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