Page 244 - Fundamentals of Magnetic Thermonuclear Reactor Design

First Wall Components Chapter | 7 225

7.2.3.6 Strength and Fatigue Lifetime
The stress–strain state of the FW panels is the result of a number of loads, the
most important being the following:
l thermal mechanical stresses
l electrodynamic loads
l baric stresses caused, for example, by coolant’s pressure
l process loads
Thermal mechanical loads occur in compound structures whose parts have
different thermal expansion coefficients, and also can be generated by nonuni-
form temperature fields. These stresses are proportional to the elastic (Young’s)
modulus and a material’s thermal expansion coefficient and are inversely pro-
portional to its thermal conductivity. Therefore, structures using forced cooling
systems should have thin walls, because, with increasing thickness of the wall,
load-bearing capacity grows slower than the thermal mechanical stresses, while
the surface receiving the heat flow gets hotter. In addition, a material’s strength
and thermal conductivity decrease with increasing temperature. The remaining
constraint is the coolant’s pressure of 3–4 MPa. Generally, coolant channel wall
is in the range of 1–5 mm.
A free deflection of the wall towards the heat flow could reduce the thermal
mechanical stresses. However, this option is unacceptable for the FW compo-
nents for two reasons: the panels’ load-bearing element must be sufficiently
bulky to resist electrodynamic loads and sufficiently strong to prevent any un-
controllable deformation of components arranged around the toroidal circum-
ference. This is important to ensure a uniform distribution of heat flows over the
wall surface.
An important point about the optimisation design of multilayer fragments is
that contacting materials have close thermal expansion properties. An example
is the Be-CuCrZr pair. Where it is impossible, dissimilar materials are sepa-
rated by a high-plasticity intermediate layer, such as pure copper. This is how
tungsten-armoured FW components are made. The copper intermediate layer
absorbs the stresses that are developed at the border with tungsten and reduces
them drastically at the junction with the heat sink panel.
The electrodynamic loads are due to interaction between eddy currents
occurring in the FW elements and tokamak magnetic fields. They can be simu-
lated using, for example, the TYPHOON code package (Chapter 4). We know
that electromagnetic forces depend on the rate of transient magnetic events,
structure face layer’s electric conductivity and the contour size. For ITER,
the maximum electromagnetic force pressure is within ∼4 MPa. One way
to decrease them is to make structures electrically segmented by making full
thickness incisions.
Residual process stresses occur in multilayer elements subjected to mechan-
ical deformation, such as bending, or to cooling down after a high-temperature
operation. Gravity loads on the FW components are negligibly small.