FREE ESSAY ON NUCLEAR ENERGY

NUCLEAR ENERGY

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.