Plasma Cutting in the Multi-purpose Research Reactor (MZFR) – Underwater Use at Steel Thicknesses of up to 130 mm
W. Pfeifer, B. Eisenmann, Forschungszentrum Karlsruhe, Fr.-W. Bach, R. Versemann, H. Bienia, Institute of Materials Science, University of Hanover, V. Krink, F. Laurisch, Kjellberg Finsterwalde Elektroden und Maschinen GmbH
1. Introduction
The Multi-purpose Research Reactor (MZFR) of the Forschungszentrum Karlsruhe (FZK) was a pressurized water reactor cooled and moderated with heavy water. The plant was constructed from 1961 to 1966 and taken into operation on 29 September 1965. Following a successful operation for
nearly 19 years, the reactor was shut down for the last time on 3 May 1984. The reactor had a thermal output of 200 MW and an electrical output of 57 MW.
The purpose of the plant was to gather experience in the planning, construction, and operation of heavy water reactor systems as well as to test fuel elements and materials. While a safe enclosure of the plant was envisaged initially, it was then decided in favor of its complete dismantling due to clear advantages.
The decommissioning concept for
Figure 1: Aerial view of the MZFR on the premisesthe complete dismantling of the
of the Forschungszentrum Karlsruhe plant down to the “green field”
comprises eight decommissioning steps. According to the current schedule, the “green field” will be reached by mid-2009.
Ongoing work under the 7th decommissioning license takes place above all in the reactor building.
2. Dismantling of the Thermal Shield and Moderator Tank
Under the 7th decommissioning license, the thermal shield and moderator tank among other components will be dismantled under water due to their high activation. The thermal process to be applied for this purpose is plasma cutting. Due to the radiological inventory, the plasma torch has to be controlled remotely down to a water depth of four meters. In addition, the following boundary conditions have to be taken into account:
-Wall Thicknesses of the Components to be Dismantled
Thickness of the cylindrical walls of the moderator tank: 25 mm Thickness of the upper and lower dished heads of the moderator tank: 35 mm Thickness of the thermal shield, sections 1 - 4: 70 mm Thickness of the 5th section of the thermal shield: 70 to 130 mm
-Materials:
Moderator tank: X10 Cr Ni Nb 18 9 Thermal shield: X10 Cr Ni Nb 18 9 with 0.7% B
-Dimensions and Weights:
Moderator tank: 21 Mg, Øexternal 3.94 m, H= 4.91 m
Thermal shield: 29.79 Mg, Øexternal 4.06 m, H= 4.36 m
A critical point of underwater dismantling is the dismantling of the 5th section of the thermal shield (Fig. 2). This section has a continuously increasing material thickness of 70 to 130 mm. Other aspects to be considered are the very small joint gap of 20 mm between thermal shield and reactor pressure vessel wall as well as the necessity of cutting having to be carried out under a minimum water cover of four meters due to the radiological conditions.
Figure 2: Setup of the thermal shield, detailed view of the 5th section
When preparing the concept for dismantling the reactor pressure vessel with its internals in 1996, plasma cutting was selected for underwater dismantling.
3. Cutting Technology
The system applied above all consists of a 5-axes handling device to guide the plasma torch, a visual process control unit, and the plasma cutting system
3.1 Plasma Cutting System
To dismantle the moderator tank and thermal shield, sheet metal thicknesses ranging from 25 to 130 mm have to be cut reliably by the plasma cutting system. The system conceived for this purpose (Fig. 3) is based on plasma gas supply with both air (moderator tank) and a mixture of Ar/H2/N2 (thermal shield). Both air and nitrogen may be applied as secondary gas. It ensures the necessary process stability under water and contributes to the constriction of the arc. In the MZFR, only air is used as secondary gas.
Figure 3: Schematic setup of the plasma cutting system
The lines for supplying the cutting media to the handling device are designed separably at specific interfaces. All lines are connected by quick coupling systems. Apart from the gas lines, these are control lines, cooling water lines, and the power supply lines of the plasma torch. Both the ignition unit and the gas adjustment unit are located on the manipulator bridge so as to minimize the control paths during cutting and to ensure a safe pilot arc ignition.
