Super-saturated hydrogen effects on radiation damages in tungsten under the high-flux divertor plasma irradiation

Tungsten is a prime candidate as the divertor material of the ITER and DEMO reactors, which would be exposed to unprecedentedly high-flux plasmas as well as neutrons. For a better characterization of radiation damages in the tungsten under the divertor condition, we examine influences of super-saturated hydrogen on vacancies in the tungsten. The present calculations based on density functional theory (DFT) reveal unusual phenomena predicted at a super-saturated hydrogen concentration: (1) strongly enhanced vacancy concentration with the super-saturated hydrogen concentration is predicted by a thermodynamics model assuming multiple-hydrogen trapping, i.e. hydrogen clusters formation, in the vacancies; and (2) DFT molecular dynamics revealed that hydrogen clusters can prevent a vacancy from recombining with the neighboring crowdion-type self-interstitial-atom. This suggests that neutron damage effects will be increased in the presence of the hydrogen clusters

Characterization of Ion Cyclotron Wall Conditioning Using Material Probes in LHD

The ion cyclotron wall conditioning (ICWC) is one of the conditioning methods to reduce impurities and to remove tritium from the plasma facing components. Among the advantages of ICWC are the possible operation under strong magnetic field for fully torus area based on the charge exchange damage observed in thin SS samples arranged on a hexahxedron block holder with three different facings, the areas influenced by ICWC is estimated. On the plasma facing area of the material holder, high density of helium bubbles is observed by transmission electron microscope (TEM). But the other areas show no observable damage. The fact that the bubble were observed only in a sample facing the plasma implies that the effective particles, most probably charge exchange neutrals come to the wall straightly Thus, cleaning of the surfaces un-exposed to plasma directly and those in shadow area is difficult by ICWC

Reactivity with water vapor and hydrogen storage capacity of Be2Ti compound

Beryllium intermetallic compounds show a variety of excellent properties such as neutron multiplication, refractory function, hydrogen storage, and superconductivity. Be12M compounds (M = Ti, V, and Zr) have been investigated as neutron multipliers for fusion reactors, while Be17M2 compounds have been explored as refractory materials. Furthermore, Be2Ms are known to have Laves phases, which are characterized by an A2B type compound having high H2 gas storage potential. Because of its low density, the hydrogen properties of Be2Ms have attracted great interest from viewpoints of reactivity with H2O, trap site of hydrogen, and amount of H2 gas in this compound. However, few studies have dealt with Be2M, and its database remains unsatisfactory. Preliminary synthesis of a beryllium intermetallic compound (¼Be2Ti) as a hydrogen storage material was conducted to clarify its reactivity with water vapor at high temperatures and high hydrogen storage capacity. X-ray diffraction profiles and electron-probe microanalysis results confirmed that the preliminary synthesis of single-phase Be2Ti by homogenization treatment and plasma sintering was successful. The hydrogen generation rate of Be2Ti by reaction with 1% H2O increased as the test temperature increased. High temperature exposure to H2O led to the formation of TiO2 on the surface. Furthermore, the hydrogen gas storage concentration of Be2Ti, evaluated using the pressure-concentration-temperature curve, was 0.56 wt.% (¼0.125 H/M) at 298 K, which is relatively low considering that H2 pressure was increased up to 13 MPa. Based on additional pressure-composition-temperature measurements, it does not appear to have particle size dependence with regard to hydrogen capacity. A simulation based on first-principles calculation indicated the presence of twohydrogen trap sites, tetrahedral and center of triangle with solution energies of 0.52 and 0.05 eV, respectively, implying that the maximum trap site of hydrogen with 5.4 wt.%. This dissimilarity might be attributed to the fact that the Be2Ti sample contained a large fraction of surface oxide layer, which disturbed the surface penetration of hydrogen

Interstitial Diffusion of C Interacting with Ambient H in Tungsten Crystals

Negative binding energies between interstitial C (octahedral) and H (tetrahedral) in a bulk crystal of W (bcc) were obtained with the first-principle calculations, which indicate repulsive interaction in the interstitial C-H pair. The electron cloud associated to the each interstitial atom was analyzed with Bader\u27s method. This analysis gives negative fractional charges of ?0.35 and ?0.37 for the interstitial C and H, respectively, supporting the repulsive interaction between them. Interstitial diffusion of C was studied including influences of ambient H atoms in the mean field approximation and the ergodic assumption. The calculated diffusion coefficients are significantly increased by the repulsive interaction with the H atoms

Interaction of primary precipitates in reduced -activation ferritic/ martensitic steel F82H with hydrogen atoms: Atomistic calculation based on the density functional theory

To understand the interaction of hydrogen isotopes with VC and Cr23C6 precipitates, in reduced-activationferritic/martensitic steel (F82H), we have performed first-principles calculations based on density functionaltheory. Energy calculations and electron density analysis were performed with a focus on the hydrogen retentionby vacancies in precipitates. The H atoms in the C vacancy in the VC crystal are bound to the surrounding Cratoms by relatively weak covalent forces and Coulombic attraction, and up to four H atoms are trapped. In thecase of a Cr vacancy, H atoms are strongly covalently bonded to neighboring C atoms as well as to an interstitialH atom, capturing up to six H atoms. H atoms in vacancies in Cr23C6 have a weak bonding force with the Cr atombut a strong repulsive force with the C atom. As a result, H atom is not trapped in the Cr(4a) vacancy, and Hatoms are trapped only at a distance from the C atom in the Cr(48H) and Cr(32f) vacancies. The Cr(8c) and Cvacancies are relatively far from the C atoms and have higher hydrogen trapping energies. The Cr23C6 precipitatecontaining vacancies may be a dominant trapping site in the temperature range when H atoms immediatelydissociate from vacancies. In addition, the VC precipitate may be the only trapping site for hydrogen isotopes athigher temperatures when Cr23C6 precipitates cannot trap them

nteraction of Primary Precipitates in F82H steel with hydrogen atoms: Atomistic Calculation Based on the Density Functional Theory

A recent experiment for hydrogen retention in F82H steel by thermal desorption spectroscopy after deuterium-ion irradiation shows the primary precipitates of metal carbide such as M23C6 and MX might play a role of remarkable trap site for hydrogen isotopes. However, the trapping and diffusion behaviour of hydrogen isotopes is not understood well. Since it is well known that vacancies in crystals are major trapping sites for hydrogen, the purpose of this study was to theoretically understand fundamental feature of the interaction between vacancies and hydrogen in carbides by density functional theory calculations.20th International Conference of Fusion Reactor Material