WO2004028713A1

WO2004028713A1 - Surface treatment of concrete

Info

Publication number: WO2004028713A1
Application number: PCT/GB2003/004158
Authority: WO
Inventors: Stephen Christopher Neil Brown; Julian Timothy Spencer; Andrew William Benjamin; Tristan Goronwy Thomas
Original assignee: Bnfl (Ip) Limited
Priority date: 2002-09-26
Filing date: 2003-09-26
Publication date: 2004-04-08
Also published as: AU2003299080A1; GB0222338D0

Abstract

A method of treating a surface for the removal of a surface portion by scabbling comprising the steps of oscillating, in a first direction, a laser beam incident upon the surface and superimposing a nett direction of travel of the incident laser beam over the surface in a second direction such that the incident laser radiation traverses the surface so as to effect surface removal. The amplitude of oscillation of the laser beam incident upon the surface may be less than a width of the incident laser beam. The method avoids areas adjacent to a scabbled area being effected so as to no longer be amenable to scabbling and provides a more even scabbled surface.

Description

Surface treatment of Concrete

Field of the invention

The present invention concerns surface removal from inorganic non-metallic structures, in particular concrete structures, primarily though not exclusively for the purpose of removing radioactive contamination contained in surface layers .

Discussion of the Prior Art

In the nuclear industry surfaces of concrete structures may become contaminated with radionuclides . Common contaminants include uranium oxide, plutonium oxide, strontium-90 , caesium-137 and cobalt-60. Typically this contamination is only present in a surface layer of concrete. Such layers may be around 1 to 4mm or more in thickness . By removal of such a surface layer the degree of contamination of a surface and of the structure as a whole may be greatly reduced. However, simple mechanical methods may be unsuitable for use where a potential for contamination makes it desirable for an operator to be remote from a surface to be operated upon.

Various techniques are known for the surface removal of concrete, stone and similar surfaces. One such method is the heat treatment of a surface such as to degrade that surface and release a surface layer.

JP 3002595 describes the removal of a concrete surface layer by crushing due to the heat generated by the use of microwaves to irradiate a contaminated surface layer.

GB 2316528 describes a process for decontaminating a surface using a pulsed laser beam. Pulsed laser beams emit very high energy in very short pulses. The document describes pulses of 28ns duration, and a maximum repetition rate of 250Hz (i.e. a pulse every 4ms), hence the time between pulses is approximately 140,000 times that of the duration of each pulse. Such high energy pulses often have peak power densities which are of the order of several tens of MW/cm², but which only last for the duration of the pulse. Such high peak power densities give rise to ablative processes, such as vaporisation, sublimation or plasma formation, to effect removal of a very thin surface layer. Pulsed laser processes are characteristic in that the time interval between the very high energy, short pulses is several orders of magnitude longer than the duration of each pulse. This gives a very high instantaneous energy intensity during the short duration of each pulse, but a lower power density when the output is averaged continuously over time.

EP-A-0653762 describes a method, termed scabbling, of modifying concrete by ejecting solid surface fragments, chips or flakes of material of significant size (e.g. several grams) and volume, thus causing surface removal. In the method described a laser is used which is scanned across the surface in a raster scan. In such a method energy from the laser light heats the surface and causes surface fragments to break away or be ejected, often violently, due to the generation of either steam or thermal stresses below the surface. This latter phenomenon of ejecting solid surface fragments, chips or flakes of material by use of a laser is known in the art as laser scabbling.

However, it has been found that some surfaces, including some types of concrete having aggregate therein, whilst able to be successfully laser scabbled on a first scan of the laser beam over the surface, may fail to scabble, or do so only with reduced efficiency, on areas which are either within or adjacent to a first scanned track when subjected to laser irradiation on either a subsequent re-treatment of the same area or on an adjacent scanned track.

Summary of the invention

According to the present invention there is provided a method of treating a surface for the removal of a surface portion comprising the steps of oscillating, in a first direction, a laser beam incident upon the surface and superimposing a nett direction of travel of the incident laser beam over the surface in a second direction such that the incident laser radiation traverses the surface so as to effect surface removal. The method results in a more even incident laser beam power density distribution over the surface being treated than methods hitherto described. Thus the proportion of an area being treated with a laser beam, of a given power distribution, which fails to reach a threshold power density for effecting scabbling is lower than with known methods of laser surface treatment .

