First-principles Study On Properties And Hydrogen Atoms Diffusion Of Zirconium-hydrogen System

With the increase in the development and utilization of new energy sources,the demand for electricity continues to grow.The safe use and technological innovation of nuclear energy are particularly urgent.Compared with traditional fuels,proper use of nuclear energy can greatly reduce environmental pollution.Zirconium alloys are commonly used as cladding materials for nuclear fuels due to the excellent corrosion resistance.However,excessive hydrogen concentration generates hydride precipitation,reduces the integrity of fuel cladding and affects the service life.When the situation is serious,an explosion nuclear leakage accident may even occur.Research on improving the performance of cladding materials has become important,and experiments require comprehensive simulation calculations to provide theoretical guidance.The first principles based on density functional theory and the nudged elastic band method of transition state search were used.The matrix phaseα-Zr and the hydrogen content in different hydridesα-Zr,ζ-ZrH_0.5,γ-ZrH,δ-ZrH_1.5 andε-ZrH₂ mechanical properties,ideal tensile strength,electronic properties and thermodynamic properties were calculated.For the first time,we established the diffusion model of hydrogen（H）in the body centered tetragonal（bct）structure ofε-ZrH₂,in which the interstitial position of the tetragonal structure is more complicated than that of the cubic structure.The model takes into account the first nearest neighbor of all types of self-interstitial atoms（SIA）diffusion.Finally,the influence of pressure on the structure ofε-ZrH₂ and the diffusion mobility of the optimal diffusion path was discussed.The effect of solute elements doping on bonding and diffusion of hydrogen atoms also were studied.Atomic scale theoretical basis for future experiments or phenomena interpretation of zirconium hydrogen systems is provided by our work.The constructed diffusion model provides an image thinking for future research on zirconium hydrogen diffusion.It is found that the four hydrides show good ductility.The anisotropy of the hydrides is greater than that of the matrixα-Zr,which is likely responsible for that the zirconium hydrogen precipitates reduce the overall fracture toughness.Furthermore,there are many metal bonds existed in the hydrides and Zr-H bonds are covalent.ζ-ZrH_0.5 andδ-ZrH_1.5 have the strongest tensile strength in the[100]direction.However,γ-ZrH andε-ZrH₂ can withstand greater tensile stress in the[111]direction.When the strain increases,the bond becomes weak.And the stable system gradually changes to unstable.The average atomic volume V of the five phases decrease with the increase of the hydrogen content,and the bulk modulus B increase accordingly.The increase in hydrogen content can enhance the Debye temperatureΘ_D of the phase and the strength of the covalent bond.However,covalent bonds can affect the brittleness of materials.Their coefficient of linear thermal expansionαincreases with the increase of the hydrogen content,while the C_V decreases.C_P decrease with the increase of hydrogen content at 0～500K,and when the temperature increases from 700K to 1200K,the order is exactly the opposite.ε-ZrH₂ is bct structure.There are two types of octahedral and tetrahedral gaps,and they are stable.Hydrogen atoms preferentially occupy tetrahedral interstitial positions and tend to diffuse along the c-axis.The lowest barrier diffusion path of hydrogen atoms inε-ZrH₂ is T→T2→T1,which are all tetrahedral gaps.The pressure changes the structure to tetragonal distortion,which reduces the stability of the crystal structure,but the tetrahedral gap structures are still stable.The diffusion coefficient is greatly affected by high temperature.Pressure increase the energy barrier of diffusion and decrease the diffusion coefficient.This trend can decrease as the pressure increases.The doped solute atoms Y and Gd inε-Zr₁₅H₃₁X showed obvious repulsion.Y and Gd are weakly involved in the bonding of the structure,which makes the system unstable.The solute elements Cr,Fe,Nb and Si increase the diffusion coefficient,while Ti,Ni,Y,Pt and Gd decrease.The best diffusion-inhibiting doping element is Pt,the best diffusion-promoting metal elements are Cr and Fe,and the non-metallic element is Si.