Fibre Lasers for Surface Removal of Contaminated Concrete in the Nuclear Sector
Paper presented at 2010 ICALEO Proceedings Anaheim, CA, USA, 26 - 30 Sept. 2010, Paper 1305
Contaminated concrete presents a major nuclear decommissioning challenge in terms of the total area of material to be treated, the radiation levels present and the number of facilities affected. A number of concrete decontamination techniques have previously been evaluated and whilst water-jetting or mechanical scabbling tend to be the favoured options, each has drawbacks, such as significant secondary wastes or the need for extensive control and deployment systems. Concrete decontamination by means of laser scabbling has the potential to overcome many of the above drawbacks. However, whilst this technique has been demonstrated in the laboratory, to date, no representative scale demonstration has been provided which would give industry confidence in the technique. Until recently, industrial lasers have often been seen as too unreliable for use in nuclear decommissioning environments and hence their usage has been limited. The advent of solid state 'fibre' lasers has, however, provided a robust and reliable industrial tool, capable of performing not only concrete scabbling but other laser based decommissioning processes, such as cutting. This paper will describe the development of a laser scabbling process head, equipped with an integrated debris collection device and adaptive control system for automatic maintenance of the attitude and stand off distance of the head from the concrete surface. The results of using optical fibre delivered laser power to remove the surface of a range of representative and large scale concrete samples will be described. Using 5kW of laser power, removal rates up to 7000cc of concrete per hour have been recorded at a removal depth of the order 10mm.
This paper describes the results of a recently completed project to demonstrate the potential of high power lasers in aspects of nuclear decommissioning. In March 2009, the UK's Nuclear Decommissioning Authority awarded TWI a contract to develop prototype equipment, in order to effectively demonstrate the twin processes of concrete scabbling and tube cutting and how these technologies might be implemented for remote use in nuclear decommissioning environments.
Contaminated concrete and pipework present major decommissioning challenges in terms of the total volumes of material to be treated, the radiation levels present and the number of facilities affected. This paper describes the results of the work on concrete scabbling. The results of the work on tube cutting can be found in paper #707 in these proceedings.
A number of concrete decontamination techniques have so far been evaluated and whilst water jetting or mechanical scabbling are potential options, each have drawbacks such as the generation of significant secondary wastes or the need for extensive control and deployment systems. Concrete decontamination by means of laser scabbling has the potential to avoid many of these drawbacks. As most of the contamination resides in a surface layer only a few mm thick, removing this means that the remaining concrete (often up to a metre thick) can be removed/demolished much more cheaply using conventional techniques. However, whilst laser scabbling has already been demonstrated in the laboratory, see for example,[1-5] to date, no representative demonstration has been provided which would give industry confidence in using the technique.
The major reason for the limited use of lasers in decommissioning is that historically, industrial lasers have been considered unreliable and not suited to on-site nuclear decommissioning environments. However, the advent of robust, high power (4+kW) lasers, whose beams can be transmitted down optical fibres, has provided a more realistic opportunity for use of lasers in decommissioning applications. Power from a fibre or disc laser can be transmitted via several hundred metres of fibre optic cable, hence the laser unit can be located some distance from the active area of operations. As a result there is no risk of contamination of this high value asset, which can therefore be reused on a number of decommissioning tasks so spreading the capital cost of the equipment.
Chosen laser source
A key parameter in most laser processes is of course the power density in the beam applied to the surface of the material in question. The two processes of concern in the project reported here were unusual in that laser cutting requires a very high power density in the beam, whereas laser scabbling requires a relatively modest power density. For this work, because of the former requirement, the need for an efficient, robust and compact laser source and the need, in remote applications, for optical fibre delivery of the laser beam power, a 5kW multimode industrial fibre laser was chosen. The performance of the laser was monitored using a laptop computer, which also provided detailed information about the operating status of the laser. Control of the laser was routed to the controller of the deployment system in use, in this case an articulated arm robot. It should be pointed out that for the concrete scabbling process, a direct diode laser would be another good solution, offering even better efficiency than the fibre laser.
