MANAGEMENT OF RADIOACTIVE WASTES 637
contaminants. If properly chosen such a resin will remove
99.9% of most radionuclides (Figure 7). Certain minerals—
clinoptilolite, greensand and vermiculite are examples—are
also efficient ion-exchangers. They are much cheaper than
synthetic resins but they require longer contact times for
maximum effectiveness.
Glass The very high-level “self heating” wastes—the
primary wastes held in stainless steel tanks—are too active
to be treated by flocculation or ion exchange. Storage in
liquid form is seldom regarded as a permanent solution—
somehow these wastes must be fixed in a nonleachable
solid form which can be stored safely without danger of
leakage or constant maintenance costs. One of the most
promising ways to fix high level waste is to incorporate it
into a glass.
Glass is a leach-resistant material which can be made from
simple ingredients. Its quality varies with composition but it
is not usually sensitive to changes in minor constituents. Its
low melting point makes it convenient for casting in various
shapes and sizes for different disposal procedures. Glasses aresupercooled, very viscous, liquid solutions of silicates. Soda
glass is made by melting together silica, calcium carbonate
and sodium carbonate. Other varieties contain potassium or
potassium plus lead instead of sodium, and phosphate or
borate in place of part of the carbonate. Metallic oxides are
incorporated to form coloured glasses. Such a mixture might
well be suitable for fixing the radioactive metallic oxides
which form the major proportion of “mixed fission products”,
after the nitric acid has been removed and the residue ignited.
Successful fixation of radionuclides in glass has been
reported from the USA, UK and Canada. British and
American practice has concentrated on borate and silicate
glasses, or fusing the waste oxides with glass frit, whereas
the Canadians have used a natural silicate, nepheline syenate,
instead of a glass mix (Figure 8).
Glass fixation is now being done on quite a large scale
at the Pacific Northwest Laboratory (Hanford) Washington,
USA. By the end of July 1970 nineteen million Curies had
been solidified, representing waste from about ten tons of
irradiated fuel.(^00)
20
40
60
80
100
5 10 15 20 25 30
PHOSPHATE/CALCIUM RATIO
STRONTIUM REMOVED, %
pH-12
pH-11
pH-10
pH-9
FIGURE 6 Decontamination with lime and phosphate—effect
of pH and lime/phosphate ratio on removal of strontium-90.
G
H
F
E
D
B
C
A
I
(^00)
10
20
30
40
50
60
70
80
90
100
450 900 1350 1800 2250 2700
RESIN CONCENTRATION, MG/LITER
REMOVAL, %
FIGURE 7 Decontamination with ion exchange resin—
efficiency for various radionuclides; A,^182 Ta; B,^144 Ce^144 Pr; C,
(^95) Cr (^95) Nb; D, (^140) Ba (^140) La; E, (^131) I; F, fission product mixture; G,
(^32) P; H, (^115) Cd; I, (^137) Cs (^137) Ba.
C013_001_r03.indd 637C013_001_r03.indd 637 11/18/2005 10:38:21 AM11/18/2005 10:38:21 AM