GB2374198A - Method and apparatus for decontamination - Google Patents

Abstract

A method of removing a contaminated surface layer of material from a structure comprises the steps of: directing a high intensity focussed light beam from an arc lamp onto a surface which is to be removed from the structure; producing relative movement between the surface and light beam so as to cause heating of an area of the surface; generating a temperature sufficient to effect at least one of: drive off physically associated water absorbed in said surface; remove chemically associated water of hydration in said surface layer; and melting or causing degradation of the said surface; arranging for a combination of parameters comprising pattern of heating, speed of relative movement and heat input rate to cause removal of material; and collecting said treated surface layer material. The method is of particular use in removal of surface contamination of cementitious structures in nuclear installations.

Description

METHOD AND APPARATUS FOR DECONTAMINATION The present invention relates to a method and apparatus, particularly though not exclusively, for the decontamination of structures comprising concrete and having contaminants such as radionuclides embedded in the surface layer or layers thereof.

In the nuclear industry surfaces of structures such as buildings, rooms and storage ponds for example, may become contaminated with radionuclides. Common contaminants may include one or more of uranium oxide, plutonium oxide, strontium-90, caesium-137 and cobalt-60, for example. Such contaminants may be present in the form of fine solid particulates or may have originated from solutions containing them such as in storage ponds for example. Where such contaminants are deposited upon concrete structures, the porous nature of concrete means that the contaminant may be present up to a considerable depth below the surface. However, most of the contamination, about 90%, is present within a few millimetres of the surface. Hence, the safe removal of the surface layer or layers may contribute to a significant reduction in the overall degree of radioactive contamination of the structure.

Many techniques have been proposed such as chemical washing, fluid shear blowing, paste/stripping, mechanical abrasion, for example, which have the disadvantage that

been decontaminated and which also needs to be safely collected, dealt with and stored. JP 3002595 describes the removal of a concrete surface layer by crushing due to the heat generated by the use of microwaves to irradiate the contaminated surface.

DE 3500750 describes inductively heating steel reinforcing bars within a structure to cause large pieces of concrete to spall away.

The thickness of concrete surface removed by the above two processes are not easily controlled, and may result in more concrete being removed than necessary, the bulk of which has no or low levels of contamination, giving rise to associated increased collection and storage costs.

PCT/GB90/02404 describes the use of a laser heat source passed across a contaminated concrete surface to seal or fix the contaminants therein. In this process the contamination is merely sealed into the structure to prevent its spread or dispersion rather than actually being removed.

EP-A-0 653 762 of common ownership herewith describes the use of laser heating to remove a surface layer or layers from concrete or other hydraulically bonded material. Depending upon the precise heating regime employed and the heating regime's effect upon the water content, physically and/or chemically associated with the material, the surface layer may be made to either spall away in flakes or chips of material which may be collected or the surface layer may become detached in relatively thick, large area sheets of material. However,

disadvantages of the laser heat source relate to the fact that only small surface areas may be treated at any one time due to the relatively small area of the laser beam impinging on the material surface.

W097/05630, also of common ownership herewith, describes the use of a high intensity heat source comprising an arc lamp for processing waste in order to vitrify the waste or to combine waste in a vitreous matrix for long term storage. In this document waste arising from any source including nuclear or chemically contaminated material is transported to a facility where the apparatus is sited, the waste is tipped into a suitable container together with vitrifiable matrix material, if appropriate, and treated with the arc lamp beam. The principal teaching of this reference is that waste could be directly vitrified into a suitable storage container without the generation of further intermediate waste in the form of melting crucibles for example. Such melting crucibles as used in existing vitrification processes, themselves need to be treated as intermediate or high level waste when at the end of their lives.

It is an object of the present invention to provide a method and apparatus for removing contaminated surface material from structures and which is able to treat a broader area than lasers.

According to a first aspect of the present invention, there is provided a method of removing a contaminated surface layer of material from a structure, the method comprising the steps of: directing a high intensity focussed light beam from an arc lamp onto a surface which is to be removed from said structure; producing relative movement between said surface and said light beam so as

to cause heating of an area of said surface ; generating a temperature sufficient to effect at least one of : drive off physically associated water absorbed in said surface; remove chemically associated water of hydration in said surface layer ; and, otherwise degrade the surface layer ; arranging for a combination of parameters comprising pattern of heating, speed of relative movement and heat input rate to cause removal of material; and collecting said treated surface layer material.