Figure 4: Plasma cutting sources with control board, cooling unit, and supply lines
Central parts of the plasma cutter are three parallel plasma sources supplying a total current of 900 A (Fig. 4).
3.1.1 Plasma Cutting Sources
The sources were modified so as to guarantee safe ignition of the pilot arc, a reliable first cut into the sheet metal edge, and cutting through a maximum sheet metal thickness of 130 mm.
High-voltage ignition was adapted accordingly. Here, high voltage lies on the cathode, while it is preferably supplied to the nozzle of the plasma torch in other cases. Thus, discharge of the high voltage via the water and, hence, misignitions are avoided. In addition, power of high-voltage ignition was increased.
Cutting tests performed on materials of more than 80 mm in thickness demonstrated that the maximum cutting voltage of 220 V supplied by standard power sources for plasma cutting was not sufficient for the safe cutting of material with a maximum thickness of 130 mm. For this reason, open-circuit voltage of the power sources was increased to 280 V. The specially developed transformers possess accordingly increased secondary voltages. Due to the associated increased power consumption, their cooling is optimized. When connecting three power sources in parallel, a total current of 900 A is made available for cutting, the cutting voltage being 250 V at the working point. This corresponds to an electric power of up to 225 kW.
3.1.2 Gas Adjustment Unit
In the gas adjustment unit, the plasma and secondary gas pressures are controlled as a function of the material to be cut. In addition, volume flows of the individual gases are monitored by mass flow meters (calorimetric measurement) and displayed on the central control board. Thus, the mixing ratio of the gas can be controlled when cutting is carried out with a gas mixture, e.g. argon/hydrogen. In case of deviations, gas pressures may be changed. Adjustment of the gas mixture by pressure control alone, however, cannot be reproduced reliably, as small pressure differences between the individual gases already may cause considerable variations of volume flows. Pressures of the individual gases may be set in the range of 4 to 10 bar. For the plasma gases air and argon, hydrogen, and for the secondary air, volume flows of up to 4000 l/h, 2500 l/h, and 5000 l/h can be measured, respectively.
3.1.3 Modular Torch and Parts Subjected to Wear
In subsequent tests, flow was further optimized by modifying the secondary gas cap and in particular by a new construction of the plasma nozzle for cutting a sheet metal thickness of 130 mm. It was aimed at considerably increasing thermal wear resistance. For this purpose, various sealing systems and the connection of the cutting nozzle and secondary gas nozzle by brazing were tested.
To minimize radiation exposure of the dismantling staff, optimization of the exchange of wear parts of the plasma torch had to be considered. Major wear parts of the plasma torch are the already mentioned nozzle and the torch cathode. Depending on the material to be cut, various cathodes and nozzles are applied. If oxygen is present in the plasma gas, hafnium pins pressed into a base structure with a maximum current load of about 300 A are used. In case of Ar/H2/N2 gas mixtures, tungsten cathodes are applied.
optimized modular torch
modular torch
• bayonet of the new modular torch is protected against dirt • optimized sealing in the connection area
Figure 5: Modular torch (left), torch shaft and head (right)
Using the modular torch represented in Figure 5, the Kjellberg company succeeded in reducing the external diameter and designing the torch in a decontamination-friendly manner. The bayonet construction at the interface for the disconnection of the head allows for the manual exchange of the torch head within less than three minutes.
The interface of the plasma torch between head and shaft is designed for the transfer of all media required for torch operation:
- Cutting current 900 A
- Cooling medium
- Pilot current/ high ignition voltage 12 kV
- Plasma gas
- Secondary gas
A major task of construction work consisted in the torch head and shaft being designed such that they can be assembled and disassembled rapidly with a small force being required. This was achieved by a special design of current/water couplings.
