Patent Number: 042776880
Section: summary

The present invention relates to a device comprising a bag for covering a cask for transporting radioactive fissionable materials such as a used nuclear fuel to prevent the outer surface of the cask from contamination with the contaminated water of a fuel storage pool. The nuclear fuel used in a nuclear reactor (hereinafter referred to simply as "fuel"), for example, is transported to a fuel reprocessing plant as accommodated in a special container to prevent the radioactive contamination of the environment during the transport. The fuel transport container is usually called a "cask." To assure safety against the radioactivity, the fuel is placed into the cask as submerged in the pool in which the fuel is stored. The water of the fuel pool, which is in direct contact with the fuel, usually has a considerably high level of radioactivity, which invariably contaminates the outer surface of the cask. Consequently there arises the necessity of removing the contaminant from the cask outer surface after the cask has been withdrawn from the pool with the fuel contained therein. This procedure, however, is difficult and requires much labor and time since the cask has a large number of cooling fins on its outer surface as is well known. To simplify the removal of the contaminant to the greatest possible extent, it is known to render the cask free from contaminant before immersion into the fuel pool, for example, by covering the cask with a bag of impermeable flexible sheet. As such a bagging method, it has heretofore been proposed to bag a cask and fasten the edge of the bag opening with a stainless steel band into pressing contact with the outer surface of the cask to seal off the interior of the bag. The proposed method nevertheless involves the problem of failing to provide perfect liquid tightness and being inefficient to practice. It has also been proposed to inject clean water into the bag under pressure to prevent the contaminated water from penetrating into the bag, but this method still has the drawback that after the cask has been withdrawn from the pool, the water within the bag must be disposed of by a cumbersome procedure which leads to a reduced efficiency. The main object of this invention is to provide a cask bagging device which affords a reliable seal between the opening edge of the bag and the outer surface of a cask and which ensures an efficient operation. To fulfil this object, the present invention provides a cask bagging device comprising a bag and a flexible annular tube disposed at the opening end of the bag. A pressure gas is fed to the annular tube to cause the pressure to hold the opening end in pressing contact with the outer periphery of the cask to seal off the bag. According to a preferred embodiment of the invention, the annular tube is adapted to fit into the space between a pair of adjacent fins among a large number of annular fins on the outer surface of the cask, and/or a member of high rigidity is held in contact with at least part of the outer surface of the annular tube except where the tube is in contact with the cask outer surface, the annular tube thus providing a reliable seal. When the cask bagged in the device is immersed into the fuel pool with the pressure gas fed to the annular tube to seal off the bag, the pressure of the water will act on the tube in accordance with the submerged position of the cask, thus reducing the sealing function of the tube. While it appears useful to give the tube such initial pressure that the liquid tightness will not be impaired when subjected to the water pressure, a problem will then arise in respect of the strength of the fins because the thinner the fins, the higher are the characteristics thereof so that it is not desirable to increase the thickness of the fins for reinforcement. Furthermore the bag will be subjected to the external water pressure in corresponding relation to the submerged position. The bag will not be liable to damage despite the water pressure if made from a material of high strength, but this is in conflict with the requirement that the bag be made of thin combustible material so as to be disposable with ease after use. According to another preferred embodiment of the present invention, the device is provided with means disposed close to the annular tube for detecting the external water pressure and a system for controlling the pressure of gas to be supplied to the tube in accordance with the external water pressure detected, whereby a constant pressure difference is maintained at all times between the inside and outside of the annular tube to assure liquid tightness with stability without any adverse effect on the strength of the fins. Additionally the pressure gas is also fed to the interior of the bag under similar control so that the external pressure on the bag is limited to a specified range, rendering the bag serviceable free of any damage notwithstanding that it is made of combustible lightweight material. Preferably the interior of the bag is maintained at a specified negative pressure to keep the bag in intimate contact with the cask before the cask is immersed into the pool so that the bag will not be damaged by engagement with part of some other article or apparatus during handling. When the cask has been placed into the body of water, the interior of the bag is usually adjusted by the control system to a predetermined negative pressure relative to the external pressure of water, whereby the bag can be held in its initial state as fitted to the cask throughout the whole process of immersion into the fuel pool and withdrawal therefrom. On the other hand, the interior of the bag, which is at the specified negative pressure before immersion into the pool, may be controlled to a slightly positive pressure relative to the external water pressure on immersion into the pool to preclude the penetration of contaminated water into the bag more effectively. In this case, however, the positive pressure in the bag will inflate the bag when the cask is withdrawn from the pool. Further in this case, the pressure line for feeding the pressure gas to the bag must be provided with an intermediate start valve which opens the line on detecting the immersion of the cask into the water.