Investigation Of Microstructure Evolution In Tungsten Under Neutron Irradiation Using Kinetic Monte Carlo Method

Tungsten is considered to be the best candidate for the plasma facing materials of fusion reactors due to its high melting point,high sputtering threshold,good thermal and electrical conductivity,low tritium retention and low thermal expansion coefficient.During service,tungsten is not only bombarded by high-energy neutrons but also irradiated by high-flux helium plasma.High-energy neutrons generate a large number of vacancies and interstitial atoms in tungsten.Helium will accumulate in tungsten to form high-density overpressure helium bubbles,which will affect the thermal and mechanical properties of the materials,and the presence of helium will affect the thermodynamic and dynamic properties of vacancy and interstitial atoms,thus affecting the evolution of defects.PKA energy,helium concentration,and temperature all have effects on the evolution of these defects,which can lead to degradation of material properties and pose a threat to the steady state operation of the plasma.At present,the synergistic effects of helium and neutron irradiation on neutron irradiation damage are not clear.In this paper,Kinetic Monte Carlo(KMC)method is used to simulate the cumulative process of irradiation damage in pure tungsten and tungsten materials containing helium impurities.The defect evolution in pure tungstun and tungsten containing helium was compared.The effects of different conditions such as temperature,helium concentration and PKA energy on defect evolution were studied.According to the results,the following can be concluded:(1)In pure tungsten,the interstitial atoms migrate much faster than vacancies,and the vacancies only move at high temperatures.Therefore,only the vacancies and vacancy clusters are remained in the simulation box,and the interstitial atoms are more likely to be absorbed by defect sinks,such as grain boundaries.Single vacancies are exclusive to each other and not easily clustered.(2)The larger the PKA energy in pure tungsten,the more Frenkel pairs would be generated,so the total number of defects would be larger.Meanwhile,the larger the PKA energy is,the larger the average size of the cluster will be.(3)Helium migrates quickly and easily to form stable helium-vacancy clusters.Even at high temperature,helium-vacancy clusters hardly move and do not dissociate,which reduces the probability of recombination with interstitial atoms and increases the number of vacancy clusters and interstitial clusters in tungsten.(4)The vacancy clusters can continuously absorb other defects to become bigger,and the size of the cluster generated by the cascade containing helium is larger than that of pure tungsten.Higher helium concentration leads to larger size of cluster and higher proportion of He-vacancy defect complex.