Storage Of Radioactive Wastes

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 now, new ways of disposing and
storing these wastes have been developed to prevent them
from being 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. The waste solution is mixed with:
 
1) borosilicate: any of several salts derived from both
boric acid and silicic acid and found in certain minerals
such as tourmaline. These additives are necessary for the
production of borosilicate glass, calcination and
elaboration of the glass. The operations are carried out in
a metallic pot that is heated in an induction furnace. The
vitrification of one load of wastes comprises the following
stages: feeding, calcination and glass evaporation, and
glass casting.
 
In the 'feeding' stage, 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
liters 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 resulting in 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 a
solid rather than a 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 vapor 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 hydrolyzed 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 = Qt2 mn 

R is the waste dispersion radius (meters)
 Q is the flow rate (m/day )
 t is the solution pumping time (days)
 m is the effective thickness of the formation (meters)
 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 into
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 favorable, the efficiency
of removal will be low. 

Column application is essentially a large number of batch
operations in series. 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
absorption 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.