Diffusion Reaction Barrier Between UO2 and Zr Shell
UO2 ceramic nuclear fuel is characterized by high hardness,a high melting point,and radiation resistance.Consequently,UO2 fuel has been widely used in pressurized water reactors and various research and experimental reactors.UO2 ceramic nuclear fuel pellets are encapsulated in a Zr cladding material to produce fuel elements that are now the mainstream fuel elements for pressurized water reactors owing due to their high strength,plasticity,and thermal conductivity.However,under reactor operating conditions,the UO2 fuel and Zr shell are thermodynamically unstable,and the O atoms in UO2 can diffuse into Zr,forming ZrO2 and other brittle phases that affect the safety and service life of nuclear fuel assemblies.Following the Fukushima accident,higher standards for reactor safety and longevity are demanded worldwide.Fuel assembly safety studies have shown that the formation of an oxygen diffusion barrier layer between the UO2 fuel phase and Zr shell to prevent the diffusion of oxygen atoms from the UO2 phase into the Zr shell is a method for improving the safety and service life of nuclear fuel assemblies.The calculations indicated that Nb and Cr are potential materials for diffusive barrier layers.To investigate the ability of Nb and Cr as diffusive barrier materials for preventing the diffusion of oxygen atoms,Nb,Cr,and Nb/Cr coatings were fabricated on Zr plates via arc-ion plating.Subsequently,the Zr plates with different coatings were tightly bonded to the UO2 plates to form a sandwich diffuser.The diffusing couple was placed in a mold,filled and packed with Zr powder,and placed in a sintering furnace to prepare for the diffusion reaction.Vacuum thermal pressing was used to accelerate diffusion.The experimental conditions of diffusion reaction were as follows:pressure of 50 MPa,reaction temperature of 800 ℃,hold temperature time of 4 hours,and furnace cooling were employed.After hot-press sintering,the diffusing couple was cut in the middle to observe and analyze the cross section.Scanning electron microscopy and energy dispersive spectroscopy were used to observe the morphology and analyze the elements.The results showed that the metallic coating effectively prevented diffusion between UO2 and Zr.Comparative studies have found that Nb coatings have more optimized oxygen resistance;however,Nb and Zr diffuse and form a two-phase structure under experimental conditions.The mutual diffusion of Nb and Zr atoms results in a large number of Zr atoms in the Nb coating and a large number of Nb atoms in the Zr shell.Cr was compatible with UO2 and Zr,and no diffusion reactions were observed between UO2 and Zr under the experimental conditions.However,Cr atoms have a greater affinity for O atoms than for Nb atoms,and the concentration of O atoms in the Cr coatings is higher than that in the Nb coatings.If the Cr coating is thin,O atoms can pass through the Cr coating,enter the Zr shell,and affect its properties.In summary,at this limit,a metallic coating can effectively prevent the diffusive reaction between UO2 and Zr.However,a single Nb or Cr coating does not completely solve the problem of diffusion between UO2 and Zr.Based on these results,a Nb/Cr composite layer to solve the diffusion problem between the UO2 fuel and the Zr shell may be a better approach.The side near the UO2 fuel was coated with Nb and that near the Zr shell was coated with Cr.The study of the diffusive reaction barrier layer between the UO2 fuel and Zr shell provides reference for improving the safety and service time of nuclear fuel assemblies.
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