Research On High Temperature Steam Oxidation Behavior Of Zr-0.75Sn-0.35Fe-0.15Cr-xNb Alloy In Nitrogen-containing Atmosphere

In the nuclear power plant accident beyond the design basis,such as loss of coolant accident,the reactor core will be exposed to a mixed atmosphere of steam and air under extreme conditions,causing the fuel cladding tube to undergo high-temperature steam oxidation in the air/steam mixed atmosphere.It is known that about80%of the air is nitrogen,which can promote the oxidation of fuel claddings.Therefore,it is important to study the high-temperature steam oxidation behavior of fuel cladding materials in a nitrogen-containing atmosphere,which may contribute to the formulation of severe accident regulations.In this dissertation,5 kinds of Zr-0.75Sn-0.35Fe-0.15Cr-x Nb(x=0,0.15,0.30,0.50 and 1.0,wt.%)were prepared and designated as 1#,2#,3#,4#and 5#alloys,respectively.The high temperature steam oxidation tests were performed using a thermogravimetric analyzer to study the transformation behavior of the oxides formed in the atmosphere with 20%N₂ and 80%steam and with 20%air and 80%steam at 800,1000 and 1200?.The size,crystal structure and composition of the second phase particles(SPPs)in the alloys before oxidation were analyzed by using TEM and SEM equipped with an INCA energy dispersive X-ray spectrometer.The cross-sectional microstructures of the samples after oxidation were observed by using SEM and optical microscope.The oxygen and nitrogen contents of the oxide film were examined by using EPMA with WDS from the cross section of the samples after oxidation.The main results and conclusions can be obtained as follows:(1)As the Nb content increased,the number of SPPs in the Zr-0.75Sn-0.35Fe-0.15Cr-x Nb alloys was increased while the average size of SPPs was decreased.Two kinds SPPs,namely,Zr(Fe,Cr)₂ with a fcc structure and Zr(Fe,Cr,Nb)₂ with a hcp structure were detected in both 1#and 5#alloys.(2)In N₂/H₂O atmosphere,the high-temperature steam oxidation resistance at800?was decreased with the increase in Nb content.However,at 1000?and 1200?the high temperature steam resistance of 1#and 4#alloys were better than that of the other alloys,indicating that the influence of Nb on the high-temperature steam oxidation of zirconium alloys in N₂/H₂O atmosphere is very complicated,and there is no monotonic change with the variation of Nb content.(3)In Air/H₂O atmosphere,1#alloy without Nb addition had higher oxidation resistance at three temperatures than the other four Nb-containing zirconium alloys,suggesting that adding Nb may degrade the high temperature steam oxidation performance of the Zr-0.75Sn-0.35Fe-0.15Cr alloy in Air/H₂O atmosphere.Similar to the N₂/H₂O atmosphere,however,the effect of Nb content on the oxidation behavior of zirconium alloys in Air/H₂O atmosphere is also very complicated,with no monotonic changes by altering the Nb content.(4)The oxidation of 5 alloys at 3 temperatures was more serious in Air/H₂O atmosphere than in N₂/H₂O atmosphere,which indicated that the presence of O accelerated the oxidation kinetics of zirconium alloys in nitrogen-containing vapor atmosphere.The effect of Nb on the high temperature steam oxidation behavior of zirconium alloys varied depending on the atmosphere and temperature.At 800?,the effect of Nb was generally enhanced with the increase in Nb content,while at 1000 and1200?on obvious change was observed by increasing the Nb content.(5)When oxidized in N₂/H₂O atmosphere,all the alloys had a parabolic or parabolic-linear oxidation kinetics at 800?with no obvious oxidation transition in the oxidation weight gain curves.However,the oxidation kinetics at 1000?followed a linear or power exponential law,and the oxidation transition occurred in some alloys.As for 1200?,all the oxidation kinetics curves changed from a linear law to a parabolic-linear law with an evident oxidation transition.When oxidized in Air/H₂O atmosphere,the alloys presented parabolic,parabolic-linear or linear oxidation kinetics at 800?without any transition.The oxidation kinetics of the alloys at 1000?followed the parabolic-linear,linear or power exponential law,and some alloys even undergo a secondary oxidation transition.At 1200?,The oxidation kinetics of the alloys followed the parabolic-linear or linear rule,and an oxidation transition occurred.(6)Some alloys had different oxidation reaction activation energy(Q)values in different temperature ranges regardless of the oxidation atmosphere,but there was no obvious relationship between the Q value and Nb content.At the temperature range of800?1000?,the Q values of the alloys in N₂/H₂O atmosphere and Air/H2O atmosphere were within 54?76 k J/mol and 49?132 k J/mol,respectively.At1000?1200?,however,the Q values of the alloys in N₂/H₂O atmosphere and Air/H₂O atmosphere were in the range of 193?213 k J/mol and 99?197 k J/mol,respectively,which w 2?5 times the Q value at 800?1000?.(7)The cross-sectional structure of the oxide film formed at 800?was mainly composed of a Zr O₂ layer and a?-Zr(O)layer,while at 1000?it consisted of a Zr O₂layer(columnar oxides and porous oxides containing Zr N),a?-Zr(O)layer and an original?-Zr layer.The cross-sectional structure at 1200?is a Zr O₂ layer(columnar oxides+porous oxides containing Zr N)together with a?-Zr(O)layer.