MATERIAL REMOVAL BY LASER ABLATION

Abstract: A method and apparatus for the ablation of a contaminated surface is described. The method comprises the steps of providing ablation chamber means adjacent the surface to be ablated, the ablation chamber means having a laser beam access aperture therein to allow a laser beam to pass through the chamber means and strike the contaminated surface to ablate said surface, passing a continuous gas stream through said chamber means to carry the ablated material to collection means and providing means to prevent said ablated material escaping from said ablation chamber means into the surrounding atmosphere.

Description: MATERIAL REMOVAL BY LASER ABLATION The present invention relates to a method and apparatus for the ablation and removal of material from a contaminated surface by laser ablation- Methods currently used for the removal and/or sampling of contaminated surfaces of metal and concrete structures such as are used in the chemical and nuclear industries for example, have tended to be relatively crude physical techniques including sticky pads, abrasive discs, drills and core samplers. The actual analysis of the removed sample will depend upon what type of contamination is present. For reasons of safety it is desirable to be able to remove a contaminated surface or a sample therefrom for testing remotely and to be able to transport that sample away from the contaminated surface without the necessity of people having to approach the contaminated area. It is an object of the present invention to provide a method and apparatus for the remote removal or sampling of a contaminated surface by laser ablation without causing further contamination due to escape of contaminated ablated material. According to a first aspect of the present invention there is provided a method of sampling or decontamination of a contaminated surface by laser ablation, the method comprising the steps of providing ablation chamber means adjacent the surface to be ablated, the ablation chamber means having laser beam access means therein to allow a laser beam to pass through the chamber means and strike the contaminated surface to ablate said surface, passing a continuous gas stream through said chamber means to carry the ablated material to remote collection means; said ablation chamber means further including means to prevent said ablated material escaping from said ablation chamber means into the surrounding atmosphere. It is pointed out that the present invention is concerned with either or both of the sampling of a contaminated surface for testing purposes and with the decontamination of a surface by laser ablation and removal of the contaminated ablated material. In one embodiment of the method of the present invention the ablation chamber means is provided with gas sealing means to prevent the ambient atmosphere from intruding into the chamber means and also to prevent the ablated material from leaking into the surrounding atmosphere and causing further contamination. Preferably, the gas is such as to prevent reaction with the ablated material and may, for example, comprise nitrogen, argon or any other suitable gas. The method of the present .invention may also include the step of positioning the chamber means on the surface to be sampled by remote handling means such as a robotic arm. The method may also further include the step of applying the laser beam to the surface to be ablated via fibre optic means. According to a second aspect of the present invention, there is provided ablation chamber means, said ablation chamber means having means to allow a laser beam to pass therethrough and to strike a contaminated surface and ablate the material of said surface, said chamber means further including sealing means to prevent egress of said ablated material into the surrounding atmosphere and to prevent ingress of ambient atmosphere into said chamber means, said chamber means further including gas inlet means and gas outlet means. According to a third aspect of the present invention there is provided an apparatus for the ablation of a surface, the apparatus comprising laser means, ablation chamber means having means to allow a laser beam from said laser means to pass therethrough and to strike said surface and ablate the material of said surface, said chamber means further including sealing means to prevent egress of said ablated material into the surrounding atmosphere and to prevent ingress of ambient atmosphere into said chamber means, means to provide a continuous gas flow to and from said ablation chamber means, said continuous gas flow carrying said ablated material to collector means from said ablation chamber means. The means to allow a laser beam to pass through the chamber means may be an aperture therein. Alternatively, the means to allow a laser beam into the chamber may be a fibre optic. The fibre optic may be provided with