Abstract
Abstract In the present work, the role of plasma facing components protection in driving the EU-DEMO design will be reviewed, focusing on steady-state and, especially, on transients. This work encompasses both the first wall (FW) as well as the divertor. In fact, while the ITER divertor heat removal technology has been adopted, the ITER FW concept has been shown in the past years to be inadequate for EU-DEMO. This is due to the higher foreseen irradiation damage level, which requires structural materials (like Eurofer) able to withstand more than 5 dpa of neutron damage. This solution, however, limits the tolerable steady-state heat flux to ~1 MW/m2, i.e. a factor 3–4 below the ITER specifications. For this reason, poloidally and toroidally discontinuous protection limiters are implemented in EU-DEMO. Their role consists in reducing the heat load on the FW due to charged particles, during steady state and, more importantly, during planned and off-normal plasma transients. Concerning the divertor configuration, EU-DEMO currently assumes an ITER-like, lower single null (LSN) divertor, with seeded impurities for the dissipation of the power. However, this concept has been shown by numerous simulations in the past years to be marginal during steady-state (where a detached divertor is necessary to maintain the heat flux below the technological limit and to avoid excessive erosion) and unable to withstand some relevant transients, such as large ELMs and accidental loss of detachment. Various concepts, deviating from the ITER design, are currently under investigation to mitigate such risks, for example in-vessel coils for strike point sweeping in case of reattachment, as well as alternative divertor configurations. Finally, a broader discussion on the impact of divertor protection on the overall machine design is presented.
Highlights
The European (EU) DEMO is mentioned in the EU-Roadmap [1] as the first device able to reliably demonstrate a net electricity production of few hundred MW, breed its fusion fuel and exhibit a suffi ciently high availability, a long lifetime of its components
The thickness of the first wall is appreciably less than the ITER one, since DEMO has to allow for T breeding, which requires the neu trons to be able to stream across the Plasma Facing Components (PFC) and to be absorbed in the breeding region
Concerning the first wall (FW), the EU-DEMO solution is characterised by the presence of protruding limiters, which play the role of absorbing the largest fraction of heat carried by charged particles, both during steadystate and during planned and unplanned transients, protecting the thin breeding wall, especially during disruptions
Summary
The European (EU) DEMO is mentioned in the EU-Roadmap [1] as the first device able to reliably demonstrate a net electricity production of few hundred MW, breed its fusion fuel (tritium) and exhibit a suffi ciently high availability, a long lifetime of its components. The thickness of the first wall is appreciably less than the ITER one, since DEMO has to allow for T breeding, which requires the neu trons to be able to stream across the Plasma Facing Components (PFC) and to be absorbed in the breeding region. This “weakness” of the EU-DEMO wall requires that, essentially, no plasma-wall contact can happen, especially at plasma currents close to the nominal flat-top value.
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