1. Improving confidence in the materials choices for ITER
Session 6
Improving confidence in the materials choices for ITER
A synthesis of material pros & cons, unresolved issues and open questions

2. Some personal reflections to ‘set the scene’…...

3. The currently defined set of ITER materials must be the starting point for any discussion of materials choices

4. The currently defined set of ITER materials must be the starting point for any discussion of materials choices
CFC at the strike-points
No melting during disruptions/large Type I ELMs
Good thermal conductivity
Intrinsic radiation impurity
Assumption: transient power loads sufficient to melt high Z

5. The currently defined set of ITER materials must be the starting point for any discussion of materials choices
CFC at the strike-points
No melting during disruptions/large Type I ELMs
Good thermal conductivity
Intrinsic radiation impurity
Assumption: transient power loads sufficient to melt high Z
W at the baffles
Good power handling
Low erosion
Assumption: no transient power loads sufficient to cause melting

6. The currently defined set of ITER materials must be the starting point for any discussion of materials choices
CFC at the strike-points
No melting during disruptions/large Type I ELMs
Good thermal conductivity
Intrinsic radiation impurity
Assumption: transient power loads sufficient to melt high Z
W at the baffles
Good power handling
Low erosion
Assumption: no transient power loads sufficient to cause melting
Be covering the first wall, upper target and limiters
Low Z
O2 getter
Assumption: limited power loads only (mostly CX fluxes)
high Z would result in unacceptible core impurity

7. These were the best choices at the time of the EDA
These were the best choices at the time of the EDA. However, new experiments, modelling and changes to the equilibrium configuration since the ITER physics basis have raised questions -

8. These were the best choices at the time of the EDA
These were the best choices at the time of the EDA. However, new experiments, modelling and changes to the equilibrium configuration since the ITER physics basis have raised questions -
Significant uncertainty now exists over the validity of some of the assumptions driving the material choices

9. These were the best choices at the time of the EDA
These were the best choices at the time of the EDA. However, new experiments, modelling and changes to the equilibrium configuration since the ITER physics basis have raised questions -
Significant uncertainty now exists over the validity of some of the assumptions driving the material choices
Some of the material choices have unwanted features which could introduce new problems

10. These were the best choices at the time of the EDA
These were the best choices at the time of the EDA. However, new experiments, modelling and changes to the equilibrium configuration since the ITER physics basis have raised questions -
Significant uncertainty now exists over the validity of some of the assumptions driving the material choices
Some of the material choices have unwanted features which could introduce new problems
To reduce the uncertainty in extrapolation of PMI issues in ITER, help optimise the H phase and influence the design choice for future (or perhaps even initial) plasma facing components:
Our ITPA needs to quickly and clearly identify the key unknowns, propose and carry out co-ordinated research and development to fill these gaps - a basis for decisions to be taken

11. These were the best choices at the time of the EDA
These were the best choices at the time of the EDA. However, new experiments, modelling and changes to the equilibrium configuration since the ITER physics basis have raised questions -
Significant uncertainty now exists over the validity of some of the assumptions driving the material choices
Some of the material choices have unwanted features which could introduce new problems
Reminder: The EDA materials mix is an attempt at a compromise - there is probably no ‘perfect’ solution for ITER
Any change at this time would require a rock-solid case and an alternative, holistic solution (i.e. all the impacts must be assessed)
We have concerns but no such case and alternative yet exists

12. Some of the questions …...

13. CFCs and the strike point region
For the strike-point region, is the assumption that ‘transient power loads near the strike-points will be sufficient to melt high Z materials’ valid?
Open ‘questions’:
Evidence that Dh broadens by factor ~10-30 during disruption
Wth released in >O(10ms), 75% radiated (JET) - peak divertor power loads ‘similar’ to Type I ELMs
but
Power load could still be sufficient to melt high Z material
Some evidence for ‘stepped’ Wloss - steps can be rapid O(100ms) and release 10% of Wth
Dh broadening may not occur at beginning of disruption, some Wloss may be into pre-disruption Dh

14. CFCs and the strike point region
CFC specific issues (both for existing devices and ITER):
Where are the primary sources of carbon, poloidally and toroidally? What is the impact of co-deposit re-erosion?
What is the role of ELMs in carbon erosion?
How much divertor carbon will re-deposit on first wall surfaces?
What is the impact of Be+ target fluxes on carbon erosion?

15. CFCs and the strike point region
CFC specific issues (both for existing devices and ITER):
What determines the D/C ratio in co-deposit?
How does Be alter co-deposition? What is the trapping mechanism of D in carbonised Be (or W)?
What is the role of surface temperature in co-deposition?
How toroidally symmetric will the co-deposit be?
Under what conditions does carbon migrate to shadowed regions and non-LOS? Does the divertor geometry play a role?
How much co-deposit will be in tile gaps? Which gaps?, to what depth? and thickness? What is the impact of surface/gap ratio?
Why are estimates of global fuel retention so different? What role does pulse length play? What is the impact of gas puffing?
What is the significance of SOL flows and drifts in carbon (and Be) transport? What is the relative impact of transport to and residence at a location?

