Nuclear Power


        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  urnace. 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.      
 



 