lens focusing means so as to focus the laser beam at the point of entry of said beam into said fibre optic. The laser beam delivery system may also further include lens focusing means so as to focus said laser beam emerging from the fibre optic onto the surface to be ablated in said chamber means. The laser delivery system comprising a fibre optic as described above may be sealed to the ablation chamber means so as to prevent egress of gas with contaminated ablated materials. The chamber means may be a chamber which is held adjacent the surface to be ablated. The chamber may have an interior volume through which the gas flow passes, the face of the chamber adjacent the surface to be sampled being at least partially open. The open face may be sealed against egress of ablated material and ingress of ambient atmosphere by seals formed by the gas flow provided to the chamber for carrying the ablated material to the collecting means. The chamber walls may be provided with gas flow passages therein to provide gas seals to prevent ingress of atmosphere and egress of ablated material. The open end of the chamber may also or alternatively include flexible skirt means formed from plastics and/or rubber materials to sit on or adjacent the surface to be ablated to assist in the functioning of the gas seals where they are additionally provided. The chamber may also be provided with gas flow outlet conduit means connected to the collector means. The gas flow outlet conduit means may comprise a flexible tube allowing the ablated material or sample to be carried a considerable distance from the ablation or sampling chamber. Tests have been carried out with the ablated sample being carried by the gas flow for many metres from the ablation or sampling chamber through narrow- bore tubes without significant loss of material. Distances of up to 30m have been achieved. An advantage of the ablation chamber of the present invention employing gas seals for the prevention of egress of contaminated ablated material and ingress of ambient atmosphere is that it may be used on rough surfaces such as concrete, for example, where mechanical contact seals would be relatively ineffective. The collector means may comprise an in-line filter for example, the ablated material then being tested or disposed of in any desired manner. Alternatively, the collector means may itself comprise testing means, the ablated material being fed directly inτo apparatus for performing an analysis thereof. The analysis apparatus may comprise an inductively coupled plasma (ICP) mass spectrometer or an optical emission spectrometer for example. The gas flow to the chamber means may be provided by a pressurised bottled gas source of nitrogen or argon for example. The gas flow may be further assisted by vacuum pump means connected to the outlet side of the ablation chamber. The laser means may be a neodymium YAG laser, or any other suitable laser for the purpose of ablation, maintained in predetermined relationship to the ablation chamber. Alternatively, the laser energy may be conducted through an optical fibre to the ablation chamber. This has the advantage of making the apparatus more convenient to use, being able to gain access to more confined spaces such as the inside of pipes, for example, and also to reduce the weight which needs to be manipulated. As explained above, a further advantage of conducting the laser energy through an optical fibre is that this may be sealed into the top of the sampling chamber, thus simplifying the task of sealing the ablation chamber against egress of ablated material and ingress of atmosphere. A yet further significant advantage of a fibre optic laser delivery system is that as with the remote sampling where the ablated material may be transported for a considerable distance away from the contaminated site, the laser device itself may also be situated a considerable distance from the contaminated site. Experiments employing a fibre optic of 10m in length have been made without appreciable losses of laser power. Distances in excess of 10m may also be employed where necessary, thus further increasing the safety aspects of the method and apparatus of the present invention. The apparatus may further include remote handling means such as a robotic arm for example, to place the chamber in any desired position on a surface to be ablated. Any such robotic arm may be programmed to move the sampling chamber in a predetermined manner over the surface. The laser means may be operated in a continuous or pulsed mode as desired and as required to produce the conditions needed to ablate the surface of the contaminated material in question. The apparatus may be used for sampling an area or may be used for "depth profiling" of a surface. An advantage of the method and apparatus of the present invention is that the minimum quantity of, for example, radioactive material may be remotely removed