1. plasma-facing wall protection in EU DEMO IAEA FEC 25th October 2018
Technologies for
plasma-facing wall protection
in EU DEMO IAEA FEC 25th October 2018
Tom Barrett, B. Chuilon, M. Kovari, D. Leon Hernandez, M.L. Richiusa, E. Rosa Adame, R. Tivey, Z. Vizvary and Y. Xue (CCFE)
F. Maviglia (EUROfusion consortium)

2. Summary Wall shaping Discrete limiters
Outer equatorial port limiter
Upper port limiter
Preliminary tritium breeding ratio (TBR) assessment
Tom Barrett | Technologies for plasma-facing wall protection in EU DEMO | IAEA FEC 25th October 2018

3. EU DEMO plasma-facing wall
Vacuum vessel
EU DEMO key missions include tritium self sufficiency and net electric power [1]
Breeding blanket lines the main chamber
RAFM (Eurofer) structural material
2-3 mm tungsten PF surface armour
~1 MW/m2 heat flux limit (He or water) [2]
Breeding blanket segments
Segment edges and gaps
Most of the power to the wall is by thermal radiation but
Peak power densities arise due to thermal charged particles (conducted power) [3]
Divertor
One EU DEMO sector (22.5°),
2017 baseline
Tom Barrett | Technologies for plasma-facing wall protection in EU DEMO | IAEA FEC 25th October 2018

4. Transient events start-up and ramp-down limited configuration
Addressed here
start-up and ramp-down limited configuration
plasma displacement events and disruptions
confinement transients (e.g. H-L)
ELMs
loss of detachment in the divertor
… NOT an exhaustive list
These transients could cause heat fluxes far in excess of the blanket FW limit
Protection of the blanket FW will be vital
Representation of start-up equilibrium, Ip=6MA.
Bold red line: X-Point
Bold green line: LCFS
Image data courtesy of Consorzio CREATE
J. Gerardin et al., presented at PSI-23,
June 2018
Tom Barrett | Technologies for plasma-facing wall protection in EU DEMO | IAEA FEC 25th October 2018

5. Wall design
In ITER a HHF wall-limiter [4] is used to handle the conducted power (static and transient phases). Not deemed suitable for DEMO
Baseline blanket material Eurofer has low thermal conductivity, and need for TBR>1 limits the potential for additional HHF plasma-facing components
Requirement for high temperature coolant for power cycle efficiency
Alignment accuracy, cost…
But like in ITER, a shaped wall may be essential in DEMO to
Spread the power arriving following field lines, especially power in the ‘far’ SOL
Because of the considerable size and cost of the blanket segments, we must allow for modest misalignment of the wall (in steady-state plasma conditions 20mm or more radial displacement of a shaped wall maintains tolerable wall HF [5])
The blanket FW has been shaped using an approximation of the constant power shaping used for ITER [6]
Tom Barrett | Technologies for plasma-facing wall protection in EU DEMO | IAEA FEC 25th October 2018

6. Transients exceed HF limits  Discrete limiters investigated
Wall shaping
Outer blanket modules
Rooftop profile
Small radius central ridge
But with shaping alone, heat fluxes in certain areas are close to the blanket FW limit, even in normal flattop conditions [7]
Transients exceed HF limits
 Discrete limiters investigated A A
SECTIONAL VIEW ON A-A
Tom Barrett | Technologies for plasma-facing wall protection in EU DEMO | IAEA FEC 25th October 2018

7. Discrete Limiters
Aim to protect the rest of the FW from plasma transients
Use HHF technology
Maintenance via VV ports
Few in number to minimise TBR impact
2 types of limiter are considered here
Other types of possible limiters are being studied to protect against a more complete list of transients, including for example downward VDEs and a H-L transition
Outer equatorial port limiter
Receives the wall heat load during the plasma start-up phase (60-100s)
Tom Barrett | Technologies for plasma-facing wall protection in EU DEMO | IAEA FEC 25th October 2018

