SYNTHESIS OF RESULTS OBTAINED WITH LASER CUTTING, A PROMISING DISMANTLING TOOL

Transcription

1 Proceedings of the 8th International Conference on Nuclear Engineering ICONE8 May 72, 20, Xi'an, China ICONE829 SYNTHESIS OF RESULTS OBTAINED WITH LASER CUTTING, A PROMISING DISMANTLING TOOL Guy Pilot Institut de Radioprotection et de Sûreté Nucléaire BP GifsurYvette Cedex, France ABSTRACT Several experiments have been carried out with a Nd YAG laser as a future dismantling cutting tool. At the beginning, industrial lasers could cut only small thicknesses due their small power ( kw) but now, with the rise in industrial power (8 kw), it is possible to cut plates up to 0 mm thickness. This technique is practical with the use of optical fibers which allow to maintain the laser generator in a nonradioactive zone. This paper provides a synthesis of the measurements of aerosols and gases produced in different configurations (cutting in air and underwater) and some comparisons with other cutting tools. INTRODUCTION Among the different possible thermal cutting tools for the dismantling of nuclear facilities and particularly for highly activated components, the laser has been studied in the frame of European fiveyear programs on the decommissioning of nuclear installations. Firstly, the CO 2 laser has been tested with the use of mirrors for the propagation of the beam [], but it has been replaced by the NdYAG laser which avoids the use of mirrors and capitalizes upon the property of optical fibers for the beam propagation. It is thus possible to maintain the laser generator and an optical fiber transporting the power to a coupler in a non radioactive zone and to use a second fiber bringing the laser power to the cutting head in the radioactive zone. DEFINITION OF SECONDARY EMISSIONS During the laser cutting in air or underwater, some secondary emissions are produced. It is possible to classify these solid and gaseous emissions as follows: the aerosols which are exhausted in the ventilation duct of the cutting place and can reach the filtration barriers where they are in major part stopped, the deposits on the walls of the cell which are due to aerosols captured by the walls, the sedimented dross which fall on the floor of the cutting place. The sedimented dross can be reduced by the formation of attached slags on each side of the kerf at the back side of the cut piece, the particles in suspension during underwater cutting, gases which are due to the vaporisation of constituents of the cut piece and other gases which are similar to welding gases (CO, CO 2, NO, NO 2, O 3 ). LASER EQUIPMENTS, TEST FACILITIES AND CUT PIECES The experiments have been carried out in different conditions on the sites of Saclay and Arcueil (Paris region, France) as it is shown in table for the equipments and table 2 for the test facilities. The cut pieces were stainless steel (4L and Uranus 65) and mild steel (table 3). SOLID SECONDARY EMISSIONS A synthesis of the solid secondary emissions measured during the abovementioned experiments has been compiled (table 4). During the cuttings in air, the major part of solid secondary emissions are sedimented dross (between 90 and 99% of the total collected mass), the deposits on the walls of the cell vary from 0.0% to 0.5% and the aerosols drawn in the exhaust ventilation duct represent from 0.8% to % of the total collected mass for the stainless steel plates. Copyright 20 by ASME