at a controlled rate. A further advantage of the method and apparatus is that no reaction forces are involved thus, the apparatus may be light and easily manipulated. The method and apparatus may also be used for the removal of surface contamination without significantly affecting the properties of the substrate material. In order that the present invention may be more fully understood, examples will now be described with reference to the accompanying drawings, of which: Figure 1 shows a schematic view of apparatus according to the present invention and for carrying out the method of the present invention; Figure 2 shows a schematic representation of a cross-section through an ablation chamber in more detail; Figure 3 shows a schematic part-cross sectional plan view of the ablation chamber of Fig. 2; Figure 4 shows a cross section through an alternative ablation chamber having a fibre optic means of delivering a laser beam to a surface to be treated; and Figure 5 which shows a detail of apparatus used with the apparatus shown in Figure 4. Referring now to the drawings and where in Figure 1 an apparatus according to the present invention is shown generally at 10. The apparatus comprises an ablation chamber 12 mounted on a robotic arm 14 for the purpose of manipulation. A laser energy source 16 is connected to the chamber 12 by a fibre optic 18 so as to allow a laser beam 20 to impinge on a contaminated surface 22 adjacent which the chamber 12 is held. The chamber 12 is provided with a carrier gas supply 24 through a flexible conduit 26 and a gas outlet conduit 28 which is connected to material collection means 30 such as a filter. Transport of the ablated material particles in the carrier gas is assisted by a pump 32. Contaminated ablated material is prevented from escaping from the chamber 12 by seals, generally indicated at 40, and which will be described in more detail below with reference to Figures 2 and 3. Part of the apparatus is situated in a contaminated area 34 whilst the personnel (not shown) operating the apparatus, and the remainder of the apparatus are situated in a non-contaminated operating area 36 some distance away and as schematically indicated in Figure 1. The ablation chamber 12 as shown in Figures 2 and 3 is slightly different to that in Figure 1 in that access for the laser beam to the contaminated surface 22 is via an aperture 42 rather than a fibre optic 18 sealed into the top of the chamber 12 as in Figure 1, the laser beam either coming direct from the laser device or being diverted through the aperture 42 by a mirror or mirrors (not shown) outside the chamber aperture. The ablation chamber 12 as shown in Figures 2 and 3 comprises a generally cylindrical body 50 with a top closure portion 52 having a laser beam access aperture 42 therein. The wall of the body 50 has two generally annular gas flow passages 54 and 56 therein, these gas flow passages being connected to the gas supply conduit 26 (see Fig. 1). The gas supply is divided between the two annular passages 54 and 56 such that part of the gas supply is directed into the interior 58 of the chamber 12 from the passage 54 and indicated by the arrow 60, and part of the gas supply is directed out of the passage 56 into the atmosphere 62 and indicated by the arrow 64, the gas emerging from the lower face 66 of the body 50 through a series of gas jets 68 and 70 in the lower face 66 and formed either side of a flexible plastics and/or rubber skirt 72 which is in contact with the contaminated surface 22. The gas flow 60 prevents, in combination with the skirt 72, egress of ablated material into the atmosphere 62, the ablated material being transported away by the carrier gas flow 60 to the collection means 30 through the conduit 28 in the body wall which communicates with the chamber interior 58. The gas flow 64 prevents, in combination with the skirt 72, ingress of ambient atmosphere gas into the chamber interior 58. The top portion 52 also has two gas supply conduits 74 and 76 which are also connected to the supply conduit 26. The conduits 74 and 76 have outlet jets 78 and 80, respectively in the aperture 42, the jets being mutually offset axially. Interaction of the gas flows from the two jets creates an outflow into the atmosphere indicated by arrow 82 and an inflow into the interior of the chamber 58 indicated by the arrow 84. This interaction between the gas jets into the aperture prevents escape of contaminated material into the atmosphere and ingress of atmosphere into the chamber. An outlet conduit 28 communicating with the interior chamber 58 is also provided and as shown in Figure 1. In tests carried out with the apparatus described above, a Spectron (trade mark) Nd YAG laser having a 1 Joule per pulse maximum output was used and employed mirrors to deliver the laser beam through the aperture 42 onto the surface 22 to be ablated. The output wavelength of the laser is 