Nuke Waste Term paper

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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

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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

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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

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