Overview of decontamination processes for decommissioning · PDF fileOverview of decontamination processes for decommissioning Sergey Mikheykin ... pipes, valves etc. can be ... Disadvantages

IAEAInternational Atomic Energy Agency

Overview of decontamination processes for

decommissioning

Sergey Mikheykin

D-R PMU, Kozloduy NPP site

[email protected]

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

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Washing-up VS. Decontamination

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• Relatively fresh

contaminations

• Radioactivity (protection)

• Strong deposits/corrosion

IAEA Workshop, Visaginas, Lithuania, 24-28.08.2015

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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

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Contaminations

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Contaminations

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Interaction of contaminations with

surface

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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).


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Interaction of contaminations with

surface

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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


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Composition of deposits

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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.


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Chemical decontamination

Chemical

• Detergents

• Acid solutions

• Alkali solutions

• REDOX

• Strippable coatings

• Gels

• Molten salts


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Chemicals

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Chemicals 1 – dissolution of

contaminations

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Chemicals 2 – dissolution of oxides

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• 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


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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).


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Chemicals

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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.


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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.


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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


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Polymeric films/Gels

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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.

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Used films

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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

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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.


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Foams/Surfactants

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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


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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.

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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

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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.


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Physical Decontamination 1

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Physical Decontamination 2

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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.


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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


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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.


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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.


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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.


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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.

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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.


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Decontamination Factor

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• 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


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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


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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


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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


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Thank you!

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Documents

Overview of decontamination processes for decommissioning · PDF fileOverview of decontamination processes for decommissioning Sergey Mikheykin ... pipes, valves etc. can be ... Disadvantages