Page 44 - Radiochemistry and nuclear chemistry

Nuclei, Isotopes and Isotope Separation 33

amount of product. Small changes in ct can have a large economic impact in isotopic
separation.

2.8.2. Chemical exchange

As an example of an industrial isotope exchange process, let us consider the production
of heavy water by the chemical reaction

k=2.32(32~
H20(l ) + HDS(g)= HDO(I) + H2S(g ) (2.54)
k= 1.80(138~

From the values of the equilibrium constants k we see that the enrichment of deuterium in
water increases with decreasing temperature. Use is made of this property in the
two-temperature H20-H2S exchange process known as the G-S process (Girdler-Sulphide),
which is used in many countries to produce heavy water. A typical plant consists of several
units as shown in Fig. 2.9. Through the upper distillation tower natural water flows
downward and meets hydrogen sulfide gas streaming upwards. As a result of the exchange
between H20 and H2S, heavy hydrogen is enriched in the water. In the lower tower, which
is operated at a higher temperature, the equilibrium conditions are such that deuterium is
enriched in the hydrogen sulfide and moves with that gas to the upper tower. No catalyzer
is required in order to achieve rapid equilibrium in this reaction. The product of the process
is water which is enriched in deuterium from the top tower and water which is depleted in
deuterium from the bottom tower. The hydrogen sulfide circulates through both towers with
no net loss.
Plants capable to produce a total of more than 1200 tons annually are in operation in
Canada, India and the US. The largest exchange towers are 60 m high and have a diameter
of 6 m. In 5 units (only one unit is indicated in Fig. 2.8) the D20 concentration is raised
from 0.014 % to about 15 %. The final concentration to 99.97 % D20 is then usually made
by distillation of water. The 1990 price for pure D20 was ~ US $400 per kg. It is
important to recognize in tracer applications that commercially available D20 always
contains some tritium, which is co-enriched with deuterium; 2 - 7 kBq kg-1 D20.
As another example, the lithium isotope 7Li (used as 7LiOH for pH control in some
nuclear power plants because of its small neutron cross-section) is produced in 99.99 %
purity by countercurrent chemical exchange of lithium between an aqueous solution of
LiOH and lithium amalgam. A separation factor of 1.06 to 1.07 is reported. Reflux of
lithium is obtained at one end by electrolytic reduction of LiOH to Li(Hg) at a mercury
cathode and at the other end by spontaneous oxidation of Li(Hg) on graphite by water
producing hydrogen gas and LiOH.

2.8.3. Electrolysis

Electrolysis of water produces hydrogen gas at the cathode, which contains a lower
proportion of deuterium than the original water. The isotope effect stems from the