Radioactive Wastes

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 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 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 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.