| Silicon carbide(SiC)is used as an important structural material in advanced nuclear energy systems due to its stable chemical properties,low expansion rate,and low neutron absorption cross-section.The mature application of SiC in high-temperature gas-cooled reactors is used as a cladding material in TRISO fuel to prevent the overflow of metallic fission products.SiC is also an important first-wall candidate structural material in fusion reactors.However,whether in fission or fusion reactors,SiC will be irradiated by high flux neutrons,and the material will inevitably trigger a transmutation reaction under neutron bombardment to produce impurities such as H and He,which will lead to bubbles and cracks.Pd metal is often used as an important tritium storage material in nuclear applications due to its excellent hydrogen absorption and release characteristics.With the increase of tritium storage time,tritium will continue to decay into 3He.It can be found that both SiC and Pd need to consider the influence of H,He,and other atoms on their microstructure in nuclear energy applications.In addition,Pd is one of many metallic fission products in fission reactors,and Pd has been reported to corrode the SiC layer in TRISO fuel.Therefore,this paper also studies the synergistic effect among Pd,H,and He in the SiC layer of TRISO fuel.In this paper,density functional theory is used to investigate whether He atoms can aggregate and nucleate in 3C-SiC due to the self-trapping behavior.By analyzing the changes in the structure and formation energy of He clusters with the number of He atoms,the compact structure of He clusters is selected as the research object of this work.We found that the larger the cluster,the looser the structure at low temperatures.While He clusters tend to dissolve at high temperatures.Therefore,the self-trapping behavior of He atoms cannot occur in 3C-SiC whether at low or high temperatures.In order to explain the phenomenon of platelet bubbles appearing in SiC under He ion irradiation at room temperature,we selected the single vacancy with the highest content after irradiation as the research object based on the previous simulation results.By analyzing the dissolution energy of He atoms around the vacancy,we found that a single C vacancy can significantly reduce the dissolution energy of He atoms located at its third and fourth nearest neighbor site,while a single Si vacancy can only reduce the dissolution energy of He atoms within its second nearest neighbor site.Besides,both a single C and Si vacancy can provide He atoms with a low migration energy ring channel with a radius of about 0.2 nm.However,He atoms need to overcome a high potential barrier to diffuse from the low migration energy channel to a position far away from the vacancy.For this reason,we studied the coupling of two single vacancies and found that two C vacancies that are close to each other can connect their low migration energy channels,thereby realizing the long-distance migration of He atoms under low-temperature conditions.Combining with the characteristics of materials that the vacancies tend to form clusters during the irradiation process,we believe that a group of adjacent vacancies promote the longdistance migration of He atoms at room temperature.Interstitial atoms can be produced during preparation and irradiation in SiC,also these interstitial atoms may have many different metastable structures in the material.For this reason,we systematically studied a variety of possible self-interstitial structures in 3C-SiC and discovered a series of interstitial structures that have not been reported before.We obtain that the activation energies of neutral C and Si interstitial atoms for full-space rotation are 0.48 eV and 0.73 eV,respectively.Besides,the minimum energy barriers for long-distance migration of C and Si interstitial atoms are 0.71 eV and 0.73 eV,respectively,which are consistent with previous simulation results and the temperature range of defect annealing recovery.In addition,to compare the changes in the rotation and migration behavior of the defect structure under charge state,we selected the C2+and Si4+interstitial atoms,whose formation energies are lowest in a larger Fermi level region that closed to the valence band maximum,as our research objects.The research shows that the activation energies of C2+and Si4+for full-space rotation are 0.59 eV and 0.43 eV,respectively,which means that they can rotate at lower temperatures.However,the energy barriers for long-distance migration of C2+ and Si4+are 1.57 eV and 3.15 eV,which are significantly greater than that of corresponding neutral interstitial atoms.In the process of storing tritium,3He produced by tritium decay can easily accumulate in the material to form He bubbles in Pd,which affects the macroscopic mechanical properties of materials.For this reason,we studied the influence of H and Pd interstitial atoms on the nucleation process of He clusters.We found that He atoms in a pure Pd system will nucleate through selftrapping.Since the interaction between H and He clusters is almost zero,the low concentration of H has little effect on the nucleation process of He atoms.However,the self-interstitial Pd atoms and He atoms exhibit mutual attraction,a Pd interstitial can promote the aggregation of He atoms and accelerate the nucleation of He clusters.Since the SiC layer contains not only impurity atoms such as H,He produced by transmutation but also metal fission products such as Pd in the TRISO fuel,we have studied the synergistic behavior of H,He,and Pd atoms in SiC.We found that H can promote the migration of He atoms around it,and Pd can promote the migration of He atoms away from Pd atoms.Under the synergistic effect of H and Pd impurity atoms,the influence of Pd on the migration behavior of He is dominant,that is,He atoms tend to migrate away from Pd atoms.However,the migration barrier of He atoms under the combined influence of H and Pd has been further reduced when compared with the case that only being affected by a single impurity atom. |
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