Limitations of the prior art

The method of the invention, of laser scabbling using an oscillating spot of laser light, significantly reduces the creation of areas treated by the laser light which subsequently resist further laser scabbling treatments, when compared to other known methods of applying laser light of substantially continuous power. In addition the method results in a flatter scabbled surface than with other known methods of laser scabbling.

The method of the invention, of laser scabbling using spots of laser light, requires an average power density on the surface at the point of application of an oscillating spot of laser light of above 30W/cm2 or thereabouts and below around 200 /cm2 or thereabouts. Above an average power density of around 200 /cm2 the concrete may scabble for a short time, but this will usually result in melting of the concrete matrix which hinders further scabbling. Below an average power density of around 30W/cm2 the threshold powder density, which gives a heating rate sufficient to cause scabbling is unlikely to be reached.

A preferred average power density range for laser scabbling according to the invention is 50W/cm2 to 150W/cm2. An average power density of an oscillating spot of laser light found to produce optimum scabbling results is of the order of 100W/cm2. The precise optimum average power density used will depend upon the characteristics of the surface to be scabbled such as concrete type, reflectivity etc.

The term average power density refers to power as averaged over the area of an oscillating spot of laser light. This is in contrast with a power density as averaged over time, which is not meant .

The scabbling of a concrete surface by a laser of substantially continuous power may be compared to the treatment of a surface by the pulsed laser typically used in ablative processes, and the resulting effects are significantly different. When scabbling by applying a continuous laser, such as the oscillating spot of the invention, to a given surface location being swept by an oscillating spot of laser light, the heat has sufficient time, and therefore the opportunity, to diffuse below the surface and build up the stresses and/or steam in an appreciable volume of material, resulting in the ejection of solid fragments, chips or flakes of material of significant size. However, when using a pulsed ablative system, because of the short duration of the laser pulses (nanoseconds to milliseconds) in relation both to the interval between the pulses and to the time taken for heat to diffuse below the surface of the concrete, the energy input from the short pulses is of insufficient duration to heat the concrete beneath a very thin surface layer. Thus neither mechanical stresses nor steam can build up within a significant volume of material below the surface layer, and scabbling by ejection of solid fragments, chips or flakes of the material cannot therefore occur.

The energy delivered to a to a given surface location being swept by an oscillating spot of laser light which uses a laser of substantially constant intensity, lies between approximately 200 and 1350 J/cm2, compared to around 0.7J/cm2 for an ablative system, with a pulsed laser. The time over which laser light energy is delivered to a given surface location being swept by an oscillating spot of laser light in laser scabbling is in the order of 1 to 30 seconds for the present invention. The equivalent time for ablative systems using a pulsed laser is typically in the order of 20ns.

The power density of a typical oscillating spot of laser light required for scabbling in the method of the present invention, which is tens or hundreds of W/cm2, is many orders of magnitude lower than that required for ablative processes such as are typical for pulsed laser treatment which is in the order of several tens of MW/cm2 The use of pulsed laser ablative processes would often be considered disadvantageous for the removal of radioactive contaminants, as either the creation of plasmas or the vapourisation or sublimation of contaminants would lead to further airborne spread of contamination. Scabbling avoids these problems because relatively large (mm to cm) and solid surface fragments, chips or flakes are forcibly ejected from a surface being scabbled and can be readily collected using conventional solids collection methods, such as air extraction and filters.

However, Problems have been found in the prior art with surface removal by laser scabbling which are associated with a non-uniform power distribution, for example, quasi- Gaussian, across a laser beam spot, and in particular with the presence of lower power density regions of an incident beam of laser light on the surface to be treated. The part of a beam of laser light incident upon a surface that is above a critical, or threshold, power density able to effect scabbling causes scabbling of the surface to occur. Any part of an incident laser beam, usually a peripheral part, which is below that threshold power density level, i.e. a part not subjecting a surface portion on which it is incident to a power density above a critical value, does not give rise to surface removal by scabbling, but the element of the surface treated by it is nevertheless heated. Such heating without scabbling may cause a number of processes to occur in the material, such as relaxation, dehydration and chemical change. These processes can result in the heat-affected surface no longer being amenable to subsequent surface removal by laser scabbling, even when that subsequent treatment is above the threshold power density to achieve scabbling. That is at least when laser light is used in a manner whereby surface removal would otherwise be effected by scabbling, i.e. the ejection of surface chips, rather than by ablation.

