Computational Simulation Of The Structure And Properties Of Nuclear Waste Immobilization And Nuclear Radiation Detection Materials

The development of nuclear science and technology has brought great benefits to human society.In today's world,whether in the military or nuclear energy industry,or even in industry and medicine,the use of radioactive materials is very extensive,but the production and use of radioisotopes inevitably causes some direct or potential harm.Both nuclear science and technology and public safety have put forward higher and higher requirements for nuclear safety.The closely related research of nuclear waste immobilization and nuclear radiation detection materials has also become a hot topic in the scientific community.Based on the density functional theory,we have conducted a series of studies on the structure and properties of nuclear waste immobilization and nuclear radiation detection materials that have been closely related to radiation protection in the scientific community in recent years.The main contents are shown as follows:1.The solubility of Pu in Gd₂Zr₂O₇ has been investigated by the density functional theory Hubbard U correction.It is found that the formation of PuGdZr₂O₇,Gd₂PuZrO₇and Gd₂Pu_1.5Zr_0.5O₇ are exothermic,whereas the formation of Pu_0.5Gd_1.5Zr₂O₇,Pu_1.5Gd_0.5Zr₂O₇ and Gd₂Pu_0.5Zr_1.5O₇ are endothermic.The calculations show that both the Gd and Zr lattice sites can be substituted by the Pu,and Pu immobilization at Zr site enhanced the radiation resistance of Gd₂Zr₂O₇.The site preference of Pu in Gd₂Zr₂O₇ is found to be dependent on the chemical environment,i.e.,Pu prefers to substitute for Gd-site under Gd-rich and O₂-rich conditions and for Zr-site under Zr-rich and O₂-rich conditions.2.A systematic density functional theory study is performed to investigate the mechanical stability,elastic moduli,Debye temperature,thermal conductivity and electronic structures of Gd_2-yTh_yZr₂O₇ and Gd₂Zr_2-yTh_yO₇.All the Th-doped Gd₂Zr₂O₇compositions are found to be structurally and mechanically stable.As compared with the pure Gd₂Zr₂O₇,Th incorporation into both Gd-site and Zr-site results in generally better ductility,lower Debye temperature,and reduced thermal conductivity.The reduction in thermal conductivity can be as high as 25-32%,depending on the content of the Th substituent.Our calculations suggest that Th-doped Gd₂Zr₂O₇,especially Gd₂Zr_2-yTh_yO₇,exhibits better mechanical and thermal properties that are beneficial to its application in extreme conditions than the pure state.3.The structural,mechanical,thermal,and electronic properties of a series of Gd-site and Zr-site substituted Gd₂Zr₂O₇ pyrochlores have been investigated by first-principles calculations.Substitution of Gd³⁺with smaller and heavier Yb³⁺introduces strong phonon scattering effects and results in Gd_1.5Yb_0.5Zr₂O₇ exhibiting the lowest thermal conductivity.Substitution of Gd³⁺with larger and lighter La³⁺does not give as much reduction in the thermal conductivity as in Yb³⁺doped Gd₂Zr₂O₇.As for Ti⁴⁺,Hf⁴⁺and Ce⁴⁺substitution for Zr⁴⁺site in Gd₂Zr₂O₇,slight changes in thermo-physical properties are observed for Gd₂Zr_2-yTi_yO₇ and Gd₂Zr_2-y-y Hf_yO₇ pyrochlores,while the substitution of Ce⁴⁺for Zr⁴⁺site results in significantly smaller Young's modulus,better ductility,smaller Debye temperature,and lower thermal conductivity.With the increasing Ce content,the Young's modulus subsequently decreases by 22.2-59.9 GPa.The thermal conductivity of Gd₂Zr₂O₇ is significantly reduced by 21%with complete Ce doping at the Zr-site based on the calculation model.This is mainly caused by the fact that larger guest ions incorporation weakens the bonds and enhances the phonon scattering in the host oxides and thus effectively lowers the thermal conductivity.4.To predict how the bandgap changes of CdS/ZnS heterostructures with the composition,density functional theory calculations are conducted.All the heterostructures are found to be energetically and mechanically stable.The band gaps of CdS and ZnS are reduced by up to 14.5%and 43.3%in the heterostructures,respectively.The content of the compositions in heterostructures plays a vital role in tuning the band gap and conduction band edge level.With the increasing number of CdS layers,the band gap first decreases and reaches a minimum value at?CdS?₅/?ZnS?₅,and then increases slightly.As a result,the?CdS?_m/?ZnS?_n?m?1,m+n=10,or?10%of CdS?heterostructures attain band gaps in the range of 2.06-2.72 eV.Our findings indicate that integration of CdS and ZnS semiconductors into layered heterostructures is an effective strategy for band gap engineering.5.Using hybrid density functional theory method,we demonstrate that Ag dopant can affect the structure,energy and electron properties of CdSe nanocrystals?NCs?significantly.It is found that Ag atoms tend to relax inwards CdSe NCs regardless of the doping position.However,the electronic structures are affected greatly by the Ag dopant location.Incorporation of Ag into the core region leads to metallic-like electronic characteristics,while NCs with Ag doped at surface exhibit a semiconducting behavior.In addition,electrons will be transferred from Ag impurities to CdSe nanocrystals.This study also found that increasing the number of Ag impurities will weaken the stability of the nanocrystals.In CdSe nanocrystals smaller than 2 nm,the number of Ag atoms should be less than 3.The present work provides the structural information of Ag substitution and explores the underlying mechanisms of how Ag dopants influence the structural and electronic properties of CdSe NCs,which may have important implications for the application of CdSe NCs in nuclear radiation detection devices.6.The effects of Cu impurities on the structure,electronic properties and Cu oxidation state of CdSe nanocrystals are studied based on the hybrid density functional theory method.It is found that incorporation of Cu dopants into the surface of magic sized Cd₃₃Se₃₃ NCs leads to non-magnetic Cu 3d orbitals distribution and Cu⁺¹ oxidation state,while doping Cu atoms in the core region of NCs can lead to both Cu⁺¹ and Cu⁺22 oxidation states,depending on the different distribution positions of Cu 3d orbitals caused by the local environment of Cu atoms in nanocrystals.Here,the effects of reducing agent such as sulfur(S^2-)on the oxidation state of Cu ions in the doped NCs are examined as well.The present results enable us to use the science-based principles to control the doping of Cu impurity in CdSe NCs and achieve special physical properties for their applications in high-efficiency electronic devices.The methods used to resolve the electronic properties of Cu-doped CdSe NCs can be extended to other II-VI semiconductor NCs by incorporating a transition-metal ion with a variable valence.