Underwater laser cutting as a decommissioning tool Ali Khan and Paul Hilton TWI Granta Park, Great Abington, Cambridge CB21 6AL Abstract There are several potential benefits of using laser cutting in aspects of nuclear (and other) decommissioning processes. These include the speed of the cut, the lightness of the cutting head, the flexibility offered by optical fibre beam delivery, the minimal reaction force on the part being cut and the high degree of remote automation possible with this process. Such benefits have already been described, for cutting in air, by TWI and others. This paper will focus on laser cutting underwater, where it will be shown that some of these benefits are just as applicable underwater as in air and indeed, particularly for the application of nuclear decommissioning, some additional benefits accrue for the case of underwater cutting. A cutting head will be described which uses a series of jets of compressed air to create a local dry zone at the point of interaction of the laser beam and the material being cut, to perform underwater cutting. A series of trials are described to investigate the effects of parameters used for cutting C-Mn steel and stainless steel underwater, using a 5kW laser beam. The parameters investigated primarily involved the relative pressures in the gas jets and the positions of the laser beam focus and the cutting nozzle tip with respect to the surface of the material being cut. The results indicate that with an appropriately designed underwater cutting head, the cutting performance for both C-Mn steel and stainless steel is very close to that achievable in air with the same laser power. In underwater cutting, the height of dross (re-solidified metal and metal oxide) left adhering to the cut edges, was found to be significant, particularly when cutting C-Mn steel. During size reduction of contaminated material, maximising the dross adhesion may be important, as it means less radioactive material from the kerf will be released as secondary waste which reduces the risk of loss of containment of radionuclides during the size reduction operation. 1. Introduction One motivation for dismantling intermediate and high level nuclear waste underwater is to significantly reduce the amount of secondary waste produced during size reduction operations, escaping into the atmosphere. Underwater cutting could also eliminate the logistics of handling and transporting contaminated product from a pond environment to a dry dismantling facility. There is an increasing interest in the nuclear sector to acquire an underwater size reduction technology that could be versatile enough to dismantle and decommission such waste. The nuclear industry is very conservative in its approach to decommissioning and the techniques currently in use for size reduction encompass abrasive water jet cutting, diamond wire sawing and mechanical shearing. The only thermal process which has had some use is plasma arc cutting. Amongst these size reduction technologies, laser technology, especially with recent developments in fibre delivered beams, could offer the nuclear sector alternative size reduction tool, capable of cutting various materials both in air and underwater and has the potential to minimise the production of secondary waste and reduce the complexity of remote operation. Several examples of laser cutting in air (as opposed to underwater) have been demonstrated in published literature. The most applicable to actual use in decommissioning all are more recent and involve the use of 1 micron laser beams delivered by optical fibre. A good example is the paper [1] Pilot, which is a review of research into the use of laser cutting in decommissioning, for work up to 2010 and references work conducted in France and Japan. Since this time, TWI have published several papers on the use of a 5kW fibre laser and a range of purpose built decommissioning specific cutting heads, for cutting a range of stainless steel plate and tube material and C-Mn steel plate, in air. Ref [2] discusses mainly the potential for tube cutting, where the laser demonstrates a particularly useful benefit for decommissioning, in that it can cut tubes with a linear pass from only one side of the tube. With a 5kW laser, TWI demonstrated the cutting of 170mm diameter stainless steel tube of 8mm wall thickness, using this single sided technique. Ref [3] describes the cutting of C-Mn and stainless steel plate material, at thicknesses up to 50mm, again with a 5kW laser source. In this work the relationships between the cutting gas pressure, the beam focus position and the distance between the material surface and the tip of the cutting nozzle, were established for optimum cutting speeds. The work demonstrated that for applications where the resultant edge quality is not important, a large tolerance in stand-off distance is available in producing a cut, all other cutting parameters remaining constant. For example, 25mm thickness stainless steel plate was cut at constant gas pressure, laser power and cutting speed, while the stand-off distance was changed from 15 to 70mm, in a cyclic fashion, along the cut length. Such tolerances are important for real applications on active components which of course must be undertaken remotely.
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Underwater laser cutting as a decommissioning tool
Ali Khan and Paul Hilton
TWI Granta Park, Great Abington, Cambridge CB21 6AL
Abstract
There are several potential benefits of using laser cutting in aspects of nuclear (and other) decommissioning processes.