Scope of Work
For the work reported here, 1 micron wavelength laser sources were used to develop scabbling processing parameters on a range of concrete containing different aggregates. In addition, a special process head was developed with remote operation in a contaminated environment in mind. This head included, as well as the beam forming optics, a system to remove the concrete debris during processing and deposit this into a container (for future removal). The process head was also equipped with its own vision system which adaptively maintained the stand off distance from the concrete surface and the attitude of the head normal to the surface during operation. This was achieved by feedback to the control system of the robot onto which the scabbling head was mounted. This system was used to demonstrate the scabbling of large areas of concrete and also to demonstrate the effects of surface contamination on the concrete such as paint and grease.
The concrete scabbling process
In the laser scabbling process, the laser beam is applied to the surface of the concrete and its energy is absorbed, heating the concrete matrix and the concrete aggregate. Expansion of residual water vapour, probably in both the matrix and aggregate, and differential expansion between aggregate and matrix, causes the concrete to break up in a highly energetic fashion, leaving a rough scabbled surface, consisting of matrix and aggregate. High speed video shows the energetic nature of the process. Figure 1 is a still image, taken from such a high speed video of the scabbling process. In this case, the concrete contained limestone aggregate. In the video image, taken through a polycarbonate tube, the laser beam is incident downwards and has a diameter of about 60mm when its hits the concrete surface. From successive images, it was possible to calculate the speed of the concrete particles leaving the surface to be about 18m/sec.
Figure 1. High speed video image of the laser scabbling process
Effects of concrete aggregate
Concrete is composed of a mix of hydraulic cement, water, a fine aggregate, sand and a coarse aggregate. A broad, typical formulation is: cement 350kg/m3, water 170kg/m3, sand 750kg/m3, coarse aggregate 1150kg/m3 and nominal density ~2400kg/m3. The cement chemically reacts with the water to produce an inorganic complex matrix of calcium silicates with smaller amounts of calcium aluminates and alumino-silicates, all of which are essentially inert and durable materials; this binds the aggregates to form a hard durable material. This process also results in the formation of up to about 20% weight in the cement of hydrated lime (calcium hydroxide); the amount depends on the specific cement type used. There is always a proportion of free water and loosely bound water in new and aged concrete, the quantity depending at least, on time, the amount initially added and the quantity of cement in the mix. The sand and aggregate less than 4mm size (approximate diameter) is normally natural material, with over 90% siliceous content, although some concretes have a proportion of sand replaced by a crushed, fine limestone.
The coarse aggregate, which forms the largest volume fraction of the concrete, up to 80%, generally varies with the regional location and availability. This is size graded in increasing proportions between 5 and 20mm, (approximate diameter) although some early massive nuclear structures are believed to include a proportion of 40mm material. Three generic types of aggregate are used, namely: limestone, basalt (an igneous rock) and quartzite (high in silica and mixed gravels and also known as siliceous).
Results - Siliceous aggregate
For the series of trials reported in this section, the laser power was fixed at 4000W. Three different laser spot sizes were used, of 65, 43 and 23mm diameter, corresponding to laser power densities from 120 to 960W/cm2 incident on the concrete surface. Single passes were made with the surface of the samples both dry and wet. Figure 2, shows the results of a series of affected areas with single passes of the laser beam at two spot sizes of 43 and 23mm at a range of speeds for a dry surface.
Figure 2. Siliceous aggregate. Laser passes at 4000W on a dry surface
As can be seen, at the higher power density, all speeds between 100 and 1400mm/min produced different degrees of surface vitrification, with speeds below 200mm/min producing particularly hard surfaces which tend to be a mixture of white and black in colour. At the lower power density very little scabbling occurred for any speed between 100 and 1400mm/min. At the lowest speed a certain amount of vitrification of the surface could be seen, parts of which were very hard.
