Nuke Waste Essay, Research Paper

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

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.