MANAGEMENT OF RADIOACTIVE WASTES 639
on disposal of liquids into cavities cut in the salt suggests
that hot liquid waste could also be placed in such a site.SolidsAs with liquid wastes, the most intractable problem is the safe
management of the high-volume, low activity waste. The high
activity waste is at first sight more dangerous, but although
safe custody may be expensive it is not technically difficult.
Low-level waste consists mainly of “garbage”—
contaminated clothing, equipment and structural material;
broken glassware, cleanup materials such as cloths and
mops; and a large amount of “potentially contaminated”
material such as packing and paper which must be treated as
active simply because it originates in an active area.
Much of this material can be reduced in volume by incin-
eration or baling under high pressure. Fumes and smoke from
incinerators and the dusty air from baling plants are cleaned
up by methods dealt with under Gases (pp. 717–718), but the
ash and baled waste remain to be dealt with.
In some countries geographical or legal circumstances
restrict the possibility of burial of radioactive material in the
ground. Elsewhere, ground burial is regarded favourably. In
the latter case bales and non-combustible waste are likely
to be buried in sparsely populated regions. Where land is
cheap, low-level wastes may be buried without any volume
reducing process.
Conditioning Pre-treatment of waste before final dis-
posal is called “conditioning”. The aim is usually immobi-
lization of radionuclides together with, if possible, volume
reduction. There is very wide variation in practice from one
country to another. For example, in France quite low level
solid wastes are put into concrete containers which are then
filled with cement mixture so that the end product is a large
concrete block. These blocks are stored, under a roof, on a
concrete floor. In Canada, on the other hand, similar wastes
are put into open trenches at Chalk River and covered with the
local sandy soil. Practical measurements seem to show that
both procedures are equally safe in the local circumstances.
A very effective conditioning process is fixation in bitu-
men or asphalt. Bitumen is very resistant to radiation, has a low
melting point, is impermeable to water and has some mechani-
cal flexibility. Radionuclides enclosed in, or even mixed with,
bitumen leach very slowly into water. Sludges are dewatered
when mixed with melted bitumen, which helps considerably in
restricting the volume of the disposals. In general, bitumen is
beginning to be favoured over concrete as the method of choice
for “fixing” otherwise mobile waste radionuclides.
Ground Disposal In some countries direct burial of
contaminated material in the ground is forbidden at any
level, whereas in others the amount and nature of ground
disposals is left to the discretion of the operator.
Nearly all cations move through soil more slowly than
the ground water although some anions—ruthenate and
iodide for example—are retarded very little. In the case of
the average “mixed fission products” usually of concern
in waste management the fastest moving radionuclide is
ruthenium, usually followed by Sr, Cs and Ce in that order.Relative rates of movement are affected by the nature and
pH of the soil and the ground water, but even in acidic sandy
soil^90 Sr moves through the soil at only 1/25 to 1/100 of the
rate of movement of the ground water.
If the site of the waste management area is selected with
care in relation to potable water supplies, so that the time of
transit between the point of disposal and the point of human
consumption is prolonged in relation to the half-life of the
critical radionuclides, direct ground disposal of low level
waste is effective and safe. There are a great many places
where knowledge of the rate and direction of movement of
the ground water, together with the distribution coefficients
of radionuclides between water and soil, make it apparent
that no significant discharge into the environment would be
credible as a result of direct disposal into the ground.
When simple burial is unacceptable, disposal trenches
and areas can be drained, with processing of the drainwater,
or the area can be covered with asphalt and protected from
encroachment of ground water by circumferential drainage.
A further step in the direction of safety is the “engineered
enclosure.” This is a structure built like a concrete house base-
ment. It usually takes the form of a long concrete-lined trench
divided into sections by concrete cross-walls. The section in
use is covered by a temporary roof (Figure 9). The object of
the structure is to prevent the ingress of water, so joints inFIGURE 9 Concrete trench. Double trench, for medium-level
solid wastes, is covered with a light roof when in use. The filled
trench is levelled with sand and a concrete roof is poured. Note
galvanized steel seals for joint.C013_001_r03.indd 639C013_001_r03.indd 639 11/18/2005 10:38:21 AM11/18/2005 10:38:21 AM