2 During the underwater cuttings, the sedimented dross are also the major part ( 94%), the aerosols account for around % and the particles in suspension in the water the remaining part ( 5%). For the mild steel A42, a more important proportion of attached slags has been noted [3]. The recent increase of power of the industrial NdYAG lasers has allowed to cut thicker plates but also to cut them faster. The cutting speed is an influent parameter for the production of aerosols [4]; it is thus advised to cut close to the maximal possible cutting speed if the objective is to cut with the minimal quantity of aerosols drawn in the ventilation exhaust duct. The aerosol mass in mass per cut length (g/m, for example) in the abovementioned experimental conditions: decreases versus the cutting speed [4], decreases when the cutting takes place underwater [2], decreases with the water depth [2], does not vary with the standoff (between and mm), thus does not vary versus the kerf width [4], does not vary with the couple pressureflowrate of the assistant gas [4], increases with the laser power [4] between 2 and 4 kw, is dependent upon the nature of the workpiece [4], mild steel or stainless steel, is independent of the position of the workpiece [4], horizontal or vertical. The size distribution of the aerosols is often multimodal with a main mode situated between 0. and.5 µm for in air and underwater cuttings of mild steel and stainless steel plates [2, 3, 4, 6]. For underwater cutting compared to cutting in air, the main mode is slightly shifted towards smaller dimensions [2]. Chemical analyses of the sampling filters have underlined an enrichment in chromium (factor:.4 to 2.7), in manganese (factor:.4 to 2.) and in nickel (factor: 4) during stainless steel 4L plate cutting [2]. This enrichment has been noticed in another study [3] but only in manganese (factor: 5). For some underwater cuttings, an enrichment in manganese (factor: 20) and chromium (factor: 2) was underlined by Chida and al with a depletion in nickel (factor: 2.6) [6]. These results must be taken into account mainly in the case of cutting of activated pieces since the ratio Mn/Fe will be higher in the HEPA (High Efficiency Particle Air) filters than in the cut piece. The ratio of the total mass of aerosols on the analysed elemental metal masses is superior to.7, which suggests a strong oxydation of the aerosols [3]. The removed mass per cut length varies versus the cutting speed, the standoff, the nature and the pressure of the assistant gas, the laser power [4], the nature and the position of the plate [3, 4] but is independent of the ventilation flowrate. The mean diameter of the sedimented dross is dependent upon the thickness of the workpiece, the environment cutting place and the water depth: it has a value around 0.35 mm [2]. For some underwater cuttings of 36L stainless steel 4 mmthick plates, it was found that, according to a result of XRay analysis of sedimented dross, Cr 9 Fe 7 Ni alloy is mainly detected and Fe 3 O 4 is rarely detected [6]. The median volumic diameter of the particles in suspension in the water ranges between 36 and 90 µm with a span (ratio of the volumic diameter at 90% minus the volumic diameter at % on the volumic diameter at %) varying from.4 to.8 [2]. GASEOUS SECONDARY EMISSIONS The productions of ozone, oxides of nitrogen, oxides of carbon and hydrogen have been measured for cutting in air and for underwater cutting with a laser power of 0.8 kw on the workpiece for stainless steel plates with thicknesses between and 3 mm and for E24 mild steel plates with thicknesses between and 9.5 mm [2]. The production of NO is around 3. 4 l/min (2. 3 to. 2 l/m) for the stainless steel plates, 7. 5 l/min (5. 4 to. 3 l/m) for mild steel plates and these of NOx respectively 4. 4 l/min (2. 3 to. 2 l/m) and 2. 4 l/min (7. 4 to 3. 3 l/m). These weak productions do not seem to vary strongly as a function of the environment of the cutting place, the thickness of the plate and the water depth. The production of ozone is not constant and varies from 6 l/min to some 5 l/min (maximum: 5. 3 l/m). The production of CO is not in evidence during the cutting of stainless steel plates (< 5. 4 l/min, < 5. 3 l/m), whereas it can vary for the E24 mild steel plates, from 5. 4 to 5. 3 l/min (5. 3 to 2. 2 l/m) The production of hydrogen for underwater cutting was not measurable (< 0.02 l/min, < 0.2 l/m). COMPARISON WITH OTHER CUTTING TOOLS Several cutting tools have been studied in the same cutting conditions (same cutting cell, ventilation flowrate, measurement devices, thickness and nature of the cut workpiece) except the cutting power and the cutting speed which are dependent upon the tool used [3, 4, 5]. Some results have yet been presented [4]; they are completed particularly by a comparison of the gaseous emissions and by a figure gathering several size distributions of aerosols. For the stainless steel 4L, the 4 kw NdYAG laser produces less removed mass and, in particular,.7 times less than the plasma torch A, a result that is in concordance with the kerf widths. The 4 kw laser produces more aerosols than the (factor: ), but less than the other tools and 2 times less than the A plasma torch. The kw pulsed laser, used without assistant gas, produces 8.7 times more aerosols that the 4 kw laser; this result can be explained by the cutting speed, which is thirty times faster for the 4 kw laser (figure ). 2 Copyright 20 by ASME

