overview of decontamination processes for decommissioning · pdf fileoverview of...
TRANSCRIPT
IAEAInternational Atomic Energy Agency
Overview of decontamination processes for
decommissioning
Sergey Mikheykin
D-R PMU, Kozloduy NPP site
Workshop on development of specific decontamination techniques for RBMK dismantlement and/or highly
active material from contaminated areas from accident conditions,
Visaginas, Lithuania, 24-28.08.2015
IAEA
Washing-up VS. Decontamination
2
• Relatively fresh
contaminations
• Radioactivity (protection)
• Strong deposits/corrosion
IAEA Workshop, Visaginas, Lithuania, 24-28.08.2015
IAEA
Definitions
Decontamination:
The complete or partial removal of contamination by a deliberate physical, chemical or biological process.
This definition is intended to include a wide range of processes, but to exclude the removal of radionuclides from within the human body, which is not considered to be decontamination.
http://www.iaea.org/ns/tutorials/regcontrol/intro/glossaryd.htm
3IAEA Workshop, Visaginas, Lithuania, 24-28.08.2015
IAEA
Interaction of contaminations with
surface
6
There are two main types of adsorption:
• Physical sorption between a solid and a molecule having
electrostatic polarization forces and the London dispersion
forces (binding energy - 4-42 kJ/mol),
• Chemical - when the physical and chemisorption -
covalent forces or electrostatic interaction and exchange
that gives rise to a strong chemical bond (the binding
energy of 42- 420 kJ/mol).
IAEA Workshop, Visaginas, Lithuania, 24-28.08.2015
IAEA
Interaction of contaminations with
surface
7
In real systems usually contains various kinds of impurities.
As the contact time with the surface radioactive substance
reduced the share non-fixed pollution and increases the
proportion of chemisorbed forms bond with the surface
hardened
I.e. – old contaminations are fixed contaminations
IAEA Workshop, Visaginas, Lithuania, 24-28.08.2015
IAEA
Composition of deposits
8
The inner layer of surface equipment from Stainless Steel enriched
with chromium.
External oxide layer is nickel ferrite composition
(for steam generator tubes made from nickel alloys) or magnetite
Fe3O4 (for steam generator tubes made of stainless steel).
Depending on the nature and pH of coolant (hydrogen
concentration), the outer metal oxide may include metallic nickel.
IAEA Workshop, Visaginas, Lithuania, 24-28.08.2015
IAEA
Chemical decontamination
Chemical
• Detergents
• Acid solutions
• Alkali solutions
• REDOX
• Strippable coatings
• Gels
• Molten salts
9IAEA Workshop, Visaginas, Lithuania, 24-28.08.2015
IAEA
Chemicals 1 – dissolution of
contaminations
11
IAEA Workshop, Visaginas, Lithuania, 24-28.08.2015
IAEA 13
• Ion desorption. The process involves an active replacement of surface ions by solution ions.
• Metal Dissolution. Radioactive contaminants can penetrate deep into the interior of metals
• Dissolution of oxide films and corrosion deposits. Based on the dissolution behavior of Fe and Cr oxides
IAEA Workshop, Visaginas, Lithuania, 24-28.08.2015
IAEA
Decontamination
The dissolution of metals.
Radioactive contaminants may be associated directly
with the metal and diffuse into the bulk of it. The
problem of decontamination includes removing surface
layers of metal.
Some anions activate the dissolved metal: Cl ~> Br-> F-
> NO3-
Chemicals added in water media for removing of oxides layer from metal and for slight
corrosion of metal upper layer.
Chemical solution consist of one reagent is the simplest for decontamination. It may be
acid solution or alkali; mineral or organic. In this case DF vary from 2 to 2000
(approx).
14IAEA Workshop, Visaginas, Lithuania, 24-28.08.2015
IAEA
Chemicals
15
A multistep process, namely, the application of a strongly oxidizing solution followed by a complexing agent in an acid solution, is a technique commonly used for removing the contaminated oxide layer from metal surfaces such as stainless steel.
• The first (alkaline) stage is intended to oxidize the chromium oxides in order to yield soluble chromate ions. Alkaline permanganate is the most common reagent used at the first stage.
