Study On Permeation And Retention Behavior Of Hydrogen Isotope In Tungsten And Tungsten-based Materials

In order to enter the new ranks of the world's fusion research and development and solve the energy crisis,China launched the major scientific project of the China Fusion Engineering Test Reactor(CFETR)in December 2017,and plans to realize its tritium self-holding and power generation output functions around 2030.The material directly facing the deuterium-tritium plasma in the fusion reactor is called the"first wall",which is in a very harsh environment,so the requirements for the first wall material are very strict.Tungsten(W)material is considered to be the most promising first wall material in the future because it has many properties satisfying plasma orientation.The permeation and retention of hydrogen isotopes in tungsten-based materials of fusion fuel is one of the important problems to be considered,which is related to the reasonable evaluation of tritium in fusion reactor and the safety design of the whole reactor.At present,the research on hydrogen isotope penetration and retention in tungsten and tungsten-based materials at home and abroad has relatively few and scattered experimental data,especially the lack of mixed gas penetration data corresponding to the actual working conditions.In this paper,the permeation and retention behavior of single and mixed hydrogen isotopic gases in tungsten matrix materials have been systematically studied by means of gas driven permeation,gas thermal charging and thermal desorption,combined with X-ray diffraction(XRD),X-ray photoelectron spectroscopy(XPS)and scanning electron microscopy(SEM).The deuterium permeation behavior of pure tungsten and tungsten doped with two rolling ratios of Y₂O₃ was studied.The quantitative data of deuterium permeability,diffusion coefficient,solubility and diffusion activation energy in pure tungsten and W-Y₂O₃ were obtained.The results show that the defect density and the doping of Y₂O₃ have no significant effect on the steady state permeability of deuterium in tungsten,but the doping of Y₂O₃ and the rolling process affect the diffusion activation energy and the dissolution heat of deuterium in tungsten.The diffusion activation energy of deuterium in W is decreased and the dissolution heat of deuterium in W is increased by element doping and increasing the rolling ratio.With the increase of the rolling ratio,the diffusion activation energy of deuterium in W is further decreased and the dissolution heat is further increased.The increase of Y₂O₃ doping and rolling ratio promotes the diffusion of deuterium in tungsten,and thus reduces the solid solubility of deuterium in tungsten.The microstructure analysis shows that the doping of Y₂O₃ and the increase of the rolling ratio increase the defect density and defect density in tungsten,which makes the deuterium diffusion interface increase and promotes the deuterium diffusion.It can be seen that appropriate element doping and increasing the rolling ratio are effective means to reduce the solid solubility of deuterium in the material.Deuterium permeation behavior of pure tungsten and tungsten-copper composites with different copper contents was studied.The results show that the permeability of deuterium in W-Cu composites is 2?3 orders of magnitude larger than that of pure W-Cu composites,and the diffusion coefficient is about 5 orders of magnitude higher.With the increase of copper content in the composites,the permeability and diffusion coefficient of deuterium in the composites will increase.The solubility of deuterium in tungsten-copper composites is much lower than that in pure tungsten,and the heat of dissolution is also higher.The phase interface between tungsten-copper composites promotes deuterium diffusion in tungsten and reduces the solid solubility of deuterium in tungsten.Interestingly,the gas phase thermal charge-thermal desorption experiments show that the deuterium retention in W-Cu composites is 10²⁴atom/m³,while the deuterium retention in pure tungsten is 10²³atom/m³,and the deuterium capture sites in the composites are more than that in pure tungsten.The rapid cooling makes the deuterium trapped in the composite too late to be released,indicating an increase in apparent deuterium retention in tungsten.Permeability of pure deuterium and mixture of hydrogen and deuterium with different deuterium contents in pure tungsten was compared.It is found that the presence of hydrogen partial pressure will affect deuterium permeation.When the hydrogen partial pressure is greater than 30%,the permeation of deuterium gas will be suppressed with the increase of hydrogen partial pressure.When the partial pressure of hydrogen is 50%,the permeability of deuterium gas decreases by about 40%,the diffusion coefficient increases by nearly 700%,and the solubility decreases by nearly 95%.The microstructure analysis shows that the single gas will cause damage to the material surface when the gas phase driven infiltration occurs,and the damage caused by the mixture(including deuterium-helium mixture)will be more serious,which can not only make the material surface grain refinement,but also cause damage on the grain boundary and grain surface.