3 Exhausted aerosols A plasma 4 L Figure : aerosols exhausted in the cell ventilation duct during the 4 L stainless steel mmthick plate cutting by various tools The 4 kw laser produces less deposits on the walls of the cell than the other cutting tools (figure 2). For A42 mild steel, the 4 kw laser, like the kw pulsed laser, creates an important proportion of attached slags and thus a smaller removed mass, in comparison with the other tools. deposits on the cell walls A plasma 4 L Figure 2: deposits on the walls of the cell during the 4 L stainless steel mmthick plate cutting by various tools The 4 kw produces notably less aerosols than the kw ; this can be explained mainly by the cutting speed which is 37 times faster. It produces 29 times less aerosols than the plasma torch (figure 3). Exhausted aerosols A plasma Mild steel A 42 Figure 3: aerosols exhausted in the cell ventilation duct during the A42 mild steel mmthick plate cutting by various tools The deposits on the walls of the cell by the 4 kw laser cutting are small (figure 4). Deposits on the cell walls Mild steel A42 A plasma Figure 4: deposits on the walls of the cell during the A42 mild steel plate cutting of mm thickness by various tools During the cutting of stainless steel plates (thickness = mm), the size distribution of aerosols in the exhaust ventilation duct is often multimodal: the main mode is around 67 µm for the, the and the arcair, around 0.7 µm for the, around 0.2 µm and 0.04 µm respectively for the 200A and A plasma torches, with secondary modes around 0.4,.2 and 7 µm (figure 5). The aerosol size distribution for the kw has a main mode around.5 µm [3] and for the 4 kw laser presents two main modes around 0.2 µm and 0.45 µm (figure 5). 3 Copyright 20 by ASME

4 dm/m.dlogd 4 L = mm 0.0E03.0E02.0E0.0E+00.0E+0.0E+02 Aerosol aerodynamic diameter (µm) torch A plasma torch Grinder Figure 5: size distribution of the aerosols exhausted in the cell ventilation duct during the 4 L stainless steel mmthick plate cutting by various tools The aerosols are, in major part, submicronic for the laser and the plasma torches, whereas the produces aerosol sizes superior to µm. The 4 kw laser produces less aerosols superior to µm than the plasma torches (figure 5). The gases (ozone, nitrogen and carbon oxides) have been measured [2, 5]. The steadystate values are indicated in table 5 (the values for the laser have been extrapolated from results [2]), where they are compared with the permissible limits in a workshop atmosphere, based on INRS (Institut National de Recherches en Sécurité) data. It may be seen that certain values exceeded the permissible limits, notably for NO and NO 2 during the plasma torch cutting tests when nitrogen was used as the plasma generating gas. Similarly, the carbon monoxide production reached seven times the permissible limit during the arcair cutting tests. Manual thermal cutting operations must therefore be performed either with considerably higher ventilation rates or with the use of ventilated suits by the worker. The laser produces very few nitrogen and carbon oxides, much less that the plasma torch (table 5). PROSPECTS Some laser cutting tests were recently carried out up to 8 kw. The laser power was delivered through a specific power supply chain: a 0.4 mm fiber was transporting the power from the laser to a coupler; then a second 0.6 mm fiber was bringing the power laser to the cutting head. The cutting capability was about mm by kw; a steel plate of 0 mm thickness has been cut with a cutting speed of 7.5 mm/min [7]. Measurements of secondary emissions (solid and gaseous) produced by in air and underwater cuttings in the Research and Development facility tackled in [7] are presently in progress. REFERENCES [] J. Geffroy, J. Gonnord, J.P. Alfille, P. de Beaucourt, M. Dobler, G. Ermont, D. Galey, P. Garrec, M. Hofman, M. Marceau, M. Mergy, J.P. Noël, J.P. Nominé Téléusinage par laser pour site nucléaire en cours de déclassement (programme ROLD) Rapport EUR 4497 (993) [2] J.P. Alfille, D. de Prunele, G. Pilot, J. Schildknecht, J. Raoux, P. Frederik, V.S. Ramaswami, P. Muys Application des procédés lasers CO 2 et YAG à la découpe dans l air et sous eau de structures métalliques. Etude expérimentale et analyse comparative Rapport EUR 6854 (996) [3] G. Pilot, J.P. Alfille, J.P. Grandjean Remote cutting of steel plates by a wihtout assistant gas Euradwaste 999 Radioactive waste management strategies and issues. Fifth European Commission Conference on Radioactive Waste Management and disposal and Decommissionning, Luxembourg, 5 to 8 November 999 Report EUR 943, pages 952 [4] G. Pilot, S. Fauvel, X. Gosse, G. de Dinechin Dismantling of evaporators by laser cutting. Measurement of secondary emissions 4 th International Conference on Nuclear Engineering, ICONE 4, July 720, 2006, Miami, USA [5] J. Bernard, G. Pilot, J.P. Grandjean Evaluation of various cutting techniques suitable for the dismantling of nuclear components Rapport EUR 799, 998 [6] I. Chida, K. Okazaki, S. Shima, K. Kurihara, Y. Yuguchi, I. Sato Underwater cutting technology of thick stainless steel with YAG laser First International Symposium on HighPower Laser Macroprocessing Proceedings of SPIE Vol 483, p (2003) [7] C. Chagnot, G. de Dinechin, G. Canneau, J.M. Idasiak Dismantling nuclear plant with new industrial cw Nd YAG high power lasers Proceedings of Global 2009, Paris, France, September 6, Paper Copyright 20 by ASME

