Conventional Corrosion Behavior And High Temperature Steam Oxidation Behavior Under Simulated LOCA For Zr-1Nb-xM Alloys

Zirconium alloy has good corrosion resistance in high-temperature and high-pressure water,and has sufficient high-temperature strength and other characteristics.It is the only fuel element cladding material currently used in water-cooled nuclear reactors.Zirconium alloy must not only meet the corrosion resistance requirements under normal conditions,but also need to exhibit excellent performance under accidental working conditions.LOCA?loss of coolant accident?refers to a water loss accident that occurs during the operation of a nuclear reactor.The behavior of high-temperature steam oxidation and quenching brittleness of a nuclear fuel cladding zirconium alloy during a water loss accident is called LOCA behavior.Studying the oxidation behavior of zirconium alloy in high-temperature steam is helpful to understand its specific oxidation mechanism under accident conditions and provides a theoretical basis for the development of cladding materials that maintain good performance under accident conditions.In this paper,8 kinds of Zr-1Nb-x M?M=Cu/Bi/Ge,x=0.05/0.2;M=Si,x=150 ppm,wt.%,the same as follows?alloys prepared by melting were selected.The corrosion behavior of these 8alloys in 360?/18.6 MPa deionized water and 400?/10.3 MPa superheated steam was studied by using a static autoclave corrosion test.The oxidation behavior of the eight alloys at 800??1200? high temperature steam?relative humidity,RH=70%?under the LOCA condition was studied using thermogravimetric analysis instrument equipped with a 100%water vapor generator.SEM and TEM equipped with EDS were used to analyze the crystal structure and composition of the?SPPs?before oxidation.SEM equipped with EDS and Optical Microscope were used to investigate the cross-section of oxidized specimens respectively.The main results and conclusions are as follows:?1??-ZrSPPs exist in the Zr-1Nb-x M alloys;Zr₂Cu SPPs precipitate when the Cu content increases to 0.2%,and Zr?Fe,Cr,Nb?₂ and Zr₄Si SPPs also precipitate when 150 ppm Si is added,but no other SPPs are found when other alloying elements are added.Adding 0.2%Cu or 150 ppm Si changes the size distribution of the SPPs of the alloy;adding 0.2%Bi reduces the average size of the SPPs of the alloy,while adding 0.2%Ge has the opposite effect.?2?In lithiated water at 360?/18.6 MPa and in superheated steam at 400?/10.3 MPa,the addition of different amounts of alloying elements has different effects on the corrosion resistance of Zr-1Nb.Among them,in the deionized water environment,the corrosion resistance of the alloy after adding 0.2%Bi is the best,and the corrosion resistance of the alloy after adding 0.2%Ge is the worst;In the superheated steam environment,the corrosion resistance of the alloys with 0.2%Cu,0.2%Bi,and 150 ppm Si is close to each other,and they all belong to the best series,while the alloy with 0.2%Ge has the worst corrosion resistance.The hot working process used in this paper produces a large amount of?-Zr,which deteriorate the corrosion resistance of Zr-1Nb-x M alloy under 2 kinds of water chemistry conditions.?3?Zr-1Nb-x M alloys with different compositions have different resistance to high temperature steam oxidation at 800??1200?.After adding Cu and Bi elements,high temperature steam oxidation resistance of alloys is not as good as Zr-1Nb alloy at 5 temperatures,and the higher the alloy element content when oxidized at 900??1200?,high temperature steam oxidation resistance of alloys the worse;when oxidated at 800??900?,the addition of Ge is not conducive to the high temperature steam oxidation resistance of Zr-1Nb alloy,but when the oxidation temperature exceeds 1000?,the addition of an appropriate amount of Ge can improve the high temperature steam oxidation resistance of Zr-1Nb alloy,and the higher the temperature,the more obvious the improvement effect;and adding 150ppm Si can significantly improve the high temperature steam oxidation resistance of Zr-1Nb alloy at 5 temperatures.?4?During the steam oxidation at 800?1200?,as the oxidation temperature increases,the evolution of the cross-sectional structure of the alloy after oxidation is as follows:equiaxed crystal?ZrO₂layer+?-Zr?O?layer+prior?-Zrlayer?ZrO₂layer+prior?-Zrlayer?ZrO₂ layer+?-Zr?O?layer.During oxidation at 800?1000?,with increasing temperature,the proportion of?-Zr?O?layer gradually decreases,and the proportion of prior?-Zrlayer gradually increases;at 1100?,mainly ZrO₂ layer+prior?-ZrLayer,the?-Zr?O?layer disappears;at 1200?,the ZrO₂layer and the?-Zr?O?layer account for about 50%,and the prior?-Zrlayer disappears.?5?As the oxidation temperature increases,the hardness of both the?-Zr?O?layer and the prior?-Zrlayer gradually increase,which is related to the accelerated diffusion of oxygen leading to more dissolved oxygen in the alloy.The high temperature steam oxidation resistance of Zr-1Nb-150 ppm Si and Zr-1Nb-0.05Ge alloy are the best and the worst,respectively,because the addition of 150 ppm Si can suppress the diffusion of oxygen in the Zr-1Nb alloy during high temperature steam oxidation;but the addition of 0.05%Ge can promote the diffusion of oxygen inside the Zr-1Nb alloy during high-temperature steam oxidation.