Page 240 - Fundamentals of Magnetic Thermonuclear Reactor Design

First Wall Components Chapter | 7 221

l select the material for the heat-sink panel and armour,
l compute the temperature conditions and determine the armour’s highest ac-
ceptable thickness and erosion lifetime, and
l estimate the strength and functional fatigue of the structures.

7.2.3.1 Heat Load Estimation
A stationary surface heat load on the first wall includes plasma’s electromag-
netic radiation and charge-exchange atoms. Its total value is usually taken to
be 20% of the fusion power (excluding fusion neutrons power). The average
specific heat load (total power load on the wall divided by the wall area) is
generally taken to be at least 2× greater, considering the heat flows’ potential
spatial non-uniformity.
The vertical divertor targets and the limiter face the plasma directly. Plasma
heat is transferred to the wall along magnetic field lines, and the specific load
depends on the longitudinal (along a field line) heat flux density and the target
tilt angle.
Heat distribution across the field lines is determined for physical reasons.
In a simple case, including a low-recycling fusion experiment, when energy
losses from the edge plasma are low, this distribution can be set analytically.
In the case of a gaseous divertor target, when the inert gas density near the
target is high, computational codes can be used for analysis purposes. The
changing poloidal angle between the wall and a field line is the main tool for
controlling the heat flux density, as shown in Fig. 7.3. The smaller the angle,
the less is the density (and the greater the space needed for the target), but
practically, the range of these variations is limited. Both the target with flat
areas and gaps between the cassettes and a rippled magnetic field are non-
uniform. At a poloidal angle less than 10–15 degrees a peaking heat load den-
sity increase due to this factor may surpass the benefit of the average density
reduction.

7.2.3.2 Determination of Coolant’s Parameters
With a given coolant type and a given heat load, there is some ambiguity about
the selection of parameters for the optimum coolant as different factors come
into play. For the sake of distinctness let us consider ITER, which uses water
as a coolant. We start with estimating the minimum acceptable inlet water tem-
perature. This provides headroom for the temperature acceptable for different
parts of the structure, and thereby increases the structure’s functional longevity.
For water, the minimum temperature could be 30°C, but for ITER, it has been
raised to be 100°C. The reason is that at lower temperatures, the main candidate
materials for the cooling panels (steel and copper alloys) experience radiation-
induced brittleness—even at small radiation doses. A high inlet temperature
helps avoid this risk. It is also for this reason that the inlet temperature of a
coolant (helium or a liquid metal) is increased to 600°C–700°C, if the heat-
removing panels are made of molybdenum or tungsten.