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The classification of radioactive waste for short-term management
For short-term management (and in particular for processing and temporary
storage), radioactive waste (both conditioned and non-conditioned) is
split into three main categories. This categorisation is carried out mainly
on the basis of the waste's activity level (i.e. the number of nuclear
disintegrations per unit of time) and the nature of the radiation emitted
(for example, whether or not there is alpha contamination may affect the
way in which the waste is processed). For the transportation and storage
of radioactive waste, a third factor is taken into account: the surface
contamination level.
These
various factors determine the means to be used to ensure protection against
the radiation emitted by the waste (shielding and confinement), the protective
clothing needed to guarantee the safety of workers and how long workers
may remain exposed to radiation without running the risk of being contaminated.
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Low-level
waste is radioactive waste whose contact dose rate (dose
to which an individual is exposed when in contact) is less than
5 milliSieverts per hour. Its production and processing require
only minimum shielding measures. Those involved in its processing
wear protective clothes, which generally include a plasticized suit,
gloves and sometimes a filter mask to ensure the purity of the air
they breathe.
This
category of waste comes from the generation of electricity by means
of nuclear energy (operating of nuclear power plants and production
of unspent nuclear fuel), from the use of radioactive sources in
medicine, agriculture and industry, and in particular from the decommissioning
of nuclear facilities that have been closed down.
These
include filters, resins, protective equipment (clothing, gloves,
and so on), paper, biological waste, activated concrete, residues
from the processing of waste water in nuclear power plants and also
decommissioning waste that has been in contact with radioactive
substances.
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Waste
produced by
nuclear power plants
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Protective
mask nozzles
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Syringes
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Medium-level waste is radioactive waste whose contact dose
rate is between 5 milliSieverts and 3 Sieverts per hour. It must
be processed in an enclosed and shielded area. Workers are protected
by concrete walls and armoured glass windows. All operations are
performed using remote-controlled devices.
Most
of this category of waste comes from the production and reprocessing
of nuclear fuel and from decommissioning activities. Some filters
and resins from nuclear power plants also belong to this category.
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High-level
waste is radioactive waste whose contact dose rate is greater
than 2 Sieverts per hour. This type of waste emits heat. It is subject
to the same precautions as medium-level waste. The only difference
is the thickness of the shielding, which is always calculated so
as to reduce as much as possible the amount of radiation to which
operators are exposed.
This
category of waste consists of fission products from the reprocessing
of spent nuclear fuel, the spent fuel itself, if it has not been
reprocessed, and any other excess fissile materials.
(Photo
: the fuel assembly)
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In
terms of waste volumes, around 75% of radioactive waste is low-level waste
and around 1% is highly radioactive (although it represents 95% of total
radioactive waste activity, all categories included). The rest is medium-level.
The classification of radioactive waste for long-term
management
For
long-term management (disposal), ONDRAF/NIRAS has adopted a method for
classifying conditioned radioactive waste based on three categories. This
classification method is compatible with those of the International Atomic
Energy Agency (IAEA)
and the European Union. It categorises radioactive waste according to
its activity level and life span.
The
half-life of radionuclides present in the radioactive waste (the time
needed for its level of activity to reduce by half) determines the solutions
to be applied for the long-term management of this waste. The objective
of radioactive waste management is to at all times prevent such waste
from harming man and his environment. The waste is therefore isolated
from the biosphere until its activity level has fallen to a level comparable
with that of background radiation.
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Category
A waste
(low or medium-level and short-lived waste)
Category
A waste is conditioned waste containing radioelements of a sufficiently
low activity level and with a sufficiently short life span to allow
surface disposal. It includes low or medium-level and short-lived
waste, i.e. waste made up of radioelements of which the majority
have a life span of less than or equal to 30 years. These radioelements
may contain minute quantities of long-lived alpha emitters.
This category of waste must be isolated from man and the environment
for 300 years, which is the time necessary for their activity level
to decrease by at least a factor of 1,000.
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Category
B waste
(low or medium-level and long-lived waste)
Category
B waste consists of conditioned low and medium-level waste contaminated
by long-lived alpha emitters in quantities that are too great for
this waste to be classified as category A, but which emit too little
heat to belong to category C. This waste may also contain varying
quantities of beta and gamma emitters.
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Category
C waste
(high-level short or long-lived waste)
Category
C waste includes all conditioned high-level waste containing significant
amounts of beta and gamma emitters with a short life span and large
quantities of long-lived alpha emitters. Because of the high activity
level, most waste in this category emits considerable amounts of
heat (more than 20 Watt/m³).
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Category
B and C waste, which contains significant quantities of radioelements
of a half-life greater than 30 years, must be isolated from man and the
environment for more than 300 years. The time needed for its activity
level to decrease by at least a factor of 1,000 may indeed be several
tens of thousands of years.
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