Takuji Oda

Comparison of deuterium retention for ion-irradiated and neutron-irradiated tungsten

The behavior of D retention for Fe2+-irradiated tungsten with a damage of 0.025–3 dpa was compared with that for neutron-irradiated tungsten with 0.025 dpa. The D2 thermal desorption spectroscopy (TDS) spectra for Fe2+-irradiated tungsten consisted of two desorption stages at 450 and 550 K, while that for neutron-irradiated tungsten was composed of three stages and an addition desorption stage was found at 750 K. The desorption rate of the major desorption stage at 550 K increased as the displacement damage increased due to Fe2+ irradiation increasing. In addition, the first desorption stage at 450 K was found only for damaged samples. Therefore, the second stage would be based on intrinsic defects or vacancy produced by Fe2+ irradiation, and the first stage should be the accumulation of D in mono-vacancy and the activation energy would be relatively reduced, where the dislocation loop and vacancy is produced. The third one was found only for neutron irradiation, showing the D trapping by a void or vacancy cluster, and the diffusion effect is also contributed to by the high full-width at half-maximum of the TDS spectrum. Therefore, it can be said that the D2 TDS spectra for Fe2+-irradiated tungsten cannot represent that for the neutron-irradiated one, indicating that the deuterium trapping and desorption mechanism for neutron-irradiated tungsten is different from that for the ion-irradiated one.

The deuterium depth profile in neutron-irradiated tungsten exposed to plasma

The effect of radiation damage has been mainly simulated using high-energy ion bombardment. The ions, however, are limited in range to only a few microns into the surface. Hence, some uncertainty remains about the increase of trapping at radiation damage produced by 14 MeV fusion neutrons, which penetrate much farther into the bulk material. With the Japan-US joint research project: Tritium, Irradiations, and Thermofluids for America and Nippon (TITAN), the tungsten samples (99.99 % pure from A.L.M.T., 6mm in diameter, 0.2mm in thickness) were irradiated to high flux neutrons at 50 C and to 0.025 dpa in the High Flux Isotope Reactor (HFIR) at the Oak Ridge National Laboratory (ORNL). Subsequently, the neutron-irradiated tungsten samples were exposed to a high-flux deuterium plasma (ion flux: 1021-1022 m-2s-1, ion fluence: 1025-1026 m-2) in the Tritium Plasma Experiment (TPE) at the Idaho National Laboratory (INL). First results of deuterium retention in neutron-irradiated tungsten exposed in TPE have been reported previously. This paper presents the latest results in our on-going work of deuterium depth profiling in neutron-irradiated tungsten via nuclear reaction analysis. The experimental data is compared with the result from non neutron-irradiated tungsten, and is analyzed with the Tritium Migration Analysis Program (TMAP) to elucidate the hydrogen isotope behavior such as retention and depth distribution in neutron-irradiated and non neutron-irradiated tungsten.

Study of intrinsic defects in 3C-SiC using first-principles calculation with a hybrid functional.

Density functional theory (DFT) with a tailored Hartree-Fock hybrid functional, which can overcome the band gap problem arising in conventional DFT and gives a valence band width comparable with experiment, is applied to determine formation energies and electronic structures of intrinsic defects in cubic silicon carbide (3C-SiC). Systematic comparison of defect formation energies obtained with the tailored hybrid functional and a conventional DFT functional clearly demonstrates that conventional DFT results are not satisfactory. The understanding on intrinsic defects, which were previously investigated mainly with conventional DFT functionals, is largely revised with regard to formation energies, electronic structures and transition levels. It is found that conventional DFT functionals basically lead to (i) underestimation of the formation energy when the defect charge is more negative and (ii) overestimation when the defect charge is more positive. The underestimation is mainly attributed to the well-known band gap problem. The overestimation is attributed to shrinkage of the valence bands, although in some cases such band shrinkage may lead to underestimation depending on how the defect alters the valence band structure. Both the band gap problem and the valence band shrinkage are often observed in semiconductors, including SiC, with conventional DFT functionals, and thus need to be carefully dealt with to achieve reliable computational results.

MONTE CARLO SIMULATION ON PERMEATION OF HYDROGEN ISOTOPES THROUGH bcc Fe

Abstract A code to model permeation behavior of hydrogen isotopes through bcc Fe was developed by means of a Monte Carlo technique. This code enables correlation of atomic-scale information such as diffusion barrier or adsorption energy with macroscopic quantities such as solubility or permeability. Model parameters were derived from results of ab initio calculations in density function theory. To validate the code, both temperaturedependent permeability and pressure-dependent solubility for hydrogen were evaluated. Simulation results provided reasonable permeability and solubility compared with experimental data, and adequately showed their temperature/pressure dependence.

UPS study on the interaction of hydrogen isotopes with L2O surface

Abstract The electronic states of Li2O were observed using ultraviolet photoelectron spectroscopy (UPS) and X-ray photoelectron spectroscopy (XPS). By exposing to the different pressures of H2O vapor, the peak attributed to the surface hydroxyl groups was observed. After exposing to the enough pressure to compose LiOH, an obvious change has not been observed in UPS spectra by the precision of the present study although the change was observed in XPS. The density of states (DOS) of Li2O was calculated using CASTEP code, and the obtained spectra were discussed using the results of the calculation.