1064nm. The laser was used in the Q-switched mode at an output power of 300mJ per pulse at a pulse length of 9ns and a frequency of 15 Hz. These laser operating conditions were established to give an ablated particle size in the range from about O.Olμm to about lOμm for concrete and steel which are the two most common structural materials used. Under these conditions, the size range for steel is about O.Olμm to about lμm whilst the size range for concrete is about O.lμm to about lOμm. With the size of gas outlet 28 conduit used, the above particle size range was optimised and considered to be the most appropriate for transportation purposes over relatively long distances. In order to keep the particles moving and to be transported in the carrier gas, it was found that the gas flow should be non-turbulent. If the particle size is less than O.Olμm their flow becomes turbulent whilst if the particle size is above lOμm, the particles tend to drop out of the gas flow due the effect of gravity. An overpressure of gas was maintained in the ablation chamber 12 which, in addition to the gas flow 64, assisted in the prevention of ingress of ambient atmosphere into the chamber interior 58. In the ablation chamber the gas flows 60, 64, 82 and 84 were all supplied from a single pressurised gas bottle source, the relative gas flows being controlled by valves (not shown) at the inlets to the chambers 54, 56 and conduits 74, 76. However, more than one gas source may be used. Individual, separately controlled gas supplies may be used for each of the required gas flows or combined as appropriate in order to achieve the desired gas pressures and/or flow rates. The gas flow through the apparatus was assisted by a Roots-type pump 32 downstream of the material collection means 30. The pump 32 creates a vacuum of just under atmospheric pressure whilst an overpressure of just above atmospheric pressure is maintained in the chamber interior 58. The pump is of high capacity and able to deal with high volume gas flow rates. A pressure gradient is, therefore, established between the chamber and the collection means which helps to provide non- turbulent flow and maintain particle transportation over long distances. Under the experimental conditions described, particles have been transported for over 30m without significant loss. The robotic arm 14 may be programmed in known manner to cause the chamber 12, and hence the laser beam, to move in any desired manner across the surface 22 to achieve either sampling or decontamination. The collected material may be dealt with in a numbers of ways. It may merely be collected and safely disposed of or some or all of it may be analyzed to determine the level and physical extent of contamination for example. Such methods of disposal and analysis are well known in the art and do not form part of this invention. Figures 4 and 5 show an apparatus detail where an ablation chamber 100 (which may equally well be replaced with the chamber described with reference to Figure 1 or Figures 2 and 3) has a lens device 102 sealed thereto at the laser access orifice 104, thus preventing egress of contaminated material or ingress of ambient atmosphere as before. The lens 102 has a fibre optic 106 attached thereto for delivering the laser beam 108 which is focused by the lens 102 onto the surface 110 to be ablated. At a site (not shown) remote from the ablation chamber 100, a laser device (not shown) has its beam focused by a second lens arrangement 112 so as to be parallel when entering the fibre optic 106. At the point of entry of the laser beam 114 at the end of the fibre optic 106, a T-piece 116 is provided and which is supplied with a nitrogen gas flow indicated by arrow 118. The purpose of this nitrogen gas atmosphere at the laser beam entry point is to provide a gas with a higher ionisation potential than air. If the focused power density of the laser is high enough, breakdown by ionisation of the air will occur forming a plasma and thereby inhibiting passage of the laser beam into the fibre optic. Passage of the laser beam into the fibre optic may, therefore, be made more efficient with less power loss by employing a gas with higher ionisation potential than air. In the apparatus described, a fibre having a pure silica core of 1000 μm diameter and having a Tefzel (trade name) cladding was employed. Because the apparatus operators may be positioned at a considerable distance from the contaminated site, the method and apparatus of the present invention is safer to use than earlier proposals. Similarly, because all of the ablated material is contained within the apparatus, no further contamination is produced. It is emphasised that the above conditions were those established with the apparatus employed, and that with different items of equipment, gas flow rates, conduit bore sizes, the optimum values of the various parameters may be significantly different.

