Page 416 - Fundamentals of Magnetic Thermonuclear Reactor Design

394 Fundamentals of Magnetic Thermonuclear Reactor Design

These data have been obtained in mockup experiments, in which helium
−3
accumulation rate was around 2 × 10 particles per one displacement. In
fusion reactor conditions, the accumulation rate is expected to be higher by an
order of magnitude. As the swelling and the helium-induced embrittlement are
due to this factor, both are sure to be more intense in the MFR environment.
A heat-resistive refractory metal, tungsten, is a priori susceptible to LTRE
when exposed to temperatures of 700–900°C when employed as a material for
divertor plates. However, this statement has not been proved experimentally, as
the information about the behaviour of neutron-irradiated tungsten is scarce.
Only the results of mockup experiments to explore its strength characteristics at
300–800°C have been published.
Irradiated tungsten specimens, no matter how produced, are absolutely
brittle at a stress of just ∼300 MPa. This is a classic case of the LTRE effect
with one peculiar exception: the ductile–brittle transition temperature is shifted
to 700°C. The causes of such an intense embrittlement are yet to be estab-
lished. One probable cause is an increase in transmutation product concentra-
tion (tungsten is believed to accumulate up to 0.1% of rhenium and osmium
at ∼5 dpa). In the MFR environment, the transmutation rates are much higher.
Another possible embrittlement intensification factor is hydrogen absorption.
To sum it up, despite the recent endurance experiments proving the ability
of mockup divertor plates to handle extreme heat loads, the actual durability of
tungsten tiles is likely to be limited by their susceptibility to cracking.

13.5 HEAT-CONDUCTIVE MATERIALS

13.5.1 High-Strength Copper Alloys
The key requirement placed on these materials is high thermal conductivity.
Plates designed to remove heat from in-chamber components must be ∼10 mm
thick, if made of material with a thermal conductivity close to that of copper.
A temperature gradient across the cross-section of such plates will be within
acceptable range of a few tens of kelvins.
High-strength copper alloys have the best combination of properties in this
context. As the proportion of elements used to alloy them is small, their physical
and chemical properties are in most cases close to those of pure copper. This
does not hold for their electrical and thermal conductivities, which are critically
dependent on impurity concentrations.
Copper and copper alloys are widely used in electrical engineering due to
their high electrical and thermal conductivity, corrosion resistivity, workabil-
ity and owing to the availability of a well-developed industrial infrastructure.
Their worldwide consumption is greater than 10 million metric tons. More
than 1000 t of high-strength copper alloys for special applications are pro-
duced annually.
Currently, two types of high-strength copper alloys are commercially avail-
able, namely, the Cu–Cr–Zr and Cu–Ni–Be precipitation-hardened alloys and
the alumina (Al O ) dispersion-strengthened alloys (Table 13.2).
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