GB2328783A

GB2328783A - Gel type decontamination agent

Info

Publication number: GB2328783A
Application number: GB9724630A
Authority: GB
Inventors: Byung-Kil Ahn; Hwi-Jun Won; Byung-Jik Lee; Won-Jin Oh
Original assignee: Korea Atomic Energy Research Institute KAERI
Current assignee: Korea Atomic Energy Research Institute KAERI
Priority date: 1997-08-26
Filing date: 1997-11-21
Publication date: 1999-03-03
Anticipated expiration: 2017-11-21
Also published as: GB9724630D0; GB2328783B; KR100274703B1; KR19990017848A

Abstract

A composition for decontaminating surfaces, which comprises at least one gel-forming component and an ion-exchange component. The composition has the properties of a gel when stationary but decreases in viscosity on subjection to high shear forces allowing it to be jetted onto a contaminated surface from a distance. The gel-former and the ion-exchanger may be a smectite clay such as montmorillonite. The surface is contaminated with radioactive materials.

Description

GEL-TYPE DECONTAMINATION AGENT The present invention relates to a method for purifying a surface which is contaminated with a radioactive material. In particular, the present invention relates to a gel-type composition for radioactive decontamination, a method of manufacturing the same and a method of removing radioactive contamination, in which clay is used to purify a contaminated surface in an effective and economical manner.

At nuclear power plants or in places where radioactive materials are handled, such facilities can become contaminated with radioactive materials during disassembly, repair, maintenance and replacement operations. Also, building surfaces can become contaminated due to leakage of radioactive materials, as in the case of the Chernobyl nuclear power plant accident. In such cases, removal of radioactive contamination is important for the protection of workers, for the protection of residents, and also for restoration of the environment. Conventionally, a number of decontamination methods have been used, including dry methods, wet methods, methods using high molecular weight solutions, and the like.

Dry methods include vacuum suction methods and a sand blasting methods.

In vacuum suction methods, adhered contaminant particles are physically removed by suction. However, the disadvantage of such methods is that ionically fixed nuclides cannot be removed. In sand blasting methods, sand is sprayed or jetted in air at high pressure. In this way, the contaminated surface is ground away and, thus, both the contaminating materials and the contaminated surface are simultaneously removed. Consequently, the decontamination efficiency is low and the contaminated surface is damaged.

Furthermore, fine dust containing radioactive materials is produced in large amounts, which means that a dust-capturing facility is required and the cost of such a facility can be very high. Wet decontamination methods were used at the Chernobyl nuclear power plant accident. In such methods, water or a chemical solution is spouted or jetted onto the contaminated surface to wash off any radioactive materials adhered to the surface. In such methods, however, after removal of contaminants, the washing solution itself becomes a liquid radioactive nuclear waste, which can recontaminate surrounding areas.

In decontamination methods using a high molecular solution (strippable coating), the dried polymer film which remains after decontamination is difficult to remove. Furthermore, the waste so-produced is combustible and, therefore, accidental fires may occur, in addition to the high cost of such methods. In summary, therefore, conventional decontamination agents which have been developed for removing local contamination are very expensive and are harmful to the ecological system. The present invention is intended to overcome some of the aforementioned disadvantages of conventional techniques.

It is an object of the present invention, therefore, to provide a gel-type radioactive decontamination agent, a method of manufacturing the same, and a radioactive decontamination method, in which clay is utilized to purify large scale or locally-contaminated surfaces of raw materials, buildings or other structures in an effective and economical manner.

Accordingly, in a first aspect of the invention, there is provided a composition for decontaminating surfaces, which comprises at least one gel-forming component and an ion-exchange component.

In this aspect of the invention, the gel-forming component and the ionexchange component are preferably the same. In an embodiment, the gelforming component comprises a naturally-occurring mineral. Preferably, the naturally-occuring mineral is a clay, such as a smectite and, most preferably, the smectite is selected from the group consisting of montmorillonite, beidellite, nontronite, saponite, sauconite, hectorite and mixtures thereof. In an embodimemt, the ion-exchange component is capable of undergoing cationexchange. In a further embodiment, the inventive composition further comprises an excess of exchangeable cations, especially ammonium ions. In a preferred embodiment, the inventive composition further comprises a second gel-forming component, preferably a clay. In an especially preferred embodiment, the inventive composition comprise 1-15 wt% dried clay powder, 0.1-7 wt% dried hectorite powder, 0.001-8 Wt% of a source of exchangeable ammonium ions and 0.001-0.8 wt% of a source of sodium ions.

