Molecular Dynamics Simulations & First-principles Calculations Study On Nucleation And Growth Of Helium Bubbles In Tungsten

The material of key components plays an important role in the long-term stable operation of advanced nuclear power system.Tungsten are potential spallation target for ADS and plasma-facing material for fusion reactor,due to it has the characteristics of high temperature and irradiation in service environment.Based on the previous studies,this paper simulated the nucleation and growth behavior of helium in tungsten and the defects evolution process caused by helium bubbles.At the same time,the evolution mechanism of helium bubbles nucleating and growing in tungsten is also discussed from the perspective of energies(formation energy,migration energy and Gibbs free energy of defects).The main conclusions of this paper are as follows:1.In this paper,the loop-punching mechanism and the phase transition induced by helium bubbles in single crystal tungsten are verified by molecular dynamics simulation.The charge density difference between tetrahedral interstitial atoms and octahedral interstitial atoms in bcc tungsten and fcc tungsten was calculated by first principle,and it was found that the stability of tetrahedral interstitial helium atoms in bcc tungsten was higher than that of octahedral interstitial helium atoms,while in fcc tungsten it is just the opposite.2.The mechanism of helium bubble growth at grain boundary is different from that in the bulk in tungsten.Helium bubbles in single crystal tungsten grow by punching dislocation loops.The mechanism of helium bubble growth at the grain boundary of ∑3[211](110)is as follows: firstly extrude and emit a small amount of tungsten atoms from the gap,then extrude 1/2<111> dislocation line,and then the dislocation line will migrate along the grain interface [111].At the grain boundary of sigma ∑9[110](411),no emission of tungsten intersitial and dislocation extrusion were observed in helium bubbles in our simulated time scale.3.Helium bubble can promote itself growth by intermittently releasing pressure both in bulk tungsten and at grain boundaries(∑3[211](110)and ∑9[110](411)).4.The punching-loop mechanism of helium bubbles in a linear temperature gradient field of <111> direction has a specific directivity.In other words,during nucleation and growth,The helium bubble can punch dislocation loops along the temperature gradient field in the direction of temperature rising.