STATE OF THE ART OF THERMAL AND HYDRAULIC CUTTING
TECHNIQUES FOR DECOMMISSIONING TASKS
IN NUCLEAR INDUSTRY
Prof. Dr.-Ing. Fr.-W. Bach and Dipl.-Ing. J. Lindemaier
Institute Of Materials Engineering
University Of Dortmund
Germany
ABSTRACT
This paper introduces thermal and hydraulic cutting techniques which are most suitable for nuclear decommissioning tasks, their newest developments referring to cutting performance, process safety, remote controlled application and secondary waste emission.
INTRODUCTION
Decommissioning and dismantling of nuclear facilities requires cutting techniques which are easily remote operable, which may be used under water and which do not effect an increase of the radiological risk for the operators. Almost every industrially common cutting technique - using thermal or strain energy - has already been transferred to remote operated use for nuclear decommissioning tasks. Experiences show that there is no single cutting technique which is suitable for all cutting tasks. New cutting techniques have been developed and commonly used techniques have been adapted to nuclear requirements.
The principle advantages of thermal cutting techniques and high pressure abrasive water jet cutting for decommissioning tasks are
The emission of gas and aerosols using thermal cutting techniques requires air filtration. Abrasives used for high pressure abrasive water jet cutting are additional secondary waste.
FLAME CUTTING
Flame cutting is a commonly used cutting technique since many years for workpieces made of non-alloyed steel, low-alloyed steel and cast steel. It can be used for cutting of mild steel workpieces with a plate thickness up to 3000 mm. Figure 1 shows the process principle and the tool set up for the use either in atmosphere and under water. Only few modifications of the cutting unit are necessary to realize underwater flame cutting (Internal ignition torch with shielding gas cap).
Figure 1. Principle and Tool Set Up for Flame Cutting
Up to now most of the cutting tasks for decommissioning of nuclear facilities were applications in atmosphere, which were mainly done by conventional flame cutting equipment /1/. In nuclear decommissioning the flame cutting technique is interesting for segmentation of the reactor pressure vessel, even if it is plated with stainless steel, because the energy of the liquid slag is used to melt the remaining plating layer.
The state of the art of flame cutting is embossed by the development of a flame sensor system which allows monitoring and adjustment of the primary flame, measuring the material temperature during hole piercing and edge founding, cut monitoring and scanning of material edges /2/. It ensures the highest process performance for every cutting task.
The achievable cutting speed is approx. 120 mm/min higher by using flame cutting in atmosphere compared to its under water use. Nozzle developments allow the use of high pressure cutting oxygen jets which improve the cutting speed.
PLASMA ARC CUTTING
Originally the plasma arc cutting technique was exclusively developed to cut metallic materials which could not be flame-cut, such as high-alloyed steels and non-ferrous metals as aluminium and copper. Depending on the type of material to be cut and the required cutting depth, several different methods are available today, which essentially differ with regard to the process control, arc power, plasma gas, secondary gas and cathode material. Presently, the maximum material thickness in atmosphere is 150 mm at a maximum amperage of 1000 A. Using plasma arc cutting under water is possible for a workpiece thickness up to 80 mm.
For decommissioning tasks plasma arc cutting is the most frequently used thermal cutting technique. In the german Nuclear Power Plant Gundremmingen the steam dryer apron – a 5 mm thick stainless steel construction - was cut by plasma arc with a cutting speed of 1000 mm/min in a water depth of 4 m. During the de-commissioning project JEN-1 in Spain segmentation cuts at 80 mm thick components made of aluminum were achieved with a cutting amperage of up to 500 A /3/. Further applications for under water plasma arc cutting were cutting of the thermal shields in NPP Rheinsberg, Germany /4/ (water depth up to 0,8 m) and in BR 3 Mol, Belgium (maximum thickness 76 mm, cutting amperage 800 A).
A very interesting application of the plasma arc cutting technique for decommissioning tasks in nuclear industry is the automatic pipe cutting and sampling system called RAMSES, developed by the University of Hanover in Germany. The device is manually guided to the workpiece. Two grapples fix pipe and cutting device to eachother. A plasma cutting torch then cuts around the pipe and is turned reverse afterwards. Moving RAMSES to a new cutting position at that pipe and repeating the cutting process allows to take the pipe piece by means of the grapples and transport it for deposition.
