Page 238 - Fundamentals of Magnetic Thermonuclear Reactor Design

First Wall Components Chapter | 7 219

stat, the toroidal and poloidal field coils, the shields, etc.), which are partially or
completely closed. It is impracticable to withdraw it for maintenance or replace as
a whole structure, but it is possible to repair local damage of the armour or replace
several panels with the help of a robot. This aim may be achieved by drastically
decreasing the heat load on the discharge chamber’s first wall and increasing the
heat load on sacrificial (replaceable) divertor targets. Let us explain why.
The divertor targets receive around 50% of the α-particles’ power, which
significantly reduces the density of the heat flux acting on the discharge cham-
ber walls. In addition, the targets receive charged particles responsible for sput-
tering, slowing up the ion erosion. Finally, there are reasons to believe that the
divertor protects the walls, at least partially, against heat loads caused by run-
away electrons and plasma current disruptions.
The heat and particle fluxes diverted from the discharge chamber walls are
directed towards the divertor targets. The targets’ surfaces, being much smaller
than the walls’ surfaces, experience many times greater specific loads. There are
still no technical or technological solutions to divertor targets that can withstand
such loads throughout the reactor lifetime. Therefore, the ITER design provides
for a remote replacement of the targets. The targets’ small size and location in
the lower part of the reactor facilitate their handling.
The ratio between the discharge-wall-bound and divertor-targets-bound heat
fluxes can be varied using simple procedures, such as an automated program-
mable fuel and impurities mix introduction into the edge plasma. The variation
interval is 20% to 80%.

7.2.2 Initial Stage Design
The design of the first wall or any other complex engineering system calls for
a stage-by-stage approach to solving the optimisation problems. At the initial
stage, one has to take conceptual decisions, while the methods for their engi-
neering implementation, as well as relevant parameters and characteristics are
developed/defined at later stages.
The first step to be made at the initial stage is to determine the amount and
the most likely distribution of the heat fluxes coming from the plasma to the FW
components. In tokamaks, this amount is the sum total of ohmic and additional
heating powers. In ITER, its peak value is determined by the α-particles and ad-
ditional heating, while in DEMO, the main constituent is the α-particles power,
accounting for ∼20% of the fusion power.
Different FW components are exposed to peak loads at different times dur-
ing the operation cycle. The divertor target and the discharge chamber walls are
subjected to the highest loads during the stationary discharge stage, while for
the limiter the loads peak during the discharge initiation and shut-down.
The initial design stage is mostly concerned with quasi-stationary processes
that are longer than 5–10 s and strongly affect the FW’s composition. The effects
produced by disruptions and other pulse and transient processes are accounted