Claims: CLAIMS 1. A method of sampling or decontamination of a contaminated surface by laser ablation, the method comprising the steps of providing ablation chamber means adjacent the surface to be ablated, the ablation chamber means having laser beam access aperture therein to allow a laser beam to pass through the chamber means and strike the contaminated surface to ablate said surface, passing a continuous gas stream through said chamber means to carry the ablated material to collection means, said ablation chamber further including means to prevent said ablated material escaping from said ablation chamber means into the surrounding atmosphere. 2. A method according to claim 1 further including the step of moving said ablation chamber by remote handling means. 3. A method according to either claim 1 or claim 2 wherein said laser beam is applied to said surface via a fibre optic. 4. Ablation chamber means, said ablation chamber means having means to allow a laser beam to pass therethrough and to strike a contaminated surface and ablate the material of said surface, said chamber means further including sealing means to prevent egress of said ablated material into the surrounding atmosphere and to prevent ingress of ambient atmosphere into said chamber means, said chamber means further including gas inlet means and gas outlet means. 5. An ablation chamber according to claim 4 wherein said means to allow a laser beam to pass therethrough is an aperture in said chamber. 6. An ablation chamber according to claim 4 wherein said means to allow a laser beam to pass therethrough is a fibre optic in said chamber. 7. An ablation chamber according to any one preceding claim from 4 to 6 wherein the sealing means to prevent egress of ablated material from said chamber includes a gas seal. 8. An ablation chamber according to claim 7 wherein said gas seal comprises gas jets which direct gas into the interior of said chamber. 9. An ablation chamber according to claim 8 wherein said gas carries said ablated material into said gas outlet. 10. An ablation chamber according to either of claims 8 or 9 wherein said gas jets are provided in said chamber adjacent said contaminated surface. 11. An ablation chamber according to any one of claims 7 to 10 further including a gas seal which directs gas away from a region between said chamber and said contaminated surface. 12. An ablation chamber according to claim 11 wherein said gas seal comprises gas jets which direct gas away from said region. 13. An ablation chamber according to any one of claims 7 to 10 further including a skirt member outwardly of said gas seal. 14. An ablation chamber according to either claim 11 or 12 further including a skirt member between said gas seal which directs gas into said chamber and said gas seal which directs gas away from said chamber. 15. An ablation chamber according to any one of preceding claims 5 or 7 to 14 further including a gas seal in said aperture. 16. An ablation chamber according to claim 15 wherein said gas seal comprises gas jets directed into said aperture such as to interact to prevent ingress of atmosphere into said chamber and egress of said ablated material from said chamber. 17. An ablation chamber according to claim 16 wherein said gas jets are axially offset from each other with respect to said aperture. 18. An ablation chamber according to any one of preceding claims 7 to 17 wherein said gas seals have a common gas supply. 19. An ablation chamber according to any one of preceding claims 7 to 17 wherein said gas seals have individual gas supplies. 20. Apparatus for the ablation of a surface, the apparatus comprising laser means, ablation chamber means having means to allow a laser beam from said laser means to pass therethrough and to strike said surface and ablate the material of said surface, said chamber means further including sealing means to prevent egress of said ablated material into the surrounding atmosphere and to prevent ingress of ambient atmosphere into said chamber means, means to provide a continuous gas flow to and from said ablation chamber means, said continuous gas flow carrying said ablated material to collector means from said ablation chamber means. 21. Apparatus according to claim 20 and including the ablation chamber means of any one of preceding claims 4 to 19. 22. Apparatus according to either claim 20 or 21 wherein said laser means is a Nd.YAG laser. 23. Apparatus according to any one of claims 20 to 22 wherein said collector means is a filter. 24. Apparatus according to any one of preceding claims 20 to 23 further including remote handling means for the ablation chamber means. 25. Apparatus according to any one preceding claim from 20 to 24 further including analysis means for said ablated material. 26. Apparatus according to any one of preceding claims 20 to 25 wherein said gas flow carrying said ablated material away from said chamber is a flexible conduit.

Inventors: CHANDRATILLAKE MALIYADDE RANKO, ROBINSON VINCENT JOSEPH, SHUTTLEWORTH STEPHEN, CHANDRATILLAKE, MALIYADDE, RANKOTHGE, ROBINSON, VINCENT, JOSEPH, SHUTTLEWORTH, STEPHEN
Applicants: BRITISH NUCLEAR FUELS PLC, CHANDRATILLAKE MALIYADDE RANKO, ROBINSON VINCENT JOSEPH, SHUTTLEWORTH STEPHEN, CHANDRATILLAKE, MALIYADDE, RANKOTHGE, ROBINSON, VINCENT, JOSEPH, SHUTTLEWORTH, STEPHEN
Application year: 1995
Main classification: G21F9
IPC codes: B01J19, B08B7, B08B15, B23K26, G01N1, G21C17, G21F9