D. Kato

H. Iwakiri

K. Morishita

M. Miyamoto

N. Yoshida

National Institute for Fusion Science (NIFS-Repository)

For the neutron multiplier, beryllium metal (Be) is a reference material in the blanket design. However, it may not be applicable to the DEMO blanket, which requires high temperature and neutron dose, because of high reactivity and large swelling. Therefore, it is necessary to develop an advanced material for a neutron multiplier that has high temperature resistance and high radiation resistance. Berylides such as Be12Ti and Be12V have been suggested as promising candidates for advanced neutron multipliers from the viewpoints of high melting point, high beryllium content, fast decay of gamma dose rate and good chemical stability, etc. The behaviour of hydrogen isotopes in the neutron multiplier will influence the safety of the reactor. Furthermore, the generated hydrogen isotope causes problems for the neutron multiplier with respect to tritium retention. In this work, the interaction of atomic hydrogen with Be12Ti has been studied by means of first principles electronic-structure methods.  All the calculations were performed using the density functional theory (DFT) method as implemented in the SIESTA code1). The electron exchange and correlation are treated within the generalized gradient approximation (GGA) of the Wu-Cohen modification of Perdew−Burke−Ernzerhof (PBE-WC) function2). The electron-ion interactions were described by normconserving Troullier  Martins pseudo potentials factorized in the Kleinman-Bylander form. The valence wave functions were expanded in a basis set of localized atomic orbitals, and both single- (SZ) and double- polarized (DZP) basis sets were used for the purpose of comparison. The calculations were carried out in a supercell containing 52 atoms. The convergence of the calculations has been tested, and we found a 4  4  4 k-point grid and a 200 Ry cutoff for the real space mesh can give well converged results.  Relaxation is performed until all the Hellmann−Feynman forces are converged to 0.01 eV/Å. Electronic convergence is achieved when the difference between the total energies of the last two consecutive steps is less than 10−4 eV. To build the lattice energy–volume curves, the lattice energy was computed as a function of the lattice volume. Fig. 1 shows the lattice energy–volume curves for Be12Ti, and Table 1 lists the parameters of the corresponding fitted Murnaghan equations of state for the sake of comparison3). It was fitted by the Levenberg-Marquardt algorithm to the resulting lattice energy–volume curves, yielding directly the parameters E0, V0, B0, and B0’. The calculated lattice parameter, a =b= 7.625 Å, c=4.319 Å were slightly large as compared with experimental values, 3.6% and 4.7% respectively. The bulk modulus, B = 100 GPa, were found to be in good agreement with the experimental values of 117 GPa in the framework of the density functional theory. The solution energy   of a H atom was calculated as follows:            (1) were  is the total energy of the system with hydrogen in Be12Ti,   is that of Be12Ti perfect crystal, and  is that of an H2 molecule. Hydrogen atoms accommodated with in twenty different sets of lattice site, resulting in solution energy of +0.1 eV +0.7 eV. These results indicate that Be12Ti intermetallic alloy is endothermic hydrogen occluding metal. Since the endothermic alloys have “positive” solution energy and fast diffusion, hydrogen reemission occurs earlier with increase temperature. Consequently the hydrogen retention coefficient decrease. Method a=b () c() a/c B (Gpa)This work 7.625 4.391 1.7365 100.1 Exp. 7.360 4.195 1.754 117 Table 1. The comparison of the equilibrium structural parameters from this work and from experiments. 1) E. Artacho, Phys. Status Solidi B 215 (1999) 8092) Z. Wu and R.E. Cohen, PRB 73 (2006) 235116 3) C. Li and P. Wu, Chem. Mater. 13 (2001) 4642-4648 Fig.1. Total energy as function of the volume calculated in this work. The points are from the SIESTA calculations; the dashed line is the fit of the Murnaghan equation of state.      	323§36. Determination of Solution Energy of Hydrogen in Be12Ti by First Principles Electronic-structure MethodsIwakiri, H., Yoshida, N. (Univ. Ryukyus),  Miyamoto, M. (Shimane Univ.), Morishita, K. (Kyoto Univ.),  Kato, D.

Determination of Solution Energy of Hydrogen in Be12Ti by First Principles Electronic-structure Methods

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