Decontamination. What Is This and Why Is It Needed?
Decontamination is the removal of radioactive substances from surfaces of buildings, equipment, household items, clothing, water, other liquids, etc.
In particular, radioactive contamination occurs through direct contact with the environment containing radionuclides, including through their deposition from the air.
It is possible to reduce the level of radioactive contamination in a natural way without human participation and in an artificial way with active human participation. The natural way is to reduce radioactivity level through the natural decay of radioactive elements. This explains the rapid decrease in radioactivity during the first hours after the incident with the release of radioactive substances. This is because a significant number of isotopes have a short half-life. The presence of long-lived isotopes requires special decontamination measures.
The issue of decontamination for Ukraine has become especially acute after the Chornobyl accident. However, even now, despite the fact that the third part of a century has passed since those events, it remains relevant, because a number of radioactive substances that have spread after it have a significant half-life. This means that radiation hazard remains in many cases. In addition, radioactive waste is still generated resulting from the use of radiation sources and use of nuclear technologies, such as, manufacturing, operation and reprocessing of nuclear fuel, equipment maintenance and refueling at NPPs.
Radionuclides contained in radioactive waste have different physical and chemical properties and affect people and the environment in different ways causing harm. Therefore, contaminating radioactive substances require the minimum possible amount, most reliable isolation and processing.
Decontamination can be considered one of the most promising ways to reduce the amount of radioactive waste.
The experience of decontamination was largely gained during the mitigation of the Chornobyl accident consequences. Before the Chornobyl accident, industrial decontamination and decontamination of the areas contaminated with radionuclides were common decontamination methods. Due to the fact that nuclear weapons were tested in the middle of the twentieth century, there was also the experience of military decontamination. However, when after the ChNPP accident, there was a threat of large-scale contamination for large areas and the public, the decontamination methods tested at that time became not effective enough and therefore new technologies were developed, such as the KPI Technology for water decontamination, which was developed and successfully implemented by researchers of the Radiochemistry Laboratory of the Faculty of Chemical Technology of the Kyiv Polytechnic Institute.
Now experts on decontamination in Ukraine are trained by the Sikorsky Kyiv Polytechnic Institute, Odessa National Polytechnic University, nuclear power plants, specialized enterprises for radioactive waste management. Previously, such experts were trained also by the Sevastopil Institute of Nuclear Energy and Industry in Crimea, but now it is under Russian occupation. Experts who would be able to competently implement decontamination measures are much needed and not only during or after radiation incidents.
Today, there are 15 power units in operation in Ukraine. The life cycle of each of them includes decommissioning and complete decontamination of the territory from radioactive materials: when the decommissioning and dismantling of NPP equipment is conducted, according to one of the accepted concepts, a clean green lawn, which is safe for people and the environment, should remain at the NPP site
However, there is still no standard generic decontamination method that would be equally effective for any surfaces and materials.
During the operation of nuclear facilities, a certain amount of radioactive waste is accumulated, which can be in liquid, gaseous and solid conditions. To reduce its volume to the minimum possible level, special methods are used to minimize radioactive waste, which are covered by the IAEA as low as reasonably achievable concept for reducing the amount and activity of radioactive waste. Obtaining the lowest reasonably achievable level of activity and volume of radioactive waste is one of the main principles of the state policy of Ukraine in radioactive waste management.
To decontaminate gaseous waste, either holding in gasholders is used, which can be mounted directly in the lower part of the ventilation pipe during the time required for decay of radionuclides, or purification in absorbing installation.
The methods of ion exchange and multi-stage evaporation in special evaporators, which allows reducing the volume of liquid contaminated with radionuclides, and provides a high degree of its purification are most often used in practice for liquid radioactive waste.
In addition to contaminated reactor components and equipment, contaminated overalls and footwear, solid radioactive waste is waste generated after solidification of liquid radioactive waste. Solid radioactive waste is disposed in specially designed places. In particular, abandoned salt and limestone mines, formations in permafrost and deep geological formations can be used for this purpose.
Decontamination is performed using chemical, electrochemical, mechanical and other special methods that allow, if not completely eliminate radioactive contamination, then reduce it to the level when it will no longer pose a threat. Comprehensive decontamination is often conducted by applying several different decontamination methods to the same object.
Considering that the amount of radioactive waste will only increase with the gradual decommissioning of nuclear facilities, the issue of decontamination is very relevant, since it can significantly reduce its amount.
The decontamination process is implemented in two stages: the first is to remove connection between the carriers of radioactive contamination and the contaminated surface, and the second is to transfer radioactive contamination from this surface.
