Page 414 - Fundamentals of Magnetic Thermonuclear Reactor Design

392 Fundamentals of Magnetic Thermonuclear Reactor Design

ratio is the same as before irradiation. A long exposure to mechanical loads does
not result in their embrittlement either before or after irradiation. Unfortunately,
there are no sufficient experimental data to predict their behaviour at a fluence
27
−2
higher than 0.5 × 10 m . Not only these limitations, but also the susceptibil-
ity to hydrogen embrittlement, low heat-resistance and reduction of strength at
elevated temperatures make us assess the titanium alloys’ application potential
with caution.
Aluminium alloys have a high thermal conductivity and a small atomic num-
ber. Nevertheless, their applicability is limited by temperature restraints.
Copper alloys feature a high thermal conductivity, adequate strength and
plasticity and highest thermal resistance [3]. The following estimates illustrate
their strength performances.
.
.
−2
A tolerable first wall load, in neutron equivalent, will be 25–30 MWyearm
2
.
for a Cu–Cr–Zr alloy, 3–4 MWyear/m for a Cr–Ni austenitic steel, and
2
.
6–8 MWyear/m for a ferritic stainless steel.
Refractory metals are extraordinarily resistant to heat and wear, but unfortu-
nately, have limitations, such as a large atomic number, swelling and low work-
ability and weldability characteristics. Currently, they are only regarded as a
good choice for the most ‘high heat flux’ components, such as divertor plates.
The prospects for their more extensive use are remote.
Beryllium and beryllium alloys have strong thermal conductivity and elastic-
ity and high thermal expansion coefficients. They are extensively used in fission
reactors as a reflector material.
As one can see, austenitic stainless steels, and copper and beryllium alloys
have properties that are more or less consistent with the FW specifications.
These materials have been used to fabricate structural components of fusion
machines designed for use in different operating conditions [8]. For this rea-
son, fusion structural components may be grouped as follows: (1) plasma-facing
materials, (2) heat-conductive equipment and (3) vacuum chamber and ancillary
structures. Materials used for each group of structural components are exposed
to more or less the same operating conditions.
Plasma-facing materials must withstand cyclic thermal flows of up to
2
10 MW/m , a cross-section temperature gradient of 300–700°C and an intense
ion bombardment. In addition, they must feature a high thermal conductivity
and erosion resistivity. The highest radiation load is 20 dpa for the FW and
5 dpa for the divertor plates. Currently, all the mentioned criteria are best met
by beryllium alloys (candidate materials for the discharge chamber walls and
plasma-facing parts) and tungsten alloys (divertor plates).
Copper alloys are the material of choice for heat-conductive structures,
while for the vacuum vessel, special stainless austenitic steels, for example,
316L(N) IG, are proposed (IG means “ITER grade”).
We will mostly focus on the resistivity to neutron damage of materials fur-
ther in this chapter. The problem of tritium accumulation in plasma-facing mate-
rials will not be discussed because of the shortage of experimental statistics.