First Principle Study On Pyrochlore As High-Level Nuclear Waste Immobilization Matrix

The rapid development of nuclear energy has met people’s growing demand for energy systems and also alleviated the ecological problems caused by the over-exploitation of fossil fuels.However,the proper disposal of high-level radioactive waste faces huge challenges,which restrict the further development of fission energy utilization.Pyrochlore is considered as a potential immobilization matrix for high-level waste due to its good mechanical properties,excellent chemical stability,excellent radiation resistance,and low leaching rate.Gd₂Zr₂O₇ pyrochlore is considered as a potential immobilization matrix for high-level waste because of its excellent physical and chemical properties.The helium accumulation in the pyrochlore ascribed to alpha decay has important effects on its performance during service.This work adopts first-principle method to investigate the structural evolution,discrepancy in mechanical properties and radiation resistance before and after He doping in Gd₂Zr₂O₇ pyrochlore.By calculating each configuration of pure and He-doped Gd₂Zr₂O₇,it is found that the unit cell expands and distorts due to the doping helium.A single helium atom prefers to occupy the octahedral interstitial site and Gd vacancy.The variation of elastic constants and elastic modulus implies that helium doping has no obvious damage to mechanical stability.He-Gd₂Zr₂O₇ systems have lower antisite defects formation energy and higher vacancy defects formation energy,indicating better radiation resistance.Moreover,the electronic structure analysis shows that<Zr-O>bonds have a greater influence on radiation resistance than<Gd-O>bonds.This study demonstrates that Gd₂Zr₂O₇ pyrochlore would exhibit excellent stability in long-term service.High-entropy pyrochlore has become a promising immobilization matrix due to its ability to immobilize multiple nuclides especially for high-level radioactive waste.In this work,the synthetic possibility of the high-entropy pyrochlore sample(Lu_0.25Y_0.25Eu_0.25Gd_0.25)₂Ti₂O₇（HTP-1）is analyzed by using the empirical range of pyrochlore radius ratio and size disorder,and the first-principle calculation method is used to analyze the conditions for its synthesis.By calculating the Gibbs free energy of possible chemical reactions,it is predicted that the reaction driving force for the synthesis of HTP-1 is greater than that for the synthesis of single-component pyrochlore when the temperature exceeds 1225 K.The characterization result of XRD shows that HTP-1 sample is successfully prepared by high-temperature solid-phase method under predicted conditions.The lattice distortion caused by the high entropy effect can be clearly observed in the optimized structure,especially the severe distortion of the BO₆octahedron.The analysis results of x _O48f and anti-defect formation energy suggest that the radiation resistance of HTP-1 is between Eu₂Ti₂O₇ and Y₂Ti₂O₇,which is consistent with the results of the in-situ TEM investigation.