Managing Tritium Inventory and Permeation in In-Vessel Systems

1. Managing in-vessel tritium inventory and permeation presents a significant technical challenge for both ITER and DEMO • Scope: • Each 400 s ITER pulse uses 120 g of T2 (1.2×106 Ci) • Release limit: 1 Ci/day; Release fraction <10-6 for one shot • Limited Experience: 99 g processed during TFTR D-T campaign • Self-fuel sufficiency requires retention < 0.01% [Tanabe, J. Nucl. Mater. (2013)] • Fuel retention closely coupled with several other factors: • Plasma conditions (flux, fluence, surf. temp., etc.) • Material microstructure • Concentration and type of intrinsic defects within material • Plasma-induced damage (H and He precipitates) • Defects introduced through neutron bombardment • Tritium/Radiation-specific effects: • Enhancement of T diffusion in a radiation environment. • Atomic T can circumvent permeation barriers that block T2 dissociation / chemisorption • 3He accumulation in material / 3He(n,H)Trecoils with E = 100’s keV • Different trap energies, diffusion constants, etc. Tritium inventory in ITER for various PFC options [J. Roth et al., Plasma Phys. Control. Fusion (2008).]

2. Laboratory experiments have provided insight into physics governing hydrogen trapping in un-damaged W • Fluence and temperature dependence extensively explored since the 1990’s. • Absolute values vary considerably between laboratories, but several clear trends can be discerned. Fluence Dependence*: PISCES-B exposures confirm t1/2 scaling, but no saturation* • Temperature Dependence*: • (a) Low D retention at T < 200 °C • Slow kinetics, i.e. permeation. • (b) Max. D retention at 300 - 400 °C • Traps fill and diffusion is fast enough to extend to greater depths. • (c) Low D retention at T > 400 °C • Weak binding of D to traps. a b c T=640 K *W. R. Wampler, Int. H Workshop (2010.) Summary of retention data (temperature dependence) • H plasma flux and ion energy: • Affect near-surface concentration at the end of range • Have a less pronounced effect on hydrogen diffusion and retention *R. Doerner et al, Nucl. Mater. Energy (2016)

3. Retention will be strongly influenced by evolving material microstructure • Effect of specimen microstructure: • Different intrinsic defects for trapping • Strongly coupled to blister and bubble nucleation and growth (trap H2 gas, reduce H permeation into the bulk) • Diffusion enhancement and trapping along grain boundaries • Modification by low energy H plasma: precipitate growth suppressed blistering loop punching blistering Retention in different tungsten grades varies ~ 20x M. Balden, et al J. Nucl. Mater. 2014 rolled W ultra-fine grained W R. Kolasinski, et al Int. J. Ref. Met. & Hard Mater. (2016) recrystallized W ITER-grade W S. Lindig, et al Phys. Scr. (2011)

4. Exposure with He strongly reduces D retention and permeation Molecular dynamics predicts hydrogen decoration of He bubbles (1 – 2 ML deep into matrix), clarifies growth mechanisms N. Juslin & B. D. Wirth, J. Nucl. Mater. (2013); F. Sefta, et al. Nucl. Fusion (2013). D2+He exposure reduces D retention to detection limit Mechanism: (1) He bubble network provides a pathway for H isotope release, or (2) He bubble induced stress acts to trap diffusing H isotopes R. Doerner et al, Nucl. Mater. Energy (2016); M. Baldwin et al, Nucl. Fusion (2011) W. R. Wampler et al, Nucl. Fusion (2009) Y. Uemura et al., presented at ICFRM-17.

5. Many aspects of neutron damage can be investigated using high E ions • Retention too high for TBR > 1 in low fluence damaged tungsten; need damage study at higher fluencew/wo He 20x reduction 643 K data from PISCES-B 0.2 dpa 380 K 0.2 dpa 1200 K Projected 1000 K retention 0.3 dpa 320 K neutrons Upper limit On trapping probability 643 K data – PISCES-B 1 week FW fluence 10 weeks FW Fluence • Recent ion damage + simultaneous exposure of material to low energy D (Markelj et al, PSI 2016). • D may stabilize defects during exposure. G. Tynan, et al., PSI 2016.

6. Data on retention / permeation of neutron damaged materials is limited • TITAN and PHENIX program providing first results • Retention at 500 °C increased dramatically by neutron irradiation M. Shimada, et al., Nucl. Fusion 2015.

7. Potential Topics for Priority Research Directions Near Term Topics • Characterize the details of defects produced by ion / neutron irradiation to compare with simulations • Investigate observed reduction of retention / permeation due to concurrent D/He plasma exposure for ion and neutron damaged W • Measure temperature gradient effects on H diffusion (Soret effect: Q*) Longer Term Topics • Resolve discrepancies between H isotope retention in neutron and ion damaged W • Develop methods for the measurement of tritium retention / permeation needed at high temperatures and longer length scales • Investigate increased permeation in a radiation field (neutron / gamma) • Incorporate improved understanding of damage/diffusion/trapping into integrated PFC model