Microstructure,hardening and deuterium retention in CVD tungsten irradiated with neutrons at temperatures of defect recovery stagesⅡandⅢ

Samples of ultra-high-purity tungsten prepared using chemical vapour deposition(CVD)technique were irradiated with neutrons at temperatures T_(irr)=373-483 K(stage Ⅱ of defect recovery)and T_(irr)=573-673 K(stage Ⅲ)up to 0.15 displacements per atom(dpa)in the Belgian reactor(BR2).The study of the microstructure of neutron-damaged samples using transmission electron microscopy(TEM)revealed visible defects with a predominance of dislocation loops.With an increase in the neutron irradiation temperature,the spatial distribution of the loops acquired pronounced inhomogeneity,and their average size moderately increased.Cavities and voids were not observed.Irradiation-induced hardening was found and a linear correlation was obtained between Vickers microhardness and nanohardness for undamaged and neutron-irradiated CVD-W samples.Irradiation of tungsten with neutrons led to a significant increase in the retention of deuterium,which accumulated mainly in vacancy-type traps.Furthermore,the influence of the columnar grain structure in low-dose neutron-irradiated tungsten seemed to be non-trivial upon deuterium retention.

Influence of recrystallization on tungsten divertor monoblock under high heat flux

Extremely high heat flux reaching 20 MW·m^(−2) can be foreseen for the future fusion reactor.Such high heat flux would induce recrystallization of tungsten(W)material,leading to significant strength loss of tungsten material and increment of ductility at high temperature,in particular when the temperature is much higher than its ductile-to-brittle transition temperature(DBTT).In this paper,an International Thermonuclear Experimental Reactor(ITER)-like tungsten divertor monoblock is modeled,and benchmark has been done first to get consistent results with ITER.Then,the monoblock structure has been optimized in order to get a lowest possible temperature and stress during heating and cooling phase separately compared to the baseline structure.Structural analysis of two kinds of states:stress-relieved tungsten and recrystallized tungsten using finite element method has been performed,aiming at finding out the recrystallization impact on the mechanical behavior of tungsten in divertor monoblock under cyclic high heat flux.Damages due to progressive deformation and time-independent fatigue lifetime of these two states of tungsten have been assessed and compared according to criteria The Structural Design Criteria for ITER In-vessel Components.Finally,the impact of different material recrystallization temperature on mechanical behavior has been explored under stationary heat load.The result shows that after recrystallization,thermal stress of tungsten material can be released by the larger plastic deformation compared to the stress-relieved tungsten.However,it is easier for recrystallized tungsten to get damaged due to progressive deformation as well as fatigue under 20 MW·m^(−2) cyclic heat flux than stress-relieved tungsten because of its relatively lower yield strength and larger plastic strain,which would cause low-cycle strain fatigue.Furthermore,tungsten with lower recrystallization temperature distorts more seriously,and therefore,it can be predicted the cracks would be initiated more easily.