Current is transferred via two contact discs, through which the cooling medium (forward and reverse flow) flows at the same time. Thus, a high current load of 450 A per contact can be reached. Special emphasis was put on large distances and, hence, long creep paths between the various electric potentials (cathode and nozzle potential) to reach the required stability against the high ignition voltage.
4. Laboratory Cutting Tests
Parallel to system development and as a basis of further optimization, laboratory cutting tests were carried out. These tests were conceived in line with the tasks of cutting the moderator tank and thermal shield. Initially, plate material was used. Then, cutting tests were executed on mock ups.
4.1 Test Rig
The test rig used mainly consists of a pressure chamber with a volume of 6.7 m3, a cutting pool, and a Kuka robot of the type KR 15/2. The maximum water depth simulated in the pressure chamber is 10 m (Fig. 6).
Figure 6: Test rig at the Underwater Technology Center, Hanover
4.2 Cutting Tests
4.2.1 Moderator Tank (35 mm Sheet Metal Thickness)
Tests first concentrated on the cutting of a wall thickness of 35 mm (moderator tank). Air was applied as plasma cutting and secondary gas. Cutting was carried out with a current source of 300 A only. Tests covered the cutting of classical tub-shaped structures, cutting of vertical walls (in horizontal and vertically descending direction), and overhead cutting under an angle of 45° (Fig. 7).
Figure 7: Overhead cutting under an angle of 45° (sheet metal thickness: 35 mm, 1.4031)
Safe cutting was accomplished at every cutting position, even in case of enhanced melt formation
overhead. During cutting, the pressure and volume flow of the plasma and secondary gases, distance between torch and material, secondary gas caps used, water depth, and cutting speed were varied.
As obvious from Figure 8, variation of the cutting speed is of particular relevance to the “L” cut.
Figure 8: Cutting strategy for “L” cuts
Upon the ignition of the pilot arc and movement onto the sheet metal at reduced speed, the torch stops for about 0.5 s (at position 1) to reliably cut the work piece and stabilize the arc. Until a change of direction (position 2), cutting takes place at a speed that depends on sheet metal thickness. When changing the direction, the torch has to be stopped for 1.5 s again to ensure reliable cutting of the corner and prevent strong rounding of the edges. Then, cutting is continued at a thickness-dependent cutting speed again. To avoid material bridges in the lower third of the thickness in particular, cutting speed has to be further reduced towards the end of cutting (position 3).
It is confirmed by the cutting edge of an “L” cut that 35 mm can be cut in a burr-free manner with air as plasma gas and a cutting speed of 0.45 m/min (Fig. 9). Apart from the absence of burrs, accurate cutting out of the rectangular segments along the contours is important for their removal, as is obvious from the cutting edge. The modular design of the system allows for the use of a single plasma current source of 300 A as master source for cutting a thickness of 35 mm. Figure 9: Cutting edge 35 mm, “L” cut, plasma and secondary gas: Air, cutting current: 300 A,
cutting voltage: About 215 V, cutting speed: 0.45 m/min, cutting position: Horizontal –
vertically descending.
In parallel, current/voltage analyses were made using the analyzer of the type Hannover AH19. The high-resolution measurement system is preferably applied for quality assurance and documentation of arc welding processes. It does not only allow for a time-dependent current/voltage analysis, but also for a statistical evaluation of electric process parameters. The following section deals with the analysis of current/voltage characteristics versus time. Cutting started from the edge of the sheet metal. Measurement started upon the ignition of the pilot arc only, as high-frequency high-voltage ignition would have destroyed the measurement system. Current supply was divided equally between water supply and the reverse flow of water. Only water supply was measured, such that the measured currents made up 50% of total current.
Figure 10 shows the current/voltage characteristic of an L cut in 35 mm sheet metal under a water cover. In analogy to 8 m water depth, the phases of pilot arc burning (up to t = 4 s) and main arc burning (from t = 4 s) can be identified. It is also evident that the analyzer of the type Hannover AH19 allows to detect in homogeneities of the process due to the cutting strategy selected – here, the stopping of the torch for 1.5 s at the point of change of the direction of movement. In analogy to the behavior of the arc when stopping the movement, cutting voltage also increases in this case. The stop for 1.5 s does not have any significant impact on the cutting current. Also the cutting voltage reaches its initial value within a minimum period of time after having reached the preselected cutting speed.