8. Outer equatorial port limiter
Cooling conditions: water at 150ºC, 5MPa
Tom Barrett | Technologies for plasma-facing wall protection in EU DEMO | IAEA FEC 25th October 2018

9. Discrete Limiters
Aim to protect the rest of the FW from plasma transients
Use HHF technology
Maintenance via VV ports
Few in number to minimise TBR impact
Upper port limiter
Attempts to sacrificially protect the blanket system in the event of a postulated upward- VDE
Outer equatorial port limiter
Receives the wall heat load during the plasma start-up phase
Tom Barrett | Technologies for plasma-facing wall protection in EU DEMO | IAEA FEC 25th October 2018

10. Upper limiter PFC development
Objectives:
protect the remainder of the FW in the event of an upward VDE
shadow the rest of the FW near to the secondary (inactive) X-point
Energy deposited due to an unmitigated disruption leading to a VDE  based on present knowledge [7] 1.3 GJ for 4 ms (thermal quench) and ~0.5 GJ for ms (current quench)
Extensive melt damage likely, but the limiter could be sacrificial
Priority is to avoid failure of a water cooling boundary
Very unusual load case and requirements  novel PFC technology
Tom Barrett | Technologies for plasma-facing wall protection in EU DEMO | IAEA FEC 25th October 2018

11. Upper limiter PFC Hypothesis – force heat further around the cooling pipe and exploit tungsten thermal inertia
Reduced CuCrZr pipe temperatures
Reduced severity and duration of excursions above CHF
Reduced temperature gradient reduces bending stress
Wider blocks lead to lower total water flow and pressure drop and fewer pipe welds
+ Swirl tape (not shown)
Tom Barrett | Technologies for plasma-facing wall protection in EU DEMO | IAEA FEC 25th October 2018

12. Transient thermal results
Finite element analysis using ANSYS using a quarter-model of the monoblock
Water cooling: 150C and 5 MPa. On reaching CHF, the wall heat flux at that point remains constant, regardless of increases in wall temperature
Postulated VDE heat load: 100 MJ/m2 over 250 ms (400 MW/m2 as a uniform HF)
Reference block - narrow
Reference block - wide
Upper limiter PFC concept
Peak CuCrZr temperature after transient
652C
725C
454C
Tom Barrett | Technologies for plasma-facing wall protection in EU DEMO | IAEA FEC 25th October 2018

13. Transient thermal results
Tom Barrett | Technologies for plasma-facing wall protection in EU DEMO | IAEA FEC 25th October 2018

14. Comparison of stress at the W-Cu interface Steady state heat flux – conservative 4 MW/m2
Upper limiter PFC concept
Reference block - narrow
Reference block - wide
Tom Barrett | Technologies for plasma-facing wall protection in EU DEMO | IAEA FEC 25th October 2018

15. Research needs for limiter PFCs (not a complete list)
Manufacturing and materials technology
Manufacturing procedure for the monoblock with embedded thermal barrier
Novel PFC armour – tungsten foams which reduce stress and so rate of damage
Manufacturing within required tolerances of the shield block
Materials database and design criteria
Data on damage tolerance of Cu alloys in the DEMO main chamber
Design criteria for PFCs intended for extreme transients  water cooling regime above CHF and gross plastic deformation of the structure may well be acceptable
This key challenge must be tackled on multiple fronts, including development of plasma scenarios, magnetic configurations and control systems
Tom Barrett | Technologies for plasma-facing wall protection in EU DEMO | IAEA FEC 25th October 2018

16. Preliminary TBR impact assessment
22.5° DEMO baseline 2017 model with limiters added
Preliminary TBR impact assessment
Tom Barrett | Technologies for plasma-facing wall protection in EU DEMO | IAEA FEC 25th October 2018