In this embodiment, the balance of the composition is usually made up of water, although further components and/or solvents may also be present. In a preferred embodiment, the composition exists in the form of a gel when at rest but becomes fluid when stressed, to allow it to be applied by spraying or jetting. Preferably, the viscosity of the composition is less than 0.2 dyne.s/cm2 at shear rates above 1,000 s-1. In another embodiment, the composition forms adry film on standing, so that it may be easily removed, along with the adhered or exchanged contaminants.

In a second aspect of the invention, there is provided a method of forming a composition as claimed in any one of the preceding claims comprising mixing at least one gel-forming component, an ion-exchange component, and a liquid phase. Preferably, the liquid phase is water.

In a third aspect of the invention, there is provided a method of decontaminating a surface, which comprises applying a composition according to the invention in its first aspect to a contaminated surface, drying the composition and recovering the dried composition and contaminants to leave a decontaminated surface. Preferably, the composition is applied to the contaminated surface using a jet, a roller and/or a brush. In a preferred embodiment, the dried composition is recovered by suction means.

In an embodiment, the present invention uses natural clay minerals of the smectite group as the base component. Such clays contain ions and added ammonium ions which are exchangeable with radioactive nuclides. At normal temperatures, compositions containing such clays can exist in the form of gels that do not undergo phase separation. If a shearing force is applied, however, the viscosity of such gels is decreased and they can be sprayed like liquids.

After removal of contaminants, the loosely-attached dry clay film containing radioaaive materials can be easily recovered by vacuuming.

The objects and advantages of the present invention will become more apparent from the following detailed description of a preferred embodiment of the present invention with reference to the attached drawings in which: Figure 1 is a graph showing the number of applications of a gel-type decontamination agent according to the invention versus radioactive decontamination efficiency, the contamination source being Cs-137; and Figure 2 is a graph showing the shearing rate of a gel-type decontamination agent according to the invention versus variations in viscosity.

In Figures 1 and 2, the gel-type decontamination agent includes: 1-15 wt% of a dried clay powder; 0.1-7 wt% of a dried hectorite powder; 0.001-8 wt% of ion exchangeable ammonium; 0.001-0.8 wt% of sodium ions; and balance of water.

The gel-type decontamination agent according to the present invention substantially solves conventional problems. The gel-type decontamination agent is spread onto contaminated surfaces using a spraying apparatus, a painting roller, or a brush. The surfaces are then left to dry naturally, and the dried decontamination agent layer is recovered by means of a vacuum suction device. Thus, a radioactive contaminated surface can be purified without causing a damage to the surface. The amount of decontaminating agent applied is preferably 0.5-2 litres per square meter of contaminated surface.

Particularly if a large area is contaminated with radioaaive material, the decontamination agent usually has to be sprayed from a long distance,and the spraying operation should be completed within as short a time as possible, so that the operator is not unduly exposed to radiation. In this regard, although the decontamination agent according to the present invention remains in a gel phase when stationary, if the shearing rate exceeds 1,000 s-l (which is the usual rate) during jetting or spouting, it shows the rheological property that its viscosity decreases to less than 0.2 dyne.s/cm2. Consequently, longdistance spraying is possible even at low spraying energies, using conventional fire-fighting equipment.

In the present invention, an ionexchange material having a large ionexchange capacity can be deposited on the radioactive ions, thereby doubling the purifying effect and improving the rate of removal of contaminated nuclides.

Thus, in a preferred embodiment, the invention provides a gel-type decontamination agent containing a clay mineral of the smectite group as the main component, in which surplus ions capable of ion-exchanging with the radioactive nuclides on the surface are also added. If cations such as ammonium ions are added as exchangeable ions to a dispersed clay solution, electron charges on the corners and surfaces of planar montmorillonite ( which is the main component of clay) are changed, with the result that clay is precipitated. Furthermore, the liquid and solid phases separate and, therefore, a stable gel cannot be obtained and the mixture cannot serve as a decontamination agent.

In order to prevent such a phenomenon, heaorite is added. In this way, even if ammonium ions showing a cationexchange capacity many times that of clay are added, the gel-type decontamination agent has the property that its viscosity decreases in a steep curve as the shearing rate is increased.

Accordingly, the viscosity of such agents is greatly lowered by the shearing force applied during jetting. Thus, long-distance spraying is possible even at low energies and, therefore, workers can be protected from radioactive contamination. Furthermore, during purification of large contaminated areas, large scale application of such gels is possible and, accordingly, the decontamination operation can be completed in a shorter period of time.

