Radioactive Waste Disposal


Radioactive wastes, must for the protection of mankind be
stored or disposed in such a manner that isolation from the
biosphere is assured until they have decayed to innocuous
levels. If this is not done, the world could face severe
physical problems to living species living on this planet. 
 Some atoms can disintegrate spontaneously. As they do,
they emit ionizing radiation. Atoms having this property
are called radioactive. By far the greatest number of uses
for radioactivity in Canada relate not to the fission, but
to the decay of radioactive materials - radioisotopes.
These are unstable atoms that emit energy for a period of
time that varies with the isotope. During this active
period, while the atoms are 'decaying' to a stable state
their energies can be used according to the kind of energy
they emit. Since the mid 1900's radioactive wastes have
been stored in different manners, but since several years
new ways of disposing and storing these wastes have been
developed so they may no longer be harmful. A very
advantageous way of storing radioactive wastes is by a
process called 'vitrification'. Vitrification is a
semi-continuous process that enables the following
operations to be carried out with the same equipment:
evaporation of the waste solution mixed with the
1) borosilicate: any of several salts derived from both
boric acid and silicic acid and found in certain minerals
such as tourmaline. additives necesary for the production
of borosilicate glass, calcination and elaboration of the
glass. These operations are carried out in a metallic pot
that is heated in an induction furnace. The vitrification
of one load of wastes comprises of the following stages.
The first step is 'Feeding'. In this step the vitrification
receives a constant flow of mixture of wastes and of
additives until it is 80% full of calcine. The feeding rate
and heating power are adjusted so that an aqueous phase of
several litres is permanently maintained at the surface of
the pot. The second step is the 'Calcination and glass
evaporation'. In this step when the pot is practically full
of calcine, the temperature is progressively increased up
to 1100 to 1500 C and then is maintained for several hours
so to allow the glass to elaborate. The third step is
'Glass casting'. The glass is cast in a special container.
The heating of the output of the vitrification pot causes
the glass plug to melt, thus allowing the glass to flow
into containers which are then transferred into the
storage. Although part of the waste is transformed into a
solid product there is still treatment of gaseous and
liquid wastes. The gases that escape from the pot during
feeding and calcination are collected and sent to ruthenium
filters, condensers and scrubbing columns. The ruthenium
filters consist of a bed of 2) condensacate: product of
condensation. glass pellets coated with ferrous oxide and
maintained at a temperature of 500 C. In the treatment of
liquid wastes, the condensates collected contain about 15%
ruthenium. This is then concentrated in an evaporator where
nitric acid is destroyed by formaldehyde so as to maintain
low acidity. The concentration is then neutralized and
enters the vitrification pot. Once the vitrification
process is finished, the containers are stored in a storage
pit. This pit has been designed so that the number of
containers that may be stored is equivalent to nine years
of production. Powerful ventilators provide air circulation
to cool down glass. The glass produced has the advantage of
being stored as solid rather than liquid. The advantages of
the solids are that they have almost complete insolubility,
chemical inertias, absence of volatile products and good
radiation resistance. The ruthenium that escapes is
absorbed by a filter. The amount of ruthenium likely to be
released into the environment is minimal. Another method
that is being used today to get rid of radioactive waste is
the 'placement and self processing radioactive wastes in
deep underground cavities'. This is the disposing of toxic
wastes by incorporating them into molten silicate rock,
with low permeability. By this method, liquid wastes are
injected into a deep underground cavity with mineral
treatment and allowed to self-boil. The resulting steam is
processed at ground level and recycled in a closed system.
When waste addition is terminated, the chimney is allowed
to boil dry. The heat generated by the radioactive wastes
then melts the surrounding rock, thus dissolving the
wastes. When waste and water addition stop, the cavity
temperature would rise to the melting point of the rock. As
the molten rock mass increases in size, so does the surface
area. This results in a higher rate of conductive heat loss
to the surrounding rock. Concurrently the heat production
rate of radioactivity diminishes because of decay. When the
heat loss rate exceeds that of input, the molten rock will
begin to cool and solidify. Finally the rock refreezes,
trapping the radioactivity in an insoluble rock matrix deep
underground. The heat surrounding the radioactivity would
prevent the intrusion of ground water. After all, the steam
and vapour are no longer released. The outlet hole would be
sealed. To go a little deeper into this concept, the
treatment of the wastes before injection is very important.
