Microstructure Analysis And Corrosion Investigation Of Al/B₄C Neutron Absorbers Used As Interim Storage Of Spent Fuel

Al/B4C MMCs are being considered to be the mainstream neutron absorber materials used as interim dry/wet storage of spent fuels in the future based on their excellent mechanical properties,good heat-conducting properties andhigh neutron absorption cross-section.Neutron absorber materials will suffer irradiation damage and corrosion in storage environment.The characteristics of the microstructures of Al/B4C MMCs with different B4C mass percent(marked as Al-15%B4C,Al-25%B4C and Al-30%B4C)were investigated in this paper.The samples including the non-irradiated,irradiated and corroded at 100°C for 0～250 h in 5000 ppm boric acid solution were analyzed by FIB、SEM、GIXRD and TEM(EDS、STEM、EELS、HRTEM and Element Mappings).On the basis of conventional metallographic polishing methods,this paper developed a polishing method of focused Ga+ ion beam,which was used to obtain an ideal sample surface of Al/B4C MMCs with very low roughness to characterize the distribution and size factor of B4C particles in aluminum matrix.The size and area percentage of B4C particles were statistically analyzed.The B4C particles were not homogeneous in the aluminum matrix,ranging from several hundred nanometers to several micrometers.The area of B4C particles mainly concentrated on the range from 0 to 0.5 μm2,which was approximately 64.29%,86.99%and 76.86%for the Al-15%B4C,Al-25%B4C and Al-30%B4C MMCs,respectively.A large number of plane dislocations including twin and stacking faults were observed in the B4C particles with micron scale.The size of aluminum grain varied from tens of nanometers to several hundred nanometers,and it has a large number of dislocations.The results of elemental surface scanning at the HAADF-STEM model showed that there were elliptical geometrical shape alloy precipitates with several hundred nanometers in the aluminum matrix,which contained Cu,Cr,Fe,Si and other elements.The surface scanning results of B element with EELS technique confirmed that the polygonal nanoparticles distributed in the matrix were B4C particles.The crystal plane(021)of B4C phase and(111)of A1 phase were grown in parallel geometrical relation,and the lattice mismatch was about 1.68%.The partially amorphous B4C particles were found after helium ion irradiation,and the amorphous proportion increased with the increasing irradiation dose.However,the crystal structure of Al phase was still preserved after helium ion irradiation,and the crystal structure was not amorphous.The diffraction peak intensities of B4C and A1 phase decreased gradually with the increasing ion dose,and the lattice constant of B4C and Al phase were also increased.That means the irradiation caused the crystal swelling of B4C and Al matrix.The helium atoms migrated,gathered,grew and then formed helium bubble belts in both aluminum and B4C particles.Helium atoms were much more likely to be concentrated in the grain boundary and the phase boundary,and the average size of helium bubbles was larger than that the helium bubble inside the crystal.As the corrosion time increasing,the pH value of the corrosion solution increaseed gradually.Microvoids were formed on the sample surface,including microvoids between boron carbide particles,the gaps between the boron carbide and aluminum substrate interface,cracks of boron carbide particles and aluminum matrix,microporous formed after boron carbide particles flake,pitting corrosion pits in the aluminum substrate,and corrosion channels formed after He+ ion irradiation damage in the sample surface.He+ ion irradiation reduced the corrosion resistance of Al/B4C composite materials.With the same corrosion conditions,the corrosion weight gain was more significant in the He+ ion irradiated samples than those of the corresponding non-irradiated samples.And at the same time,with the increasing B4C content,the corrosion resistance of Al/B4C composite materials was decreased for both irradiated and unirradiated samples.On the surface of the corrosion sample,the corrosion products were formed,and their size and content increased with the increasing corrosion time and the helium ion irradiation dose.