4.2.1 Thermal Shield (70 – 130 mm Thickness)
After connecting three current sources in parallel to generate a maximum current of 900 A, laboratory cutting tests were carried out on mock ups with sheet metal thicknesses increasing from 70 to 130 mm. The cutting voltages occurring varied between 215 and 220 V. As plasma gas, a mixture of argon and hydrogen was applied. Air was used as secondary gas. At first, the thickness of 70 mm allowed for a cutting speed of 240 mm/min. For safe cutting at a thickness of 130 mm, cutting speed had to be reduced linearly to 90 mm/min. As obvious from Figure 11, cutting direction can also be changed at this thickness.
| Figure 11: Cutting edge of the mock up 70 – Figure 12: | Melt adhesions, gap width: 20 mm, |
| 130 mm; 5th section of the thermal | sheet metal thickness: 70 mm |
| shield, plasma gas: Ar/H2, secondary | |
| gas: air, cutting current: 900 A, | |
| cutting voltage: 215 – 220 V, cutting | |
| speed: 240 mm/min (70 mm) – 90 | |
| mm/min (130 mm), cutting position: | |
| vertically descending – horizontal |
As a whole, the parameter fields determined for a reliable cutting of 70 to 130 mm lie in a narrower range than those determined for a thickness of 35 mm.
The gap between thermal shield and reactor pressure vessel in the MZFR turned out to be of particular importance. This gap with a mean width of 25 mm obstructs the flow-off of molten joint material (Fig. 12). If deposits are formed in the gap, plasma cutting is disturbed considerably. As a result, the nozzle or secondary gas cap of the plasma torch may be damaged.
This problem was studied at the Underwater Technology Center, Hanover. A critical gap width of 40 mm was determined. Attempts to solve this problem by varying the attack angle and using different air and water nozzles did not produce any significant success.
5. Tests under Real Conditions at the Kahl Experimental Nuclear Power Plant (VAK)
The tests that have been carried out since November 2003 at the VAK test rig in the presence of an inspector are rather promising. Figure 13 shows the rig used with the carrying and rotating ring of the handling device on the left. The control board for the operation of all wet dismantling systems, including the tool carrier and video monitoring systems, is shown on the right.
Using the technology described in Section 3, the parameter ranges obtained from the laboratory cutting tests were verified for various mock ups. These cutting tests under real conditions that mainly served to analyze the interaction of plasma cutting and handling technology were completed successfully in the 8th calendar week of 2004.
6. Summary and Outlook
Using the wet dismantling system available, safe and reproducible cutting of most difficult geometries of the moderator tank and thermal shield with the plasma cutting method has been demonstrated at the VAK test rig. This system has been developed by the Institute of Materials Science of the University of Hanover in cooperation with the Kjellberg company for high thicknesses in particular. It is suited for cutting sheet metal thicknesses of up to 130 mm at 4 m water depth and a maximum cutting current of 900 A. The modular torch developed for use under the MZFR radiological boundary conditions allows for the rapid remote exchange of the torch head and its wear parts. The tests of all wet dismantling systems and components started in the middle of November 2003 in the presence of an inspector. They were completed in mid-February 2004. Installation of these systems in the MZFR will take about three to four months. Then, these systems will be commissioned in the presence of an inspector. Cutting and packaging of the moderator tank, including the down- and ring pipe, are planned to be completed in the course of
| this year. | |
|---|---|
| Other companies and persons involved: | |
| RWE Nukem, Alzenau | Dr. Leffrang, Mr. Stanke, Mr. Arnold |
| Studsvik-ifm, Pforzheim | Mr. Süßdorf |
| IABG | Mr. Hammer |