First-principle Study On Ion Irradiation Performance Of Ceramics Matrix Materials For Solidifying High-level Radioactive Waste

The continuous development of nuclear energy has led to the production of a large amount of nuclear waste.Among them,the high-level nuclear waste's safety disposal problem has been the focus of attention.At present,the main method to deal with nuclear waste is to deep geologically bury after solidification the high-radioactive nuclear waste.Therefore,the materials' properties of radiation resistance are an important basis for nuclear waste form materials.Due to its excellent physical properties and its inclusiveness to high-level radionuclides,apatite?M,o?AO₄?₆X₂?and Titanate fluorite structure derivative?Ln₂Ti₂O₇ and Ln₂TiO₅?has been extensively studied as a potential solid waste material for nuclear waste.Apatite has good thermodynamic stability and is one of the most abundant natural minerals on the earth's surface.The most common apatite is fluoroapatite(Ca₁₀?PO₄?₆F₂),chloroapatite(Ca₁₀?PO₄?)₆C₁₂)and hydroxyapatite(Ca₁₀o?PO₄?₆?OH?₂).In this paper,the effects of different anions?F^-,Cl^-,OH^-?on the properties of radiation resistance of apatite structure ceramic materials were studied by experiments and first-principles calculations.Titanate fluorite structure derivative Lu₂Ti₂O₇ and Lu₂TiO₅ are cubic structures.Previous studies have studied the similarities and differences between Lu₂Ti₂O₇ and Lu₂TiO₅.This paper explains the difference in irradiation performance between these two kinds of ceramic materials under irradiation of Kr ions by first-principles calculation.The apatite sample used in this paper is Ca₁₀?PO₄?₆X₂.We irradiated the apatite ceramic material with 800 keV Kr²⁺ at different doses at room temperature.The irradiated samples were characterized by GIXRD and TEM.we found that these three kinds of apatite have all undergone an amorphous transformation under Kr²⁺irradiation,but there are some differences in the properties of resistance to irradiation induced amorphous transformation.Specifically,fluorophosphatE exhibit the strongest resistance to irradiation induced amorphization.while the hydroxyapatite is easiest to undergo an amorphous transformation under irradiation.We calculated the charge density map and the Mulliken population of these three kinds of apatite in the basis of First principle calcultion.By analyzing their charge density maps and the Mulliken population,we found that the differences in chemical bonds among these three kinds of apatite are mainly due to the difference between the Ca2-X bonds,in which the ionicity of Ca2-F is the strongest,and the Ca2-OH bond has begun to exhibit a certain covalent property.Since the covalent bond in the material makes the defects generated after the irradiation difficult to migrate.The defects were difficult to move or recombine after the irradiation,which result in the accumulation of defects and then make the material undergo an amorphous transformation.Among the three kinds of apatite,the ionic bond of Ca2-OH in hydroxyapatite is the weakest,so the resistance of irradiation induced amorphization of hydroxyapatite is the worst.While the Ca2-F bond of fluoroapatite is the strongest,so fluoroapatite exhibit excellent resistance to irradiation induced amorphization.Meanwhile,we further calculated the migration energy of the X vacancies in the channels formed by Ca2 atoms.The calculated migrate energy of F vacancies,Cl vacancies,and OH vacancies in the channel structures are 0.98 eV,1.14 eV and 1.19 eV,respectively.This means that the F vacancy is the easiest to migrate in the channel,and the OH vacancy is the most difficult to migrate.This result verifies the idea that covalent bonds make defects difficult to migrate.Finally,we also calculated the lattice constant change of the pyrochlore structure Lu₂Ti₂O₇ with increasing anti-site defects.The calculation results show that both Lu₂Ti₂O₇ and Lu₂TiO₅ were undergoing lattice swelling under irradiation,but the lattice swelling percentage of Lu₂Ti₂O₇ is significantly larger than that of Lu₂TiO₅.The calculations show that as the number of anti-sites in the unit cell increases,the lattice swelling of Lu₂Ti₂O₇ gradually increases.The calculated curve of the lattice swelling of Lu₂Ti₂O₇ with the anti-sites is consistent with the experiment result,so we believe that the anti-sites generated during irradiation is an important factor leading to the swelling of Lu₂Ti₂O₇,However,Lu₂TiO₅ is originally a cation disorder,and its swelling has no contribution of anti-sites,so the swelling percentage of Lu₂Ti0₅ is much lower than Lu₂Ti₂O₇.