To avoid breakdown of the rock that constitutes the
formation, the acidity of he wastes has to be reduced. It
has been established experimentally that pH values of 6.5
to 9.5 are the best for all receiving formations. With such
a pH range, breakdown of the formation rock and
dissociation of the formation water are avoided. The
stability of waste containing metal cations which become
hydrolysed in acid can be guaranteed only by complexing
agents which form 'water-soluble complexes' with cations in
the relevant pH range. The importance of complexing in the
preparation of wastes increases because raising of the
waste solution pH to neutrality, or slight alkalinity
results in increased sorption by the formation rock of
radioisotopes present in the form of free cations. The
incorporation of such cations causes a pronounced change in
their distribution between the liquid and solid phases and
weakens the bonds between isotopes and formation rock. Now
preparation of the formation is as equally important. To
reduce the possibility of chemical interaction between the
waste and the formation, the waste is first flushed with
acid solutions. This operation removes the principal
minerals likely to become involved in exchange reactions
and the soluble rock particles, thereby creating a porous
zone capable of accommodating the waste. In this case the
required acidity of the flushing solution is established
experimentally, while the required amount of radial
dispersion is determined using the formula: R = Qt 2 mn R
is the waste dispersion radius (metres) Q is the flow rate
(m/day) t is the solution pumping time (days) m is the
effective thickness of the formation (metres) n is the
effective porosity of the formation (%) In this concept,
the storage and processing are minimized. There is no
surface storage of wastes required. The permanent binding
of radioactive wastes in rock matrix gives assurance of its
permanent elimination in the environment. This is a method
of disposal safe from the effects of earthquakes, floods or
sabotages. With the development of new ion exchangers and
the advances made in ion technology, the field of
application of these materials in waste treatment continues
to grow. Decontamination factors achieved in ion exchange
treatment of waste solutions vary with the type and
composition of the waste stream, the radionuclides in the
solution and the type of exchanger. Waste solution to be
processed by ion exchange should have a low suspended
solids concentration, less than 4ppm, since this material
will interfere with the process by coating the exchanger
surface. Generally the waste solutions should contain less
than 2500mg/l total solids. Most of the dissolved solids
would be ionized and would compete with the radionuclides
for the exchange sites. In the event where the waste can
meet these specifications, two principal techniques are
used: batch operation and column operation. The batch
operation consists of placing a given quantity of waste
solution and a predetermined amount of exchanger in a
vessel, mixing them well and permitting them to stay in
contact until equilibrium is reached. The solution is then
filtered. The extent of the exchange is limited by the
selectivity of the resin. Therefore, unless the selectivity
for the radioactive ion is very favourable, the efficiency
of removal will be low. Column application is essentially a
large number of batch operations in series. Column
operations become more practical. In many waste solutions,
the radioactive ions are cations and a single column or
series of columns of cation exchanger will provide
decontamination. High capacity organic resins are often
used because of their good flow rate and rapid rate of
exchange. Monobed or mixed bed columns contain cation and
anion exchangers in the same vessel. Synthetic organic
resins, of the strong acid and strong base type are usually
used. During operation of mixed bed columns, cation and
anion exchangers are mixed to ensure that the acis formed
after contact with the H-form cation resins immediately
neutralized by the OH-form anion resin. The monobed or
mixed bed systems are normally more economical to process
waste solutions. Against background of growing concern over
the exposure of the population or any portion of it to any
level of radiation, however small, the methods which have
been successfully used in the past to dispose of
radioactive wastes must be reexamined. There are two
commonly used methods, the storage of highly active liquid
wastes and the disposal of low activity liquid wastes to a
natural environment: sea, river or ground. In the case of
the storage of highly active wastes, no absolute guarantee
can ever be given. This is because of a possible vessel
deterioration or catastrophe which would cause a release of
radioactivity. The only alternative to dilution and
dispersion is that of concentration and storage. This is
implied for the low activity wastes disposed into the
environment. The alternative may be to evaporate off the
bulk of the waste to obtain a small concentrated volume.
The aim is to develop more efficient types of evaporators.
At the same time the decontamination factors obtained in
evaporation must be high to ensure that the activity of the
condensate is negligible, though there remains the problem
of accidental dispersion. Much effort is current in many
countries on the establishment of the ultimate disposal
methods. These are defined to those who fix the fission
product activity in a non-leakable solid state, so that the
general dispersion can never occur. The most promising
outlines in the near future are; 'the absorbtion of
montmorillonite clay' which is comprised of natural clays
that have a good capacity for chemical exchange of cations
and can store radioactive wastes, 'fused salt calcination'
which will neutralize the wastes and 'high temperature
processing'. Even though man has made many breakthroughs in
the processing, storage and disintegration of radioactive
wastes, there is still much work ahead to render the wastes
absolutely harmless. 


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