It has been found that treatment with an arc lamp is capable, depending upon the combination of parameters employed, of effecting one or more of: melting; spalling of the surface layer; repeated spalling of subsequently generated surface layers to various controlled depths; or degrading the surface layer so that the whole of the treated area may be readily cleaved off in relatively thick, large area sheets.

Depending upon the combination of parameters, material removal may be by one or more of the following mechanisms: 1) generating a temperature sufficient to expand or vaporise either physically associated water absorbed in said surface or chemically associated water of hydration in said surface layer, the expanded liquid or vapour then causing a pressure within the pores of the material, e. g. concrete, which bursts or spalls the surface; 2) if the rate of heating is high enough the differential thermal expansion of the surface layers may cause shear stresses which exceed the shear stress capability of the material, thus causing the surface layers to shear off. This may be manifested in violent spalling;

3) certain rates of heat input may give rise to differential thermal expansion and thermal stresses which may cause the fracture of aggregate particles within the material and thus may initiate the release of other stresses; 4) heating up the surface layers to a temperature at which concrete material melts and flows ; 5) re-hydration of a surface in which the surface has previously been melted as in 4) above, causes spontaneous disintegration of the previously melted material; and 6) degrading the entire surface layer so that the whole of the treated area may be readily cleaved off in relatively thick, large area sheets.

A significant advantage of arc lamp heating compared with laser heating is that where structures contain metal components close to the surface and which components may be contaminated, and therefore, desirably removed, that the wavelength of the arc lamp light couples efficiently with the steel, for example, and may be cut or melted as desired where the power of the arc lamp is sufficiently high.

Further significant advantages of arc lamp heating which have been discovered relate to its high heat-flux capability which may be applied over a relatively very broad area compared with a laser; the very low generation of dust when surface material is being removed; and the controllability of the level of heat intensity.

Control of the power density may be effected by: the design of the reflector; control of the arc current; or control of the focus of the heating beam which may be varied from a focussed line of narrow width at the upper

extreme of power density to lower power densities as the beam is de-focussed to wider lines.

Since relatively much larger areas of material are treated in one pass of an arc lamp beam being traversed across a surface, where the heating conditions are such as to cause spalling of the surface, the spalled pieces can be arranged to be of greater area depending upon the processing parameters chosen. Consequently with the method according to the present invention there is less dust generation than with prior art laser-based processes. However, the objective of removing only material adjacent the surface which has the highest contamination level is nevertheless achieved.

Tests have been carried out with a Model 112 (trade name) arc lamp manufactured by the Vortek Industries Ltd.

(trade name) company having a 150kW single quartz arc tube in which a DC arc is struck. The tube is pressurised with argon and cooled with de-ionised water which flows in a spiralling film along the tube in contact with the inner surface of the tube. Thus, the cooling water is in direct contact with the arc. Other types of known arc tube have two concentric tubes where cooling water flows through the space between the inner and outer tubes. An advantage of the type of arc tube design used in the tests is that there is much less attenuation of the arc by the tube and cooling water and a smaller temperature gradient through the thickness of the arc tube. This difference permits arc powers approximately an order of magnitude greater than the other type of arc lamp having the twin concentric tube construction. Light from the tube is collected and focussed by a rhodium-plated, water cooled aluminium alloy reflector. The optical surfaces of the reflector are coated with low absorption glass with

mirrored rear surfaces adhered with heat-sink compound. Depending upon the heating regime intended to be applied to a particular surface to be decontaminated, the reflector may be designed to provide at least one of optimised: focus distance; beam breadth; sharpness of focus; uniformity of heating over the target area ; and, depth of field.

The wavelength of the light emitted from the lamp is in the range from 0.2 to 1. 4m with a peak at about 0. 8gm.

Doped quartz tubes can be used to reduce emission in the UV band if desired. These wavelengths are absorbed well by most engineering materials. Quartz, however, has the added advantage that it hardly absorbs any of the lamp emission band and can, therefore, be used for windows, mirrors and containers in the target area when processing contaminated surfaces.

At full power the input power is 150kW which results in approximately 75kW of continuous optical power output from the arc tube, of which, after losses from attenuation by the reflector still results in more than 15kW of available power on the target area. Thus, the power utilisation of the arc lamp is approximately 10%.

