Nuclear Energy
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 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 filters consist of a bed of
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 equired
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.