These include the speed of the cut, the lightness of the cutting head, the flexibility offered by optical fibre beam
delivery, the minimal reaction force on the part being cut and the high degree of remote automation possible with this
process. Such benefits have already been described, for cutting in air, by TWI and others. This paper will focus on laser
cutting underwater, where it will be shown that some of these benefits are just as applicable underwater as in air and
indeed, particularly for the application of nuclear decommissioning, some additional benefits accrue for the case of
underwater cutting. A cutting head will be described which uses a series of jets of compressed air to create a local dry
zone at the point of interaction of the laser beam and the material being cut, to perform underwater cutting. A series of
trials are described to investigate the effects of parameters used for cutting C-Mn steel and stainless steel underwater,
using a 5kW laser beam. The parameters investigated primarily involved the relative pressures in the gas jets and the
positions of the laser beam focus and the cutting nozzle tip with respect to the surface of the material being cut. The
results indicate that with an appropriately designed underwater cutting head, the cutting performance for both C-Mn
steel and stainless steel is very close to that achievable in air with the same laser power. In underwater cutting, the
height of dross (re-solidified metal and metal oxide) left adhering to the cut edges, was found to be significant,
particularly when cutting C-Mn steel. During size reduction of contaminated material, maximising the dross adhesion
may be important, as it means less radioactive material from the kerf will be released as secondary waste which reduces
the risk of loss of containment of radionuclides during the size reduction operation.
1. Introduction
One motivation for dismantling intermediate and
high level nuclear waste underwater is to significantly
reduce the amount of secondary waste produced during
size reduction operations, escaping into the atmosphere.
Underwater cutting could also eliminate the logistics of
handling and transporting contaminated product from a
pond environment to a dry dismantling facility. There is
an increasing interest in the nuclear sector to acquire an
underwater size reduction technology that could be
versatile enough to dismantle and decommission such
waste. The nuclear industry is very conservative in its
approach to decommissioning and the techniques
currently in use for size reduction encompass abrasive
water jet cutting, diamond wire sawing and mechanical
shearing. The only thermal process which has had some
use is plasma arc cutting. Amongst these size reduction
technologies, laser technology, especially with recent
developments in fibre delivered beams, could offer the
nuclear sector alternative size reduction tool, capable of
cutting various materials both in air and underwater and
has the potential to minimise the production of
secondary waste and reduce the complexity of remote
operation.
Several examples of laser cutting in air (as opposed to
underwater) have been demonstrated in published
literature. The most applicable to actual use in
decommissioning all are more recent and involve the use
of 1 micron laser beams delivered by optical fibre. A
good example is the paper [1] Pilot, which is a review of
research into the use of laser cutting in
decommissioning, for work up to 2010 and references
work conducted in France and Japan.
Since this time, TWI have published several papers on
the use of a 5kW fibre laser and a range of purpose built
decommissioning specific cutting heads, for cutting a
range of stainless steel plate and tube material and C-Mn
steel plate, in air. Ref [2] discusses mainly the potential
for tube cutting, where the laser demonstrates a
particularly useful benefit for decommissioning, in that it
can cut tubes with a linear pass from only one side of the
tube. With a 5kW laser, TWI demonstrated the cutting of
170mm diameter stainless steel tube of 8mm wall
thickness, using this single sided technique. Ref [3]
describes the cutting of C-Mn and stainless steel plate
material, at thicknesses up to 50mm, again with a 5kW
laser source. In this work the relationships between the
cutting gas pressure, the beam focus position and the
distance between the material surface and the tip of the
cutting nozzle, were established for optimum cutting
speeds. The work demonstrated that for applications
where the resultant edge quality is not important, a large
tolerance in stand-off distance is available in producing a
cut, all other cutting parameters remaining constant. For
example, 25mm thickness stainless steel plate was cut at
constant gas pressure, laser power and cutting speed,
while the stand-off distance was changed from 15 to
70mm, in a cyclic fashion, along the cut length. Such
tolerances are important for real applications on active
components which of course must be undertaken
remotely.
The capability to laser cut underwater was first
demonstrated using a CO2 laser many years ago. In 1996
[4] a comparison of underwater cutting using CO2 and
Nd:YAG lasers was published by Alfille et al and more
recently, there has been interest in this subject in Japan
[5], [6], and India[7], using 1 micron sources and fibre
optic delivery. The lasers used were COIL, 4kW
Nd:YAG and 250W average power Nd:YAG,
respectively. The cutting heads for these systems all
employed cutting gas of some sort to create a local water
free region in the area of the cut. It is probable that
various types of underwater plasma torch designs
influenced the design of the early underwater laser
cutting nozzles.
This paper will describe the design and operation of an
underwater cutting head for use with a 5kW fibre laser
system and water depths up to a few metres, in line with
the typical ‘pond’ applications mentioned earlier. It has
been used to cut plates of both C-Mn steel and 304L
Stainless steel, with the laser beam directed vertically
downwards and also with the beam directed horizontally.
The cutting head uses a series of jets of compressed air
to remove water and maintain an effective dry area in the
region of the interaction of the laser beam and the
surface of the material being cut. An additional, more
conventional, central gas jet, is used to blow molten
material from the kerf of the cut. The experimental