Such fluid behaviour is a physical phenomenon which appears in the viscoelastic phase and is an important property of high molecular weight fluids. This means that the present invention can provide a stable gel-type decontamination agent having a shear-thinning property like a high molecular weight fluid.

Example 1 A gel-type decontamination agent according to the invention was prepared using clay minerals of the smectite group as the basis component, so that the agent would have the following characteristics. That is, the agent is capable of ion-exchanging with radioactive nuclides and exists as a gel without phase separation at normal temperatures. If a shear force is applied, the viscosity of the agent is greatly decreased. The radioactive residues can be easily recovered, after removing soluble or particle-type nuclides and removing contamination from the surfaces.

In order to obtain such properties, the gel-type decontamination agent according to the present invention includes: 1-15 wt% of a dried clay powder, 0.1-7 wt% of a dried hectorite powder, 0.001-8 wt% of exchangeable ammonium ions, 0.001-0.8 wt% of sodium ions, and a balance of water.

In preparing the gel-type agent, a dried clay powder is added to a hectorite powder with mixing and an amount of water is then added. A source of ammonium ions and sodium ions is then added into the above mixture and more water is added thereby forming the gel-type decontamination agent according to the present invention.

Example 2 A gel-type decontamination agent according to the present invention can be used in cases where surfaces of raw materials or surfaces of buildings are contaminated during disassembly, repair, maintenance or replacement at a nuclear power plant or at places handling radioactive substances. Here, the source of contamination is soluble or particle-type radioactive nuclides and heavy metals. After completion of the decontamination operation, the final product exists in the form of a dried solid inorganic material and, thus, contaminants can be removed without damaging the surface. This operation is carried out either at short range using a brush or a roller brush, or by using jetting equipment at a distance of 5 m or more. This decontamination agent shows a gel-phase at normal temperatures and in a stationary state. At a shearing rate of 1000 s-1, however, its viscosity is 0.2 dyne.s/cm2.

According to the present invention as described above, localised surfaces or large scale surfaces which are contaminated with soluble or particle-type radioactive materials can be purified and contamination of workers can be reduced during purification. The overall procedure is simple and, therefore, contaminated areas can be purified more easily and effectively. Moreover, the decontamination agent according to the invention maintains a stable phase that does not separate during storage and transportation and, therefore, reliable purification can be ensured. During drying, adherence of the inventive decontaminating agent is better and, thus, in cases where the surface is contaminated by particle-type radioactive materials, the decontaminating effect is improved.

Claims

1. A composition for decontaminating surfaces, which comprises at least one gel-forming component and an ionexchange component.

2. A composition as claimed in claim 1, wherein the gel-forming component and the ion-exchange component are the same.

3. A composition as claimed in claims 1 or 2, wherein the gel-forming component comprises a naturally-occurring mineral.

4. A composition as claimed in claim 3, wherein the naturally-occuring mineral is a clay.

5. A composition as claimed in claim 4, wherein the clay is a smectite.

6. A composition as claimed in claim 5, wherein the smectite is selected from the group consisting of montmorillonite, beidellite, nontronite, saponite, sauconite, hectorite and mixtures thereof.

7. A composition as claimed in any one of the preceding claims, wherein the ion-exchange component is capable of undergoing cationexchange.

8. A composition as claimed in any one of the preceding claims, further comprising an excess of exchangeable cations.

9. A composition as claimed in claim 8, comprising an excess of exchangeable ammonium ions.

10. A composition as claimed in any one of the preceding claims, further comprising a second gel-forming component.

11. A composition as claimed in claim 10, wherein the second gel-forming component is a clay.

12. A composition as claimed in any one of the preceding claims, comprising 1-15 wt% dried clay powder, 0.1-7 wt% dried hectorite powder, 0.001-8 wt% of a source of exchangeable ammonium ions and 0.001-0.8 wt% of a source of sodium ions, the balance being water.

13. A composition as claimed in any one of the preceding claims, wherein the composition exists in the form of a gel when at rest.

14. A composition as claimed in any one of the preceding claims, wherein the composition becomes fluid when stressed.

15. A composition as claimed in claim 14, wherein the viscosity of the composition is less than 0.2 dyne.s/cm2 at shear rates above 1,000 s-1.

16. A composition as claimed in any one of the preceding claims, wherein the composition forms a dry film on standing.

17. A composition substantially as hereinbefore described in the foregoing examples.

18. A method of forming a composition as claimed in any one of the preceding claims comprising mixing at least one gel-forming component, an ion-exchange component, and a liquid phase.

