DECOMISSIONNINING AND CUTTING METHODS FOR THE NUCLEAR INDUSTRY #cuttingmethods #nucleardismantling #nuclear #decomissioning

DECOMISSIONNINING AND CUTTING METHODS FOR THE NUCLEAR INDUSTRY

#cuttingmethods #nucleardismantling #nuclear #decomissioning

Up today, almost 100 nuclear power plants have been shut down worldwide after an operational lifetime of approximately 30 years. All those nuclear sites have been scheduled to be decommissioned.

?The term "decommissioning" may be defined as being the step following the permanent closure of an industrial site, either nuclear or not, to ensure ongoing safety and progressively improve the quality of the local environment. Therefore decommissioning starts immediately further to a final and permanent closure while targeting to leaving a clear site where the facility had once stood

The problems that arise in decommissioning nuclear power plants are in radiation protection. Shutting down also means that each of the components and materials of the nuclear installation needs to be characterized, cut and sized in order to be packed into specific drums

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and containers. Since the cost for the disposal of the nuclear waste is approximately 6 / month per cubic meter, the amount of secondary waste generated is also to be seriously taken into account.

Many studies and trials have been undertaken in order to determine the optimum cutting method. The cutting of metallic material in a nuclear environment has to deal with very strict safety and health issues. The main topics that should be brought up and seriously considered when making the definition of the cutting method along with the proper equipment are:

? The material to be processed

? The range of wall thickness

? The accuracy required

? The material finish required

? The production rate targeted

? The cost of technology

? The associated operating cost

? The operator skill requirement

? The compatibility with the immediate environment

? The secondary waste generated

DECOMISSIONNINING AND CUTTING METHODS IN THE NUCLEAR FIELD

The metal cutting techniques that are regularly used in the field of decommissioning can be divided in 3 families.

? The cold cutting methods

? The hot cutting methods

? The others

MILLING

The milling process is certainly one of the most versatile cutting method. It basically can be used on any configuration for any steel, stainless steel, inconel, zircaloy, aluminum, cast iron etc. Theoretically the optimum cutting speed could go from 30m/mn up to 200m/mn which is so called conventional machining and from 500m/mn up to 2000m/mn when using the technologies of high speed machining. In the first configuration both of the chips and the tool itself are getting the heat due to energies generated by the cut. In the second case more than 80% of the heat is absorbed by the chips, since the energies generated by the cut don’t have time to diffuse to the milling cutter. To machine at such a high speed an ideal configuration is required:

? stability and rigidity of the equipment (fixed and bench mounted machine),

? heavy weight for the stability of the cutting process,

? cooling or cutting fluid to ease the cut and preserve the lifetime of the cutter,

? easy material to cut,

? the segment to be cut well clamped onto the machine

In reality, almost none of those favorable conditions are available onsite, the weight and clearance of the machine being used are limited, no cutting fluid is allowed, and the carrier of the cutting process is always in an “aerial” position. However what really matters in a decommissioning phase is the possible speed for cutting a specific segment, and not the cutting speed of the tool itself.

Therefore taking in account that:

? Milling can still perform cuts up to 80 mm in one pass with a feed rate ranging from 5 to 300 mm/mn in difficult positions,

? Techniques of surface treatment and specific shape of the cutters along with their own cutting parameters, can keep a temperature of the tool as cool as 50° C

? It can be used for longitudinal or orbital cuts This process should be considered as a good option.

Advantages:

1 Can be carried out under water or in air

2 Can be remote

3 Low emission of active materials

4 Thickness to be cut can be precisely adjusted

5 Secondary waste can be collected

6 Thickness up to 80 mm in 1 pass

7 Can follow any profile

8 Can be adapted to a tracking system

Disadvantages:

1 Gives reaction force

2 Therefore need of a rigid mechanical structure

3 Generates secondary waste

4 Specific shape of cutting bit could be required depending on material and thickness

CIRCULAR SAW

This process cumulates some of the advantages of the band saw and the milling. It can be used in different positions and follow different types of contour. It can feed directly into the work piece as a milling bit.

Since the blades are much more rigid than a band, deviations of the cut are limited. Depending on the size of the blade it can get through heavy wall and can almost cut any type of wall thickness that could be found in a power plant during decommissioning.

As a major difference with milling it can also machine thin walls, while reducing the vibration phenomena related to the lack of rigidity of thin wall work pieces. Maintenance can be remote, and the cost of technology will mainly depend on the carrier. The carrier needs to be stiffen to compensate the reaction force, Cutting speed average in between 20 to 100 m/mn when mounted onto a “flying machine”. As almost all the mechanical cutting process it generates secondary waste, which are quite difficult to collect during the machining phase.

Advantages:

1. Can be carried out under water or in air

2. Can be remote

3. Low emission of active materials

4. Thickness to be cut can be precisely adjusted

5. Can also be used for thin wall

6. Can be used for radial or orbital cuts

Disadvantages:

1. Gives reaction force

2. Potential for dispersal of secondary waste

3. Secondary waste difficult to collect

CUTTING WHEELS

This process is very specific. It is a mechanical cold cutting process. The main interest in using cutting wheels is that there are no significant

secondary wastes generated. (Only few particles or dusts which are difficult to quantify…) Cutting wheels are being used with success in many cutting or dismantling operations as the opening of plutonium boxes, decladding operation final cutting of the primary piping when dismantling or replacing a steam generator, the opening of light radioactive waste drums in a productio cell etc.

The main restrictions are the wall thickness and the cutting time. Depending on the size of the cutting wheel, the rigidity of the equipment, the load applied to the cutting wheel, and the hardness of the material, the wheel can be fed into the material by sequences of 0,05mm up to 0,5mm We have experienced cuts up to 13mm wall thickness with this process. It is certainly possible to perform cuts up to 20 mm wall thickness when the cutting time is not a major criteria.

Its versatility can be found in orbital cutting while longitudinal cuts are a bit more technical to implement. The ideal configuration when using this process is to always have a counter load opposite to the cutting wheel (especially in the case of walls o 5mm). This counter load can either be applied by an other cutting wheel or a pressure wheel mounted in a symmetrical position.

In the case of thin wall cutting it could be necessary to reinforce the work piece (from the ID in the case of thin wall tubing for example) in ord to avoid the ? squeezing ? of the work piece due to the heavy torque applied on the cutting wheel.

When mastering this process, it can be very reliable and cutting tool change period can be suitable to remote operations. Since the process can easily be remote with a low cost of technology (no waste emission or solid secondary waste to manage) this technology should seriously be considered for pipe works .

Advantages:

1 Can be carried out under water or in air

2 Can be remote

3 Low emission of active materials

4 No significant secondary waste

5 No need of tool drive. Only the axis carrier needs to be driven

6 Low cost of process

Disadvantages:

1 Slight deformation of the material

2 Slow cutting process

3 Limited wall thickness

4 More efficient for orbital cut

RECIPROCATING SAW

Advantages:

1. Can be carried out under water or in air

2. Low emission of active materials

3. Low cost of process

Disadvantages:

1. Difficult to remote

2. Generates secondary waste

3. Secondary waste difficult to collect

4. Need to start cutting on an extremity

5. Limited applications

HYDRAULIC SHEARS

Advantages:

1 Can be carried out under water or in air

2 Low emission of active materials