Arc lamps having the basic design of that used in the tests are commercially available up to 300kW in power input. However, higher powers still may be achieved if desired by using arrays of arc lamps.

The type of surface removal produced, i. e. spalling, repeat spalling, melting or cleaving following degradation is governed by the nature of the heating regime applied to the surface. Factors such as: the arc

lamp current which controls power output and the peak heat flux which may be applied to the surface ; the stand off distance; the traverse speed of the beam over the surface; the frequency of repeated traverse across the same area; and the size of the selected area which is traversed all influence the manner in which material is removed.

Conditions to cause spalling require that the power input is high enough for the moisture which is physically and/or chemically associated with the material to be heated rapidly, but not too high so that the material in the surface layer region becomes overly dried out of moisture. Tests using the Vortek Model 112 (trade name) arc lamp described above indicates that spalling may be achieved under processing conditions lying within the following broad limits. Spalling has been shown to occur at a peak flux, i. e. the flux in the centre of the heated area, of between about 25 to 130 W/cm2 but this is dependent upon the traverse speed which may be between about 50 and 800 mm/min and also upon the traverse length, which needs to be short enough so that the surface does not cool too much by conduction into the material before the next traverse by the light beam.

Repeated spalling, i. e. a surface treated once to cause spalling and then treated again to cause spalling of the freshly generated surface after the first treatment, requires that the surface should not be overheated. It is believed that spalling is caused by build up of hydraulic pressure within closed pores in the material from heated water which was either free, or chemically combined as water of hydration but released after dissociation by heating. Spalling is the result of release of energy by sudden release of thermally induced shear stresses built

up under the surface of the concrete. Explosion of aggregate particles due to thermal shock is also believed to contribute to spalling. To achieve repeated spalling it appears to be important that some moisture remains in the material below the previously spalled layer to enable the spalling mechanism described above to operate. Thus, repeated spalling is most likely when the surface is traversed again quickly after first spalling treatment so that residual heat in the material does not cause moisture in the surface to evaporate or become dissociated and evaporate. Overheating of the original surface whilst causing spalling initially may prevent repeated spalling due to having overly dried out the substrata of the material. It is believed that there is an optimum heat flux and area of treatment where the heat input is sufficient to cause spalling but drives off the minimum amount of water from the underlying material thus, water remains available to produce repeat spalling. Such an optimum set of parameters will vary depending upon the type of concrete or other hydraulically bonded material in question. Furthermore, the parameters may also be influenced by the addition of further externally supplied water, the history of the contaminated structure and the environment in which it is located and on the type of structure itself, for example, an old storage pond.

Melting of the surface requires a relatively high peak heat flux and relatively slow traverse speed. Under suitable conditions, the matrix of concrete having a cement matrix and aggregate filler can be seen to melt and subsequently flow on a vertical surface and pieces of aggregate fall from the surface. On horizontal surfaces, aggregate may be seen to dissolve in the molten matrix.

If the molten material is allowed to solidify,

decomposition due to absorption of atmospheric moisture causes the previously melted layer to become very friable and be easily removed over a period of about a day. If water is physically added by sprinkling or spraying to accelerate the decomposition, the previously melted layer virtually spontaneously disintegrates. Thus, this technique may be useful in view of the fact that such techniques involving low volumes of water sufficient only to dampen the material also suppress dust formation and permit collection of the contaminated material without spreading the contamination. Conditions to achieve melting may fall under the following broad conditions in that melting can begin at slow traverse speeds at about 28-40 M/crn depending upon the area being treated. Any heat flux above this may be used if the traverse speed is chosen accordingly. As an example, melting was achieved at a relatively fast speed of 720-800 mm/min at a flux of 95 W/cm2 after repeated passes.

Heating a surface using a combination of parameters sufficient to induce melting, but for an extended period of time using repeated passes, can produce a form of degradation of the concrete material at a depth below the surface such that the entire contaminated surface effectively becomes detached from the parent material and may be cleaved off in thick, large area sheets with minimal effort.

According to a second aspect of the present invention, there is provided apparatus for removing a contaminated surface layer of material from a structure, the apparatus including arc lamp means; means for holding and moving said arc lamp means relative to said contaminated surface on the structure or moving the structure relative to said arc lamp; power supply and service facilities for said

arc lamp ; and, means for collecting contaminated debris removed from said structure.