Mechanical methods of decontamination presents removal of radioactive substances from contaminated surfaces using brushes, powerful jet of air (for example, from an aircraft engine) or water, at the same time, surface water tension is reduced by heating or adding surfactants, such as soap, washing powder or other detergent. The method of mechanical decontamination is used to decontaminate equipment, vehicles, clothing, and personal professional equipment. Moreover, mechanical decontamination stations are arranged within checkpoints from contaminated areas, because it is considered the simplest and most affordable.
When radioactive substances are on the surface and are weakly bound to it, contaminants are collected from the surface using a vacuum. In some cases, mechanical decontamination is considered the best, in particular for porous surfaces. The mechanical method includes the abrasive technique: removal of surface contaminated layer using special abrasive agents (sandblasting, cleaning with crushed fruit kernels, small ice crystals, etc.), jet technique: is based on water properties to dissolve some chemical substances and clean smooth surfaces of almost all materials from contamination by jet washing. However, water is ineffective in cleaning old contamination and contamination chemically bounded with surface. The force of water jet pressure is also of great importance.
The decontamination method using Freon-413 is also used, because it enters pores and crevices better than water, actively interacts with oils, etc. In addition, most radioactive contaminants cannot be dissolved in Freon, and therefore are easily filtered out.
In some cases, the only method of decontamination is brushing, i.e. removing surface contaminated layer of metal, concrete or other material of which the object requiring decontamination is made using special brushes or devices.
Fusion decontamination also belongs to mechanical decontamination methods. The importance of this method is very significant because radioactive metal is accumulated after dismantling of nuclear facilities. In some countries, according to adopted metal management strategies, such material is disposed in special storage facilities, but, for example, in France, Germany and Sweden there are general management strategies for radioactive metal, which include construction of centralized highly economical smelting units for processing metal waste. The metal obtained after remelting has lower radioactivity levels and can be reused. If, due to volume reduction, the concentration of radionuclides increases, such metal should be disposed.
Methods of mechanical decontamination using ozone, microbiological decontamination, supercritical fluid extradition in carbon dioxide, plasma decontamination and thermal decontamination are also known and used.
However, due to the fact that radioactive substances seep into the depths of objects, which cannot be simply mechanically decontaminated, a physicochemical method is also used. It consists in the use of special preparations that increase decontamination efficiency.
The chemical decontamination method is the use of special chemical agents to remove radioactive contamination from surfaces. During interaction with the contaminated surface, they dissolve the contaminating film without damage of the material of a decontamination object. The use of the chemical method is the most effective for treatment of large objects such as walls or floors; at NPPs: for decontamination of equipment within circuits, for cleaning of a steam generator (washing of slots, pipe bundles, cleaning by water monitors and filling with decontamination solutions).
In some cases, chemical decontamination is performed by immersing a contaminated object in a bath with chemical decontamination solution, and the course of a chemical reaction is catalyzed by solution displacement or heating. Efficiency increase of decontamination activities is possible with the use of ultrasound. The use of this technology started in 1954, and now it is widely applied because ultrasonic waves propagate well in an elastic medium and contribute to a more complete removal of radioactive substances.
Today, one of the most promising methods of decontamination is decontamination with gels, which allows reducing the amount of liquid radioactive substances generated during decontamination due to the ability of gels to retain chemical agents on decontaminated surfaces. Gels have a lower fluidity than conventional solutions of chemical agents; they are better retained on the surface. Due to this, a longer contact is provided between decontamination solutions and an object to be decontaminated. Decontamination with a gel developed in Germany was used during the mitigation of ChNPP accident consequences. This method was used in France to decontaminate spent nuclear fuel storage pools, in Japan to treat the control room.
In 1998 in France, STMI decontamination gel was developed. It can be applied directly to the components of NPP equipment, and when, after a few hours of treatment, the gel is washed off with a water stream under pressure, radiation doses and waste generation are minimum.
One of the low-waste methods to take decontamination measures is decontamination with foam. This method is also marked by a relatively small amount of radioactive substances. Water serves as a solvent during foam formation. The composition includes a decontaminating agent, foaming agent, foam stabilizer, complexing agents that prevent reverse sorption of radionuclides on the surface, components that increase adhesion, etc.
The steam emulsion method of decontamination is also known. It consists in supplying hot decontamination steam in the form of emulsion using saturated steam under pressure. Etching of surfaces using chemical agents also belongs to the chemical method of decontamination.
