Effects Of Neutron Poison Gd On Mechanical Properties,Nitric Acid Corrosion Resistance And Neutron Absorption Properties Of Zirconium Alloys

In order to meet the needs of closed-loop treatment of spent fuel and neutron shielding in the nuclear chemical industry,on the basis of Zr-702 alloy with good mechanical properties and excellent corrosion resistance,Zr-Gd alloy with corrosion resistance and neutron absorption properties was prepared by adding neutron poison Gd.Based on the above requirements,this thesis takes Zr-Gd alloy as the research object,and introduces the cluster method to design the composition Gd₁Zr₆₃,Gd₁Zr₄₇,Gd₁Zr₃₅,Gd₁Zr₃₃,Gd₁Zr₃₁,and the empirical method to design the composition 1 wt.%,3 wt.%,5 wt.%,7 wt.%,9 wt.%for evidence.Taking Zr-702 as a reference,the effect of Gd on the mechanical properties,corrosion resistance and neutron absorption properties of the alloy was studied by controlling the content of Gd element.The mechanical properties and corrosion resistance of Zr-Gd alloy welded samples were also tested.The mechanical properties and microstructure of Zr-Gd alloy were tested.The experimental results show that the grain size of Zr-Gd alloy increases with the increase of Gd content.When the Gd content of the alloy exceeds 7 wt.%,the precipitation of hcp-Gd occurs,and it can be found by EDS characterization that the 7 wt.%alloy has obvious Gd element enrichment,but it is not enriched along the grain boundary.With the increase of Gd content,the ductility of the alloy decreases and the hardness increases.The fracture type of the alloy is ductile fracture→ductile plus brittle fracture→brittle fracture.The corrosion resistance test and surface characterization of Zr-Gd alloy were carried out.The results show that in the long-term corrosion experiment,the corrosion rate of the alloy before 7 wt.%alloy changes little.From 7 wt.%,the corrosion rate of the alloy increases sharply with the increase of Gd content,which proves that the corrosion resistance of the alloy decreases seriously at this time.The number of surface pores in the microstructure of the alloy after corrosion increases with the increase of Gd content,and the pores tend to deepen.In the electrochemical test,the corrosion resistance of the alloy is roughly stable when the alloy composition is below 7 wt.%.When the alloy composition exceeds 7 wt.%,the corrosion resistance of the alloy decreases sharply,and a porous and unstable passivation film is formed on the surface after polarization.The XPS characterization analysis shows that the proportion of Zr O₂on the surface decreases with the increase of Gd.The neutron absorption properties of Zr-Gd alloy were simulated by Monte Carlo particle transport program（MCNP）.The results show that the thicker the alloy plate is,the better the neutron shielding performance is under the same Gd content.In the case of the same thickness of the alloy plate,the more Gd content,the better the neutron shielding performance of the alloy.When the Gd content is 3 wt.%,99.9%of the thermal neutrons can be absorbed without 2 mm thick plate.With the increase of Gd content,the thermal neutron absorption effect of the same thickness plate is more obvious.However,Gd absorbs some thermal neutrons while shielding thermal neutrons,resulting in the harm of secondaryγ-rays.The mechanical properties and corrosion resistance of Zr-Gd alloy welded samples were tested.The experimental results show that compared with the annealed alloy,the ductility of the welded alloy decreases significantly,and the fracture type transformation still follows the law of ductile fracture→ductile plus brittle fracture→brittle fracture.After the full immersion corrosion of the boiling simulated liquid,there are obvious weld marks in the center of the alloy.When the Gd content exceeds 3 wt.%,two color bands appear at the weld mark position.The positive center is the weld,and the two sides of the weld are the heat affected zone.The corrosion resistance law is base metal>weld>heat affected zone,and the corrosion rate increases in the ninth cycle.Through the analysis of the microstructure of the Zr-Gd alloy weld mark position,it can be seen that there are a large number of black pits on the surface of the passivation film at the weld mark position of the alloy and some of the passivation film near the black pit is upturned.The passivation film is an unstable loose passivation film,which reduces the corrosion resistance of the alloy.