The arc lamp is able to provide a continuous high heat flux. The 150kW lamp used in experiments was capable of applying a heat flux of about 130 W/cm2 at full power to the centre of a target area. There is between about 85 and 120 W/cm2 available on an area of the target of about 140 x 50mm and between about 60 and 85 W/cm2 available on an area of the target of about 200 x 70mm. The wavelength emission band produced, stated above, is absorbed well by concrete. These figures were obtained with a standard commercial arc lamp not tailored to the particular task in question. By suitable design of the reflector, these figures may be greatly improved. Furthermore, as noted above, arc lamps of much greater power are commercially available and greater powers still may be achieved by utilising arrays of arc lamps.

In the experiments carried out, a water cooled window was interposed between a target concrete block and the arc lamp to protect the arc lamp and reflector from flying spalled debris particles. This window reduced the power able to be transmitted to the target by attenuation by approximately 50%. A similar commercially available 300kW lamp without a water window could achieve power densities

2 of up to 400 W/cm2. Higher power lamps may be sited further away from the target surface and employ light beams which need not be normally incident on the surface but obliquely incident. This would afford protection without the need for power absorbing protective windows.

However, it is possible to transmit the arc lamp light to the required target using mirrors and through windows if desired according to the environment in which the lamp is to be used.

It should be noted that the figures for heat flux given in this specification, unless otherwise stated are with the use of an attenuating water window which as noted above decreases available power by about 50%.

Arc lamps have low thermal mass which provides a virtually instant switch off and small time constant for control purposes when it is desired to change the power setting of the lamp. The control of the arc lamp may be by computer means which is able to ramp up or down power settings as required and hold power settings at will. The degree of control of the actual power output is very high with the ability to apply any power level from 1 to 100% of the lamp's power capability. The arc lamp has high power resolution with an arc current variable from 12 to 600A in steps of 1A thus permitting extremely precise power output control.

Further advantages of the arc lamp include the fact that there are no reaction forces unlike prior art mechanical methods. The arc lamp is comparatively compact and lightweight and may be sited remote from its service cabinets with an umbilical supplying the services of electricity, gas and cooling water. Bearing in mind the fact that decontamination must be effected on site and the hostile nature of the structures which are to be decontaminated these are very important advantages.

Since the arc lamp is compact and lightweight and that the service cabinets having power supply and cooling water supplies may be sited remotely, the arc lamp head may be carried on any suitable handling and manipulating device such as a robot arm for example which may be programmed to traverse a contaminated surface as

appropriate to the type of surface removal which it is desired to achieve.

Whilst the capital costs of laser and arc lamp systems are generally comparable, when judged on the rate of removal of contaminated material, the costs of arc lamp processing according to the present invention are low compared with laser processing costs.

I Collection means for the removed debris may be any that are suitable and may include enclosures traversed with the arc lamp head adapted to receive debris spalled, for example, from the surface. Suction extraction means may be used as appropriate to prevent spread of contamination.

In order that the present invention may be more fully understood, examples will now be described by way of illustration only with reference to the accompanying drawing which shows a schematic arrangement of apparatus used for experimental tests.

The drawing shows a schematic arrangement of apparatus 10 for performing heating experiments on concrete blocks 12 comprising a cement matrix and aggregate filler. A target concrete block 12 was suspended by a chain (not shown) driven by a motor (not shown) and located within a guide frame 14 such that the block 12 could be raised or lowered vertically as desired at a chosen rate in the directions indicated by the arrow 16. An arc lamp 20 in a reflector 22 was positioned horizontally from the face 24 of the block at a distance of approximately 220mm (which is the optimum focus distance for the arc lamp and reflector used in the experiments) therefrom. The arc lamp and reflector was provided with a controllable power

supply (not shown), argon gas and cooling water (not shown) as required by the lamp. A water cooled 30 window 32 was interposed between the face 24 and the arc lamp to protect both the arc lamp and reflector from debris such as spalled flakes for example.

In experiments under controlled conditions to determine the parameters under which various types of surface removal may be achieved it was expedient to move the material to be treated rather than the arc lamp. In apparatus according to the present invention where structures must be decontaminated on site this will clearly not apply and apparatus where the arc lamp head at least is moved with respect to the surface will be employed.

The arc lamp used was the 150kM Vortek Model 112 (trade name) unit described hereinabove. The material being treated was concrete comprising cement with aggregate reinforcement. The Vortek Model 112 arc lamp was arranged so as to traverse a concrete block by means of the block being moved along at a fixed distance from the lamp.