• The second (acid) stage is primarily a dissolution reaction for the complexing of dissolved metals. Sulphuric, phosphoric, hydrochloric and hydrofluoric acids and other reagents have been successfully used separately as aggressive decontaminants, generally at concentrations of 2–15%.
• At the second stage, a variety of reagents such as ammonium citrate, or ammonium citrate followed by EDTA, oxalic acid, a mixture of citric and oxalic acids, sulphuric acid, etc., have been used successfully for various applications in the decontamination of stainless steel, carbon steel, Inconel, Zircaloy cladding, etc.
IAEA Workshop, Visaginas, Lithuania, 24-28.08.2015
IAEA
Chemical decontamination
Advantages
• Chemical decontamination is relatively simple and it may also be
relatively inexpensive where additional equipment or personnel
skills are not required.
• With proper selection of chemicals and cycles of treatment, almost
all radionuclides may be removed from contaminated surfaces that
it means very high decontamination efficiency.
• Chemical decontamination is very effective even for complex
geometry and internal surfaces
• Chemical decontamination can apply for processing equipment
without dismantling, i.e. complex objects like primary circuits,
heat exchangers, extraction columns, pipes, valves etc. can be
decontaminated before dismantling and cutting.
16IAEA Workshop, Visaginas, Lithuania, 24-28.08.2015
IAEA
Chemical decontamination
Disadvantages
• The main disadvantage of chemical decontamination is the
generation of secondary liquid waste in the form of aggressive
solutions (acids and alkali with complex composition). The
treatment and conditioning of this secondary waste requires
additional efforts.
• Decontamination with acids and alkalis usually are under
temperature up to 70 to 95°C that may requires additional
protection and operating cost
• High corrosive danger of the process and secondary wastes
17IAEA Workshop, Visaginas, Lithuania, 24-28.08.2015
IAEA
Polymeric films
• Aqueous acryl and vinyl solutions, aqueous emulsions of polymers
(polyvinyl acetate), aqueous dispersions of rubber, water-based
double and triple copolymers are in common practice as coating
formers.
• Effective decontaminating coatings call for aggressive ingredients
like mineral and organic acids, oxidants and alkaline metal
hydroxides.
• Contaminants penetrating 1-50 μm in metal an be removed using a
number of special additives including aggressive ones for different
kinds of surface and different nature of contaminants.
19
IAEA
Gels
• Chemical gels are used as a carrier of chemical decontamination
agents. Gels are sprayed onto a component, allowed to work, and
then scrubbed, wiped, rinsed or peeled off. An airless compressor
can be used for spraying the gel, and with a change in heads, for
rinsing.
• Typical reagent combinations are a nitric-hydrofluoridric-oxalic acid
mixture and a nonionic detergent mixed with a carboxy-methy-
cellulose gelling agent, with aluminium nitrate used as a fluoride
chelating agent
22
IAEA
Foams
• Cleaning agents use foam such as detergents and as a
carrier of chemical decontamination agents.
• They can be applied in a thin layer to a surface in any
orientation, even to overhead surfaces.
• The foam decontamination method can effectively
decontaminate metallic pieces and parts of complex
components.
• Surfactants in the foaming agent enhance the effect by
increasing contact with the surface.
23IAEA Workshop, Visaginas, Lithuania, 24-28.08.2015
IAEA
Foams
Advantages:
• small amount of chemicals and secondary wastes
• possibilities to decontaminate equipment and constructions with
complicated profiles.
Disadvantages:
• transportability of foam is insignificant.
• limited life -time of foam. After destruction of foam it is possible to have
secondary
• contamination of a surface with radionuclide
25IAEA Workshop, Visaginas, Lithuania, 24-28.08.2015
IAEA
Molten salts
• The molten salt stripping process relies on chemical oxidation of the
coating by a molten salt bath
• The organic content of the coating or paint (hydrocarbons) will be oxidized
by reaction with air and the salt bath and will form only CO2 and H2O.
During molten salt stripping process, by-products of the reaction of the salt
and the coating, as well as the radioactive contaminants present mainly in
the corroded and oxidized areas of the metal surface, accumulate in the
bath.