19. A method as claimed in claim 18, wherein the liquid phase is water.

20. A composition substantially as hereinbefore described in the foregoing examples.

20. A method of decontaminating a surface, which comprises applying a composition according to any one of claims 1-17 to a contaminated surface, drying the composition and recovering the dried composition and contaminants to leave a decontaminated surface.

21. A method as claimed in claim 20, wherein the composition is applied to the contaminated surface using a jet, a roller and/or a brush.

22. A method as claimed in claims 20 or 21, wherein the dried composition is recovered by suction means.

23. A method of decontaminating a surface substantially as hereinbefore described in the foregoing examples.

24. A gel-type composition for decontaminating surfaces according to claim 1, characterized in that the composition comprises a natural clay mineral of the Smectite group as a base component for permitting ion-exchange with radioactive nuclides, wherein the composition exists in the form of a gel without exhibiting phase separation, the clay shows reduced viscosity if a shearing force is applied, and residual radioactive materials may be easily recovered after removal of surface contaminants in the form of soluble or particle-type radioactive nuclides has been completed.

25. A gel-type composition as claimed in claim 24, which comprises 1-15 wt% of a dried clay powder, 0.1-7 wt% of a dried hectorite powder, 0.001-8 wt% of exchangeable ammonium ions and 0.001-0.8 wt% of sodium ions, the balance being water.

26. A method of manufacturing a gel-type composition for decontaminating surfaces, comprising the steps of adding a dried clay powder to a dried heaorite powder with mixing, adding an amount of water; adding ammonium ions and sodium ions to the resultant mixture, and adding water to the mixture so obtained.

27. A method of removing a contaminant from a surface using a gel-type composition for decontaminating surfaces, characterized in that the composition is spread on contaminated surfaces using a jetting appartatus, a jetting roller or a jetting brush, the surfaces are left to dry naturally, and the dried inorganic coating layer is recovered by means of a vacuum suction device, whereby a surface contaminated with radioactivity can be purified without causing damage to the surface.

28. A method as claimed in claim 27, wherein the composition for decontaminating surfaces exists as a gel phase when stationary but, if the shearing rate exceeds 1000 ,-1 (the usual rate) during jetting, shows the rheological property that its viscosity decreases to less than 0.2 dyne.s/cm2 and, by utilizing this property, a contaminated surface may be purified by jetting the composition from a distance of 5 m or more.

AMENDMENTS TO THE CLAIMS HAVE BEEN FILED AS FOLLOWS CLAIMS 1. A method of decontaminating a surface, which comprises applying to a contaminated surface a composition including at least one gel-forming component and an ion-exchange component, drying the composition and recovering the dried composition and contaminants to leave a decontaminated surface.

2. A method as claimed in claim I, wherein the gel-forming component and the ion-exchange component are the same.

3. A method as claimed in claims 1 or 2, wherein the gel-forming component comprises a naturally-occurring mineral.

4. A method as claimed in claim 3, wherein the naturally-occurring mineral is a clay.

5. A method as claimed in claim 4, wherein the clay is a smectite.

6. A method as claimed in claim 5, wherein the smectite is selected from the group consisting of montmorillonite, beidellite, nontronite, saponite, sauconite, hectorite and mixtures thereof.

7. A method as claimed in any one of the preceding claims, wherein the ion-exchange component is capable of undergoing cation-exchange.

8. A method as claimed in any one of the preceding claims, further comprising an excess of exchangeable cations.

9. A method as claimed in claim 8, comprising an excess of exchangeable ammonium ions.

10. A method as claimed in any one of the preceding claims, further comprising a second gel-forming component.

11. A method as claimed in claim 10, wherein the second gel-forming component is a clay.

12. A method as claimed in any one of the preceding claims, wherein the composition exists in the form of a gel when at rest.

13. A method as claimed in any one of the preceding claims, wherein the composition becomes fluid when stressed.

14. A method as claimed in claim 13, wherein the viscosity of the composition is less than 0.2 dyne.s/cm2 at shear rates above 1,000 s-l.

15. A method as claimed in any one of the preceding claims, wherein the composition forms a dry film on standing.

16. A method as claimed in any one of the preceding claims, wherein the composition is applied to the contaminated surface using a jet, a roller and/or a brush.

17. A method as claimed in any one of the preceding claims, wherein the dried composition is recovered by suction means.

18. A method of decontaminating a surface substantially as hereinbefore described in the foregoing examples.

19. A composition for use in a method as claimed in any one of the preceding claims, comprising 1-15 wt% dried clay powder, 0.1-7 wt% dried hectorite powder, 0.001-8 wt% of a source of exchangeable ammonium ions and 0.001-0.8 wt% of a source of sodium ions, the balance being water.