Patent Number: 048329037
Section: summary

BACKGROUND OF THE INVENTION 1. Field of the Invention This invention relates to storage arrangements for irradiated fuel following its removal from nuclear reactors. The invention can also relate to the storage of pre-irradiated fuel and also vitrified waste after spent fuel reprocessing. 2. Description of the Related Art It is a common practice to store spent fuel under water, in what are generally known as pond stores, for periods that are long enough to allow the decay heat and radiation levels to reduce sufficiently to allow the fuel to be transported with safety. However, the use of a pond store is not entirely satisfactory where the fuel needs to be stored for any considerable length of time. Thus, the ability to store the fuel safely for protracted periods in a water environment is very dependent upon the materials of the cladding in which the fuel is accommodated, the irradiation history of the fuel and/or the cladding, the integrity of the cladding, and the quality of the water in which the fuel is stored. Thus, cooling and shielding functions can be carried out completely satisfactorily while the fuel cladding remains intact, and while the water is present. However, if the fuel cladding is perforated by corrosion or handling, then fission products can escape, and both fission products and corrosion products that are radioactively contaminated are then able to float and permeate to the surface of the water, which could result in high dose rates to operators. In addition, it is possible for these fission products and corrosion/contamination products to adhere to the walls of the pond. Variations in the pond water level, due to evaporation or leakage, could allow these products to dry out, when they could then become airborne, causing possible ingestion hazards to operators and the risk of atmospheric pollution. Moreover, in order to maintain adequate cooling and shielding, the pond integrity must be assured to very high limits. Small leaks could give rise to minor contamination problems, and larger leaks, resulting in loss of cooling water, may result in a serious district hazard. As safety requirements for nuclear installations become more rigorous, and the allowable dose rates to operators continue to decrease, the need to design storage systems and other nuclear installations to even higher orders of integrity becomes essential, particularly as for various reasons it is now becoming necessary to store spent nuclear fuel for longer periods than was originally anticipated. In United Kingdom patent application No. 2061798, there is described and claimed an alternative form of storage arrangement which substantially avoids the above-mentioned disadvantages. Such a storage arrangement comprises an enclosure for the fuel that utilizes air as its storage medium; an exhaust system for exhausting this air from the enclosure through filters so as to maintain the interior of the enclosure at subatmospheric pressure; and a transfer mechanism for transferring fuel into and from the enclosure. Maintaining a depression or underpressure within the enclosure could eliminate the need for a high integrity envelope for the enclosure, as any leakage that might occur will be into the enclosure and, accordingly, the invention provides an inherently safer store than the usual water filled pond. In addition, as the fuel is stored in air rather than water, the risk of corrosion is reduced, and consequently the need for an operator to maintain the water chemistry at precise levels in order to prevent the generation of corrosion products, and the possibility of atmospheric pollution is thereby avoided. Another form of dry storage arrangement for irradiated nuclear fuel is described in United Kingdom Patent Specification No. 1583303, such an arrangement comprising a grid having a plurality of openings for supporting respective fuel cans so that they extend downwards therefrom, the space above the grid forming an-air filled enclosure associated with an exhaust system for exhausting air from the space through filters to maintain the interior of this space at subatmospheric pressure, and the arrangement including means for producing a flow of cooling air over the exterior of the cans. In use of such an arrangement, the fuel is first enclosed in cans, and the cans placed in openings in the grid, the unused openings being sealed with lids. However, the lids need to be removed for accommodating further cans, which is inconvenient, and failure of a can could give rise to contamination of the cooling air, with the risk of polluting the atmosphere. SUMMARY OF THE INVENTION An object of the invention is to provide an alternative form of dry storage arrangement which avoids these disadvantages and has other benefits as will be apparent from the following description. According to the invention, a storage arrangement for nuclear fuel comprises a plurality of storage tubes each of which is closed at one end and is closeable at the other end by a removable plug, a pipe connecting the interior of each tube to manifolds serving a plurality of tubes, venting or flow control means connected to a manifold for maintaining the interiors of the respective tubes, when the plugs are fitted thereto, at a positive pressure above atmosphere, or atmospheric pressure, or at a subatmospheric pressure, and means for producing a flow of cooling fluid over the exterior surfaces of the tubes. The storage system allows the nuclear fuel to be stored in air or in an alternative non-oxidizing gas. The alternative gas will allow fuel to be stored at a higher temperature without damage to the