Irradiation Effect Of Zirconium Based Ceramics And Modification Of Cladding With Ceramics Coatings

In 2001,Fukushima Daiichi nuclear accident made nuclear security faces new challenges.It demonstrated the failure of Zircaloy in loss of coolant accident?LOCA?.The zircaloy reacted with water at high temperature and produced hydrogen leading to explode.Cladding materials are the most sticking point of safety guarantee of nuclear system.To improve the propeties of cladding,many reasearchers have turned their attention to Molybdenum alloys,ceramics and ceramic coatings.Ceramics own lots of properties superior to alloys including high melting point,high hardness and high strength etc.which made them to have good prospects to be used as core materials in future nuclear system.Due to the combination of high temperature,high stresses,chemically aggressive coolant and intense radiation fluxes,the core of nuclear reactors present an exceptionally harsh environment for materials.ZrC,ZrN and SiC have been proposed as promising candidates for core materials in future nuclear system,based on their superior properties for nuclear application,great potential to survive in such harsh environment and low neutron absorption cross-section.The intense production of heat in nuclear fission and fusion reactor are accompanied by the generation of energetic neutrons,He and other particles which can displacement damage materials to degrade.Au ions had been chosen to simulate neutrons irradiating micro-polycrystalline ZrN and ZrC-SiC composite with high dose in this article.There were similar characteristics of defects varying with depth in irradiated ZrN and ZrC grains.Most of energy of Au ions dissipated by inelastic collisions while the Au ions tracked the bulk materials.That made the increase of local temperature,leading to the movement of defects and long dislocation lines would be generated.Thus,low-density long dislocation lines can be seen beneath the depth of maximum of Au ions enrichment area,and density small dislocation loops in deeper depth.Oxygen had been injected with energitic Au ions,so nano-crystalline ZrO₂ can be found in ZrN and ZrC grains.Irradiated SiC became amorphous and ZrC keep structural integrity at the same depth.Volume expansion during amorphization of SiC resulted in ZrC in compression.Regards to single grain,the damage depth in SiC?¹.55?m?is found deeper than in ZrC?¹.22?m?and Zr N?¹.2?m?,which coincides to the theoretical simulation.It has been considered that grain boundaries can capture interstitial atoms and then fire them back into the lattice to fill up any vacancies,leading to improvement of irradiation tolerance.In this article,nano-crystalline Zr₃N₄,different stoichiometries ZrC_1-x,ZrC-SiC composite and ZrC/SiC double layer coatings had been obtained by magnetron sputtering.Then He ions had been chosen to irradiate these coatings.The Zr₃N₄ coatings with different thickness,ZrC-SiC composite and ZrC/SiC double layer coatings show stable structure after irradiation.The unit cell parameters of different stoichiometries ZrC_1-x coatings decrease with irradiation dosages.Besides the nano-crystalline coatings,the irradiation effect of amorphous SiC had also been investigated.The SiC keep its amorphous structure with broken C-Si bonds after being irradiated by He ions with different dose.Thus,coatings of ceramics onto Zircaloy-4 could be a practical and feasible way to improve the properties of cladding materials by taking advantages of both Zircaloy and ceramics.Here,we investigated a magnetron sputtering kinetics for the deposition of ZrC and SiC coatings on Zircaloy-4,and evaluated the density and adhesion of coatings.By optimizing variety of processing parameters,we finally obtained dense ZrC and SiC coatings with maximum values of hardness?34.39 GPa,25.61 GPa,respectively?and critical load?15.09 N,8.24 N,respectively?.Structural characterization indicates nanocrystalline?¹0 nm?structures of the ZrC and SiC coatings.In addition,the adhesion of SiC coatings can be strengthened by using Ti transition layer,and the thermal stability of SiC coatings can be increased by using Cr transition layer.