Figure 3. Basalt aggregate. Laser passes at 4000W using a 43mm diameter spot size
For the series of results reported in this section, the laser power was fixed at 4000W and a laser spot size of 43mm diameter was used, corresponding to a laser power density of 280W/cm2 incident on the concrete surface. Passes were made with the surface of the samples both dry and wet. Figure 3 shows the effects of a series of single passes of the beam over the concrete surface, at speeds from 50 to 300mm/min. For speeds at and below 100mm/min, significant vitrification of the surface was seen, producing a very hard black glassy result, so hard in fact that a wire brush could not remove it. Wetting the surface did not improve the situation, as can be seen if the two passes at 100mm/min are compared. No additional work was performed on this particular type of concrete.
Results - Limestone aggregate
For the series of results reported in this section, the laser power was fixed at 4000W. Laser spot sizes of 65, 43 and 23mm diameter were used, corresponding to laser power densities of 120, 280 and 960W/cm2 respectively, incident on the concrete surface. Passes were made with the surface of the samples both dry and wet. Clearly obvious with the first slab of this concrete, processed with the spot size of 65mm diameter, was a clear scabbling process, characterised by the spalling of significantly large pieces of concrete at high expulsion velocity, leaving a scabbled surface which showed no indication of melting. Figure 4 shows the effects after the scabbling process on this type of aggregate. In this photograph no brushing or cleaning of the surface has been performed. In particular the run on the right of the picture is a single pass at a speed of 100mm/min, while the central run corresponds to a double pass, one on top of the first, at the same conditions and a travel speed of 100mm/min.
Figure 4. The scabbling process on concrete containing limestone aggregate
The concrete scabbling head
In any effective use of the scabbling process for decontamination, clearly the laser beam must move with respect to the concrete surface and the ejected debris must be contained. In this work, the former was achieved by the use of an articulated arm robot and the latter by enclosing the process and using a large pump and filtration system to recover the debris.
A photograph of the scabbling head showing its major components can be seen in Figure 5. In the scabbling system, the laser light was fed, via an optical fibre 0.6mm diameter, to a conventional set of optics employing a 160mm focusing lens. The laser light was brought to a focus at a small aperture and then allowed to expand to a diameter of about 60mm at the base of a debris collection/recovery tube. This tube, about 150mm in diameter, was terminated round its circumference by a steel wire brush, in contact with the concrete surface. The aperture and the region through which the beam passed below the focusing lens were both protected by jets of compressed air. On this system the air pressure and any possible contamination of the optical elements were continuously monitored. If contamination occurred a warning signal was automatically generated. If the compressed air failed, the laser beam could not be released.
The top of the aluminium tube was connected to a long flexible hose and hence to a pumping system which removed the concrete debris as it left the surface of the material. The complete scabbling head was mounted on the arm of an articulated robot, which was itself mounted on a linear gantry some 6m in length. The scabbling process and effective debris removal requires the process head to be at all times roughly perpendicular to and at a constant distance from the concrete surface. The 6 axes of motion offered by the robot allows this to be achieved. However, the scabbling head was also equipped with its own vision system. A combination of low power lasers and a camera were mounted on the side of the scabbling head. The information recorded by the camera is interpreted by software and the results fed back automatically to the robot controller. In this way, once a scabbling area has been set and the head driven to its approximate start position, the vision system and its feedback to the motion controller of the robot, will automatically maintain both the attitude of the head perpendicular to the concrete surface and a constant stand off distance, as the scabbling process proceeds.
Figure 5. The scabbling head
A large 16kW motor powered the vacuum system which removed the concrete debris. Air was sucked in at the base of the scabbling head, through the wire brushes. This air draws the concrete debris into the flexible tube and down to the first stage of an enclosed separation process. Concrete particulate matter was deposited in a first container and concrete dust was collected via a filter, in a second container. The body of the pumping unit also contained two additional housings capable of containing HEPA filters.
