Encyclopedia of Environmental Science and Engineering, Volume I and II

MANAGEMENT OF RADIOACTIVE WASTES 635

but also because^85 Kr is absorbed much more powerfully by

cold charcoal. This is the only practical means we have for

removal of radioactive noble gases.

The very large dispersive capacity of a high stack usually

makes it unnecessary to remove^14 C (as^14 CO 2 ) or tritium

(mainly^3 H^1 HO) because their toxicity is very low. However,

the coolant CO 2 in a gas-graphite reactor does contain

enough^14 C to require alkaline scrubbing, which removes

radioiodine as well.

Liquids

Storage The necessity for long-term storage of very large

quantities (many millions of gallons) of high level, strongly acid

waste has led to the development of tankage and pipeline sys-

tems which have stood up to severe conditions for many years.

Failures have occurred, but good design and carefully selected

materials have prevented environmental contamination.

Tanks are constructed from material, often stainless

steel, which will not be corroded by the solutions to be

stored. Secondary containment is provided by catch tanks or

drip trays and sufficient spare tankage is kept available for

rapid emptying of a ruptured tank. Leakage is detected by

a monitoring system which alarms immediately if radioac-

tive liquid appears in the catch tank (Figure 5). Movement

of active liquid is effected by pumping rather than by gravity

to ensure that it is the result of deliberate action rather than

accident.

Evaporation The most straight-forward and apparently

the simplest method of treatment for radioactive liquid

wastes is evaporation. In a carefully designed evaporator

with an efficient droplet de-entrainment system the radio-

nuclide content of the distillate can be about one millionth

of that in the pot. There is little about the design that is spe-

cifically related to radioactivity except that shielding may

have to be provided for the operator, and off-gases must be

monitored and possibly treated in some way. Unfortunately,

evaporation is expensive because it consumes a large amount

of energy and the end product—the concentrate—is still a

radioactive liquid waste. Evaporation to dryness or to the

point of crystallization has been practised, by the residue is

so soluble in water that without further processing it is not

suitable for disposal.

Where discharge of a large volume of low-level waste

into the environment is unacceptable the cost of evapora-

tion may be justified by its many advantages. Practically

all liquid wastes are treated by evaporation in Denmark and

Sweden, and it is also widely used in Japan.

Residues from evaporation may be mixed with cement,

fused with glass frit or various ceramic mixtures, or incor-

porated with melted bitumen. The product is then handled as

a solid waste.

CONTAINMENT:

IN CLADDING

IN PRIMARY

CONTAINMENT

IN SECONDARY

CONTAINMENT

IN EXCLUSION

AREA

EMERGENCY

COOLING

FILTER–ADSORBER

SYSTEMS

ORNL– DWG 70– 9869

FIGURE 4 Reactor containment system. Any leakage from fuel must pass through the cladding, the primary containment, and

either the secondary containment or the stack filters. Contamination within the building can be removed by sprays and/or filters.

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