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Nuclear Power
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
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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 consists of
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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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