The Composition Design And Hydrogen Absorption Properties Of Yttrium Zirconium Alloy

Due to the limitation of volume and mass,metal hydride is usually used as moderator in small nuclear reactor.Since the second half of last century,zirconium hydride is the main metal hydride used as moderator material for small nuclear reactor,but it has the disadvantages of short life and low working temperature.Compared with zirconium hydride,yttrium hydride has stronger thermal stability and is suitable for long life and high power small nuclear reactor.In order to prolong the life of small nuclear reactor and increase its power,it is necessary to study and fabricate yttrium hydride.In this paper,metallographic microscope,scanning electron microscope(SEM),X-ray diffraction analyzer(XRD),gas chromatography and other detection methods were used to study the hydrides containing zirconium yttrium alloy in detail,including:?Preparing alloy matrix.Through investigation and calculation,the alloy composition was determined,and the melting method was studied.Finally,the alloy matrix with uniform composition and no pollution was obtained.?Studying on hydrogen absorption performance.PCT curves of alloy samples were obtained by constant temperature hydrogen absorption experiment,and analyzed to provide theoretical guidance for the preparation of metal hydride.?Fabricateing metal hydride.Under the experimental conditions of non-uniform and uniform hydrogen absorption,yttrium hydride was successfully fabricated,and its microstructure and morphology were studied.?Testing the hydrogen permeability.The hydrogen permeation performance of yttrium hydride was tested,and the prediction formula of hydrogen permeation rate of yttrium hydride in different working atmospheres was established,and compared with zirconium hydride.The specific research results are as follows:?There are some problems in melting Zr-Y alloy in vacuum consumable arc furnace and vacuum electron beam furnace,such as uneven composition,difficult control of composition proportion,shrinkage defects and so on.Compared with the two methods,the vacuum magnetic induction suspension furnace is the best melting method for the Zr-Y alloy system,which is of great significance to the melting of zirconium yttrium alloy system and similar alloy system.?It is found that the addition of zirconium can refine the grains.After adding zirconium,the grain size decreases from 50?60?m to 10?20?m.When the zirconium content exceeds the solid solubility,zirconium begins to precipitate at the grain boundary and forms zirconium rich phase.With the increase of zirconium content,zirconium rich phase first shows the increase of zirconium enrichment.When zirconium content reaches 50at%?60at%,zirconium rich phase no longer increases,but the areas of zirconium rich phase begin to increase.?The pressure-composition-temperature curves of yttrium and yttrium alloys in the range of 700? to 900? are obtained,which supplement the thermodynamic data of Y-Zr-H ternary system.In this temperature range,pure Y is less affected by temperature,and the change of temperature does not make the PCT curve change greatly;however,with the increase of zirconium content,the influence of temperature on the hydrogen absorption performance of the alloy becomes greater and greater,which improves the discrimination between PCT curves,proving the difference of thermal stability between zirconium and yttrium in this temperature range.?According to the analysis of phase transformation point and special inflection point in PCT curve,three kinds of phase transformation mechanisms of Zr-Y alloy system under different conditions are revealed.According to this mechanism,the order of hydrogen absorption in Zr-Y alloy system is summarized as follows:?-Zr(solid solution)>?-Y(solid solution),?-Y(solid solution)>?-Zr(solid solution),?-Zr(solid solution)>?-Y(phase transformation),?-Y(phase transformation)>?-Zr(phase transformation),?-Zr(phase transformation)>?-YHx,?-YHx>?-ZrHx.?Due to the different hydrogen contents and oxygen contents on the surface,the hydrogenated samples show different optical properties when they are just out of the oven,but after the surface is cleaned with alcohol,the yttrium hydrides all show dark blue.It is found that there are three morphologies of yttrium hydride:white a-Y single-phase region,light blue 6-YHX single-phase region and dark blue(?-Y+?-YHX)two-phase region.SEM photographs of yttrium hydride with different hydrogen contents show that O elements which are uniformly distributed at grain boundaries gather and enrich during the hydrogenation process.?The hydrogen permeation performance of yttrium hydride in 600?,CO2+He atmosphere and pure He atmosphere was examined,and the prediction formula of hydrogen permeation rate was obtained.In CO2+He atmosphere,the hydrogen permeation rate Va(kg·m-2·s-1)and time t(s)satisfy the following equation:Va=1.396×10-6x(t-71005.680)-0.5In pure he atmosphere,Va and t satisfy the following equation:Va=1.196×10-6 ×(t-51507.177)-0.5Through calculation and comparison,it is found that the hydrogen permeation rate of yttrium hydride and coated zirconium hydride are in the same order of magnitude at 600?,but slightly larger.It is proved that the thermal stability of yttrium hydride and coated zirconium hydride are at the same level at 600?,and the advantages of yttrium hydride may be displayed at higher temperature.