Phase Stability Studies Of Compounds In Several Uranium-based Fuels Based On Clustering And First Principles Calculations

Recently,with the development of accident-tolerant fuels,many efforts have been devoted to reliability of advanced metallic nuclear fuels and structural materials under normal and off-normal conditions.As for nuclear fuel,it not only needs to develop advanced nuclear fuels,but also must answer the phase stability of known nuclear fuel materials at high temperature,the stability under the irradiation and its compatibility with cladding materials.This paper investigated the metallic nuclear fuels,centering the phase stability.In this paper,the phase stability of typical compounds in U-Mo,U-Zr,U-Mo-Al and U-N systems have been discussed,giving in-depth insights on their intrinsic properties using cluster formulae and first-principles calculations;the linear relationship between Cauchy pressure over bulk modulus(CP/B)and the shear modulus over bulk modulus(G/B)was uncovered by collecting more than seven hundred of elastic constants of diversified crystal systems.The previous proposed ductile-to-brittle transition criterion suitable for cubic crystal system was improved and used to discuss the phase stability and the phase stability under irradiation for a given materials.The main contents and results were listed as following:(1)The phase stability of compound in U-Mo binary system.The crystal structures of compounds in U-Mo system are analyzed using aforementioned cluster formulae.It revealed that the principal clusters in y-U phase and U2Mo phase were identical,i.e.rhombic dodeca-hedron cluster,thus the phase transformation from y phase to U2Mo phase can easiliy occurs.The crytal structural stability,energetic stability,mechanical stability,dynamic stability and electronic stability of I4/mmm-U2Mo were discussed using first-principles calculations.The I4/mmm phase was a metastable phase,and high-throughput structure search gave several possible ground-state structures with hexagonal or orthormbic structure at low temperature,which satisied the mechanical,dynamic stability criteria.Their stability stems from the lower occupation of electrons at EF,as compared to I4/mmm phase,which may be the main reason underlying the possible stress-induced structural phase transformation.(2)The phase stability of compound in U-Mo-Al ternary system.The crystal structures of compounds in U-Mo-Al system are analyzed using cluster fromulae.It revealed that there were two principal clusters,i.e.[Mo-Al12]and[U-Al16],in UMo2Al20.The former one is icohedron-like cluster,which is similar to that in amorphous alloys.In this sense,this compound may become amorphous easily,evidenced by previous experimental findings.First-principles calculations demonstrated that this compound satisfied the mechanical and dynamic stability,exhibiting ionic bonding character.Its phase stability originated in the strong hybridization of Al-p and Mo-d electrons.The Mo-Al bonds and Al-Al bonds in[Mo-Al12]clusters were bonding states,whereas the U-Al bonds and Al-Al bonds in[U-Al16]clusters exihibits a little bonding and antibonding states.(3)The phase stability of compounds in U-Zr binary system.The crystal structures of compounds in U-Zr system are analyzed using cluster formula.It revealed that there exist Zr-centered clusters(i.e.[Zr-Zr8U6])with coordination number of fourteen in the ? phase in the U-Zr system,similar to the cuboctahedral cluster with coordination number of twelve in a-Zr phase.In this way,the chemical composition with wide range of Zr in 8 phase is deduced.The phase transformation from bcc ? phase to hexagonal ? phase can be deduced by tracking the bond length and angle changes in rhombic dodechedron cluster.The cluster proposed in this work was in agreement with earlier ab initio calculations,molecular dynamic simulations and positron annihilation spectroscopy(PAS)experiment.Our transmission electron microscope(TEM)observations confirm the existence of 8 phase in U-rich U-Zr alloys.These results reveal that the Zr-rich nanoclusters in U-rich U-Zr alloys proposed by PAS experiment originate from ? phase.For ? phase,a Zr-rich U-Zr alloy was prepared and its microstructure was explored using optical microscopy,scanning electron microscopy,TEM and X-ray photoelectron spectroscopy.(4)The phase stability of compounds in the U-N system.The crystal structures of compounds in U-N system are analyzed using cluster formulae.It revealed that the only difference in the five kinds of uranium nitrides was the differing occupations of N atoms in the cuboctehedron cluster formed by U atoms.These uranium nitrides can be seen as the cuboctehedral cluster formed by twelve U atoms with N atoms occupied either eight tetrahedral sites or six octahedral sites.Also,the phase transformations at certain temperature and pressure:from CaF2-type UN2 to Mn2O3-type U2N3,from Mn2O3-type U2N3 to NaCl-type UN and from NaCl-type UN to HgIn-type UN,were deduced by tracking bonds and angle changes of a simplified cluster[U-U6N6].The effect of O atoms on the phase stability of CaF2-type UN2 was discussed using first-principles calculations.It revealed that O atoms preferred to occupy the vacancy of N atoms,thereby resulting in the U-N-O ternary compounds.It demonstrated that some of these ternary compounds were stable from the thermodynamic point of view.(5)The ductile-to-brittle transition and its relation with phase stability and phase statbility under irradiation.The ductile-to-brittle transition criterion suitable for cubic crysta system was modified:the cauchy pressure was dimensionlessed by bulk modulus and the cauchy pressures of hexagonal,orthorhombic and tetragonal system was averaged and was made more general for hexagonal,orthorhombic and tetragonal systems,thus this cretirion can be used for more crystal systems.A universal linear relation between Cauchy pressure/bulk modulus(CP/B)and shear modulus/bulk modulus(G/B)was derived by summarizing over seven hundreds of elastic constants of materials with various crystal structures.The correlation between the Pugh criterion,Pettifor criterion and emipirical criterion revealed that the bonding character in a material can be reflected by the value of G/B.Importantly,this linear relation is robust for materials under temperature,pressure and with defects.In general,G/B of a material will gradually decreases with increasing pressure and temperature,while a significant transition of G/B against increasing temperature often occurs at a certain phase transition,such as magnetic transition,spin flipping,and structural phase transition and so on.It is suggested that a material with a value of G/B varying narrowly against temperature may exhibits high damage tolerance.The discussion of the response of a material to the deformation at the electronic scale revealed that charge may redistribution among three kinds of critical points in solids,however,the charge density at certain points did not change.These observations provide in-depth understanding of the flow of electronic density in solids under deformation.