Patent Number: 054835623
Section: summary

TECHNICAL FIELD The present invention relates to a device, during work with contaminated parts, to prevent or limit contamination of a surrounding liquid medium, especially suitable for machining, such as repair or scrapping, of radioactive components from a nuclear reactor. BACKGROUND ART, PROBLEMS In nuclear reactors it has hitherto been common to store scrap and rejected parts in their present condition in the reactor pool associated with the reactor. The space in many reactor pools has therefore been reduced and compaction has begun to be applied. It has also been normal practice to carry out repairs direct in the fuel or reactor pool, which has resulted in the water therein being contaminated. Consequently, after completed work, cleaning of large volumes of water and large areas is required. Swedish patent publication 465 236 discloses a processing vessel for scrapping rejected parts. The vessel is composed of a bottom end plate and a casing which can be mounted on the bottom end plate and consists of one or more detachably connected shell sections, arranged one after the other. Outside the processing vessel there is arranged a sealable shipping container for decomposed scrap. This container is detachably joined to the processing vessel and is arranged to communicate with the interior of the vessel via a sealable sluice opening. The shipping container is connectible to a cleaning plant for cleaning of a medium located in the shipping container. The disadvantage of this device is that a transport flask, in which the scrap is to be stored, is arranged outside the processing vessel whereby the scrap must be moved out to this flask, and this movement out takes place by means of the shipping container. Further, the processing vessel, which is usually made of stainless steel, is large, heavy and,unwieldy, which is annoying particularly during transport and storage. The processing vessel is also difficult to clean since gaps and pockets are formed between the shell sections and since metal chips of the same material as the processing vessel will easily adhere to the surfaces thereof. The processing vessel is also expensive to manufacture. SUMMARY OF THE INVENTION, ADVANTAGES According to the present invention, contamination of the surrounding environment is prevented by the use of a volume delimitation tank. The tank is arranged, for example in the fuel or reactor pool of a nuclear reactor, such that the water in the tank has a sealed or at least limited connection with the surrounding pool water during the machining phase. The volume delimitation tank comprises a sack or channel which constitutes at least the substantially vertical walls of the tank and is made of a material which is foldable, radiation-resistant and resistant to puncture due to sharp objects. The material should produce little waste during scrapping and combustion. The material consists, for example, of a fabric, or a cloth or a plate, of strong plastic fibres which are organic or synthetic and have a high strength, for example polyethylene, where the fibres have a high molecular weight and a high degree of order. An example of such a material is the fibre called Dyneema, marketed by DSM High Performance Fibers B.V. and by Nippon Dyneema Ltd. Another example is the fibre called Spectra, marketed by Allied Signals. The material stated is, in the fabric and cloth design, permeable to water to a limited extent and can be used as such or be made Watertight by lamination, either in conventional manner or by means of calandering. To increase the strength of the cloth or the fabric, several layers can be laminated, thus obtaining a more or less stiff plate. In addition to the sack or channel, the volume delimitation tank comprises a bottom part and support rods intended to support the sack or channel. The bottom part and the support rods can be made of the same material as the sack or the channel. The bottom part can either be made weak, that is, be made with only one layer of cloth or fabric, or be made stiff as the support rods, or of aluminum or of stainless steel. When using a sack, the bottom part can be integral with the sack or be arranged as a loose part therein. When using a channel, the lower part thereof is attached to the bottom part. When mounting the tank, support rods are arranged at at least the vertical walls of the tank such that these are distended, whereafter the tank is lowered into the pool and filled with water therefrom. When using a tank with walls of cloth or fabric, sinkers can be used which are arranged at, for example, the bottom part. The vertical walls of the tank are then folded out and the tank is arranged at the pool's edge. The tank is attached, for example, to a substantilly horizontal beam structure arranged at the pool edge with beams projecting over the pool. It is also possible to allow the tank to stand on the bottom of the pool. Depending on the machining that is to be carried out, a suitable frame structure may possibly be lowered down into the tank. The bottom part may possibly be arranged in this frame structure. The frame structure may be provided with a platform for tools and positions for the objects that are to be machined. Disintegration of scrap and repair of objects, respectively, take place with the aid of remotely operated members which are operated from a work platform arranged on top of the pool. During scrapping, some type of cutting device is used, for example, whereafter the separated pieces of scrap are brought, by means of the same or some other remotely operated member, to a scrap stand arranged in the tank. The water in and outside the tank constitutes a radiation shield. During such chip-forming machining where the particles are difficult to remove by means of conventional cleaning, that is, by flushing and slurry exhaustion, an inner sack is arranged in the tank. The inner sack is made of a water-transmitting material which suitably is inflammable. The inner sack is, for example, of the same material as that of which at least the vertical walls of the tank are made. The inner sack is attached to the tank wall and the bottom part, by, for example, lines and loops, before the tank is lowered down into the pool. When dismantling, the inner sack is raised at such a speed that the water has time to pass out through the limiting surfaces of the sack. To accelerate the emptying, a pump can be arranged to pump out the water from the inner sack via the cleaning plant. Thereafter, the inner sack is burnt and the ashes passed to ultimate storage. The tank can be connected to a cleaning plant for continuous cleaning of a medium located in the tank. After completed work, the entire enclosed volume of water is cleaned. When cutting methods have been used, the bottom part is slurry-exhausted to capture chips or the like. Objects which are lifted out of the tank are possibly flushed clean. Thereafter, the tank can be opened wholly or partially to insert new objects which are to be machined. This opening of the tank can take place in several ways (see the embodiments below). The tank may possibly be provided with a sluice so that it can be used repeatedly without the entire enclosed water volume having to be cleaned (see the embodiments below). When the tank is not used, it is dismantled and stored at some other location. All loose parts, such as tools, are flushed clean and raised from the tank whereupon the frame is raised, flushed clean and dismantled. Also the tank walls and the supports are flushed as they are raised and dismantled. Where necessary, the bottom part is slurry-exhausted. Any final cleaning is carried out in the reactor hall belonging to the pool, whereupon the parts are packed for storage or transport. The invention offers many advantages. The foldable and light material in at least the walls of the tank contributes to a small space requirement and to a considerable simplification of transport, installation and storage. Since the tank is relatively inexpensive to manufacture in combination with its small space requirement when not being used, it is possible to use several tanks of different sizes and shapes in parallel. In this way a volume, which is suitable in relation to the size and shape of the object in question, can be simply delimited, which permits repair and scrapping of contaminated parts to be made in a not unnecessarily small/large delimited volume. The volume delimitation tank made of a plastic material permits a construction without gaps and pockets, which makes it easy to clean. Further, it is an advantage to have a tank of plastic material since the chips are hydrophilic whereas the tank is hydrophobic, which prevents the scrap particles from adhering to the tank and further facilitates cleaning. It is also an advantage that the tank can be made so large that the scrap container can be arranged inside the tank, thus avoiding sluices for sluicing out scrap parts. The tank can be made wholly of plastic. After scrapping and combustion, the tank produces little waste. In comparison with other materials, a fabric, a cloth or a plate of a suitable fibre is strong and is not essentially damaged by falling objects or by abrasion. The friction between the fibres is low, which permits the fabric to be folded in a simple manner. The light material in the tank means that the tank has a low weight, which in turn entails a considerable simplification of suspension devices and lifting tools for the tank.