Nuclear Engineers for Society: What Education can do

Engineering education is a key factor in determining the range of engineers’ expertise, the attitude and the behavior of engineers, and the culture of the engineering professional community. This chapter is devoted to nuclear engineering education post-Fukushima Daiichi nuclear accident. Prior to education itself, knowledge and attitudes required of nuclear engineers are firstly discussed, focusing on social aspects of nuclear technology. I emphasize the importance of mutual communication with society, not only with the general public but also with experts in other fields, by referring to 3 points which are essential for appropriate advancement of nuclear engineering and can be reinforced with mutual communication: social legitimacy of nuclear technology, introspection within the nuclear professional community, and public trust in nuclear technology and the professional community. These points are not only needed for smooth utilization of nuclear technology, but also, and more importantly, needed for enhancing the safety of nuclear technology utilization and advancing nuclear technology to provide more benefits and welfare to society. Finally, I propose 4 items for education reform, which are mainly designed to make mutual communication with society more effective while maintaining a high level of technical expertise: standardization and internationalization, transparency and sharing, social-scientific literacy education, and development and evaluation of faculty.

Effect of Grain Size on Hydrogen Isotope Behavior in LiNbO3

Abstract Understanding the tritium behavior in breeder materials is essential for the safe and economic operation of fusion reactor. In this paper, the effect of grain size on bulk diffusion of hydrogen isotope in ternary lithium oxide was studied. LiNbO3 single-crystal samples of different grain sizes were used: cubic samples (a) 4.7×4.7×4.7 mm3, (b) 1.5×1.5×1.5 mm3, and (c) 0.46×0.46×0.46 mm3; powder sample (d) 45 μm (a.v). With the grain size of cubic samples decreasing, it was clearly observed that the position of TDS peak related to bulk diffusion moved to a lower temperature because of shorter diffusion path. In comparison with computational simulation based on a diffusion model, a fair agreement between experiment and calculation was obtained.

Development of Monte Carlo Simulation Code to Model Behavior of Hydrogen Isotopes Loaded into Tungsten Containing Vacancies

Abstract The behavior of hydrogen isotopes implanted into tungsten containing vacancies was simulated using a Monte Carlo technique. The correlations between the distribution of implanted deuterium and fluence, trap density and trap distribution were evaluated. Throughout the present study, qualitatively understandable results were obtained. In order to improve the precision of the model and obtain quantitatively reliable results, it is necessary to deal with the following subjects: (1) how to balance long-time irradiation processes with a rapid diffusion process, (2) how to prevent unrealistic accumulation of hydrogen, and (3) how to model the release of hydrogen forcibly loaded into a region where hydrogen densely exist already.

Experiment to Recover Tritium from Li-Pb Blanket and Understanding Chemistry of the Li17Pb83–H System

Abstract Various properties of Li-Pb eutectic alloys have been reported aiming at adopting the tritium breeder for the next-step fusion reactor, DEMO. Relations among several physical or chemical properties are reinvestigated here based on not only macroscopic views of the H isotope solubility in Li-Pb and the chemical activity of Li and Pb atoms in the alloy but also a microscopic view on the state of being of H and Li atoms in alloy based on the 1st principle molecular dynamic (MD) numerical calculation. The Sieverts’ constant of H dissolved in Li-Pb is closely related with the chemical activity of Li in Li-Pb. It is found that H dissolved in Li-Pb eutectic alloy has an ionic Li+-H− bond with a single Li atom independent of other Li or Pb atoms and the Li+-H− ionic bond is isolated from another Li atom surrounded by Pb ones. The isotope effect for the Sieverts’ constant is also understood in terms of the state of being of the Li+-H− bond in the alloy. The amounts of inert gases dissolved in the Li-Pb eutectic alloy are evaluated, and it is found that their solubilities are in proportion to the square of the molecular diameter which is estimated from exclusive volume of dissolved gas and consequently with the open space volume among Li-Pb atoms. Two experimental results of hydrogen isotopes recovery are introduced using a permeation window and a Li-Pb and inert gas direct contact method, and mass-transfer coefficients to correlate the overall hydrogen transfer process are determined as a function of diffusivity and flow velocity.

Recovery of Tritium Dissolved in Sodium at the Steam Generator of Fast Breeder Reactor

Abstract The tritium recovery technique in steam generators for fast breeder reactors using the double pipe concept was proposed. The experimental system for developing an effective tritium recovery technique was developed and tritium recovery experiments using Ar gas or Ar gas with 10-10000 ppm oxygen gas were performed using D2 gas instead of tritium gas. It was found that deuterium permeation through two membranes decreased by installing the double pipe concept with Ar gas. By introducing Ar gas with 10000 ppm oxygen gas, the concentration of deuterium permeation through two membranes decreased by more than 1/200, compared with the one pipe concept, indicating that most of the deuterium was scavenged by Ar gas or reacted with oxygen to form a hydroxide. However, most of the hydroxide was trapped at the surface of the membranes because of the short duration of the experiment.