Cluster Dynamics Simulation Of Effects Of Grain Size On The Radiation Damage In Materials

Searching and designing plasma facing materials with a long time service is the key of safe operation of the thermonuclear fusion.Studying the behaviors of radiation damage behavior and anti-irradiation mechanism in materials can provide a theoretical support for the experiment.At present,the main focus is about the radiation damage of materials under H/He and neutron irradiation.Due to high density of grain boundaries in nano-crystalline materials,the nano-crystallization is considered as an effective way to improve the anti-radiation properties of materials.By introducing the multi-scale framework of cluster dynamic methods based on the rate theory,and taking consideration of grain boundary and dislocation trapping effect,the relationship of hydrogen/helium particle retention in tungsten with grain size and the effects of diffusion bias on radiation tolerance in nano-crystalline iron and tungsten are systematically investigated.The main contents are as follows:(1)The research of tritium retention in plasma facing materials tungsten is very important.At present,the study of H/He retention is not systematically and even controversial.In view of this phenomenon,by introducing the cellular sink strength of grain boundaries into the cluster dynamics model,the behavior of hydrogen and helium retention in tungsten with different grain sizes is studied under various irradiation conditions systematically.It is found that the hydrogen/helium retention increases dramatically with decreasing grain size at typical service temperatures,due to the enhancement of hydrogen/helium capture ratio by grain boundaries.Our further study shows that,under the irradiation of low energy and low fluence ions,the total He retention is thus dominated by the competing absorption of grain boundaries and dislocations,that is,changing from the dislocation-based to grain boundary-based retention with decreasing grain size.In view of these grain size-related behaviors of H/He retention in tungsten,it is suggested that coarse-grained crystals should be selected for tungsten-based plasma-facing materials in practice.(2)We use a variety of methods to study the effects of irradiation resistance materials,including the equilibrium state equation based on the chemical rate theory,non equilibrium based on the cluster dynamics and molecular dynamics method,respectively.The effect of grain size on radiation tolerance in iron and tungsten are systematically simulated.We found that the anti-irradiation ability of nano-crystalline materials is mainly determined by vacancy diffusivity but barely by the diffusion bias in equilibrium state.Under the non-equilibrium state,by developing a long time dynamics evolution of cluster dynamics model that incorporates grain boundary sink effects,we systematically simulated the defects(especially vacancies)retained in tungsten and iron at different grain size under neutron irradiation.As expected,the irradiation tolerance of nano-crystalline iron is obviously improved at the temperature of 500 K.However,nano-crystalline tungsten dose not show obvious advantages on the annihilating vacancies even at high temperature of 900 K.At typical service conditions in future fusion devices,the level of diffusion bias dominates the radiation tolerance of nano-crystalline materials before the steady state can be reached.The molecular dynamics method is also employed to simulate the absorption capacity of grain boundaries on defects,which is consistent with that by the cluster dynamics model.We recommend that for W based PFMs,poly-crystalline materials should be selected in practice.In contrast,for Fe based structural materials,nano-crystalline materials would be selected,due to having a better anti-irradiation ability of neutron irradiation.These results provide a new insight into the behavior of nano-crystalline materials under irradiation.