fuel. This may be necessary for limited periods when fuel heat output is too great to give acceptable temperatures for storage in air. The venting system allows selection of the interior pressure of the tubes depending upon the choice of gas within the storage tube. This invention allows for the use of an alternative gas in the tubes held at a controlled positive pressure greater than atmospheric pressure or the use of air in the tube at atmospheric pressure. An exhauster connected to the venting system also allows the tube pressure to be reduced to below atmospheric pressure for routine leak checking procedures or in the event of tube leakages developing. The depression maintained within the tubes by the exhauster effectively forms a secondary containment system by causing air to flow inwards through leakage paths in the tubes. The use of a venting system connected to individual tubes by a manifold and pipe arrangment has the advantage of limiting the spread of radioactive particles throughout the storage system compared with an arrangement in which the tubes communicate with a common chamber. In addition, it enables fault conditions that might arise to be more rapidly detected; the use of a plurality of manifolds each serving a respective series of storage tubes also enables faults to be quickly traced by isolating different sections of the manifold system. Monitors for detecting any rise in radiation levels can be situated in any convenient part of the venting system. Monitors for detecting the flow of gas to or from the storage tubes can be situated in any convenient part of the venting system to allow leakage flows to be measured. When air is used in the tubes and the venting system is arranged to maintain atmospheric pressure, all pressure loads on the tube are removed, thereby reducing pressure stress in the tube walls and reducing the significance of any leakage paths. The natural atmospheric temperature and pressure fluctuations causes gas within the tubes to flow inwards or outwards via the venting system to the atmosphere via filters. It will be understood that the filters associated therewith must, of course, be of the kind suitable for preventing the escape of radioactive particles from the storage tubes. In that the fuel to be stored may have been previously stored in water, the invention deals with the unintentional retention of small quantities of this water in cavities within the fuel assembly. Such water may have a deleterious effect on the fuel during long periods of storage if not removed from the storage tubes. The venting system allows water vapor that may be generated within the tube, from the fuel, to pass to atmosphere. Air that flows back into the tubes during natural pressure and temperature fluctuations is caused to pass through an air drying system. This natural self sumping feature of the invention, whereby moist air will flow outward and dry air allowed to flow inward, is used to reduce the quantity of water vapor that may exist in the tubes, thereby removing the potential for fuel degradation caused by corrosion. The storage of fuel in storage tubes forming a part of the storage structure means that the fuel does not first have to be placed into sealed cans, which is an advantage as canning involves an additional process and moreover removes the ability to check the fuel easily. Furthermore, canning gives rise to additional contaminated waste which must ultimately be disposed of. The storage tubes can be reused after removal of the fuel. Each storage tube forms a single walled containment boundary. The tubes are conveniently supported vertically with their closeable ends fitting closely within respective openings in the base of the charging hall used for the transfer of fuel into and from the tubes, so that the tubes extend downwards into the cooling chamber through which air is caused to pass. Preferably, the chamber has a vertical air inlet volume on one side of the plurality of storage tubes and a vertical outlet volume on the opposite side so that air is caused to flow across the chamber, and over the tube surfaces, by a natural thermosyphon process that is enhanced by a chimney connected to the outlet volume. It will be seen that for a given store geometry the amount of air flow is governed by the heat generated within the store, so that the cooling is self-regulatory. Preferably, the air-flow conditions are arranged to be sufficient for the fuel within the tubes to be maintained at a temperature of not more than 180.degree. C. under all normal storage conditions when air is used within the tubes. The tubes preferably have shoulders adjacent their upper ends which, combined with ledges within the respective openings, provide a barrier to radiation from the fuel passing to the charge hall. The tubes, being supported from the floor of the cooling chamber, are free to thermally expand upwards. This method of support enables tubes to be readily withdrawn upwards from the cooling chamber into the charge hall for replacement should this prove to be necessary. Seals are preferably provided between the plugs and the walls of the tubes so that the plugged tubes effectively form gas-tight enclosures. When the plugs are in their operating position, they are preferably surmounted by removable tiles which together provide a floor to the charging hall. It will be understood that both the charging hall and cooling chamber should be surrounded by adequate radiation shielding, preferably of reinforced concrete. A storage structure can be built up in modular fashion utilizing a plurality of independently operable storage arrangements in accordance with the invention.