• Even when the bath is saturated with by-products, stripping will continue.
• The superficial radioactive contamination is located mainly in the corroded
regions and mixed with rust.
26
IAEA
Electrochemical Decontamination
Electrolytic polishing is an anodic dissolution
technique. The material to decontaminate is the
anode, the cathode being either an electrode in
stainless steel or copper (helping electrode) in an
adapted form, or the decontamination tank itself.
During decontamination, a controlled quantity of
surface metal dissolves taking with it the
contamination fixed in the surface layers
27
IAEA
Electrochemical Decontamination
• Electrolytic polishing is an anodic dissolution technique. The material to
decontaminate is the anode, the cathode being either an electrode in stainless steel
or copper (helping electrode) in an adapted form, or the decontamination tank itself.
During decontamination, a controlled quantity of surface metal dissolves taking with
it the contamination fixed in the surface layers
• The anodic dissolution of metal in the form of simple hydrated ions or complex ions
is the opposite to cathodic process in many respects. The anodic dissolution:
M + xH2O = Mz+ xH2O + ze-
or for complex ions:
M + xA- + yH2O = MAxz-x . yH2O + ze-
The method is realized in two variants:
• in baths
• an external/remote electrodes.
28IAEA Workshop, Visaginas, Lithuania, 24-28.08.2015
IAEA
Physical and Mechanical
DecontaminationPhysical and mechanical methods of decontamination based on destruction
and removal of upper layer of contaminated surface with accumulated
radioactive contaminants without distinction of chemical and physical forms of
contamination. The main principle is the destruction of connections of
contaminations with the surface.
Mechanical decontamination methods can be classified as either surface
• cleaning (e.g. sweeping, wiping, scrubbing)
• surface removal (e.g. grit blasting,scarifying, drilling and spalling).
Physical methods of decontamination maybe very easy and chip (brushing,
scrabbling etc.) or very complicated and expensive (laser, plasma etc). In any
case physical decontamination help remove contaminated layer of material,
usually thickness to removal can start from hundred microns and can be up to
few centimetres.
31IAEA Workshop, Visaginas, Lithuania, 24-28.08.2015
IAEA
Decontamination
Physical
• Electrochemical
• Ultrasonic
• Laser
• Plasma
• Melting of metals
• Thermo
Mechanical
• Mechanical
• Vacuuming (wet and dry -brushing)
• Scrabbling, cryogenics, vibrational cleaning
• Shot blasting – gaseous agents, particle impact
• Jetting – water, steam, wet reagents
32IAEA Workshop, Visaginas, Lithuania, 24-28.08.2015
IAEA
Physical and Mechanical
DecontaminationAdvantages
• The mechanical decontamination methods are easy for application;
• Processes of mechanical treatment of surfaces are well known in
common use.
• Absence of secondary wastes in form of liquids help to reduce
cost of treatment
• Possible organization of remotely controlled operation and
application of robotic systems
Disadvantages
• Formation of airborne contamination requires additional protection
and filtration;
• The contaminated surface need to be accessible in the process of
treatment;
• Relatively low speed of treatment.
33IAEA Workshop, Visaginas, Lithuania, 24-28.08.2015
IAEA
Physical Decontamination
• Physical and mechanical
• Physical and mechanical methods of decontamination are based on the destruction
and subsequent removal of a contaminated surface without distinction of the
chemical and/or physical form of the contaminant. They are generally considered
abrasive techniques as they act to remove the surface layer of contaminated
substrates in order to separate the contamination from the object, lowering the
classification status. The removed contamination can then be collected, treated if
necessary and disposed of as contaminated waste. As the volume of waste has
been greatly reduced this will act to lower the associated cost of disposal. In the
nuclear industry this is the most common method of decontamination and as such
there is a wide variety of methods available at high maturity levels.
•
• Whilst some of these types of techniques can be deployed remotely, it is often
cheaper and easier to manually deploy them and as such they are widely used
across the nuclear industry.
