Radioactive Wastes


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 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 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