Application Focus: Cutting

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Figure 5: successful validation of the process and the developed optics at a 4m depth under water

Jan Leschke and his colleagues have developed a new underwater process for the safe dismantlement of nuclear equipment

This article was co-authored by Benjamin Emde, Jörg Hermsdorf, Stefan Kaierle and Ludger Overmeyer of the Laser Zentrum Hannover

As an ever-growing number of nuclear power plants reach the end of their lifetime, their decommissioning is increasingly being brought into focus. Additionally, political programmes, such as the nuclear phase-out in Germany that will be completed by the end of 2022, are increasing the number of nuclear power plants that require dismantling.

Germany’s phase-out programme affects 23 nuclear power plants, with the decommissioning of 17 of them already underway1. Worldwide there are 442 active nuclear power plants, while an additional 54 are under construction and 119 more are planned2.

Taking into account these numbers, the dismantling of nuclear power plants will clearly continue to be a topic of concern for the foreseeable future. Therefore, with the increasing number of facilities requiring dismantlement, the need for optimised deconstruction technologies arises.

In nuclear decommissioning, dismantlement often takes place as soon as a plant shuts down to reduce any risk of contamination. Separation work Jan Leschke and his colleagues have developed a new process for the safe dismantlement of nuclear equipment takes place in the remaining cooling water of the reactor.

Several tools and processes such as saw cutting, water jet cutting or plasma cutting are currently used in this field, with each of them having their own advantages and disadvantages.

The main disadvantage of these existing methods, especially saw and water jet cutting, is the generation of secondary waste that then needs to be filtered out of the water.

In addition, in the case of water jet cutting, a considerable amount of abrasive material is added, which must also be stored.

To overcome this drawback, a project has been carried out at the Laser Zentrum Hannover (LZH) to study the feasibility of using laser cutting under water to minimise secondary waste production.

Cutting experiments and results

The laboratory studies were carried out in a 1m³ water tank, as shown in figures 1 and 2. A disc laser was deployed for the tests, with air used as the cutting gas. As would be the case with nuclear power plant components, the samples to be cut consisted of stainless steel (1.4301) or a zirconium alloy (zircaloy).

Within the cutting tests, 1.4301 samples with 3, 6 and 15mm thickness, as well as zircaloy samples with 3mm thickness, were processed. In order to gain a fundamental understanding of the process and the influence of its parameters, comprehensive testing was first carried out on the 3mm 1.4301 samples. This was because the material required a lower amount of energy per unit length to make each cut, meaning a wider range of parameters could more easily be tested compared to the other thicknesses/material.

Left: Figure 1: the test stand used includes a gantry system for the cutting motion and a hydraulic platform to position the setup in and above the water

Right: Figure 2: cutting process in laboratory conditions using the 45° head

Since the goal of the project was to minimise the generation of secondary waste, the weight loss of each sample (in g/m) due to cutting was determined for each test. In theory, as well as in the field, two aspects of laser cutting could reduce the weight loss: minimising the width of the cutting kerf and maximising the formation of dross.

The investigations showed that varying the cutting gas pressure and the laser power, as well as the interaction between these parameters3, had a significant effect on weight loss. Both parameters were tested at predefined low and high levels.

Besides the influence on weight loss, the kerf properties changed as expected when cutting 3mm 1.4301 (see the variations in dross types in figure 3). Using a high cutting gas pressure of 3.75 bar and a low laser power of 2kW, thin cutting kerfs and a medium amount of line-shaped dross were achieved, resulting in a weight loss of about 5-10g/m. By changing the parameters to a low cutting gas pressure of 0.25 bar and a high laser power of 4kW, large dross balls were formed on the back of the samples, while the kerfs gained in width. This dross formation led to the lowest amount of weight loss in the study at about 1-3g/m, confirming that the goal lies in maximising the dross formation. The applied cutting speed for this parameter was 0.9m/min.

