Page 314 - Fundamentals of Magnetic Thermonuclear Reactor Design

294 Fundamentals of Magnetic Thermonuclear Reactor Design

and radiation endurance criteria. An extension of this window is favourable in
terms of technical and economic effects. For example,

l A higher operation temperature limit enables an increase in the coolant
temperature and, hence, a greater heat-to-electrical-energy conversion ef-
ficiency;
l An increase in the maximum permissible neutron load on the blanket at a
given fusion power would allow a decrease in the reactor size and a reduc-
tion in its capital cost;
l A higher neutron fluence limit for fatigue strength and radiation creep ex-
tends the blanket’s durability before scheduled replacement, brings up the
utilisation capacity and reduces the cost of generated power.
Reduced activation ferritic–martensitic (RAFM) steels (with an operability
window of 300–550°С) and vanadium–chromium–titanium alloys (350–700°С)
are considered to be prime candidate structural materials for the nearterm. The
RAFM steels meet the activation limitation for shallow land burial. Their opera-
tion temperature must be not lower than 250°С because of the low-temperature
irradiation-induced embrittlement, and the upper temperature limit is deter-
mined by their thermal creep resistivity.
Vanadium-based alloys, such as V–Ti (4%)–Cr (4%), are low-activation ma-
terials that perform well at high temperatures, can withstand intensive neutron
irradiation, and produce little residual heat. They make an ideal combination
with a lithium coolant. But contact with a helium coolant is only allowable if
helium is oxygen-free, as even minute quantities of oxygen in the helium rap-
idly attack metals like vanadium. A major drawback of the vanadium alloys is
their high cost.
Structural materials that are considered for use on a longer term include the
dispersion-strengthened ferritic–martensitic steels (with an operating tempera-
ture of up to 600°С) and refractory materials, such as tungsten alloys and silicon
carbide (up to 1400°С).
Neutron breeding blanket materials are utilised to increase the TBR. They
must have a large fusion reaction cross-section (n, 2n) and a small absorption
cross-section. Beryllium is believed to be the best in this respect. Its drawbacks
include toxicity that requires controlled handling procedures, high cost and high
activation. In addition, beryllium has yet to be studied in depth. Lead is some-
what inferior to beryllium. It contains bismuth as a natural impurity that produc-
es long-lived isotopes, 210 Pо and 205 Pb, which complicate the waste-handling
process. In blanket designs based on the PbLi eutectic, the breeding properties
of lead are employed.
Among the various candidates for the blanket’s neutron breeding material,
zirconium appears to have the smallest reaction cross-section (n, 2n) and the
93
largest absorption cross-section. It also produces long-lived isotopes, Zr and
93 Nb, and its residual heat is greater than that of beryllium and lead.
A neutron moderator and reflecting materials are used to reduce the
neutrons’ energy and decrease their loss. The best moderators are substances