Study On The Behavior And Mechanism Of Cracking During Hydrogen/Deuterium Absorption In Zirconium Alloys

Due to their low thermal neutron capture cross-section,high strength,outstanding corrosion resistance in high-temperature and high-pressure coolant,excellent heat-conducting property and processability propert,zirconium alloys are widely used as cladding materials in nuclear reactor that is currently operable and under construction.But under some extreme conditions,such as LOCA(Loss of Coolant),zirconium will react with water quickly at elevated temperature up to 627-927? and release hydrogen during this reaction.Thus zirconium alloys will end up with a lager amount of hydrogen absortion,leading to cracking of zirconium alloys when the reactor is cooled.The investigation of hydrogen absorption cracking of zirconium is of significant importance since the craking will result in nuclear fuel leakage.However,there are limited reports about the cracking behavior and mechanism in hydrogen absorption zirconium alloys,which need to be studied father.On the other side,zirconium alloys are a kind of potential deuterium storage materials.Compared with uranium metal,one kind of traditional deuterium storage materials,zirconium alloys have good deuterium storage properties,including high storage density,low deuterium decomposition pressure and quick release of deuterium at elevated temperature.However,zirconium alloys will crack after deuterium absorption,limiting the application of zirconium alloys as deuterium storage materials.Thus how to solve the cracking issue of zirconium alloys due to deuterium absorption need to be investigated for zirconium alloys used as deuterium storage material.Thus in this study,a serial of hydrogen/deuterium absorption experiments of zirconium alloys at high temperature and high hydrogen/deuterium absorption amount were studied and then analysed with XRD(X-ray Diffraction),SEM(Scanning Electron Microscope),TEM(Transmission Electron Microscope),and EMPA(Electron Microprobe Analysis).Specificly,the effects of microsturcture,hydrogen/deuterium absorption,hydrogen/deuterium absorption temperature,interrupted cooling and alloying element(Nb and Cu)on the cracking of zirconium alloys due to hydrogen/deuterium absorption were studied in this work.And the cracking mechanisms were proposed.Four types of microstructures of Zr-4 alloy were prepared by heat treatment,namely original rolling microstructure,furnace cooling microstructure,annealing microstructure and water quenched microstructure.The hydrogen absorption behaviors of these four types of Zr-4 samples were studied at 900 ? and 2 atm.Then the hydrogen absorption amount and expansion in thickness were calculated.The phase structure of the hydriding Zr-4 samples were analysed by XRD and the macro cracks on the surface of Zr-4 samples were observed with SEM.The results show that the grain size of Zr-4 samples influences the cracking behavior greatly.The cracks are quite small for rolling samples and annealing sample whose grain size is lower than 30 ?m,namely while the cracks become very lager for furnace cooling sample and water quenched sample,whose grain size is lager than 500?m.The effect of hydrogen/deuterium absorption amount on hydrogen/deuterium absorption cracking was studied.The results show that with the increase of hydrogen/deuterium absorption amount,the expansions in thickness and cracking tendency increase.And the phase structures of hydriding/deuterated Zr-4 samples change from ? phase to ? phase with the increase of hydrogen/deuterium absorption amount.To study the influence of absorption temperatures on deuterium absorption cracking behavior of Zr-4 alloy,900?,800? and 700? were chosen.The results indicate that as the absorption temperatures drop from 900 0C to 800 0C or 700?,the deuterium absorption amount and the expansion in thickness increase.About the deuterium absorption cracking,the Zr-4 samples deuterated at 900?show flat surface expect some cracks on the edge.The samples deuterated at 800? crack serious and the cracks connect with each other,showing a net-shaped morphology.And the Zr-4 samples deuterated at 700? have the similar morphology as that at 800?? with much smaller size and greater density.About the effect of interrupted cooling,four different interrupted temperatures,namely 900?,800?,700? and 600? were selected.Before the interrupted temperature?mthe cooling is slow at a speed of 2?/3 min and after the interrupted temperature,the samples were furnace cooled to room temperature.The results illustrate that with the decrease of interrupted temperatures,the deuterium absorption amount increase and the cracking tendency decrease.The phase structures with different interrupted temperatures are the same,which is ?phase.The influence of alloying elements and fabrication technique were investigated.Zr-18%Nb alloy was prepared by arc melting and spark plasma sintering(SPS)method respectively and Zr-xCu(x=0,5 wt.%,10 wt.%,15 wt.%)was prepared by SPS method.After hydrogen/deuterium absorption,Zr-18%Nb prepared by SPS shows a smaller trendency to hydrogen absorption cracking than that of Zr-18%Nb prepared by arc melting.Zr-xCu alloys show a better ability to resist to hydrogen absorption cracking than that of Zr-18%Nb alloys.With the addition of about 5%Cu,no crack is found on the surface of the Zr-xCu alloys.Combined the above experimental results with the observation of hydride from TEM and theoretical calculation,hydrogen/deuterium absorption cracking mechanisms are proposed.It turns out that the cracking mechanisms change with the condition of hydrogen absorption that mentioned above.With regard to hydrogen absorption amount,the cracking is decided by hydride/deuteride type.For hydrogen absorption pressure,the cracking is determined by phase transformation.And the influence of interrupted cooling is controlled by the rate of martensite transformation.