17. Preliminary TBR impact assessment
Neutronic analyses using MCNP6.1
EU DEMO HCPB and WCLL blanket variants analysed (2015)
Using homogenised material compositions optimised for the 2015 blanket designs but using 2017 baseline blanket geometry
Limiters integrated and replace breeding zones
Single sector results scaled up to full reactor TBR
8 upper port limiters and
4 outer equatorial (start-up) limiters
TBR < -0.03
 In agreement with P. Pereslavtsev study of TBR impact of limiters in a DN DEMO [8]
Other types of possible limiters are being studied to protect against a more complete list of transients, which will also have an impact on TBR
Tom Barrett | Technologies for plasma-facing wall protection in EU DEMO | IAEA FEC 25th October 2018

18. Summary
Shaping of the DEMO wall in 3-D is required to manage wall heat loads, but this strategy alone is not sufficient to protect the wall against anticipated transient events
In EU DEMO wall protection using discrete limiters is pursued
Two limiter types described here, but others may be needed
Outer equatorial limiter for plasma start-up
Upper limiter for protection against an upward VDE
Both use water cooled W/CuCrZr monoblock technology
Analysis of the proposed upper limiter PFC suggests greater robustness against extreme transients (reduced risk of loss of water cooling pipe integrity)
These PFCs will require specific design criteria. Design for survival beyond CHF is a new design philosophy
The proposed arrangement of equatorial and upper limiters leads to a modest impact on reactor TBR of less than 0.03
The challenge of plasma transients must be tackled in a multi-disciplinary way
Tom Barrett | Technologies for plasma-facing wall protection in EU DEMO | IAEA FEC 25th October 2018

19. Thank you for your attention
Please see also
G. Federici FIP/ M. Gilbert SEE/2-1
M. Siccinio FIP/P H. Lux FIP/P7-2
O. Crofts FIP/P L. Zani FIP/P7-5
H. Reimerdes TH/P C. Day P3-8
[1] Federici, G. et al., Fusion Eng. Des., In press, corrected proof, Available online 5 June 2018, [2] Boccaccini, L.V. et al., Fusion Eng. Des. 109–111 (2016) 1199–1206. [3] Wenninger, R. et al., Nucl. Fusion 57 (2017) [4] Mitteau, R. et al., Fusion Eng. Des. 88 (2013) [5] Vizvary, Z. et al., “DEMO first wall misalignment study”, presented at 30th Symposium on Fusion Technology, 2018, submitted to Fusion Eng. Des. [6] Stangeby, P.C., Mitteau, R., J. Nucl. Mater. 390–391 (2009) [7] Maviglia, F. et al., Fusion Eng. Des., In press, corrected proof, Available online 23 February 2018, [8] Pereslavtsev, P. et al., Fusion Eng. Des., In press, corrected proof, Available online 9 February 2018,
Tom Barrett | Technologies for plasma-facing wall protection in EU DEMO | IAEA FEC 25th October 2018

20. Additional Slides
Tom Barrett | Technologies for plasma-facing wall protection in EU DEMO | IAEA FEC 25th October 2018

21. Wall shaping
Tom Barrett | Technologies for plasma-facing wall protection in EU DEMO | IAEA FEC 25th October 2018

22. Wall shaping
Tom Barrett | Technologies for plasma-facing wall protection in EU DEMO | IAEA FEC 25th October 2018

23. Assumptions in the postulated extreme transient heat load
Energy deposited due to an unmitigated disruption leading to a VDE  based on present knowledge [7] 1.3 GJ for 4 ms (thermal quench) and ~0.5 GJ for ms (current quench)
Current FEA simulations are not able to model melting or evaporation, and so modelling the TQ load has little value (we are using other tools e.g. RACLETTE)
We assume that 0.5 GJ is deposited on the limiter(s), over an area of 5 m2
100 MJ/m2
(8 limiters, m2 per limiter  ~0.8x0.8m wetted patch per limiter, if all perfectly aligned)
As we are focusing on the response of the Cu alloy pipe, transient load duration has little effect on results
Assume: 100 MJ/m2 over 250 ms (400 MW/m2)
Tom Barrett | Technologies for plasma-facing wall protection in EU DEMO | IAEA FEC 25th October 2018

24. Tom Barrett | Technologies for plasma-facing wall protection in EU DEMO | IAEA FEC 25th October 2018