Multiscale Simulation Of Helium Bubble Evolution In The Matrix Of The Nuclear Structural Steels α-fe Under Neutron Irradiation

The candidate structural materials of the fusion and advanced Gen-Ⅴ reactors,Ferritic/Martensitic(F/M)steels will suffer from neutron irradiation with energy up to 14 MeV.High energy neutron irradiation will not only lead to massive displacement damage,but also will produce lots of helium and hydrogen elements via(n,α),(n,p)transmission reactions.The solubility of helium in metals is very low.Helium are easy to diffuse and aggregate in the defects of the metals such as void,dislocation,grain boundary,etc.,forming into nano-size helium bubbles.Helium bubbles will lead to the degradation of F/M steels,such as irradiation hardening,irradiation embrittlement,irradiation swelling and irradiation creep,resulting in the decrease of the service lifetime.This phenomenon is called irradiation helium effect.Revealing the nucleation,formation and growth of helium bubbles in F/M steels is imperative for designing the irradiation-resistant materials.The nucleation and formation of helium bubbles could hardly be investigated by experiment methods.In contrast,multi-scale simulations could investigate the property of helium bubble in a convenient and effective way,and has been increasingly adopted to study the evolution of helium bubble in nuclear structural materials.In this dissertation,the first-principles,molecular dynamics and dynamic Monte Carlo methods,were applied to investigate the mechanisms of nucleation,formation and growth of helium bubbles in the matrix of F/M steels,BCC iron,from the following aspects:(1)The binding mechanism of two close helium atoms during the helium bubble nucleation was studied.The results showed that when the distance between two interstitial helium atoms is less than one lattice constant in BCC Fe,they would bind as helium pair with an equilibrium distance around 1.6 (?) and binding energies varying from 0.15 to 0.49 eV.Compared with far He-He configuration,helium pair occupies larger volume by further repelling surrounding Fe atoms.The charge density variation is smaller,so that helium pair is more stable than far He-He.Helium pair could migrate in BCC Fe as a unit,and the complex migration could be precisely,directly and comprehensively described by the He-He formation energy hypersurface.(2)The formation mechanisms of helium bubbles formed by the simultaneous introduction of cascade and helium atoms were investigated.The influence of temperature,dislocation line,cascade and hydrogen on the formation of helium bubble were analyzed.It was found that elevated temperature would prompt the diffusion of helium and evolution of helium bubbles,so that the sizes of the helium clusters increase and the number density decreases.The strain field of dislocation line could foster the formation of a string of helium clusters with low helium content around the dislocation line.The coalescence of large helium-vacancy clusters within the time scale of molecular dynamics simulation was discovered.The coalescence mechanisms were the spontaneously kicking out mechanism and the cascade-prompted mechanism.Dislocation could inhibit the kicking out coalescence,but could promote the cascadeprompted coalescence of helium clusters.(3)The quantitative influence of irradiation cascade on the stability,migration,coalescence of helium bubble was investigated.It was found under the stable He/V ratio of helium bubbles under cascade overlapping is around 0.5.With the increase of the He/V ratio,the mechanisms of the cascade induced migration change from vacancy migration to helium atom migration,and the migration distance under single cascade overlapping decreases from 1 nm to around 1 (?).Cascade could also lead to the coalescence of two helium bubbles with surface distance 2.8 A.With the increase of He/V ratio,the coalescence percentage induced by single cascade overlapping decreases first and increases again.It was found that the joint strain field of the two helium bubbles could significantly prompt the cascade induced coalescence.Hydrogen could significantly hinder the migration of voids under cascade overlapping and decrease the coalescence percentage of voids.But the presence of hydrogen enhances the strain field and increases the coalescence percentage of two helium bubbles.(4)The long-term helium bubble evolution under conditions close to high-energy neutron irradiation environment was studied.By OVITO(Python)scripts,the transition of input-output between the molecular dynamics simulation tool LAMMPS and kinetic Monte Carlo simulation package MMonCa was realized,so that a coupling method between the molecular dynamics and kinetic Monte Carlo program was achieved.The evolution of time scales on the order of seconds for helium bubble starting from single helium introduction was simulated and compared with the helium bubble density in structural irradiated by high-energy spallation neutron source.The results showed that this coupling method is closer to the real irradiation than MD and KMC simulation alone.All the conclusions of this dissertation are expected to provide an effective reference for understanding the mechanisms of helium bubble evolution in F/M steels,and the design&improvement of the helium-effect-resistant materials.