Multi-scale modeling of hydrogen transport in porous graphite

Transcription

1 Multi-scale modeling of hydrogen transport in porous graphite Ralf Schneider Computational Science Institute of Physics Ernst-Moritz-Arndt Universität, Greifswald

2 Hydrocarbon-codeposition Emppu Salonen, Univ. of Helsinki G F Counsell, Plasma Sources Sci. Technol. 11 (2002) A80 A85 chemical Erosion of carbon by hydrogen produces hydrocarbon species (C x H y ) dissociation & recombination's leads to amorphous hydrocarbon layer formation these act as sponge for hydrogen Tritium is retained by co-deposition with carbon, on the plasma facing sides or on remote areas.

3 Safety limit problem safety operating limit of 350 g of Tritium in ITER-FEAT Codep. rate No. Pulses to reach (g-t/pulse) limit 350 g-t co-deposition prediction by Brooks (ANL): 2-5 g-t/pulse different co-deposited films (soft, hard, hydrogen content) determined also by species distribution

4 Diffusion in graphite Real structure of the material needs to be included Internal Structure of Graphite Granule sizes ~ microns Void sizes ~ 0.1 microns Crystallite sizes ~ Ångstroms Micro-void sizes ~ 5-10 Ångstroms Multi-scale problem in space (1cm to Ångstroms) and time (pico-seconds to seconds)

5 Multi-scale approach Real structure of the material needs to be included Macroscales KMC and Monte Carlo Diffusion (MCD) Mesoscales Kinetic Monte Carlo (KMC) Microscales Molecular Dynamics (MD)

6 Plasma wall interaction Processes included in the model: H 2 molecule desorption Hydrogen atom desorption from the surface Trapping at the crystallite edges Hydrogen atom diffusion within the crystallites Hydrogen atom desorption from the internal surface Recomb. of adsorbed H-atom with a trapped H-atom at the crystallites edge Granules Hydrogen atom diffusion along crystallite-microvoid interface H 2 molecule diffusion Hydrogen atom diffusion along granule-void interface Detrapping from the crystallite edges Crystallies Void Going into the bulk

7 Max-Planck Institute for Plasma Physics, EURATOM Association

8 Parameters ( 0,E m, L ) Max-Planck Institute for Plasma Physics, EURATOM Association Parametrization of processes Hydrogen atoms Diff. channel 1 Diff. channel 2 ω = ( s -1 ) E m =2.6 ev L = 1 Å Detrapping ω = ( s -1 ) E m =2.67 ev L = 3 Å Going into crystallite ω = ( s -1 ) E m =0.9 ev L = 2 Å Desorption Hydrogen molecules ω = ( s -1 ) E m =2.0 ev L = 3 Å Simple jump ω = ( s -1 ) E m =4.45 ev L = 2 Å Dissociation ω = ( s -1 ) E m =0.06 ev L = 10 Å Desorption Recombination

9 Effect of voids A: 10 % voids B: 20 % voids C: 20 % voids Larger voids Longer jumps Higher diffusion

10 Meso/macro-scales standard graphites 2 D(cm / s) highly saturated graphite 1000 / T ( ) K 1 Large variation in observed diffusion coefficients Strong dependence on void sizes and not void fraction Diffusion coefficients without knowledge of structure are meaningless

11 Tore Supra deposits Toroidal pumped limiter Cross-section of Tore-Supra Neutraliser No signs of wall saturation (50 % hydrogen is retained) Where is the hydrogen going? Flux ~ H m -2 s -1 Temperatures up to 1500 K

12 Structure of Tore Supra deposits TS deposits analyzed: Adsorption isotherm measurements Electron microscopy SEM Parallel network of slit-shaped pores Multi-scale porosity Micropores ( < 2 nm, ~ 11 %) Mesopores ( < 50 nm, ~ 5%) Macropores ( > 50 nm, ~ 10%) TEM TEM Parallel to oval axis Perpendicular to oval axis

13 Chemical erosion of graphite Mechanism of molecule formation: CH 3 molecule formation only within the implantation zone (local chemistry, sticking) H 2 molecule formation even beyond the implantation zone (diffusion through voids)

14 Flux dependence of chemical erosion Experiment: Flux dependence of chemical erosion J. Roth et. al., Nucl. Fusion 44 (2004) L21 L25? Need for a better understanding of chemical erosion (quantitatively) Quantifying co-deposition Can we still use carbon as PFM?

15 Flux dependence of chemical erosion Release probability determined by geometrical constraints Erosion yield from 3D KMC model Only few surface layers accessible by incoming hydrogen atom Upper limit of the released carbon flux

16 Flux dependence of chemical erosion

17 Implementation of structure in the code

18 Results: H retention at meso scale Flux H m -2 s H m -2 s H m -2 s H m -2 s -1 At very high flux (10 24 H m -2 s -1 ) a lot of hydrogen is retained in molecular form within the crystallites

19 Results: H re-emission at macro scale Meso - pores Macro - pores Higher void fraction, larger internal surface area, trapping prob., therefore, more molecules and lesser atoms are re-emitted

20 Re-emitted Flux (%) Re-emitted Flux (Fraction) Max-Planck Institute for Plasma Physics, EURATOM Association Hydrogen re-emission Experiment: P. Franzen, E. Vietzke, J. Vac. Sci. Technology A12(3), 1994 Simulation: H 2 5% H 2 9% H 2 8% H 8% H 5% H 9% Temperature (K) H-atom release is limited by detrapping process,not by diffusion Temperature(Kelvin) Simulation matches very well with experiment

21 Integrated modeling physics: hierarchical models needed (upgrading, downgrading) validation (experiment, theory) mathematics: multi-scale strategies for weakly and strongly coupled systems technical: open source, standard interface, benchmarking