PMI issues beyond ITER

1. PMI issues beyond ITER Presented by R. Doerner University of California in San Diego Special thanks to J. Roth (IPP-Garching) and M. Baldwin (UCSD) for their advice

2. PISCES The missions of ITER & DEMO will force a sea change in emphasis. • The ITER mission is power generation [Q = 10] • Requires the core plasma to function • Wall is secondary, its only purpose is to allow core to operate successfully • The mission of DEMO/reactor is power conversion • Wall/blanket system must function • Core plasma will become secondary, its purpose will be to supply power to the wall

3. PISCES Even ITER barely begins to address DEMO relevant PMI issues. • PFC armor materials, Be and C, are not reactor relevant • Elevated wall temperature necessary for efficient power conversion • Tritium fuel cycle • Neutron effects will alter material performance

4. PISCES Tritium loss terms in reactors must be understood and carefully controlled. • Too much in-vessel inventory and the Tritium Breeding Ratio may drop below unity • Smaller than expected in-vessel T inventories may result in unexpected on-site tritium surplus • Best possible result for ITER, but perhaps not for DEMO • Trapped tritium inventories must be understood • Bulk retention in PFCs • Erosion of plasma-facing materials can lead to tritium trapping due to codeposition (as well as a reduction in PFC lifetime)

5. PISCES Tritium losses include retention of implanted energetic particle flux. • Different materials exhibit different retention behavior with increasing fluence • Identical elements can behave differently depending on their structure • Measurements above a fluence of 1026 m-2 are sparse • Plenty of low temperature data on W exist From J. Roth et al., PPCF 50(2008)103001.

6. PISCES Data base for retention in W: e.g. fluence dependence at 500 K Experimental scatter emphasizes lack of understanding of underlying retention principles. Possible explanations: - Material differences - Sample pretreatment - Exposure conditions - Measurement techniques

7. PISCES Fortunately, retention in W at high surface temperature is consistently reported to be low. • Low temperature peak in retention correlates with surface blister formation • More high temperature (>900K), large fluence retention data needed • For 1000 m2, 1019 m-2 T retention is ~1.5gm/min, or ~0.1 kg/hour • T permeation becomes an issue Conservative ITER estimate

8. PISCES Particle flux in DEMO will contain both T and He. TEM image of W sample exposed at ~300°C in P-B to D/He plasma (Eion ~ 60eV) from M. Miyamoto • Recent results show strong interaction of low energy He with W • At low temperature, a small He concentration (a few percent) results in nanobubble formation in the near surface region, suppressing blisters and deuterium retention D2/He plasma D2 plasma Desorption spectra from W exposed at 300°C (Eion~60eV) Fluence ~5e25 m-2

9. PISCES At high temperature He in the incident plasma also exhibits strong interactions with W surfaces. Cross section view of fractured W targets after He plasma exposure Simultaneous D/He plasma exposure produces identical W nano-structured surfaces [Consistent He plasma exposures: Ts = 1120 K, GHe+= 4–6×1022 m–2s–1, Eion ~ 40 eV]

10. PISCES At 1120 K, nano-structured layer thickness does not saturate with He plasma exposure time. From M. Baldwin and R. Doerner, NF 48(2008)035001 300 s 2000 s 4300 s 9000 s 22000 s Consistent He plasma exposures: Ts = 1120 K, GHe+= 4–6×1022 m–2s–1, Eion ~ 60 eV Growth rate follows time0.5 dependence, similar growth in D/He plasma exposures

11. PISCES Nano-structured surface may lead to several difficulties. • Thermal properties of W fuzz may lead to overheating of surface and increased vaporization of the surface • W nano-structures have little strength (can be mechanically dislodged from surfaces easily) possibly resulting in an increase in dust production during shocks (mechanical, thermal, etc) to the surface • Either, or both, of these effects may decrease the lifetime of plasma facing surfaces

12. PISCES Surface lifetime is also affected by erosion, which is often thought to be independent of temperature. • Radiation enhanced sublimation is well known for carbon-based materials. Interstitials created in the bulk migrate to the surface where they are less strongly bound and sublimate. From V. Phillips et al., JNM 179-181(1991)25. From J. Roth et al., JNM 111&112(1982)775.

13. PISCES Temperature dependent erosion is also observed from metal surfaces. • Be (left) and Au (above) data have been compared with possible models • Lighting manufacturers have used these models to explain short W filament lifetimes (~3000°C) From R. Doerner et al., JAP 95(2004)4471.

14. PISCES Liquid surfaces are not immune to temperature dependent erosion. • Temperature dependent erosion mechanisms will reduce the operational temperature window for free liquid surfaces • Temperature dependent erosion also measured for liquid gallium From R. Doerner et al., JNM 313-316(2003)383.

15. PISCES PMI at elevated temperature can influence surface material loss rate. • Material loss rates impact reactor operation • Impurity content in core • Lifetime of wall • Changing thickness will alter thermal gradients in armor which will in turn effect tritium inventory in the armor • Material mixing • Increases in material loss rates will provide more material available for codeposition with fuel • Very little PMI data at elevated (DEMO relevant) wall temperature is available

16. PISCES Different mixed-material phases form in different temperature regions (e.g. in ITER Be-W). Reaction kinetics can favor different alloy formation as temperature increases From C.R. Watts, International Journal of Powder Metallurgy 4 (3) (1968) 49. From R. Doerner et al., JNM 342(2005)63.

17. PISCES Codeposits formed at high temperature tend to retain less tritium. • Empirical scalings (T, E, r) allow predictions of retention despite the large scatter in the codeposition database • Most codeposition studies do not extend above 300-400°C • Accumulation in high temperature codeposits likely to be low • How low? • Where will codeposits form and what is their temperature From J. Roth et al., PPCF 50(2008)103001.

18. PISCES Role of neutron damage on retention in tungsten is just beginning to attract interest. • Diffusion/permeation rate of T in damaged material needs investigation at more relevant temperature • Damage may anneal during high temperature, steady-state operation • Does neutron damage differ from ion induced damage • Modeling of trap creation and filling deep (beyond micron scale) in material have started

19. PISCES Summary: PMI studies with realistic temperature are needed. • Power conversion efficiency will push toward increased temperature armor • Cost of electricity will push toward compact reactors with high power density, which will push armor temperature up • Steady-state operation will allow sufficient time for elevated temperature PMI to fully develop • Existing machines (& ITER) operate at lower temperatures, in pulsed fashion • Almost all aspects of PMI are temperature dependent • Tritium retention, diffusion, recombination, surface material loss rates, chemical reactions, damage annealing, codeposition, etc.