One of the oldest and most widely used decontamination methods is physical-chemical (hydromechanical) decontamination, which consists in the treatment of surfaces of contaminated objects with special decontamination agents using brushes, rags, etc. Such agents as Lyubomyd-105, SelORS, Barrier are most often used at Ukrainian NPPs. The advantages of this method are easy use and relatively small amount of generated waste. Hydromechanical decontamination can be used to decontaminate concrete surfaces, after which radioactivity is reduced by 30-50%
Electrochemical decontamination is also used, during which a contaminated metal part is placed in a special container and an electric current charge is applied to it. The use of electrochemical decontamination can reduce decontamination duration, consumption of reagents and amount of liquid waste, as well as improve treatment quality.
Depending on the charge that is applied to the surface requiring treatment (positive or negative charge), a distinction is made between cathodic or anodic treatment. Electrochemical decontamination is performed by electrolysis, i.e. by the release of electrons by the anode and their attachment to the cathode when an electric current passes through electrolyte solutions. The advantages of this method are high decontamination factor, improved surface quality, and the ability to remove local contamination.
Where is it used and who is engaged in decontamination in Ukraine?
Decontamination is carried out at NPPs before and after maintenance of reactor compartment equipment using special equipment and chemicals, when it refers to decontamination of heavily contaminated equipment parts, or manually.
In addition, military men of radiation, chemical and biological protection and SESU staff have decontamination skills. Only persons over 18 years after special training and testing the knowledge gained, instructed on radiation safety are allowed to perform decontamination activities.
Decontamination is also conducted by experts from special enterprises for radioactive waste management, in particular, Radon Corporation, Barrier, Central Enterprise for Radioactive Waste Management, Vector.
During decontamination, it is important to monitor constantly its efficiency. For this purpose, the level of radiation contamination is measured before decontamination, during each cycle of activities and after it.
Decontamination efficiency depends not only on the characteristics of radiation contamination, but also on the material of an object to be decontaminated. The efficiency of the measures taken is assessed by the value of the decontamination factor, which is equal to the ratio of the initial radioactivity of the contaminated object to the value after decontamination. The decontamination factor is not always accurate and unambiguous, because it does not take into account the possibility of repeated radioactive contamination.
Since decontamination process can last from several seconds to several hours, there is the notion of decontamination rate. The decontamination rate characterizes radioactivity decrease for unit of contaminated area or volume per unit of time.
Special decontamination techniques
In addition to certain proven methods of decontamination, characteristics, purpose, specificity of the material of objects to be decontaminated are of great importance in practice.
Decontamination of overalls and personal protective equipment
The surfaces of fibers that compose fabrics for workwear and personal protective equipment depending on their composition have a different ability to absorb and retain radioactive contamination. Thus, for example, wool becomes dirty at the edges of scales, cotton: in cavities and notches, acetate fibers: in channels and grooves, synthetic fibers that have a smooth surface: in small cracks. However in all cases, it will be true that the thinner the fiber or the more it is worn out, the more dirt accumulates on it, despite the fact that the fabric is practically a filter on whose surface large particles are retained, and small ones penetrate under it.
The oldest method used to decontaminate clothing and personal protective equipment during nuclear weapons tests in the United States in the 1940s was shaking out. However, this method is ineffective and can only be recommended as a preliminary decontamination
Washing with the addition of surfactants is much more effective.
Before washing radiation-contaminated clothes, dose monitoring is provided and clothes are sorted according to contamination level and fabric type. The actual decontamination by washing is carried out in two stages: the first is to remove radioactive contaminants and transfer them from clothing into the decontamination solution, and the second is retention of radioactive contaminants in this solution. Mechanical agitation (e.g. in the drum of a washing machine) helps to remove contamination more efficiently. Several complete washing cycles are provided: the first in an acidic environment, the next in an alkaline one.
Repeated dose monitoring is conducted after washing, and if the level of radiation contamination has not reached the normal value, the procedure should be repeated.
Decontamination washing is carried out in specially equipped laundries.
Decontamination of soil
Radioactively contaminated soil in most cases belongs to low-level radioactive waste. Difficulties in its management, decontamination and disposal are due to the fact that soil contains a significant amount of components with different chemical and physical properties. A special hazard of radioactively contaminated soil is forming radioactive dust in the open air, which is carried over long distances.
Today, the most common method of soil decontamination is to remove completely the surface layer and it should be twice as thick as the level of penetration of radionuclides. This is a rather complicated and laborious method, because the volume of soil to be cut is 1000 or more times greater than the mass of radionuclides themselves. Later, these places are covered with a layer of clean soil.
An alternative to cutting is setting out plants on contaminated soils that extract radionuclides from it. One of the most commonly used “green pumps” is the clover. Thanks to this technology, soil contamination can be reduced by 40-80%. After mowing, the green mass is processed in compliance with radiation safety rules.