The amplitude of oscillation of the laser beam incident upon the surface may be a maximum of a width of the incident laser beam. The width of the incident laser beam may be a total width of the incident laser spot measured in a direction or axis of oscillation. Preferably, the width of the incident laser beam may be a width of that part of the incident laser beam above a threshold power density for effecting scabbling, that is the width of an area of the laser spot capable of imparting a critical power density to a given surface such that surface removal is effected. The oscillating laser beam may be ovoid in cross-section. The oscillating laser beam may be ellipsoidal in cross-section. The oscillating laser beam may be circular in cross- section. The width of the incident laser radiation may be a diameter of an incident laser beam.

In one embodiment of the method of the invention the amplitude of oscillation may be in the range 10 to 30mm, centre to centre, for an incident spot of laser light of diameter 130mm with a 70mm diameter central part above a threshold power density for effecting scabbling when using a YAG laser at 4kW power. When using a variable output YAG laser of up to 4 kW power the total diameter of an area of a surface upon which incident laser light falls may be from 20mm to 250mm in diameter and more preferably from 30mm to 130mm in diameter. The width of the spot of laser light, which is above the threshold power density level to cause scabbling, may correspondingly be from 30 to 80 mm, or from 50 to 75 mm in the preferred range. In general, larger spot sizes are preferred as they give more reproducible results. The oscillation of the laser beam in the first direction may be transverse to the second direction, the direction of nett travel. The oscillation may be a complex oscillation comprising a primary oscillation in the first direction and a further secondary oscillation in a third direction. The third direction may be the same as the nett direction of travel, i.e. the second direction. The secondary and any further component of the oscillation may be recursive.

The oscillation, in the first direction, of an oscillating laser beam incident upon a surface may comprise bursts of movement interspersed by periods of no oscillation.

The nett direction of travel, in the second direction, of an oscillating laser beam incident upon a surface may comprise bursts of movement interspersed by periods of no nett movement . During periods of no nett movement the incident spot of laser light may describe Lissajou's figures .

A combination of oscillation in a first direction and movement in a second direction of travel may describe a sinusoidal curve or waveform on a surface being treated. The sinusoidal wave may be a sine wave.

A combination of oscillation and movement in the nett direction of travel may describe a square wave. The mark to space ratio of the square wave may be one.

A combination of oscillation and movement in a nett direction of travel may describe a triangular wave. The triangular wave may be offset to form a saw-tool wave.

The oscillation may describe a complex waveform.

A frequency of oscillation of the incident laser beam may be between 1 and 4 Hz.

A nett rate of movement in the second direction of travel of the oscillating laser beam across a surface to scabbled according to the method of the invention may be from 0.5 mm/s to 30 mm/s. Normally there will be no nett rate of movement in the first direction of travel for a given traverse of an oscillating beam over a surface to be treated.

Preferably, the laser beam may not move more than one beam width in the direction of travel until at least one oscillation perpendicular to the nett direction of travel has taken place. A laser light source for use in the invention may typically be of total continuous power from 0.5 kW to 4 kW. This power is delivered substantially constantly over time. The laser is therefore not a 'pulsed laser' in the sense used in the technical field of laser technology.

A threshold power density for surface removal by scabbling for concrete may typically be in the range of from 50W/cm² to 80W/cm², depending upon the surface to be treated, i.e. a value of 70 W/cm² or thereabouts.

The power of the laser, the size of the incident spot of laser light, the frequency of oscillation, the amplitude of oscillation and the rate of nett movement are interrelated and for any given surface will be optimised so as to achieve the most effective surface removal . The interrelationships of the above factors are governed by established physical laws known to the person skilled in the art .

A suitable laser source may be a Yttrium Aluminium Garnet (YAG) laser, a diode laser array or a fibre laser. The beams of laser light sources for use in the invention may have a non-uniform power density in cross-section.

A range of different removal depths is obtainable in a single pass of an oscillating spot of laser light according to the invention. A single pass can remove anywhere between 1 and 30mm of material at the deepest point. The laser beam may be transmitted to a delivery head by means of a fibre optic cable. The laser beam may be alternatively described as laser light or laser radiation.