Since the wearable parts of a plasma torch have a restricted lifetime, the cathode and the nozzle need to be exchanged when the cutting performance decreases. So one of the main important developments for plasma arc cutting is the plasma modular torch, shown in figure 2. The design of the torch head combines all wear parts, which can be remotely exchanged. Presently the maximum cutting amperage is 600 A.
Figure 2. Modular Plasma Torch
CONTACT ARC METAL CUTTING
The importance of the electrothermal underwater cutting technique Contact Arc Metal Cutting (CAMC) for cutting tasks in decommissioning of nuclear facilities increased since two years. The simple process principle is shown in figure 3 /5/.
Figure 3. Setup and Principle of CAMC-process
The electrode carrier guides the electrode to the workpiece until contact. A high current (up to approx. 4000 A) flows through a small contact area, due to the high energy density the workpiece material evaporates with an abrupt expansion. In the produced gap the gas becomes ionized and allows the ignition of an arc which melts more workpiece material. The molten material is rinsed out by a water scavenging which is guided down the electrode by jets. When the gap becomes too wide the arc breaks off. A continual repetition of the process is caused by moving on the electrode into the kerf which it has produced.
The cutting electrode with its application adapted shape is currently made of pure graphite, carbon-reinforced graphite, tungsten-copper or - in special cases - mild steel.
The main features of that cutting techniques are
Due to the simple cutting principle the reaction forces are neglectable, keeping the requirements on the guiding device low. Even guiding the cutting tool with a master / slave-manipulator for the application in extreme environments has already been proven. All metal materials - plated materials as well - can be cut by CAMC. The maximum thickness of the workpiece to be cut simply depends on the geometry of the electrode and the effective scavenging. The geometry of the workpiece does not influence the cutting process. Complex structures and workpieces with undercuts can be segmented with one linear cutting move.
The cutting speed of CAMC with a 2000 A power source is slightly higher as compared to plasma arc cutting.
Just as all other thermal underwater cutting techniques the CAMC process produces emissions like aerosols, hydrosols and sedimentations. The factors with the highest influence on the particle emission during the CAMC process are the electrode thickness, which defines the kerf width, and the idling voltage of the power source.
Although already today CAMC can compete in many features with other thermal and mechanical under water cutting techniques, it offers a development potential. The actual development targets are
CAMC has already been applied in nuclear industry for decommissioning tasks. It was used for cutting tasks (thermal shield) in the WWER-210 type reactor in Novoronesh, Russia /6/. Since 1997 CAMC is used in Gundremmingen, Germany to cut up complex geometry´s.
ABRASIVE WATER JET CUTTING
Nowadays, high pressure abrasive water jets are used in areas ranging from production engineering to under water applications, in which cutting tasks were performed in water depths of several thousand meters. The main difference between the use of high pressure abrasive water jets and a thermal cutting technique is the fact that the process is athermal. During the cutting process with abrasive water jets, in most cases neither thermal reaction products are produced nor does a technically relevant thermal influence of the cut edge occur /7/.
Since the cutting performance of high pressure abrasive water jet cutting increased to cut workpieces with a thickness up to 200 mm under water and workpieces up to 250 mm in atmosphere the interest on this cutting technique in order to use it for decommissioning of nuclear facilities rises continuously. To generate abrasive water jets at present, two different methods are available, shown in figure 4
Figure 4. Concepts of Abrasive Water Jet Generation
The abrasive can either be added to a plain water jet in a special mixing head (injection jet) or a premixed and pressurized abrasive water suspension can be released to the nozzle to form the abrasive jet (suspension jet) /8/. For industrial applications mainly the abrasive water injection jet cutting technique (AWIJ) is used today with operating pressures of up to 400 MPa. The abrasive water suspension jet cutting technique (AWSJ) is used in industry only at pressures of 70 MPa.
Since the volume rates of flow of water and of abrasive are very high, this process is mainly used on site and under water for repairing and dismantling of especially thick-walled components.
The main developments have been made to reduce the volume rates of abrasives, especially by using the abrasive water suspension jet to generate the cutting jet. AWSJ systems with operating pressures of up to 400 MPa have already been developed.
The first application of the abrasive water jet cutting for decommissioning tasks in nuclear facilities is part of a research and development program, supported by the german government. An abrasive water suspension jet equipment with a maximum operating pressure of 150 MPa is installed in NPP Kahl since 1997 in order to cut highly activated reactor internals.
REFERENCES