Page 313 - Fundamentals of Magnetic Thermonuclear Reactor Design

Blanket Chapter | 10 293

Liquid lithium, lead-lithium (Pb–Li) eutectic alloy (∼17 at.% Li), and
Li-containing ceramic materials (Li TiO , Li SiO , Li ZrO , Li O), and the
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Li BeF molten salt (fluorite) are candidates for the tritium breeder blanket ma-
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terial. Pure lithium is believed to have superior breeding properties due to its
wide range of operating temperatures—from ∼180°С (the melting point) to
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∼1340°С (the boiling point), low density (500 kg/m at 300°С), and ability to
easily dissolve tritium [1]. However, it may be a hazard in off-normal situations
because of its high chemical activity with respect to air and water. The Pb–Li
alloy has weaker breeding characteristics because of lower lithium content. It
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has a ∼235°С melting temperature and a density of ∼9492 kg/m (300°С), and
its tritium solubility and chemical activity are less than those of pure lithium.
The Li-containing ceramic materials are also inferior in terms of the breeding
performance. They have a temperature application window of 300–1000°С, a
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density of ∼1850 kg/m , a moderate tritium solubility, but are chemically inert.
Fluorite is inferior to lithium in terms of breeder characteristics, and has a melt-
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ing temperature of 459°С, density of ∼2000 kg/m and a low tritium solubility,
while its corrosive properties are yet to be studied.
Liquid breeding materials allow a desired durability of a high-TBR blanket
to be achieved owing to replacement of burnt-out lithium with new material. In
blankets that employ ceramic breeding materials, the tritium breeding potential
decreases as lithium burns up. It is therefore necessary to replace the blanket
while the reactor is still in operation or increase the initial TBR value. High
tritium solubility of a candidate material helps reduce tritium losses, but com-
plicates tritium recovery from the blanket and the ancillary equipment. Low
solubility produces opposite effects.
The lower the breeding materials’ density the less blanket mass for a volume
designed to ensure required tritium breeding and shielding properties. The TBR
increases with density of a ceramic pebble bed.
Water and molten metals are considered as candidate coolants for the blan-
ket. For water with an outlet temperature of 325°С at a pressure of 15 MPa,
the heat-to-electrical energy conversion efficiency is close to 35% compared
with 40% for helium (outlet temperature, 500°С; pressure, 8 MPa) and 45%
for lithium (600°С). For commercial reactors using coolants with exit tempera-
tures of 900–1200°С, the conversion efficiency is expected to be 51%–59%.
Losses due to coolant transportation in the energy conversion system account
for a ∼3%–7% efficiency decrease (the contribution of these losses is larger for
He and Pb–Li-based cooling systems, which need more power to manage the
coolant forced flow). When using a Li or Pb–Li coolant, an electrical insulation
barrier between the blanket walls and the molten metal is required to reduce the
magneto hydro dynamics (MHD) losses.
The blanket’s structural materials are exposed to high temperatures and
intense neutron radiation that affect their physical and mechanical properties.
Every material has an operability ‘window’, determined by a set of temperature