immobilize simulated TRPO-ANP waste,and the immobilization limit is above 40wt.%.Within the selected doping range,the waste form has good mechanical properties,thermal stability,density,and chemical durability.Moreover,the longer the sintering duration,the higher the overall stability of the sample.The analysis results show that the waste exists in the main crystal phases of the waste form,such as pyrochlore and fluorite,as well as in secondary crystal phases,such as perovskite and palladium alloy.Doping changes the crystal lattice configuration and melting point of the waste form,thereby affecting its properties and performance,as shown below:（1）With the doping of waste,the waste form changes from ordered pyrochlore structure to defective fluorite structure and twisted pyrochlore structure,which makes its thermal stability and mechanical properties decrease and its chemical durability increase.TRPO-ANP waste can participate in the construction of pyrochlore structure within a certain doping range,which also depends on the relative content of rare earth metal oxides and transition metal oxides in the waste.（2）During the doping process,the introduction of low melting point waste leads to a decrease in the overall melting point of the sample,while the external heat source temperature remains constant,which is equivalent to raising the solidification reaction temperature.Therefore,the surface of the doped sample is enhanced by heat,the migration of grain boundaries accelerates,and abnormal grain growth occurs.The surface of the sample becomes denser with doping,which is the main reason why samples with higher doping content have lower leaching rates at the same sintering temperature.This study has validated the feasibility of directly immobilizing simulated TRPO-ANP waste using GZO,providing some data support for the research on handling TRPO waste.