A delivery head for projecting a laser beam onto a surface to be treated may comprise focussing optics to focus the beam through a focal point. The optics may include means for changing the direction of the laser light through a right angle. The beam and any optics may be shrouded up to the focal point. The shroud may be frusto conical. After the focal point the beam may diverge before impinging upon a surface to be treated.

In one embodiment of the present invention using a YAG laser of 4kW power, the surface to be treated may, for example, be 270mm from the focal point when optics of focal length 120mm are used giving a spot of incident laser light of around 70mm on the surface to be treated.

Suitable laser equipment for practising the invention is supplied by TRUMPF GmbH and Co. KG. of Stuttgart, Germany. The laser delivery head may be deployed and guided by a robotic arm.

The laser beam may effect surface removal by the effects of thermal shock. The method of the invention may give no or negligible surface removal by melting or vaporisation of the surface being treated. The method may comprise a step of wetting the surface to be treated, with water, for example, before irradiation with the laser beam.

The method may comprise a step of coating the surface to be treated before irradiation with laser light. Suitable coatings are described in EP 0 653 762 Al .

The presence of radionuclides may affect the nature of the scabbling process and the adsorbtion or absorption of incident laser radiation. t The method of the invention may typically be used for decontamination in conjunction with fume extraction, dust or solids collection and other subsidiary protective processes to prevent the spread of any contamination, particularly where radioactivity is^' involved.

The surface for treatment according to the method of the invention may be an inorganic non-metallic surface such as a concrete, i.e. a cement, e.g. a Portland cement, matrix having aggregate therein. The surface may comprise stone such as limestone, for example, or an engineered ceramic material such as brick.

Description of the drawings

In order that the present invention may be more fully understood, examples will now be given by way of illustration only, with reference to the accompanying drawings, in which:

Figure 1 shows a schematic representation of power density across a laser beam;

Figure 2 shows a schematic representation of an incident spot of laser light;

Figure 3 shows schematically a known method of laser scabbling comprising scanning a laser beam over a surface to be scabbled;

Figure 4 shows schematically a relationship between beam width and amplitude of oscillation;

Figure 5 shows schematically a relationship between amplitude of oscillation and nett travel of a laser beam

Figure 6 shows schematically an example of a combined effect of oscillation of a laser beam in one direction with nett travel in a second direction over a surface according to the method of the invention;

Figure 7 shows schematically another example of a combined effect of oscillation of a laser beam in one direction with nett travel in a second direction over a surface according to the method of the invention;

Figure 8 shows schematically a further example of a combined effect of oscillation of a laser beam in one direction with travel in a second direction over a surface according to the method of the invention;

Figure 9 shows schematically a yet further example of a combined effect of oscillation of a laser beam in one direction with travel in a second direction over a surface according to the method of the invention;

Figure 10 shows schematically a relationship between oscillation in a first direction combined with a further oscillation in a nett direction of travel combined with that nett direction of travel;

Figure 11 shows an example of a combined effect of oscillations described in Figure 10; and

Figure 12 which shows a schematic representation of laser equipment suitable for use in the method of the invention.

In the accompanying figures like features are denoted by like reference numerals. Description of aspects of the invention

A beam of laser light may be non-uniform in power density across a width of the beam. A schematic representation of power density across a laser beam is shown in Figure 1. Figure 1 depicts a graph 2 with a y axis of laser power density (I) and an x axis of distance (d) across the beam. The curve 4 describes a quasi-Gaussian type power distribution from which it may be seen that the laser beam has a centre point 16 of highest power density. The curve may be divided into two portions, the boundary between being defined by a threshold power density 18. A portion 6 of the curve 4 above this power density threshold represents part of the laser beam capable of giving rise to scabbling of a surface, the portion 8 below represents an area not capable of giving rise to scabbling of such a surface. However, the low power density portion 8 may give rise to surface modification which makes the surface resistant to subsequent passes with a laser beam, even if that subsequent beam has a power density above the threshold 18.

Figure 2 depicts a representation of a laser beam, of uneven power density, such as described in Figure 1 when impinged, orthogonally, upon a surface. The surface, such as one to be scabbled is represented here by the plane of the paper. This representation may be termed that of a spot of laser light. The spot of laser light comprises a centre 16 of highest power density. At this point the most efficient scabbling occurs. The centre point 12, corresponding to point 16 of Figure 1, forms part of area 24 wherein the light is above a threshold power density 18 for scabbling. Towards the periphery of the spot of laser light scabbling may be less effective (i.e. less material may be removed) until a boundary 14 is reached which corresponds to the threshold power density 18 for effecting scabbling. A further area 26 of low incident power exists between the first boundary 14 and a nominal outer boundary of the beam is described by line 28 beyond which point incident power is very low, such as may be due to scattering, for example.