Figure 3: different dross types formed due to parameter variations: a) ball type, b) line type, c) inhomogeneous/none

Other parameter variations in the scope of the study resulted in dross reduction and therefore increased weight loss. Compared to conventional techniques, such as band saw cutting, weight loss can be reduced by about 95 per cent using laser cutting.

The cutting of the 3mm zircaloy samples showed equal behaviour and depended strongly on the applied cutting gas pressure. The 6mm thick samples of 1.4301 gave similar results when lowering cutting speed to adjust the energy per unit length. However, the 15mm thick samples changed in their weight loss behaviour. In their case, the lowest amount of weight loss (about 36g/m) was achieved using a comparably high cutting gas pressure of 6 bar.

Underwater laser optics

The second part of the project consisted of the development and construction of laser optics optimised for underwater nuclear power plant decommissioning applications.

The optics consisted of stainless steel and due to their cylindrical geometry (see figure 4), were able to fit through gaps ≥ 120mm, which is sufficient for most nuclear power plant components. The optics were designed with an interchangeable head that enables switching between 0° (perpendicular) and 45° working angles. While the 0° head has a higher efficiency when cutting thick materials, the 45° head allows for easier handling, since the angle of the optical system must not be changed for alternate cutting of horizontally and vertically positioned parts.

Figure 4: optics that were developed to enable optimised cutting of nuclear power plant components under water

Process validation and future plans

The project concluded with the transferring of the laser process from laboratory conditions to those reflecting industry. The developed laser optics were validated to Readiness Level 6 (system prototype demonstration in a relevant environment)4 at 4m depth in a water tank at the Unterwassertechnikum at the Leibniz University Hannover.

The tests were carried out using a mobile disc laser system set-up and operated by Laser on Demand. The movement at the 4m depth was realised by integrating the optics with a waterproof axis system, which was positioned on the ground of the tank (see figure 5 above).

To prevent the laser fibre from getting wet, the parts exposed to the water were covered by a hose. The tests included the cutting of 1.4301 sheets of 3 and 15mm thicknesses. Both sample types were successfully cut, verifying the laboratory results.

Following studies at the LZH will focus on using the developed laser process to cut 1.4301 with thickness > 15mm, as well as the cutting of multiple layers of material.

Jan Leschke is a research assistant at LZH

Benjamin Emde is head of the underwater technology group at LZH

Jörg Hermsdorf is head of the materials and processes department at LZH

Stefan Kaierle is the scientifictechnical director at LZH

Ludger Overmeyer is chairman of the scientific directorate and a member of the board of directors at LZH

The investigations were carried out in the project AZULa (Automatisierte Zerlegung von Reaktordruckbehältereinbauten mit Hilfe von UnterwasserLasertechnik). The research project (FKZ 15S9408) was supported by the German Federal Ministry of Education and Research (BMBF) through the Gesellschaft für Anlagenund Reaktorsicherheit (GRS). We would like to thank all funding organisations and our project partner, Orano.

References

[1] Deutscher Bundestag - 19. Wahlperiode, ‘Bericht nach §7 des Transparenzgesetzes - Rückbau von Kernkraftwerken für das Berichtsjahr 2019 (Drucksache 19/24770),’ Bundesanzeiger Verlag, Köln, 2020.

[2] Nuklearforum Schweiz, ‘Kernkraftwerke der Welt (www.nuclearplanet.ch),’ Olten, 2020.

[3] J Leschke, B Emde, J Hermsdorf, S Kaierle, L Overmeyer, ‘Controlling the kerf properties of underwater laser cutting of stainless steel with 3mm thickness, using an Yb:YAG laser source in nuclear decommissioning processes,’ Procedia CIRP, Bd. 94, pp. 493-498, 2020.

[4] JC Mankins, ‘Technology Readiness Levels: A White Paper,’ Office of Space Access and Technology, Nasa, 2004.

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