Example 1 Spalling tests At an arc current of 300A, which corresponds to approximately 40 W/cm2 peak heat flux in the centre of the beam, and a traverse speed of 96 mm/min, spalling of an area approximately 75 x 25mm x 4mm thick occurred after 200mm of travel.

In a second spalling test, the traverse speed was set to 200mm/min with an arc current of 550A and which caused spalling of thin pieces of concrete about 2mm thick after 25 seconds of travel. The current was then immediately increased to 600A, which corresponds to approximately 130 W/cm2 peak heat flux in the centre of the beam, which caused spalling of larger and thicker (about 6mm thick) pieces of concrete over a larger area.

Example 2 Repeated spalling tests The most consistent repeat spalling occurred using short passes of about 75mm long, at a traverse speed of about 260 mm/min and an arc current of 250A, which corresponds to approximately 32 W/cm2 peak heat flux in the centre of the beam. Once an area had been repeatedly spalled to a required depth, the lamp beam was moved on to a new area where repeat spalling was continued.

Example 3 Melting tests The arc current threshold to achieve melting of the concrete matrix material at a standstill or at a slow traverse speed of about 160mm/min appears to be about 300A which corresponds to a peak heat flux of about 40 W/cm2 at the centre of the beam. At faster traverse speeds of about 200mm/min, an arc current of 500A, which corresponds to about 100 W/cm2 peak heat flux in the centre of the beam, achieved melting very quickly. However, at faster speeds of about 750 mm/min at 500A arc current, melting was achieved only gradually after repeated passes.

Example 4 Disintegration by Hydration, and Degradation and Cleaving tests One face of a concrete block was warmed for an extended period using 14 repeat double passes at arc currents between 100 A and 150 A and using a traverse speed of 200 mm/min. The arc current was then increased to between 400A and 600 A for 3 double passes at the same traverse speed to induce deep melting of the concrete matrix.

After several days exposure to atmospheric moisture decomposition of the melted layer occurred resulting in disintegration of the treated layer.

Some weeks later, the same face was wetted with water and then re-treated at an arc current of 500A using a traverse speed of 200 mm/min. After 4 double passes, cracks appeared down the sides of the face at a depth below the surface of about 25 mm. Insertion of a tool into these cracks enabled cleaving off of the whole block face with minimal effort.

Thus, it is clear from the above examples that arc lamp conditions and relative traverse speeds may be selected to give desired forms of material removal. However, it is stressed that the geometry of the arc lamp reflector was far from ideal and that with a more suitable power density distribution on the material to be removed the conditions may change and removal may be more efficiently achieved.

Claims (7)

1. CLAIMS 1. A method of removing a contaminated surface layer of material from a structure, the method comprising the steps of: directing a high intensity focussed light beam from an arc lamp onto a surface which is to be removed from said structure ; producing relative movement between said surface and said lght beam so as to cause heating of an area of said surface; generating a temperature sufficient to effect at least one of: drive off physically associated water absorbed in said surface ; remove chemically associated water of hydration in said surface layer ; and, otherwise degrade the surface layer; arranging for a combination of parameters comprising pattern of heating, speed of relative movement and heat input rate to cause removal of material; and collecting said treated surface layer material.
2. 2. A method according to claim 1 wherein said parameters are chosen to effect material removal by one or more of: melting ; decomposition by hydration after melting ; spalling of the surface layer; repeated spalling of subsequently generated surface layers ; and cleaving of relatively thick large area sheets.
3. 3. Apparatus for removing a contaminated surface layer of material from a structure, the apparatus including arc lamp means; means for holding and moving said arc lamp means relative to said contaminated surface on the structure, or for holding and moving said structure relative to the arc lamp means; power supply and service facilities for said arc lamp; and, means for collecting contaminated debris removed from said structure.
4. 4. Apparatus according to claim 3 wherein the wavelength of the arc lamp beam is in the range from 0.2 to 1.4 m.
5. 5. Apparatus according to either claim 3 or claim 4 wherein there is a cooled window between the arc lamp and target material to be decontaminated.
6. 6. Apparatus according to any one of preceding claims 3 to 5 wherein there is an array of arc lamps.
7. 7. A method substantially as hereinbefore described with reference to the accompanying description and any one of Examples 1 to 4 and the drawing.