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14 декабря, 2021
Before describing the standard route followed by AGR fuel once it has served its useful life in the reactor, it is first necessary to give a brief account of two other fuel handling and storage facilities involved so that the sequence of events which takes place can be more clearly understood.
Buffer store Located beneath the floor of the central services block in the charge hall are several vertical storage tubes. Some of these are unpressurised and are used for storing new fuel stringers, plug units, control rod assemblies, etc., but a number are water — cooled ‘decay’ tubes, pressurised to about 21 bar of carbon dioxide. These are made specifically for the storage of irradiated fuel stringers, the closures of which are used to form the pressure seal. The buffer store at Hinkley Point В comprises 22 decay tubes and 20 unpressurised storage tubes.
Irradiated fuel dismantling cell In order that irradiated fuel stringers can be dismantled and individual fuel elements despatched to the cooling ponds, the fuel route on AGR power stations is provided with an irradiated fuel dismantling cell (IFD). Situated below the floor of the central block facilities, it comprises a cell with a hoist room above it and servicing rooms below. Adjacent to the cell is a control room in which viewing of the dismantling operations is provided with the aid of a zinc-bromide shielded window. An IFD hoist is located on a circular rotatable slab in the floor of the hoist room and a single vertical hole with fitted guide tube in the slab allows the passage of items in and out of the cell below. In the floor of the cell are 13 vertical tube lined holes arranged in circular formation so that the guide tube in the rotatable slab in the hoist room above can, by slab rotation, be positioned above any one of them. The tubes consist of a disposal tube for despatch of waste items to debris vaults, a dismantling tube containing a platform on which the irradiated fuel stack is lowered for the actual dismantling process, two pond discharge tubes, a bottling station tube used for placing fuel elements into bottles and eight storage tubes. When irradiated fuel is installed in the cell, the bottling station, storage tubes and pond tubes are supplied with cooling air which is blown from below into the cell by fans and then drawn down the dismantling tube by suction pumps.
When an empty fuel channel has been reloaded with a new fuel stringer, the charge machine is moved off the reactor and usually blown down to about 21 bar pressure so that it can be connected to a buffer store decay tube (at the same pressure) enabling the irradiated stringer to be loaded into it and stored for further cooling. If conditions permit, the stringer is sometimes taken directly to the IFD for dismantling. However, it is normal practice for the machine to revisit the buffer store at a later date, picking up the stringer en route to the IFD, whereupon the machine valve is opened and the machine depressurised, but with the IFD isolated, When the pressure approaches atmospheric, suction pumps are automatically started which draw air down through the fuel stack from an air inlet in the machine, thereby maintaining cooling flow. Eventually the IFD is de-isolated, the air How is re-directed down through the cell and its dismantling tube, and the stringer lowered by the charge machine into the cell and onto the dismantling platform (in its fully raised position), enabling the tie-bar end fittings to be removed and disposed of to the debris vaults. The machine grab and IFD dismantling platform are then lowered in ‘dual’, i. e., simultaneously and at the same speed, so that the fuel stack descends into the dismantling tube. When this operation is complete the machine lifts the stringer plug unit and tie-bar away from the fuel stack, the tie-bar is cut into pieces for disposal and the plug unit removed to an active maintenance facility for essential maintenance prior to re-use. The IFD dismantling platform is then raised and, starting with the number eight, each element is picked up and lowered down one of the pond discharge tubes. Cooling at this time is provided by air which is fan-blown up through the discharge tube, then joining the main cooling flow being drawn down into the dismantling tube by the suction pumps; However, if the fuel is required for PIE, each element is bottled at the IFD cell bottling station, and following pressurisation with nitrogen to about 0.8 bar, each bottle is despatched to the ponds via the discharge tube in the normal way. On arrival in the pond reception tube, each bottled or unbottled element receives a 20-minute wash with recycled pond — water, following which the reception tube is stood upright and the dement or bottle lifted out by the element manipulator and placed in a skip. Following a delay to accommodate cooling and a reduction in radiation levels, a full skip is loaded to a flask for transportation to BNFL in the case of standard fuel, or the Atomic Energy Establishment at Winfrith when PIE is required.