Investigation Of The Preparation And Accident-tolerant Properties Of Metal Coating On Zirconium Alloy

Since the Fukushima nuclear power plant accident of Japan in 2011,the safety of zirconium alloy cladding has become the focus of the world.Zirconium alloy fuel cladding has poor high-temperature steam oxidation resistance,and the Zr reacts violently with high-temperature steam to release a large amount of hydrogen and heat,which will cause serious radioactive material leakage.In view of the above problems,the research and development of accident-tolerant fuel?ATF?cladding materials is an important breakthrough for improving the safety performance of light water reactor?LWR?.Surface coating technology has become one of the research focuses of accident-tolerant fuel due to its mature process and compatibility with existing zirconium alloy cladding production equipment and process,which has the advantages of good economy and easy commercialization.It has the potential to improve the accident-tolerant performance of zirconium alloy in the short and medium term.In this thesis,Cr and FeCrAl coatings were prepared on the surface of Zircaloy by magnetron sputtering.The effects of deposition parameters on the microstructure and related properties of the coatings were studied,and the autoclave and steam oxidation device were used to study the corrosion resistance of the coating under normal working conditions and the high-temperature oxidation performance under accident conditions,as well as their related internal mechanisms.The research in this thesis is divided into the following two parts:1.Study on preparation process and related properties of thick Cr coating:Thick Cr coatings with a thickness of about 27?m were prepared by high speed magnetron sputtering under different biases.All Cr coatings prepared by this process were uniform and dense,and had tightly bonded to the substrate and hydrophobicity.Compared the microstructure and mechanical properties of Cr coatings under different bias,as the bias voltage increased,the?110?crystal plane of the Cr coating became the preferred orientation plane.The Cr coating with a bias of-50 V had the lowest surface roughness and nano-hardness,and the error distribution of nano-hardness and elastic modulus was the smallest,which had a relatively uniform microstructure distribution.The 1000? air oxidation screening test showed that the Cr coating with a bias of-50 V had fine air oxidation resistance.Further high-temperature 1200? steam oxidation tests were performed on the Cr coating under this process,the Cr coating with a bias of-50 V exhibited excellent high-temperature steam oxidation resistance.It was found that the oxide layer exhibited the characteristic of the layered peeling at the initial oxidation stage,and then formed a denser Cr₂O₃ layer which can effectively prevent the diffusion of oxygen.Meanwhile,lower surface roughness can effectively reduce the growth stress to suppress the formation of areas of decohesion.Based on this,a steam oxidation model of Cr coating was proposed.In addition,the autoclave tests?360?,18.7 MPa,50 d?showed that the surface of the Cr coating with a bias of-50 V was dense without obvious holes and cracks,which indicated that the coating also had good high-temperature corrosion resistance.Based on the above results,thick Cr coating prepared by magnetron sputtering technology can be used as a candidate material for accident-tolerant fuel coating,to improve the high-temperature steam oxidation resistance of Zr-4 alloy.2.Study on the preparation process and related properties of FeCrAl coating:Using the deposition temperature and deposition bias technology of magnetron sputtering,FeCrAl coatings with different process parameters were prepared,and their microstructure and mechanical properties were analyzed.All FeCrAl coatings had dense surface and were in body-centered cubic Fe-Cr phase.All FeCrAl coatings under different deposition temperatures had hydrophilic surface,and the increase of deposition temperature was beneficial to the grain growth of FeCrAl coatings.the FeCrAl coating at deposition temperature of 400? had lower average surface hardness and elastic modulus,and a narrower error distribution range.The introduction of deposition bias led to the increase of surface roughness,however,it was beneficial to improve the bonding performance of FeCrAl coatings.With the increase of deposition bias,the preferentially oriented crystal surface of the coating changed,and the nano-hardness error distribution of FeCrAl coating with a bias of-150 V was the smallest,which had a relatively uniform microstructure.The 1000? steam oxidation tests showed that the Al element in the FeCrAl coating with a bias of-150 V migrated outward to form a dense oxide film,which can effectively prevent the diffusion of oxygen elements into the interior,and it exhibited excellent high-temperature steam oxidation resistance.However,Fe and Cr elements migrated into the substrate to form a diffusion layer,which led to degradation of coating properties.In addition,the further autoclave test?360?,18.7 MPa,50 d?showed that Al element in the FeCrAl coating with a bias of-150 V was easy to form Al O?OH?and dissolved in the high-temperature water corrosion environment,and a large number of Fe₂O₃ particles were formed in the outermost layer of FeCrAl coating.But a dense Fe/Cr blend oxide layer was formed in the lower layer,which effectively prevented the further diffusion of oxygen,and it showed good high-temperature corrosion resistance.Based on the above results,FeCrAl coatings prepared by magnetron sputtering had fine high-temperature oxidation and corrosion properties.The main reasons for the degradation of coating properties were the severe diffusion and migration of alloy elements at high temperature and the dissolution of Al elements in water corrosion environment.