Page 413 - Fundamentals of Magnetic Thermonuclear Reactor Design

Structural and Functional Materials Chapter | 13 391

l resistance to radiation embrittlement, that is, a short-term or long-term deg-
radation of fracture toughness, ability to deform and thermal fatigue resistiv-
ity under a neutron irradiation;
l resistance to swelling;
l corrosion resistance, that is, compatibility with hydrogen environment and
coolants;
l resistance to erosion under plasma particle bombardment; and
l consistency with environment protection policies, including those relating to
induced activity.
Criteria, such as high workability and commercial availability, are also very
important for the selection of materials for an MFR under design.

13.3 COMPARATIVE CHARACTERISTICS OF
DIFFERENT MATERIALS
Austenitic chrome-nickel steels are offered in a wide range of varieties. They are
highly fabricable, show good weldability and are suitable for various applica-
tions [6]. A critical issue with these steels is their low resistance to swelling
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[7], which reaches 10%–15% at a fluence of ∼10 m . The swelling can be
reduced by 15%–20% by cold working, in which case the steel retains adequate
plasticity. The mechanical properties of steels strengthened by plastic defor-
mation based on cold working are largely determined by the irradiation tem-
perature. At temperatures within 500°C, the mechanical strength increases with
fluence, while at higher temperatures, some softening occurs.
Welding of cold-worked steels may involve warp of parts joined together
and softening in the heat-affected zone. Local strength impairment may lead
to a plastic deformation and subsequent destruction on the area adjacent to
a joint.
Ferritic steels have relatively low thermal expansion coefficients and high
thermal conductivity. This translates into an almost 2× decrease in thermo-
mechanical stresses compared with austenitic steels. Ferritic steels are resistive
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to radiative swelling: at a fluence of ∼10 m , the swelling is within 1% in the
most ‘risky’ 450–550°C temperature range. The main effect of neutron irradia-
tion on ferritic steel is the shift of ‘ductile–brittle transition temperature’ (up to
200–300°C).
This issue can be improved by optimising the set of elements used to alloy
a given steel and minimising impurities, such as copper, phosphorus, anti-
mony and tin. A more radical measure is to increase the operating temperature.
Exposed to neutron irradiation at 450–550°C, ferritic steels retain their room-
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temperature plasticity margin even at the 10 m fluence.
Titanium alloys feature a low thermal expansion. They are resistive to radia-
tive swelling, corrosion and thermal fatigue. Their resistivity to radiation dam-
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age is quite high: at a fluence of up to ∼2 × 10 m and temperatures of
20–350°C, their relative strain is never lower than 8%–10% and contraction