Page 241 - Fundamentals of Magnetic Thermonuclear Reactor Design

222 Fundamentals of Magnetic Thermonuclear Reactor Design

The following factor should be considered in the selection of a coolant’s
pressure and speed. When using water or another two-phase (liquid–vapour)
coolant, measures should be taken to avoid a general thermal crisis that may
lead to a thermal destruction of the structure. In other words, the channel’s wall
temperature, T , must be lower than a critical temperature, T , which depends
crit
w
on the coolant pressure. Therefore, the pressure should be selected such as to
preclude any near-wall boiling or at least make it controllable. To this end, the
jet cooling technique and devices based on the vapotron effect are employed.
Wall (coolant side) temperature is defined by the equation
Tw =Tc+q/α T w = T + q/α
c
where T is the coolant’s temperature, q is the heat flow density and α is the
c
heat transfer coefficient. This is the only parameter that can be controlled by
completely physical means and one that is the greater the better. It increases
with the speed of the coolant, and also with the flow turbulence degree and the
channel’s inside area. The most straightforward way to increase α is to speed
up the coolant flow. The limitations of this method are the increase in pres-
sure within the cooling system, the greater erosion of the pipes and the greater
consumption of energy needed to make the flow run. The erosion becomes a
critical factor at a coolant flow speed higher than 10 m/s. For these reasons, the
best way to intensify the cooling process is to add some sort of fins or spikes,
for example, on the inner surface of the channels to increase the surface area in
contact with the coolant and enhance the near-surface microturbulence.
7.2.3.3 Material Selection
The problem of structural and functional materials is addressed in Chapter 13.
We only note here that for ITER structures using forced cooling, the materials
of choice are the SS316 austenitic stainless steel (if heat loads are less than
2
1 MW/m ) and the CuCrZr alloy (if the loads are greater).
7.2.3.4 Estimation of the First-Wall Thickness and Temperature
Field
As the FW layers have different functions, their thickness criteria are different
as well. The thickness of a load-bearing element is determined by strength re-
quirements. It has practically no effect on other layers in terms of their thermal
physical ‘condition.’ The load-bearing element is only subject to relatively small
heat loads caused by fusion neutrons. For this reason, its cooling channels are
incorporated in the ‘tail’ of the cooling circuit of the heat sink panels. The largest
heat loads on those panels come from the armour side. It is therefore important
to minimise the interfacial joint temperature. As the temperature of the plasma-
facing surface is limited by the armour’s material properties, one can make the
armour thicker to improve its erosion lifetime. The interfacial joint temperature
is affected not only by the coolant temperature, but also by the thickness and
shape of the cooling channel segments closest to the plasma. The main require-