34IAEA Workshop, Visaginas, Lithuania, 24-28.08.2015
IAEA
Criogenic
• The CO2 pellets are accelerated in an injector by means of
compressed air at 18 bar and with a rate of about 21.5 m³/min. The
contamination is pulled out of the surface and carried away by the
excess of air.
• The CO2 pellets evaporate and the removed contamination is
settled, or taken away by the air filtration system. Finally, the
contamination will be located on the floor of the enclosure and/or in
the filters.
35IAEA Workshop, Visaginas, Lithuania, 24-28.08.2015
IAEA
Ultrasonic
• The process is based on the use of ultrasonic waves in a bath containing a
cleaning solution. The ultrasound is produced by a generator at a
frequency greater than 20 kHz. A transducer converts high frequency
energy into low amplitude vibrations at the same frequency. Scrubbing is
accomplished through the formation and violent collapse of thousands of
minute bubbles, which lift radionuclides from the object’s surface.
36
IAEA
Melting
• Volatile nuclides such as 137Cs will have been removed in the first
melt.
• The slag has been removed as radioactive waste.
• By melting slightly contaminated scrap, it is possible to recover
much of these valuable metals while simultaneously conserving
valuable space at final disposal facilities.
• Melting completely destroys components and as a decontamination
technique is effective only for contaminants that are volatile or that
concentrate in the slag or dross (e.g. plutonium) rather than in the
molten metal.
37IAEA Workshop, Visaginas, Lithuania, 24-28.08.2015
IAEA
Decontamination Factor
38
• Decontamination factor (DF) is the
qualitative explanation of decontamination
efficiency.
• The effectiveness of decontamination is
determined by the strength of the fixation of
radionuclides on the surface and the nature
of the decontamination agent
treatmentaftermeasuredActivity
treatmentbeforemeasuredActivityDF
IAEA Workshop, Visaginas, Lithuania, 24-28.08.2015
IAEA 39
List of decontamination techniques ( for debates)
## Decontamination
Method
Application advantages Disadvantages
1 Chemical
decontamination
Large volume of RW,
Preferable for defragmented
equipment
1a 1 reagent CS, SS, plastic High DF
1b 2 and more
reagents
CS, SS, plastic High DF, deep
penetration of
contaminants
2 Water/steam jet High aerosol formation
2a With surfactant Non-fixed contamination
onto metal, ceramic and
plastic, painted surface
Simple application Only for non-fixed
contamination
2b With chemicals Decontamination from heavy
contamination
Possibility of
decontamination of
wide list of subjects
Need high temperature of
application (60-1200C)
2c With abrasive Concrete, brick,
painted/corrosive CS and SS
IAEA Workshop, Visaginas, Lithuania, 24-28.08.2015
IAEA 40
3 Foaming Non-fixed contamination onto
metal, ceramic and plastic, painted
surface
Low RW
formation
Transport ability of foam
is insignificant. Life -time
of foam is limited, and
after destruction of foam
probably secondary
contamination of a surface
with radionuclides
4 Ultrasonic Non-fixed contamination onto
metal and plastic
Low RW
formation
Low capacity
5 Electrochemical
decontaminatio
n
5a In bath CS, SS, defragmented equipment Formation of aggressive
liquid RW
5d External
electrode
CS, SS, plastics
Possibility to decontaminate non-
defragmented equipment, floors,
walls.
Low RW
formation, wide
application
possibilities
6 Strippable
coatings
All kinds of surfaces except
Equipment with complicated
profile
Low solid RW
formation
Low mechanical strange of
polymeric film
IAEA Workshop, Visaginas, Lithuania, 24-28.08.2015
IAEA 41
7 Thermo
decontamination
Concrete, brick,
painted/corrosive CS
and SS
Low RW formation method may apply
only for horizontal
surface of concrete,
bricks and
corrosive/painted
surface of metal
8 Mechanical
decontamination
Using of special tools
and adaptive ones
8a Abrasives painted/corrosive CS
and SS
8b Scrabbles Concrete, brick,
painted/corrosive CS
and SS
9 Ice blasting Equipment with
complicated profile
Low RW formation High aerosol
formation
IAEA Workshop, Visaginas, Lithuania, 24-28.08.2015