5 Table NdYAG laser equipment Saclay (*) [2] Saclay [3] Arcueil [4] Power on the plate (kw) (and 2) Impulse energy (J) 90 0 continuous Frequency (Hz) continuous Focal length of focussing lens Nozzle diameter.4 lens 3 Gas flowrate (l/min) 80 (O 2 ) without gas 20 (air or nitrogen) (70 and > 200) Gas pressure (bar) 4 without gas 4 (2 and 7) Standoff Optical fiber Diameter 0.6 Length (m) 25 (*) This facility has been used for in air and underwater cutting Table 2 Main characteristics of the test facilities Saclay [2] Saclay [3] Arcueil [4] Containment shape Cylindrical Parallelepipedic Parallelepipedic Average dimensions (m) Ф = 0.8 h = 9 L = 3.2 l = 2.8 h = 3.5 L = 3.3 l = 3 h = 3.6 Volume (m 3 ) Diameter of the exhaust duct (m) Ventilation flowrate (m 3 /h) Velocity in the exhaust duct (m/s) Distance between the cell and the sampling point (m) R e Reynolds number Table 3 Characteristics of the cut workpieces Saclay [2] Saclay [3] Arcueil [4] Environment In air Underwater In air In air 4 L 4 L 4 L to 3 to and Nature and thickness Mild steel E24 to 20 Mild steel A and Mild steel A 42.2 Uranus (double thickness) and 3.8 (simple thickness) 5 Copyright 20 by ASME

6 Table 4 Balance of the solid secondary emissions Saclay [2] Saclay [2] Arcueil [4] Cutting environment In air Underwater (0.5 and 7 m) In air In air Workpiece nature 4L Mild steel E24 Cutting speed (mm/min) Loss of mass of the workpiece Sedimented dross Particles in suspension Deposits on the walls Aerosols Kerf width Aerosol diameter *** (main mode(s)) (µm) 0 22 * * * * * * * * * * 3.9 three modes (0.5) * * L *.72.9 *.5.4 three modes (0.5) 4L Mild steel A42 4L 2 27 * several modes 5 7 * three modes (0.5) 26 * several modes * three modes (0.6) ** * several modes (0.7) ** * several modes three modes (0.) Mild steel A ** * Uranus three modes (0.0.5) three modes (0.0.5) three modes ( ) three modes (0.45) three modes (0.0.45) two modes (0.23) * * not measured or not measurable ** important proportion of attached slags *** Mass Mean Aerodynamic Diameter or main mode(s). 6 Copyright 20 by ASME

7 Table 5 Ozone, nitrogen and carbon oxides production Tools Materials NO (ppm) NO 2 (ppm) Grinder [5] Plasma torch 200A [5] Arcair [5] Arc saw [5] Laser** [2] A42 A42 Stainless A42 Stainless A42 Stainless E24 Stainless Mild steel Mild steel steel Mild steel steel Mild steel steel Mild steel steel < * * < 0.6 < 80* O 3 (ppm) < 0.04 < 0.4 < 0.* < 0. < 80* < 0.6 < 0.02 < < 0.6 < 0.04 < 0.02 < 0.0 < CO (ppm) < CO 2 (ppm) < 680 < < 20 3 < 0.0 < 0.0 < < 0. < 0. Workshop limit threshold values (INRS Figures) Mean value Peak value (8h per day) (short time) * plasma gas: Ar (40 l/min) + N 2 (20 l/min) ** the values for the laser are calculated for the same flowrate as this in reference 5 ( 0 m 3 /h) 7 Copyright 20 by ASME