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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 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 urnace. 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 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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