Patent Number: 048329037
Section: description

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS The storage arrangement comprises a chamber 1 having walls, floor and ceiling of reinforced concrete. The ceiling 2 of the chamber is pierced by a matrix of openings 3, the upper ends of which have a slightly greater diameter than the lower ends as shown more clearly in FIG. 2, ledge 4 joining the parts of greater and lesser diameter. Each of the openings 3 has, located within it, the upper end of a storage tube 5 of steel, the top section 6 of the part of the tube within the opening 3 being of greater diameter than the remainder and being connected to it by an annular shoulder 7. The stepped annular gap so formed provides attenuation to radiation from the tube contents. The tubes 5 are supported from the floor 8 of the chamber 1 and are free to expand thermally upwards through openings 3. The tubes 5, which are closed at the bottom, serve to store irradiated nuclear fuel as at 9, and their upper ends are closed by plugs 10 which are supported within the tubes by the inner surfaces of the inclined shoulders 7. A double seal 11 is provided between the top of each plug and the tube, and each tube/plug assembly is surmounted by a removable tile 21, the tiles together forming the floor of a charge hall 12 (FIG. 3), the walls and roof of which are also of reinforced concrete, and in which there is located a charge machine and gantry 13, of any convenient construction, for introducing fuel into and removing it from the storage tubes 5. The floor of the chamber 1 has an opening 14 which communicates, via an inlet duct 15, with a louvred air inlet 16 (FIG. 3), and the ceiling 2 of the chamber has a further opening 17 leading into a discharge stact 18 extending upwards to a louvred air outlet 19 disposed a distance above the air inlet 16. The opening 17 is located at the opposite side of the chamber to the inlet opening 14, and in use decay heat from spent fuel 9 within the storage tubes 5 is transferred to the walls of the tubes by conduction, convection and radiation. The heat is removed from the tubes 5 to the atmosphere by a natural thermosyphon process, the heated air rising within the outlet stack 18 by convection, and being replaced by cooler air drawn into the chamber 1 through the inlet opening 14. The disposition of the inlet and outlet openings 14, 17 at opposite sides of the chamber 1 ensures that there is a flow of air between the tubes in a direction transverse to the tube axes, as well as vertically, as indicated by the arrows in FIG. 1, which gives rise to optimum cooling. It will be seen that the amount of air flow is governed by the heat generated within the store, and the arrangement is designed to be sure that the cooling is adequate to maintain the fuel within the tubes at a safe temperature, consistent with the gas used within the tubes. In accordance with the invention, the interior of each tube 5 communicates, by means of a pipe 20, connected to the side of the tube between the seals 11, to manifolds 22 common to a plurality of tubes, and by which the gas within the tubes can be changed and the pressure controlled. A hole in the plug 10 leads from the space between the seals 11 to a cavity in the base of the plug 10. The cavity is closed by a porous metal filter 12 that limits the passage of radioactive particles passing into the pipe 20. A suitable service point valve 23 associated with each tube allows the pipe 20 to be connected to a manifold 22 leading to an air extraction system 24 or to a second manifold 22 leading to an alternative gas system 25. The service point valve 23 also allows individual storage tubes to be isolated from the manifolds 22, if desired, in order to permit a rapid segregation of tubes should a fault condition occur, thereby enabling the fault position to be speedily traced. The air system 24 incorporates a suitable filter 26 followed by a flow measuring device 27, a one-direction valve 28 permits the outflow of gas from the air system 24. An exhauster 31 connected after the one-direction valve 28 allows gas to be drawn from the system until a depression is established in the tubes 5. The exhauster 31 is operated when leak checking of the connected tubes 5 are required or when leaks have been established because of unexpected faults. The discharge from the exhauster 31 passes to atmosphere. The flow measuring device 27 in conjunction with the operation of the exhauster 31 provides a measure of the leak tightness of the tubes 5. The depression that is maintained within the storage tubes 5 supplements the high integrity sealed enclosure for the fuel created by the tube 5 and the sealed plug 10, as any leakage that occurs will be into the tubes 5. Similarly, any leakage that occurs at the seals 11 will also be inwards. The use of an exhauster and a flow measuring device connected to pipes to the individual storage tubes has the practical advantage of enabling any fault conditions to be more rapidly detected. A gas sampling point 29 allows the radioactive and moisture content of the gas in the system 24 to be measured by suitable instruments. Air drying equipment 30 supplies dry air to the system 24 when the pressure in the tubes 5 falls below a level set by suitable valves in the drying equipment 30. The natural atmospheric temperature variations that act on the tubes 5 via the thermosyphon cooling system cause the air within the tubes to expand or contract. Expanding air passes from the system to the atmosphere via the filter 26 and one-way valve 28, carrying with it some water vapor. The inflow of air required as the air cools and contracts is supplied from the drying equipment 30. The alternative gas system 25 incorporates a suitable flow measuring device 32, a source of low pressure gas 33 and a suitable pressure relief device 34 that vents to the air system upstream of the filter 26. The gas source 33 supplies gas at the required pressure to the tubes 5 when connected via the service point valve 23. The maxium pressure within the tubes is set by the pressure relief device 34 that allows excess gas to pass from the tubes 5 to atmosphere via the filter 26. The flow of gas measured with the flow measuring device 32 provides a measure of the leak tightness of the tubes 5. If desired, portable monitors may also be used to check the conditions within the tubes 5 periodically by connection to tapping points on the individual pipes or to separate pipes communicating with the interiors of the respective tubes either through the tube walls or through the respective plugs 10. The storage arrangement described has the advantage that it can be used to store uncanned fuel, enabling inspection and monitoring to be readily carried out. Moreover, removal of fuel from the store, should this become necessary, can be speedily effected, without the need to interfere with neighboring storage tubes, simply by removing the respective tile and plug. Re-use of the storage tubes after fuel removal is an operational option. Moreover, the manner of supporting the storage tubes 5 enables a tube to be readily withdrawn upwards into the charge hall for examination or replacement should this be required. Although the tiles 21 have been shown resting on the tops of the tubes 5, the skirt portions 27 of the tiles can be extended, if desired, so that these rest upon the surface of the concrete, giving a more even floor surface to the charge hall 12. The storage tubes 5 will normally be of circular cross-section, but this is not essential, and other shapes may alternatively be employed. In some cases, the tubes may carry external cooling fins of any convenient configuration to enhance the cooling effect of the air flow. FIG. 3 shows a plurality of independent storage arrangement modules as described above, sections of which are shown at C, combined to form a nuclear storage structure having, in this case, a common receipt/dispatch building A for spent fuel or vitrified waste, and associated with a storage submodule for initially storing waste fuel before transfer to the charge hall and storage tubes. The charge hall 12 is common to all the storage modules, as in the gantry 13.