16. CFCs and the strike point region
CFC specific issues (both for existing devices and ITER):
What chance do we have of developing a scheme capable of overnight removal of 10’s-100’s g of co-deposited tritium, including from ‘hidden’ regions?
Would oxidisation techniques remove tritium co-deposited with Be and C together?
Would the impact of BeO formation and transport during oxidisation lead to an large increase in tritium trapping (perhaps in compounds with Be:C:O)?
What chance do we have of developing a scheme capable of removal of (potentially) 100’s-1000’s kg of co-deposited tritium between H/D and D/T phases?
Would it be necessary to remove co-deposit from ‘remote’ and ‘inaccessible’ regions between the H/D and D/T phases if CFCs were to be removed?
Can we forsee sufficiently accurate diagnostics to determine the fuel retention during the H (or even D) phases?

17. CFCs and the strike point region
Critical questions -
Given the current data set, how certain can we be that global tritium retention will be less than ~0.1g/shot in a device with a carbon divertor (only)?
What confidence do we have that a technology can be developed on a 10 year timescale capable of overnight removal of ~97% of co-deposited tritium?

18. W and the baffle
For the baffle, is the assumption that ‘transient power loads will not be sufficient to cause melting’ valid?
Open questions:
Evidence that Dh broadens by factor ~10-30 during disruption - could extend out of divertor onto baffle
Possibility of significant ELM power loads, ~10% DWELM on localised regions on limiter/first wall - could include baffles
but
Few direct measurements of transient power loads on baffles
Power loads likely to be dependent on the divertor geometry, difficult to extrapolate from existing devices accurately

19. W and the baffle
W specific issues (both for existing devices and ITER):
Can we extrapolate W erosion/accumulation from the H/D phase to the D/T phase
What is the erosion yield of radiating impurity ions, like neon or argon?
What tools do we need (or have) to estimate W transport in/to the core?
Will He+ bombardment (or He+/H+) lead to blistering or cracking along grain boundaries?
Could thermal cycling by ELM power loading lead to cracking?
Could melt layers lead to cracking along new grain - old grain boundaries?
Would W plasma-spaying (for repairs) lead to inferior material properties (e.g. for thermal cycling)
Do melt layers (liquid or solidified) at the strike-points really represent a problem?

20. W and the baffle Critical question -
What experiments could we do to provide confidence that a W divertor (or limiter/first wall) would not significantly reduce operational space in ITER?

21. Be and the first wall
For the first wall, are the assumptions that ‘CX fluxes will dominate erosion’ and that ‘high Z materials would result in unacceptible core impurity’ valid?
Open questions:
Possibility of significant ELM power loads, ~10% DWELM on localised regions on limiter/first wall
Mitigated disruption power loads may be sufficient to melt Be
Evidence of static limiter/first wall particle fluxes from non-diffusive transport
High d operation, 2nd separatrix at drsep~4cm  possibility of significant ELM/disruption power loads on upper target
High Z wall materials may not lead to unrecoverable impurity accumulation in divertor devices
but
Localised shallow melting may not be disasterous on first wall (worse for limiter/upper target) - although core contamination could still be problematic if steady state or transient limiter/first wall erosion high
First wall ELM power loads might be eliminated by adding more limiters?

22. Be and the first wall
Be specific issues (both for existing devices and ITER):
Would melt layers (liquid or solidified) on the first wall or upper target really represent a problem?
Would mm3 droplets expelled into the core cause a disruption?
Even if no melting, would core impurity contamination from erosion due to steady-state particle fluxes or ELM power loads be acceptible?

23. Be and the first wall Critical question -
How confident are we that Be at the first wall, limiter or upper divertor will not melt in a manner which restricts operations or limits operational space?

24. ITER material options

25. One possible scenario -
How comfortable are we with the EDA materials mix in H phase? -
Probably yes - no T, limited ELMs - but must be convinced either
1) no melting of limiter/first wall by disruptions
2) melting would not limit operational space
Is it necessary to develop an alternative divertor, available day 1? -
Probably yes. Waiting until (if) T retention proves as large as feared and no satisfactory removal schemes are devised to develop an alternative divertor would create long delays
What about first wall, limiter and upper divertor components? -
Debatable but probably yes, at least for limiter. Logically upper divertor wouldn’t need replacements (otherwise we wouldn’t have started). First wall components may cost too much for speculative spare and long repacement time may give opportunites for just-in-time manufacture

26. Any problems with this route? -
Maybe yes.
H phase will be used to ‘fine tune’operational space and identify candidates for optimising DT phase
Much of this operational experience could be wasted if there is a radical change in materials choice (eg no intrinsic impurity radiation, more high Z core impurity contamination, lower tolerance to disruptions or large ELMs etc.)