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Essay/Term paper: Nuclear power

Essay, term paper, research paper:  Science

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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
------------------------------------------------------------1)
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
------------------------------------------------------------
2) 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 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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