In the prior art an incident spot of laser light may be used to scabble a surface by traversing that surface in a known raster scan pattern 32. Figure 3 schematically depicts features 30 of such a method. The incident laser spot 20 moves over the surface in a raster scan pattern traced by the centre of the spot 12 along a line 32. The surface impinged upon by the high-power density area of the?' spot 24 scabbles. That impinged upon by the low power density part of the spot 26 does not scabble but the surface is modified to varying degrees by effects such as relaxation, dehydration and chemical change. This occurs as the incident beam 20 traverses a first track 37, area 38 of the spot leaving modified, but un-scabbled surface area 36. Other surface area traversed by the low power density region of the spot is also traversed by the higher power density region and thus scabbles . Subsequently the laser spot traverses a second, adjacent, track 39. The surface impinged upon by the high power density region of the spot 24' by and large scabbles. However surface area 36 substantially resists scabbling even though it is impinged by the high power density region 24' . This is a significant disadvantage with current laser scabbling methodology.

The method of the invention relates to an improved method of treating a surface by traversing a spot of incident laser light across the surface to be scabbled. Figure 4 schematically depicts a spot of laser light 20 incident upon a surface. Laser spot 20 has a width (W) across the region above the threshold level intensity boundary 14 with centre 12 of highest intensity. In the method of the invention the laser spot 20 moves, in a first direction 15 to position 20'', with nominal centre 12'', where the beam stops and then returns in the opposite direction. There is a distance (A) between the centres of the two locations (12, 12'') which represents the amplitude of oscillation of the laser beam. In the example, the amplitude of oscillation (A) of the laser beam is a maximum of the beam width (W) of the area of the laser beam above a threshold power density 18. In addition, either simultaneously with or at different times to the oscillatory movement the oscillating laser beam has a nett direction of travel (T) superimposed upon the oscillation as illustrated by Figure 5. Combining the amplitude of oscillation (A) and nett travel (T) gives rise to the laser beam traversing a path over a surface to be scabbled. As a result of the oscillation, the effective beam width (W) is broadened and a peripheral surface area of the beam is reduced relative to the surface area capable of causing scabbling. In addition the residence time at the extremes of travel is short and surface modification to inhibit subsequent scabbling. The potential for surface modification of areas parallel to the first direction of oscillation is related to the relationship between the nett travel and the oscillation.

A number of examples of patterns of incident laser beam movement over a surface due to oscillation of the beam in a first direction whilst traversing the surface in a second direction are described with reference to figures 6, 7, 8 and 9.

Figure 6 shows a pseudo-sinusoidal path 40 described by the laser light spot as it traverses a surface, with amplitude A in a first direction and travel T in a second direction. Using such a method little or no surface modification to resist scabbling occurs since potentially modifiable areas, parallel to the oscillation, rapidly become scabbled as the oscillating beam advances over the surface in the direction of travel (T) . As the centre 12 spot of a laser light 20

(not shown) traverses the path 40 the periphery of the laser spot (26) will affect the surface to the side of the path of travel 46 as the beam moves in direction M. However, on returning in direction M' the central portion of the laser beam 24 will scabble the area previously heated by area 26 and the surface along path 47 will now scabble before being adversely affected by the lower power density region 26 of the incident spot of laser light. In addition the incident spot of laser light is resident for only a short period of time at the extremes of travel of the oscillation 48. Thus the area potentially affected by the lower power density region 26 of the incident laser light is only irradiated for a short period of time thereby reducing the potential for modifying the surface in a non- scabbleable manner.

In a further example, Figure 7, no nett travel of the incident laser light may occur on one cycle 44 of the oscillation in a first direction whilst travel may occur in both directions on a next and on alternate cycles so as to describe a saw-tool wave 42.

In another example, Figure 8, oscillation in a first direction and movement in a second nett direction of travel may occur alternately giving rise to a rectangular wave 50 as illustrated in Figure 8.

In a yet further example, Figure 9, schematic 60 illustrates, a nett travel in a second direction 62 of the incident laser interspersed with periods of oscillation 66 in second direction about central points 64, 64' etc.

