Experimental And Simulative Research On Hydrogen Isotope Permeation And Retention In RAFM Steels

Fusion is one of the most promising technologies to meet the increasing demand for clean energy by humankind.For future fusion reactors,reduced activation ferritic/martensitic(RAFM)steels are the most preferred candidate structural materials.Structural materials will be subject to extremely hostile service environment characterized by high temperature,intense and high energy neutron irradiation,high flux of helium and hydrogen generated from transmutation as well as implanted from the plasma core,and a large inventory of tritium produced in the tritium breeding blanket.The hydrogen isotope permeation and retention properties are closely related to fuel recycling,radiological safety and material degradation.Thus,it is of vital importance to launch the research on hydrogen isotope permeation and retention properties in RAFM steels.This thesis consists of experiment and simulation part.In experiment part,7 typical RAFM steels from three categories were under investigation,including traditional RAFM steels:EUROFER97 and G91;castable nanostructured alloys(CNAs):TT3mt and FTal;and oxide-dispersion-strengthened(ODS)steels:M4 Fe10Cr,9YWTV-PM2,14YWT-SM13.Gas-driven permeation(GDP)device has been applied to obtain the deuterium transport properties.And thermal desorption spectroscopy(TDS)has been used to measure the deuterium retention properties.In simulation part,molecular dynamics(MD)method has been used to understand the H trapping effect of nano-sized helium(He)bubbles,which are representative defects in RAFM steels irradiated by energetic neutrons,in body-centered cubic(BCC)iron.Detailed contents of this thesis as following:(1)The deuterium transport parameters in these 7 RAFM steels were obtained by the GDP device,namely permeability,diffusivity,and solubility.The measurement covered the temperature range of 623 K to 873 K,and the loading pressures of 1.8×104 to 1.0×105 Pa.Experimental results indicate that the deuterium permeability has little material dependence.In contrast,the deuterium diffusivity of the studied materials shows significant variation.The deuterium diffusivity in ODS steels is one order of magnitude lower than traditional RAFM steels and CNAs,and correspondingly,has an effective solubility 2 to 10 times greater than RAFM steels and CNAs.(2)TDS measurements were performed to assess the deuterium retention and desorption of the seven studied materials following a static thermal deuterium charging at 723 K for 1 hour under the deuterium pressure of 1.0×105 Pa.It was found that ODS steels exhibit the highest deuterium retention,as well as the border and higher temperature of desorption peaks.Microstructural features are considered to contribute to deuterium retention.Therefore,sink strength theory were applied to rationalize the relationship between sink strength and the observed deuterium retention in the studied RAFM steels.(3)MD simulation method was used to evaluate the H trapping effect by nanosized He bubble effect in BCC iron.The effects of He-to-vacancy(He/V)ratio(0.5 to 1.5),bubble size(1 to 4 nm),H concentration(664 to 2641 appm),and temperature(300 to 723 K)on the H evolution were studied in detail.The simulation results suggest that H prefer to the reside at the He bubble-iron matrix interface,with only a small fraction of trapped H inside the bubble.H capture and trapping at He bubbles was found to be a reaction rate-controlled process.H2 molecules were observed inside the He bubbles and the relation between H2 formation and the He/V ratio within the bubble was discussed.The binding energy of atomic H to a 2-nm He bubble with a He/V ratio of 1.0 was calculated to be 0.73 eV at 0 K by molecular statics(MS),and the corresponding de-trapping energy was 0.80 eV.These results can benifit a quantitative estimate of the H trapping capacity of He bubbles for a variety of situations.