The efficacy of the debris removal system was high, with hardly any scabbled material remaining on the concrete regardless of its orientation. Some of the debris leaving the concrete surface was up to 20mm in size (maximum dimension per piece). However, motion through the system reduced the size of the particles and the resulting debris had a high packing density.
All results presented in this section relate to the use of concrete containing limestone aggregate. For a given laser spot size on the concrete, the main process parameters are the laser power and the travel speed. Work performed has indicated that removal rate is proportional to laser power, at least up to the 5kW of power available with the laser being used, as can be seen in Figure 6.
Figure 6. Concrete removal rate and heat input as a function of laser power at a process speed of 200mm/min, for limestone aggregate
At 5kW power, this system has removed material, to a depth greater than 10mm, over a surface equivalent to one square metre, in a time of 110min. A single pass of the process results in a scabbled 'trough', lenticular in section. This shape is related to the energy distribution in the incident laser beam which, at the concrete surface, is Gaussian in form regardless of the fibre delivery. A slower process speed will generally result in a deeper scabbled section, as shown in Figure 7. For concrete containing limestone aggregate, the deepest section has been measured at 22mm, using a laser power of 5kW and a travel speed of 100mm/minute. For removal of large surface areas, a track overlap of 50% proved to be the most effective for producing a uniform depth in the scabbled profile, as can be seen in Figure 8.
Figure 7. Scabbled profiles as a function of process speed at a power of 5kW
Figure 8.Scabbled profiles as a function of track overlap for 5kW laser power and a process speed of 200mm/min
The process was independent of the attitude of the concrete. Figure 9 is a still image taken from a video sequence showing the system operational in the removal of a 1m x1m square section of concrete to a minimum depth of 10mm, using a single pass. Note the effectiveness of the debris removal system. Also seen in this image are the laser stripes (a third is not visible in this frame) used to measure the concrete surface topography. Re-scabbling over an existing track is possible and does result in an increased removal rate, as can be seen from Figure 10. However, in multi-pass processing of the same track, the amount of concrete removed was seen to drop at each successive pass. For example, at 5kW laser power and 300mm/min. travel speed, the maximum depths of scabbling recorded for three successive passes of the beam were 10mm, 18mm and 22mm, respectively.
Figure 9. Still image from a video sequence showing the system scabbling a 1 x 1m section of concrete
Figure 10. Scabbled depth as a function of number of passes of the beam for a power of 5kW and a process speed of 300mm/min
Surface contaminants such as grease and paint, had no effect on the scabbling process, as can be seen in Figure 11. This photograph shows the effects of a single scabbled track over bare concrete, concrete painted with floor and emulsion paint and grease, the latter applied both liberally and also rubbed in.
Figure 11. Effects of a) floor paint, b) emulsion paint and c) grease
This work has demonstrated a sophisticated system for concrete scabbling. Little additional work would need to take place to implement the techniques and equipment developed in a contaminated environment. The work has also shown a significant performance dependence based on the type of aggregate in the concrete used. For concrete with a limestone aggregate, a 5kW laser will remove 1m2 of surface to a minimum depth of 10mm in under two hours. Coverage can be increased by either reducing the depth of removal or by increasing the laser power. For other types of aggregate, which show less reaction to the laser than limestone, scabbling is not as effective and further work is required to optimise performance.
The author is grateful to the Nuclear Decommissioning Authority (UK) for funding the work reported in this paper and granting permission for its publication. The assistance of Matt Spinks and other colleagues at TWI in conducting the trials is also acknowledged. Paul Evans is also thanked for his work in designing the scabbling head.
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Meet the author
Dr Paul Hilton is the Technology Fellow for Laser Materials Processing at TWI Ltd and has over twenty years of laser processing experience. He has previously been conference chair for LMP at ICALEO and is the current President of the UK's Association of Industrial Laser Users.