Encyclopedia of Environmental Science and Engineering, Volume I and II

636 MANAGEMENT OF RADIOACTIVE WASTES

Flocculation and Precipitation The cheapest and sim-

plest process for treatment of radioactive liquids is removal of

the activity on some kind of precipitate, either as an integral

part of the precipitated material, or adsorbed on its surface.

In most waste tanks a sludge settles out which may con-

tain up to 90% of the activity, and a copious precipitate of

metallic hydroxides is formed on neutralization which may

carry down up to 90% of the remainder. Further purification

of the clear effluent after separation of these sludges can be

achieved by addition of lime and sodium carbonate. Up to

99% of the remaining activity can sometimes be removed by

this treatment. Treatment with lime and sodium phosphate is

also very effective (Figure 6).

The treatment used depends upon the particular radio-

nuclides present in the waste, and also its gross composition

for example, the pH and salt content of the solution. In some

cases ferric chloride, clay or other additives are introduced

at carefully chosen points in the process. The selection of the

process, and modifications introduced as the composition of

the waste changes, require constant analysis and control by

specialized chemists.

One problem common to all flocculation processes is

how to deal with the sludge. The floc settles very slowly

and after it has been drained through filters or separated by

centrifugation it is in the form of a thick cheese-like solid

which, in spite of its appearance, still contains 80 to 90% of

water. In a successful British process the sludge is repeat-

edly frozen and thawed. The separation of pure ice crystals

leaves behind a concentrated salt solution which coagulates

the small particles of floc into a form which settles more

rapidly and is less likely to clog vacuum filters.

Ion Exchange The effluent from a flocculation process

may still contain too much activity for discharge to public

waters. It can then be passed through ion exchangers, which

are expensive but very efficient. They cannot be used eco-

nomically on a solution with a high salt content because

their ion-exchange capacity would rapidly be exhausted by

absorbing the dissolved salts.

The effluent from a well-controlled flocculation process

has a low total-solids content and after filtration to remove

traces of floc it can be passed through a cation exchanger

or mixed-bed resin suitable for removal of the radioactive

COOLING COIL

RISER INSTRUMENT RISER

CONDENSER

FILTER

(FIBERGLASS)

NOTE:

ALL WELDS ARE

RADIOGRAPHED

STEEL

WASTE

TANK

SHOTCRETE

GROUT

STEEL

PA N

CONCRETE

SLAB WATERPROOF

MEMBRANE

CEMENT

PLASTER

SUPPORT

VERTICAL COLUMN

COOLING

COIL

HORIZONTAL

COOLING

COILS

STEEL

WASTE

TA N K

INLET

FIGURE 5 Structure of high level waste tank at Savannah River.

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