Designs and technologies for plasma-facing wall protection in EU DEMO

Abstract

The EU DEMO plasma is almost completely enveloped by large breeding blanket segments for tritium breeding and power extraction. Shaping of the blanket plasma-facing wall in 3-D may prove to be essential, but this strategy alone is not sufficient to protect against anticipated transient plasma events. The high heat flux wall-limiter approach used in ITER is not thought to be viable in a tritium self-sufficient power reactor, and so in EU DEMO wall protection using discrete limiters is pursued. Two types of discrete limiter are described in this paper. One is an equatorial port limiter designed to handle the power during the plasma start-up phase making use of water-cooled tungsten/CuCrZr monoblock technology. The second is the upper limiter, featuring a plasma-facing component designed specifically for extreme transient loading due to a vertical displacement event. The heat flux channeling and thermal barrier features of this design are shown to considerably reduce CuCrZr pipe temperature, and so reduce the likelihood of catastrophic failure. A preliminary neutronic calculation has shown that the impact of these discrete limiters on overall tritium breeding ratio is relatively low.

Impact of plasma thermal transients on the design of the EU DEMO first wall protection

“…As a result it may be necessary to prescribe, to the disruption mitigation system, a lower radiation HF fraction, to avoid damages to the FW, leaving an higher fraction to the charged particles that will impact on the limiters. Both charged particles and radiation heat loads are produced on the same 3D mesh, which is used to run 3D thermo-hydraulic calculations of FW and limiter [33] designs.…”

“…For this reason, all the cases with vaporization larger than few tens of µm should be considered as a worst case. More sophisticated codes, such as TOKES, have been employed to evaluate the vapor shielding effects in ITER [32] and DEMO [33]. More extreme cases are (6) and (9) , representing the TQ of unmitigated U and D-VDEs, respectively.…”

European DEMO first wall shaping and limiters design and analysis status

“…The concept PFC features a Eurofer strip as thermal break (Figure 2) to move the heat around (to delay its transit to the coolant) and thus reduce the maximum pipe/pipe-bore temperatures. Previously a wide monoblock with complex cuts has had been proposed [12]; recently it was reversed to a simpler monoblock (Figure 2) which is easier to manufacture and allows more flexibility in geometry. This is however at the price of having more cooling pipes in the vessel as the monoblock is narrower to achieve similar capability.…”

Parametric design study of a substrate material for a DEMO sacrificial limiter

“…If the thermal diffusivity is high enough, the pipe may locally overcome the CHF limit. Even if this might not be a showstopper due to the limited VDE duration, as also suggested in [7], in absence of experimental evidence it was decided to keep the maximum temperature reached by the pipe wall among the user-defined operating requirements. The model won't lose its validity by changing this limit, because the results filtering is a post-process operation that can be easily customized.…”

“…Combined with mitigation and prevention strategies, first wall limiters seem to be unavoidable in DEMO to prevent the unshadowed first wall from plasma transients, as stated in [7]. In this context, we aim to investigate a sacrificial PFC to cope with unmitigated plasma disruptions in DEMO.…”