Soil is also decontaminated by gravitational enrichment, which consists in removing radionuclides contained in fine fractions of various density from contaminated soil. Thus, up to 95% of radionuclides can be removed from soil. This way was used to remove plutonium after the crash of B-52 strategic bomber near Palomares in the United States, when the content of three hydrogen bombs was sprayed over an area of 2.3 km2.
The methods of soil decontamination by ion extraction, ion exchange, etc. are also known. However, a sufficiently economical and efficient technology for soil radioactive decontamination have not been developed yet.
Decontamination of ion exchange resins
Ion exchange resins are widely used at NPPs to maintain coolant quality. When the service life is expired, they are transferred to radioactive waste storage facilities. Traditional technologies, such as, cementing, cause a significant volume increase because resins tend to swell in a liquid medium. The method of deep decontamination of ion exchange resins seems to be promising; moreover, during treatment, their volume is significantly reduced, and after treatment, ion exchange resins become practically safe. The essence of the method is transforming radionuclides into a liquid phase with subsequent purification using selective inorganic radiation-resistant sorbents. The spent resin is washed with a special solution, which can be reused when the chemical composition is corrected.
Decontamination of skin
Personnel working with radioactive substances are at risk of getting radionuclides on open skin that can pose a threat to health and life, so this should be avoided.
If there is a contact of radioactive substances with the skin, wash them off as soon as possible. The optimal water temperature for rinsing is not over 30-32 ° C, because hot water expands pores and promotes penetration of contamination deep into the skin. Special tool Protection can be also used. It is rubbed evenly on the affected skin area, after 2 minutes it is washed off, wiped dry and radiation monitoring is provided. If there is a significant skin lesion, the most contaminated areas are decontaminated first and then all the rest. When there is a lesion of the mucous membranes of mouth or eyes, they are washed with plenty of water and a 2% solution of baking soda.
Decontamination is considered complete if radioactivity level does not exceed the allowable level, which is confirmed by radiometer readings.
Decontamination of liquids
Several methods are used for water decontamination: settling and filtration. Using these methods, insoluble solid particles can be removed; coagulation followed by settling can remove up to 40% of strontium-90, cesium-134 and cesium-137. More complete removal is achieved by filtration, distillation or passing water through ion exchange resins.
Decontamination of transport
The most of radioactive substances is deposited on horizontal surfaces, on oil filter, radiator, individual engine components, tailgate and rear part of the body. Possible places of contamination can be working components that contact with the soil (for example, excavator buckets), wheels, engines, engine air filters, air ducts, cleaning filters, various greasy places.
Contaminated surfaces are decontaminated by wiping from top to bottom with brushes using a decontamination solution, grease parts are decontaminated with organic solvents, diesel fuel, kerosene, etc. If such treatment is insufficient, then paint and varnish coating is removed using a solvent, surfaces are etched with acids or alkalis, upper contaminated layer is mechanically removed, after which the surface is washed with water and wiped dry.
If transport should be decontaminated frequently, then special washes are arranged where decontamination technologies using jets, special pastes or other special means, steam and emulsion or hydromechanical methods are applied.
Decontamination of buildings and structures
A severe accident at NPP can cause the release of a significant amount of radionuclides into the atmosphere, some of which can settle anywhere, even on people’s houses. Due to this, residential, industrial, public and other buildings can become radioactively contaminated. Preferably, when this refers to decontamination of large objects in an open space, washing of radioactive dust with water supplied under high pressure using fire and watering machines is the most often used.
However, the method of decontamination for large objects, including buildings, is closely related to the properties of their construction materials, size, configuration, etc.
In general, decontamination activities are started from the roof preferring dry cleaning methods to prevent flow down the walls of radioactively contaminated water. The most advisable methods, in this case, would be vacuum cleaning, mechanical treatment with brushes, removal of the top contaminated layer, use of decontamination films.
Rooms are also treated inside from the ceiling to the floor, and the treatment starts from the least contaminated areas. Treatment with foams and decontamination solutions is the most effective, if liquid cleaning methods are selected. The foam is applied in a layer 2-3 cm thick, retained for 10-20 minutes and then removed, best of all using a dust extractor.
Floor is decontaminated by mechanical removal of the upper contaminated layer, insulation with polymeric materials or concreting.
Ukraine is one of the countries having the greatest practical experience in decontamination conduct and arrangement. Technologies become more sophisticated, improved and thus, more efficient, energy saving, more cost effective, friendly to the environment and people. Some of them allow reducing the contact between a person and a radioactive substance to the minimum and this is important, because human life and health are priceless