Theoretical Studies On Migration Mechanism Of Fission Products And Alloying Elements In Iron-based Alloys

Molten salt reactor?MSR?is one of the six candidates of the Generation?advanced nuclear reactor which uses molten salts as coolant and the fuel being melted directly in molten salts.The structural materials of MSR would directly contact with high-temperature molten salts in service environment,which exhibit a severe challenge to the long-term service of alloying structural materials.On the one hand,it is reported that corrosion would occur due to the dissolution of active element Cr in alloy long-term corrosion behavior of structural materials in high-temperature molten salts,in which element Cr in the alloy would migrate into the alloy-salt interface,and then dissolute into the molten salt through chemical and electrochemical processes such as electron transfer.The selective loss of Cr would lead surface pitting as well as grain boundary corrosion,which lead to the deterioration of mechanical properties of the alloy.On the other hand,a large number of fission products including ⁹⁵Nb,⁹⁹Mo,⁹⁵Tc,¹⁰³Ru,¹²⁵Sb and ¹³²Te are produced in fuel salts due to the nuclear fission reaction.These fission products would deposit on the surface of structural materials and then migrate into the structural materials,resulting in the change of microstructure and even the decrease of performance of structural materials.Especially the fission product Te has been proved to be one of the main reasons for the grain boundary embrittlement.The study on atomic migration mechanism of alloying structural materials is of great importance on understanding the behavior of materials in service environment,such as corrosion and radiation effects.In this study,by using the first-principles method combining with the newly developed high-throughput calculation method,the migration behavior of various fission products?Te,Nb,Mo,Tc,Ru and Sb?and alloying elements?Cr,Al,Mn,Fe,Co,Ni and Cu?in iron-based alloys,one of the candidate structural materials of MSR,is systematically explored at the atomic scale.The results provide theoretical support for the application of iron-based alloys in MSR.The main research contents and conclusions are as follows:1.The diffusion behavior of various common alloying elements and fission products in iron are studied in this study.In order to achieve quick calculations on structural optimizations,diffusion path calculations and energy analysis,and meet the needs of fast task submission and convenient inspection of VASP calculation,a VASP-assistant program VaspCZ is designed and developed.The program has been open-source in GitHub,which provides command-line user interface and underlying Python library,greatly improves the efficiency in material calculations.2.The stability and migration behavior of fission product Te in bcc iron are studied and compared with common solution elements such as Cr,Al,Mn,Co,Ni,Cu,Mo,Nb,Ti,V and W.It is found that due to the large atomic size,Te is energetically favorable at substitutional site with formation energy of 1.03 eV,which is much lower than that at the interstitial sites?8.10⁹.12 eV?.Strong attraction between Te and mono-vacancy is found,and Te tends to cause vacancy to gather nearby.The net diffusion behavior of Te-V₂ clusters shows that the further introduction of vacancy does not affect a lot on the migration barrier of Te in bcc iron.The diffusion barriers of Fe atom in Te-V pair and Te-V₂ clusters are greatly affected,which become more difficult.For the diffusion behavior at limited temperature,the pre-exponential factor of Te migration is similar to that of alloying elements,while the activation energy Q of Te is relatively lower,which can be attributed to the strong attractive interaction between Te and vacancy.The calculation results of diffusion coefficients based on Arrhenius formula shows that Te has a high diffusion coefficient in the temperature range of 700-1300 K.At 650?,the diffusion coefficient of Te is approximately four orders of magnitude higher than that of iron self-diffusion,as well as Nb,Ti,Al and Mo.3.The microscopic and macroscopic migration properties of various fission products?Te,Nb,Mo,Tc,Ru and Sb?and alloying elements?Cr,Al,Mn,Co,Ni and Cu?in fcc-iron are investigated.The results show that most of the fission products and alloying elements have negative binding enthalpy with vacancy,indicating the attractive interaction.With relatively lower binding enthalpy with vacancy,the fission products?Te and Sb?shows higher affinities with monovacancy when compared to the alloying elements such as Cr,Co,Mn,Ni et al..Further study shows that there are strong attractive interactions of Te?and Sb?with vacancy,but it is not enough to lead to the accumulation of vacancy nearby.It shows that the diffusion of iron would be hindered more or less by the fission products considered here,whereas the diffusion of iron does not be affected by the typical alloying elements such as Cr.The calculation results of the macro migration properties of different elements show that the pre-exponential factors of all elements are almost in the same order of magnitude,the diffusion coefficients are mainly affected by the activation energy.Among all elements studied,Nb,Mo,Sb and Te exhibit the lowest the activation energies,in which the low activation energies of Nb and Mo arise from both of the low solute-vacancy binding enthalpy and the low migration barrier,while the low activation energies of Sb and Te only arise from the very low binding enthalpy.The order of diffusion coefficient at finite temperature is as follows,Nb>Sb>Te>Tc>Mo>Ru?Cr>Ni>Fe.Most fission products exhibit higher diffusion abilities when compared to that of alloy elements in fcc iron.In summary,under reactor environment,the synergistic effect of defects,fission products as well as alloy elements,such as diffusion,aggregation,etc.,would significantly affect the microstructural evolutions and performance of materials.It is of vital importance to clarify the interaction between vacancy and Cr,Te for understanding the dynamic change of material structure under irradiation damage.In this thesis,the migration behavior of fission products and alloying elements in iron is systematically studied.The results will be helpful to understand the fundamental behavior and migration properties of those elements in ferritic,martensitic and austenitic stainless steel,and further to provide theoretical basis for the application of iron-based alloys in molten salt reactor.In addition,the VaspCZ program developed in this study greatly improves the efficiency of the calculation in this thesis,and will be expected to bring convenience to researchers especially for whom major in theoretical studies.