Encyclopedia of Environmental Science and Engineering, Volume I and II

In Canada nepheline syenite glass containing 1100 Ci

mixed fission products has been buried in a swamp, below

the water table, since 1960. Careful studies of leaching, by

sampling of the soil and ground water downstream from

the disposal, have shown that fusion products equivalent to

the dissolving of 10^ ^10 g of glass per cm^2 per day are being

removed from the disposal. Less than 1 mCi has been dis-

solved in ten years from 1100 Ci. This suggests that burial

of active glass in dry soil, or even disposal into a big body

of water, would be acceptable for quite large quantities of

wastes.

Calcination Several methods have been developed for

evaporation and subsequent calcination of wastes. Oxides

are often soluble in water, so materials are usually added that

will bind the oxides into soluble complexes. The calcination

process is done in a heated steel container, a fluidized bed or

a spray calciner.

The pot calciner is essentially an expendable piece of

steel pipe heated in an electric furnace. The waste, mixed

with flass-forming fluxes such as borax or lead oxide, is

heated to about 9000C.

The spray calciner is a heated steel cylinder with a

nozzle at the top through which the waste is sprayed. At a

temperature of 875C a fine powder is produced which must

be stored in a dry place as it is leachable by water.

It is characteristic of all waste fixation methods involv-

ing evaporation, sintering and fusion that elaborate off-gas

treatment systems are required to prevent environmental

contamination by dust and volatile radionuclides. The con-

centrating equipment itself is essentially simple and often

not expensive to build, but the glass purification plant is

always sophisticated, complex and expensive. However, it is

also very effective.

Rock Fracturing The oil industry has developed methods

for creating fissures in rock in order to encourage movement

of oil to gas through a formation towards a well. This pro-

cess has been adapted to disposal of medium-level wastes.

A horizontally bedded formation—shale has been used

up to now—is drilled to several thousand feet. A high pres-

sure jet of sand and water cuts through the well casing and

penetrates between the strata near the bottom of the hole. The

well is then sealed and water forced down under very high

pressure, splitting the rock between the bedding planes. The

water is followed up by the waste, mixed with cement, sugar

and other additives. The mixture spreads out in a thin hori-

zontal sheet, which solidifies after several hours. Typically

the sheet is about a half inch thick.

The method has been used for disposal of a very large

volume of waste at Oak Ridge, Tennessee. The equipment,

including large bins for ingredients of the cement mix,

mixing apparatus, a drilling rig and a very powerful pump is

expensive, but the method is suitable for large-scale opera-

tion because successive sheets can be injected at intervals of

a few feet through the depth of the bedded rock formation.

Salt Mines The hazard that must be met by most

radioactive waste management systems is contamination of

public waters leading directly or indirectly to intake of radio-

nuclides by man. An ideal situation for disposal would there-

fore be one where public access was impossible and contact

with water incredible. The nearest approach to these condi-

tions is found in a deep salt mine. The presence of the salt

guarantees that water has been absent for millions of years,

and geological study can produce assurance that water is not

rapidly penetrating into the salt bed. The excavated galleries

of salt mines are large and stable tunnels, suitable for storage

and roomy enough for safe work with active loads.

Major disposals of “solidified” waste are being made

in a salt mine in Kansas, where detailed investigation has

shown that eventually the creep characteristics of the salt

will seal the disposal sites of heat-producing wastes. Work

638 MANAGEMENT OF RADIOACTIVE WASTES

LIME

NEPHELINE

SYENITE

FISSION

PRODUCT

SOLUTION

PELLET-

IZER

MIXER

DRYING

FURNACE

900°C

MELTING

FURNACE

1350°C

FISSION

PRODUCTS

IN GLASS

Ru & Cs

ADSORBERS

NITRIC ACID

RECOMBINER

CAUSTIC

SCRUBBER

EXHAUST

GASES

FIGURE 8 Fixation of fission products in glass. Fission product solution is added to pelletized nepheline syenite  lime,

dried and melted at 1350C.

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