Page 182 - Academic Press Encyclopedia of Physical Science and Technology 3rd InOrganic Chemistry

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Encyclopedia of Physical Science and Technology EN008H-970 June 29, 2001 16:46

662 Liquid Alkali Metals

I. SOME MODERN APPLICATIONS the world, notably in the United States, Russia, Japan, and
Europe, to means of tapping the limitless supply of energy
Sodium and lithium have wide industrial application, and that is potentially available from the thermonuclear fusion
two examples of this are given below. Other promising fu- of light nuclei. The reaction most likely to be exploited
ture applications include the use of sodium in solar power is fusion of deuterium and tritium. The latter can be
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plants, in which the sun’s energy is concentrated by mir- produced by irradiation of the Li isotope with thermal
rors onto a central receiver, from which heat is removed neutrons, and fast neutrons (>1 MeV) can yield significant
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by flowing sodium. Potassium is finding application in additional tritium from Li. This yield can be an important
turbine technology. The thermal efficiency of a heat cycle contribution in attaining a breeding rate above unity. The
is increased by raising the peak temperature of the cycle, core of the reactor may be surrounded by a blanket of
but corrosion problems usually restrict the available upper liquid lithium, but at this stage of development many other
temperature. Efficiency can be increased by the use of an substances (e.g., Pb 4 Li) are under consideration as blanket
intermediate cycle, the topping cycle, which is stable at fluids. The unique property of lithium is therefore that it
a higher peak temperature. Potassium is the most useful can serve both as a tritium breeder and as a heat-transfer
fluid for this purpose because of its availability, compat- medium.
ibility with steels, and vapor pressure. No large-scale ap-
plications have yet been found for rubidium or cesium,
B. Rechargeable Batteries
but these may well develop when their properties are bet-
ter understood. They have been studied for use in vacuum A new generation of batteries is under development that
tubes and photoelectric cells, ion propulsion engines, and has distinct advantages over the traditional lead–sulfuric
power generation by means of magneto-hydrodynamics acid batteries both in terms of weight and energy density,
andthermionicconversion.Theseapplicationstakeadvan- and which can be adapted to road or rail transport. Most
tage of the very low ionization potentials of the rubidium attention to date has been applied to the sodium–sulfur
and cesium atoms. battery, in which liquid sodium and liquid sulfur are sep-
arated by a diaphragm of β-alumina. The cell is operated
at 300–350 C, and the cell reaction is
◦
A. Nuclear Reactors
The fast nuclear reactor provides the best example of the 2Na + 3S = Na 2 S 3
use of a liquid alkali metal on a large scale. Neutrons
Corresponding cells containing liquid lithium and sulfur
in a fast nuclear reactor are not slowed down by mod-
have not yet been developed to the same extent, though
erators, and high reactor temperatures are involved. The
higher specific energies are theoretically possible; Be-
coolant must be capable of removing large quantities of
cause of its higher reactivity toward the diaphragm, there
heat from the reactor, and only a liquid metal can satisfy all
are more problems in handling liquid lithium than with
the requirements. An experimental reactor, the Dounreay
liquid sodium.
fast reactor (DFR), was assembled during 1950–1959 and
The lithium–chlorine battery is light in weight and is
employed 120 metric tons of a mixture of sodium and
unsurpassed in the high level of energy (20 times that of
potassium. This alloy was chosen because at the eutec-
the lead–acid battery) that can be achieved. One modi-
◦
tic composition the alloy is liquid down to −12 C. After
fication of this battery can be operated at temperatures
several years experience with this alloy, the use of pure ◦
around 350 C, but its wide application is again restricted
sodium alone as coolant was no longer a problem. Rele-
by problems of corrosion.
vant physical properties of sodium are given in Section II.
In particular, its wide liquid range of temperature is ide-
ally suited to reactor conditions. A prototype fast reactor II. BASIC PHYSICAL AND
(PFR) was then built at Dounreay, in 1974, that used 1150 CHEMICAL PROPERTIES
metric tons of liquid sodium as coolant. Because of its low
density and low viscosity, the sodium can be circulated at
A. Physical Properties
rates as high as 3000 l/sec. Much larger reactors are now
envisaged; the planning and design of the first commer- Some important properties of the alkali metals are col-
cial demonstration fast reactor (CDFR) is well advanced lected in Table I. Unless otherwise stated, values are
and will incorporate up to 7000 metric tons of liquid quoted for 200 C, at which temperature all the alkali
◦
sodium. metals are liquid, and corresponding values for water
Liquid lithium may play a key role in the fusion reactors and mercury are quoted for comparison. The alkali met-
of the future. Increasing effort is being devoted throughout als clearly show the influence of increasing atomic mass