Study On Ion Irradiation Effect And Molten Salt Corrosion Of Hastelloy N Alloy After-Irradiation

Molten salt reactor(MSR),one of the six most promising Generation IV nuclear reactor systems,has received worldwide attention for its high thermodynamic efficiency,intrinsic safety,and fission fuel sustainable utilization.However,the structural materials in MSR are confronted with the challenge of the extreme harsh environments,such as high temperature,high fluence of neutron irradiationandstrong corrosive molten salt.Nickel based Hastelloy N alloy has been considered as one of promising structural materials for MSR application thanks to its good high-temperature mechanical strengthand corrosion resistance for molten salts.However,detail radiation damage and the impact of irradiation for the corrosion in molten saltstudy of Hastelloy N alloy were rarely reported.Based on the above background,this thesis investigated the irradiation damage effect and irradiation’s impact for corrosion of Hastelloy N alloy.Transmission electron microscope(TEM),atomic force microscope(AFM),grazing incident X-ray diffraction(GIXRD)and Nanoindentation were used to characterize theHastelloy N alloy samples which were irradiated by 7 MeV and 1 MeV Xe ions at room temperature with irradiation damage dose ranging from 0.5 to 27 dpa.The results show that the irradiation induced volumetric swelling and lattice expansion of Hastelloy N alloy,and both of them increase with increasing of irradiation damage dose.Moreover,no saturation is observed at 27 dpa.The irradiation induced hardening initiates at low irradiation damage dose(<1 dpa)then saturates with higher damage dose(1～10 dpa).At low irradiation damage dose(1 dpa),the main irradiation defect type is the black dot defect,at higher irradiation damage dose(10 and 27 dpa),it is dislocation loop.With increasing irradiation damage dose from 1 dpa to 27 dpa,the average sizes of the defects grew up from 5 nm to 19 nm but their total density decreased from 6.9 x 1022 m-3 to 3.0 x 1022 m-3.The irradiation induced volumetric swelling may be ascribed to excess atomic volume of defects.The irradiation induced hardening may be explained by the pinning effect where the defects can act as obstacles for the free movement of dislocation lines.And the evolution of the defects’ size and number density could be responsible for the saturation of hardness.The unirradiated and irradiated Hastelloy N alloy samples(1 and 10 dpa)were corroded in molten fluoride salt(FLiNaK)at 650℃ for 200 hours using static corrosion method.Scanning electron microscope(SEM+EDS)and GIXRD were used to characterize the corroded samples.The results show that intergranular corrosion and pitting corrosion on the surface appeared in the allcorroded the Hastelloy N samples.The specific corrosion form is that A slight depletion of Cr element on the cross section and the enrichment of Fe element on the surface and the corroded crack occurred in all samples.For the intergranular corrosion,the depths of corroded crack were all about 6pm for the unirradiated and irradiated samples and there is no difference between of them.For the pitting corrosion,the number densities of pitting holes on the irradiated samples’ surface were obviously higher than the unirradiated sample,and the number density increases with irradiation dose.It is indicated that irradiation damage exacerbated the pitting corrosion on the Hastelloy N alloy.Moreover,corrosion induced the lattice shrinking occurred in the all samples,this may be ascribed to the formation of vacancy type defect due to the depletion of alloy element in the corrosion course,and there is no apparent difference in the degree of lattice shrinking between the unirradiated and irradiated samples.