On traversing a concrete surface such example patterns of movement give rise to efficient surface removal without the creation of areas not amenable to further treatment and in addition leave a surface flatter than would be achieved had the laser spot traversed the surface in a single linear direction as in Figure 3. In further examples of patterns of an incident laser beam traversing a surface, the oscillation, in a first direction, may not be orthoganal to the nett travel in a second direction.

In addition to an amplitude of oscillation (A) orthogonal to a nett direction of travel (T) a further reciprocating oscillation, an oscillation in the second direction, i.e. in the direction of travel may be combined. Such reciprocating oscillation has an amplitude (A' ) as illustrated in Figure 10. The combined effect of the two amplitudes of oscillation (A and A' ) combined with a nett travel of the laser beam spot across a surface describes a complex waveform of a type illustrated by a curve 80 in Figure 11. The amplitude of reciprocating oscillation is variable. The variation is a function of the phase of the amplitude of oscillation perpendicular to the nett direction of travel. The amplitude of the reciprocating oscillation is smaller than the amplitude of the oscillation perpendicular to the nett direction of travel in the first direction

A schematic representation of laser equipment suitable for use in the method of the invention is shown in Figure 8. A laser light source 100, emits laser light which may be channelled along a fibre optic channel 102 to focussing optics 104. The focussing optics comprise lenses to focus the laser light to a focus 106. The laser light is shrouded by a frusto conical shroud 108, one end 110 of which surrounds the focal point of the laser light. The shroud serves to protect the optics and other components and forms part of an overall protective shroud (not shown) of the equipment. Compressed air is ejected through the end of the shroud 110 to also stop ingress of debris such as fumes and particles. Outside the equipment the laser light diverges 112 and impinges upon a surface to be treated 114.

Claims

1. A method of treating a surface for the removal of a surface portion comprising the steps of oscillating, in a first direction, a laser beam incident upon the surface and superimposing a nett direction of travel of the incident laser beam over the surface in a second direction such that the incident laser radiation traverses the surface so as to effect surface removal wherein an amplitude of oscillation in the first direction of the laser beam incident upon the surface is less than a width of the incident laser beam.

2. A method according to claim 1 wherein the surface removal is effected by scabbling.

3. A method according to claim 2 wherein the width of the incident laser beam is a width measured in the first direction of oscillation.

4. A method according to either claim 3 wherein the width of the incident laser beam is a width of that part of the incident laser beam above a threshold power density for effecting scabbling.

5. A method according to any previous claim wherein the oscillating laser beam is circular in cross-section.

6. A method according to any previous claim wherein the oscillation of the laser beam in the first direction is transverse to the second direction.

7. A method according to claim 6 wherein the laser beam incident upon the surface describes a sine wave.

8. A method according to claim 6 wherein the laser beam incident upon the surface describes a rectangular wave.

9. A method according to claim 6 wherein the laser beam incident upon the surface describes a saw-tool wave.

10. A method according to any previous claim wherein the oscillation is recursive.

11. A method according to any previous claim wherein an amplitude of a component of the oscillation varies.

12. A method according to claim 11 wherein the amplitude of a component of the oscillation is a function of the phase of another component of the oscillation.

13. A method according to any previous claim wherein the oscillation, in the first direction, of an oscillating laser beam incident upon a surface comprises bursts of movement interspersed by periods of no oscillation.

14. A method according to any previous claim wherein the nett direction of travel, in the second direction, of an oscillating laser beam incident upon a surface comprises bursts of movement interspersed by periods of no nett movement .

15. A method according to any previous claim wherein the frequency of oscillation of the incident laser beam is in the range of 1 to 4 Hz .

16. A method according to any previous claim wherein the rate of movement in the nett direction of travel of the oscillating laser beam across a surface is from 0.5 mm/s to 30 mm/s.

17. A method according to any previous claim wherein the laser is an Yttrium Aluminium Garnet (YAG) laser.

18. A method according to any previous claim wherein the surface is a concrete surface.

19. A method according to claim 18 wherein the surface is a concrete surface contaminated with radionuclides .

20. A method according to any previous claim wherein the surface portion is removed by the effects of thermal shock.

21. A method of treating a surface for the -removal of a surface portion substantially as hereinbefore described with reference to the accompanying description and any one of drawings 4, 5, 6, 7, 8, 9, 10 and 11.