Classification of dismantling techniques

In view of the wide range of dismantling tasks, many different cutting techniques have been developed so far. In some cases, techniques already used in the sheet metal manufacturing industry have been adapted to the special requirements for the decommissioning of nuclear installations. Additionally, special techniques have also been developed exclusively for such tasks. An overview is given in Table I, dividing the techniques with respect to the physi-cal mechanisms into mechaniphysi-cal/hydraulic, thermal and chemiphysi-cal/electrochem- chemical/electrochem-ical techniques. With respect to the requirements for the decommissioning of nuclear installations, for example remotely controlled applications, high process safety and efficiency, reduction of emission dissemination and applica-bility under water, the number of usable techniques, especially in the controlled area, is smaller.

TABLE I. DISMANTLING TECHNIQUES

Mechanical/hydraulic Thermal Chemical/electrochemical

Sawing Oxy-fuel cutting Explosive cutting

Shearing Lance cutting

Milling Plasma-arc cutting

Breaking Consumable electrode oxygen

Grinding jet cutting

Nibbling Consumable electrode water (Diamond) wire sawing jet cutting

Microwave spalling Oxy-arc cutting Abrasive water jet cutting Arc-saw cutting

Contact-arc metal cutting Contact-arc metal drilling Contact-arc metal grinding Laser beam cutting

Electrical discharge machining

As a result, electrochemical cutting techniques, electrical discharge machining and microwave spalling are used only for specific dismantling tasks and for decontamination purposes [1, 6, 13–16]. Furthermore, explosive cutting, used for example in Niederaichbach, Germany, for the delamination of acti-vated concrete structures, has only a few applications in decommissioning tasks, for example the dismantling of the biological shield at the Elk River reactor [1, 6]. Arc-saw cutting, i.e. working with a rotating disc, was developed in the USA and used for the dismantling of different reactor pressure vessels in the USA and for the JPDR in Japan [1]. Other arc processes are discontinuous oxy-arc cutting, consumable electrode oxygen and water jet cutting [1, 13, 17, 18].

Examples of the use of consumable electrode water jet cutting are dismantling of a pressure vessel and a steam dryer housing [1].

2.2. Thermal cutting techniques 2.2.1. Oxy-fuel cutting/lance cutting

Oxy-fuel cutting is restricted to mechanized, semi-remote as well as hand guided dismantling of mild steel or stainless steel plated mild steel structures [1, 19]. Therefore, mainly conventional cutting systems are used. Hand guided and semi-mechanized dismantling has been carried out up to plate thicknesses of 250 mm [20, 21]. With additional powder, oxy-fuel cutting is also capable of cutting stainless steel and concrete. In cutting tests, maximum cut thicknesses of 320 mm for steel and 1200 mm for concrete structures were achieved. An important disadvantage is the high quantity of aerosols produced during this process [1]. The lance cutting process can only be used for drilling and perfora-tion cutting, for example prior oxy-fuel cutting of thick structures such as pres-sure vessels. Typical features of this method is a low cutting speed, a discontinuous process, lack of suitability for automation and also a large amount of aerosols produced [1]. With regard to dismantling, combined processes were developed as combinations of consumable electrode water jet gouging/oxy-fuel cutting and plasma-arc gouging/oxy-fuel cutting [1, 22].

Research and development activities are currently being carried out for high pressure oxy-fuel cutting and mechanized oxy-fuel cutting under water, espe-cially for cutting stainless steel plated mild steel structures [19, 23, 24].

2.2.2. Plasma-arc cutting

For decommissioning purposes, plasma-arc cutting is the most commonly used thermal cutting technique for activated components, especially reactor internals. The main advantages are the high cutting speed over a wide range of

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plate thicknesses, especially for cutting stainless steel, applicability in the atmos-phere as well as under water, easy remote handling and low reaction forces. The maximum achievable cutting thickness in the atmosphere is 150 mm and under water it is 100 mm [1]. Therefore, several plasma torches based on such principles as water injection plasma-arc, dual flow plasma-arc, contact ignition, etc., were designed for fast remote controlled replacement of worn parts and modular sys-tems [1, 13].With regard to the dismantling of high activity core components, data on the quantity and size of emissions are available in Refs [1, 25, 26].

Research and development is being carried out to: reduce the kerf width in combination with the design of a personal guided ‘steady cut system’ [27];

increase the plate thickness that can be cut under water; and plasma-arc cutting up to a water depth of 20 m in Gundremmingen, Germany [28].

2.2.3. Laser beam cutting

Laser beam cutting is characterized, where applicable, by small cutting kerfs and precise cutting contours, small heat affected zones, small tolerances, little distortion of the object, stress-free treatment and high reproducibility. On the other hand, a high level of investment is necessary, while the low efficiency of lasers is coupled with high energy consumption. Laser technology can be used in many areas for the dismantling of nuclear power plants [29, 30]. For the dismantling of tanks or storage basins consisting of concrete walls lined with steel plates, cutting of the steel material is complicated. The metal sheets lie directly on the concrete, and it is rather difficult to cut them mechanically. A special nozzle technique, in combination with a hand guided laser system, was used in the nuclear power plant at Greifswald, Germany, to expel the molten material to the top surface of the sheet. Specific removal by suction of the released process emissions is also possible [31]. The mobility and flexibility of the fibre optic, hand guided Nd:YAG laser is an important reason for its appli-cation in nuclear facilities. A condition for the use of these appliappli-cations is the availability of a hand-held laser processing head and characterization data, and a suction system for the aerosols produced [32–35].

Research and development is being carried out for cutting asbestos mate-rials as well as for the design of modular laser beam cutting systems for cutting in the atmosphere and under water.

2.2.4. Contact-arc metal cutting, drilling and grinding

Contact-arc metal cutting (CAMC), drilling (CAMD) and grinding (CAMG) are electrothermal cutting techniques used to cut conductive materi-als with Joule and arc heating. CAMC, with a swordlike graphite electrode and

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a water curtain for blowing out the molten material, is a thermal cutting tech-nique currently used for the decommissioning of nuclear facilities [36]. Using this technology, complicated components like tube-in-tube objects and compo-nents with re-entry angles can be separated with a single cut. The state of the art in CAMC is the cutting of 260 mm thick components. The kerfs show widths of 4–8 mm and the wastage ranges from 20 to 25% [36]. A special CAMC tool with a turntable drive unit and integrated process control for auto-matic cutting was developed for cutting tasks in Greifswald [37]. CAMD was developed as a novel technology to drill holes or pocket holes without restor-ing forces. Furthermore, together with a warp mechanism, an automated fixrestor-ing system was built [38, 39]. Another cutting technique is CAMG, with a rotating electrode, offering new fields of application. As materials for the cutting elec-trode, steel or carbon fibre reinforced graphite can be used. The cutting speed is very high: For example, CAMG is capable of cutting objects of 15 mm thick-ness at a speed of 3 m/min. The wear of the rotating electrode can be reduced to 9% by appropriate parameter adjustments, and the maximum cutting thick-ness is 40–50 mm [39, 40]. Research and development is being carried out to reduce electrode wear and to increase the maximum cutting thickness for CAMG; a comparison of the abrasive water jet cutting and CAMC processes is also being carried out.