| The reprocessing of spent nuclear fuel（SPF）is carried out under the condition of high temperature nitric acid.Studies have shown that stress corrosion cracking（SCC）will occur in Zr702 under this condition.Whether commercial Zr702 produced in China owns a risk of stress corrosion cracking in nitric acid remains unknown.In this paper,the domestic commercial Zr702 sheet is used as the main research material,and the stress corrosion susceptibility and cracking mechanism of the material in high temperature nitric acid are studied through the slow strain rate tensile test.The fracture morphology and microstructure after stretching were characterized by optical microscopy（OM）,scanning electron microscopy（SEM）,Raman spectroscopy（Raman）,and electron backscattering（EBSD）.The strain rate tensile mechanical properties and electrochemical properties were investigated to explore the effects of different nitric acid concentrations,tensile rates,crystal orientations,and electrode potentials on their microstructure and mechanical behavior.For Zr702 under different nitric acid concentrations（0,4,8,14.4 mol/L）,different stretching rates(1×10-3 s-1,1×10-4 s-1,1×10-5 s-1)Slow strain rate stretching confirmed that domestic commercial Zr702 has stress corrosion sensitivity in high temperature nitric acid,and the stress corrosion sensitivity increases with the increase of nitric acid concentration（from 0%to 26.67%）;decreased（from 7.58%to 20.49%）.Through fracture analysis,it was found that the samples in the nitric acid environment showed the characteristics of mixed fracture of cleavage fracture and plastic fracture,and the depth of cleavage plane increased with the increase of nitric acid concentration.The RD,TD and ND planes of the Zr702 sheet were characterized by EBSD,and stretched at a slow strain rate along the RD and TD directions respectively.Under this effect,the stress corrosion susceptibility of Zr702 is anisotropic.The stress corrosion susceptibility of the RD sample under the test conditions was 26.53%,while that of the TD sample was 12.23%.The deformation mechanism of the RD and TD samples was studied by EBSD,and it was confirmed that the anisotropy mainly came from the difference in the opening difficulty of the slip system.The electrochemical analysis of the original Zr702 sample was carried out under the condition of boiling 8mol/L HNO3,and the Zr702-induced passivation potential（1.08V）and the over-passivation transition potential（1.5V）were measured.It is confirmed that the plasticity of Zr702 is better than 35%at the passivation potential,but the plasticity drops sharply at the trans-passive potential,which is only 6%.The evolution of the oxide film of the samples with different potentials is analyzed.The oxide film is dense under the passivation potential,but the oxide film is loose under the trans-passive potential,and there are a lot of cracks and the overall brittle fracture of Zr702. |
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