Lithium aluminate, LiAlO 2 , is a material that is presently being considered as a tritium breeder material in fusion reactors and coating material in Li-conducting electrodes. Here, we employ atomistic simulation techniques to show that the lowest energy intrinsic defect process is the cation anti-site defect (1.10 eV per defect). This was followed closely by the lithium Frenkel defect (1.44 eV per defect), which ensures a high lithium content in the material and inclination for lithium diffusion from formation of vacancies. Li self-diffusion is three dimensional and exhibits a curved pathway with a migration barrier of 0.53 eV. We considered a variety of dopants with charges +1 (Na, K and Rb), +2 (Mg, Ca, Sr and Ba), +3 (Ga, Fe, Co, Ni, Mn, Sc, Y and La) and +4 (Si, Ge, Ti, Zr and Ce) on the Al site. Dopants Mg 2+ and Ge 4+ can facilitate the formation of Li interstitials and Li vacancies, respectively. Trivalent dopants Fe 3+ , Ni 3+ and Mn 3+ prefer to occupy the Al site with exoergic solution energies meaning that they are candidate dopants for the synthesis of Li (Al, M) O 2 (M = Fe, Ni and Mn) compounds.Energies 2019, 12, 2895 2 of 10 Computational MethodsThe present computational study was based on classical pair-wise potential calculations to describe LiAlO 2 via the General Utility Lattice Program (GULP) code [23,24]. In the present approach, the total energy (lattice energy) is determined by long range (i.e., Coulombic) and short range [i.e., electron-electron repulsive and attractive intermolecular forces (van der Waals forces)]. The latter were modelled using Buckingham potentials (refer to the supplementary information). The van der Waals forces arising from the spontaneous formation of instantaneous dipoles are very important as the formation energy results are sensitive to those forces. The present modelling approach takes into the account of Van der Waals forces as a function of the interatomic distance (r) [23,24]. Two-body Buckingham potential mentioned in the supplementary information consists of two parts. The first part of the equation represents the Pauli repulsion (electron-electron) and the second part denotes the van der Waals interaction. Ionic positions and lattice parameters were relaxed using the Broyden-Fletcher-Goldfarb-Shanno (BFGS) algorithm [24]. Convergence criteria dictated that in relaxed configurations, forces on each atom was <0.001 eV/Å. To introduce point defects in the lattice we used the Mott-Littleton methodology [25] similarly to recent work [26][27][28]. It is established that although the present methodology may overestimate the defect formation enthalpies at the dilute limit, the trends will be unchanged [29]. In a thermodynamic perspective defect parameters can be described by comparing the real (i.e., defective) crystal to an isobaric or isochoric ideal (i.e., non-defective) crystal. Defect formation parameters can be interconnected through thermodynamic relations [30,31], with the present atomistic simulations corresponding to the isobaric parameters for the mig...
The influence of the input power, pu ng rate and neutral pressure on the fuel (hydrogen isotopes) inventory of the WEST and ITER divertors is investigated. For the chosen range of parameters (relatively low temperature at the strike points), the inventory of the WEST divertor evolves as the power 0.2 of the pu ng rate and as the power 0.3 of the input power. The inventory at the strike points is highly dominated by ions whereas it is dominated by neutrals in the private zone. Increasing the fuelling rate increases the retention in the private zone and decreases slightly the retention at the strike points. Increasing the input power increases the inventory at the strike points and does not a↵ect much the inventory at the private flux region. The inventory of the ITER divertor is not strongly dependent on the divertor neutral pressure. The inventory increases from 0 Pa to 7 Pa and then decreases slightly from 7 Pa to 10 Pa. After 10 7 s of continuous exposure, the maximum inventory in the ITER divertor was found to be 14 g. The inventory is not maximum at the strike points due to the high surface temperature of the monoblocks in this region. The maximum accumulation of H in the ITER divertor is below 5 mg per 400 s discharge and below 2 mg per 400 s discharge after 200 discharges.
Tritium transport simulations in water-cooled lithium lead (WCLL) breeding blankets have been performed with FESTIM. Fluid dynamics and heat transfer were coupled to evaluate how temperature and LiPb velocity fields affect tritium transport. The 2D WCLL model consisted of tungsten, EUROFER and liquid LiPb. Tritium inventories and permeation fluxes have been computed and the influence of trapping effects in solid domains was highlighted. It was evaluated that inclusion of trapping mechanisms increases inventory values by 15% and delays permeation to the cooling channels. A parametric study was conducted to investigate the impact of the uncertainty in the literature (2 orders of magnitude) regarding hydrogen solubility in LiPb on these quantities. Varying the lithium lead solubility over the range found in the literature was found to vary the EUROFER inventory by a factor of 25. Permeation fluxes to the coolant channels was found to vary over a factor of 3.
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