Decontamination
What is it?
Decontamination is the removal of contamination
from surfaces of facilities or equipment
by washing, heating, chemical or electrochemical
action, mechanical cleaning, or other techniques.
Why decontaminate?
- Reduce Radiation Exposure in dismantling
- Salvage Equipment and materials from
the plant
- Reduce volume of waste which must be
stored in a secure controlled waste disposal
site
- Remove loose radioactive contaminants
All plant decommissioning requires some
form of decontamination of structural surfaces.
The physical process of dismantling nuclear
plants can be done by robots (expensive)
or humans. To ensure the safety of humans
however, a certain level of decontamination
must be done to the plant.
It is also important to dispose of materials
which are radioactive in a safe and controlled
environment. This requires a large space.
If some components of the plant can be reduced
to safe levels of radioactivity, then they
will require less stringent disposal methods.
Considerations:
- Safety –The method should not add
other hazards (e.g. chemical, electrical)
- Efficiency – The method should
be capable of removing radioactivity from
a surface to the level which would enable
hands-on work instead of robotics
- Cost-effectiveness – The method
should not give rise to costs which would
exceed the costs for waste treatment and
disposal of the material
- Waste minimisation – The method
should not give rise to large quantities
of secondary waste
- Feasibility of industrialisation –
Methods should not be labour-intensive,
difficult to handle, or difficult to automate.
Methods:
- Chemical
- Electrochemical
- Melting
- Mechanical
- Other
Chemical
Chemical Decontamination requires the use
concentrated or dilute chemical reagents
in contact with the contaminated item (steel
pipes, tanks etc.) to dissolve the contamination
layer covering the base metal and eventually
a part of the base metal.
Mild chemical reagents are used if the
contaminated item is to be reused.
Aggressive reagents are used when the item
is not intended to be reused, as they operate
on the basis that the chemical will eat
away at the object.
Chemical decontamination is useful in reducing
the radioactivity of large surface area
items (e.g. drip trays).
| Advantages |
Disadvantages |
|
Simple and cheap
|
Not effective on porous
materials |
| Almost all radionuclides
can be removed |
Solutions have to be heated
to 70-90 °C |
| Decreases in activity
levels of 100:1 can be achieved |
Generation of a secondary
waste product (the chemicals) |
| Relatively
minor airborne contamination |
Corrodes the
contaminated item |
TOP
Electrochemical
Basically chemical decontamination with
the assistance of an electric field.
Can be considered as the opposite process
of electroplating metals, so rather than
adding metal to the surface, a coating is
stripped away.
The contaminated items are placed in an
electrolyte bath. Electric current causes
anodic dissolution, removing metal and oxides
from the item.
This process can only be applied to conducting
surfaces (metals).
Constant recirculation of the electrolyte
is necessary for high efficiency.
| Advantages |
Disadvantages |
|
Simple and equipment is widely available
|
Electrolyte is a secondary
waste product |
| Removes plutonium, uranium,
radium, cobalt, strontium, caesium and
americium to background levels |
Requires access to object
and a large electrolyte bath |
| Reduces objects to flat
surfaces, deterring recontamination |
Handling can lead to increased
exposure to workers |
| Electrolyte
required is lower than chemical decontamination |
|
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Melting
Most useful in reducing the size of material
in the plant.
Melting is usually the final step, often
following decontamination by chemical methods.
There are also large quantities within
a plant which are only slightly radioactive.
The volume of this scrap is best reduced
by melting.
Remaining radionuclides are distributed
homogeneously in the molten material, reducing
spread of contamination.
During melting, caesium-137 (t(1/2) = 30y)
accumulates in the dust collected by ventilation
filters and is removed.
The dominant radionuclide remaining is
Co-60 (t(1/2) = 5.3y).
| Advantages |
Disadvantages |
|
Effectively redistributes the radionuclides
hence containing
|
Safety is a major concern
in melting steel, which requires high
temperatures |
| There is no surface contamination
of the product |
The product is not reusable |
| Slag is removed and disposed
as radioactive waste |
|
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Mechanical
Mechanical decontamination can be classified
as either surface cleaning or surface removal.
Surface cleaning is done by washing, swabbing,
foaming agents, and latex-peelable coatings
Surface removal is done by wet or dry abrasive
blasting, grinding of surfaces and removal
of concrete.
Some of these methods can also be used
to decontaminate non-metallic surfaces,
such as plastics.
Secondary waste consists of dust particles
which need to be captured in the interest
of safety.
| Advantages |
Disadvantages |
|
Equipment for the methods is well
developed
|
Dust control measures
in dry blasting and large volumes of
contaminated water in wet blasting can
be hard to process and control safely
and effectively |
| Blasting techniques
have proven successful |
|
| The processes are relatively
quick |
|
TOP
Other Methods
- Ultrasonics
- Laser
- High-pressure
- Water jetting or steam spraying
- Thermal erosion
- Pastes
- Gels
- Foam
TOP
Conculsion
The process of decontaminating before dismantling
nuclear facilities can be useful in reducing
waste products and concentrating the radionuclides.
Some methods presented seem a bit excessive
and not worth the danger they pose for the
possibly small advantage in decontaminating
The cost of decontaminating must always
be weighed against the cost. In some cases,
decontamination will be necessary to enable
viable storage of radioactive waste, so
cost will not be an issue.
Source: Decontamination Techniques Used
in Decommissioning Activities
A
Report by the NEA Task Group on Decontamination
|
