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.