Patent Number: 063273209
Section: description

DESCRIPTION OF A PREFERRED EMBODIMENT An embodiment of the present invention will now be described, by way of example only, and with reference to the accompanying drawing which is a diagrammatic illustration of apparatus for loading nuclear fuel pellets into a fuel tube. Referring to the accompanying drawing, apparatus in accordance with the present invention include a stand 1 on which is located a fuel pellet insertion rig 3, the rig 3 standing on screw-threaded, adjustable, front and rear legs 5. The main body of rig 3 includes a front body portion in the form of a perspex pellet guide 7 of cylindrical shape. Attached to the rear end of pellet guide 7 is a rear body portion in the form of an aluminium fuel tube guide 9. Tube guide 9 includes a forward section 11 of circular cylindrical form with a diameter which is the same as that of pellet guide 7. Extending rearwardly from section 11 is a cylindrical section 13 of relatively reduced diameter. Pellet guide 7 has a central longitudinal hole 15 extending therethrough. In the forward part of guide 7, hole 15 is a relatively narrow bore 17 with a diameter just greater than that of the pellets 19 to be handled by the apparatus. Bore 17 includes a chamfered lead-in 21 at its front entrance. About halfway along hole 15, bore 17 opens into a counterbored chamber 23 which provides the rearward section of hole 15. Tube guide 9 also has a central, longitudinal hole or bore 25 extending therethrough. This bore 25 is of a diameter just sufficient to accommodate a fuel tube 27, as illustrated in the accompanying drawing. Bore 25 of tube guide 9 is axially aligned with hole 15 of pellet guide 7 so that a tube 27 may be fed through tube guide 9 and into chamber 23 of pellet guide 7, again as illustrated in the accompanying drawing. In use the tube 27 is inserted into the rig 3 until its open end 29 reaches a position which is spaced from but less than one pellet length from the position where chamber 23 connects with bore 15. This positioning of tube 27 within rig 3 is accomplished by means of the provision of a locking collar (not shown) on the outside diameter of tube 27, which locking collar abuts against the external rearward face of tube guide 9. As indicated above, the bore 25 through tube guide 9 is a very close fit onto the outside diameter of a tube 27. This provides accurate concentric and axial location which also ensures a partial air seal. The above-mentioned gap between the open end 29 of tube 27 and the rearward end of bore 17 should be less than the length of a fuel pellet. Typically the gap may be of the order of 3 mm. The alignment of bore 17 with bore 25 is such that, if a pellet were located in bore 17, and there is no airflow within the apparatus, then the centre lines of the pellet 19 and the fuel tube 27 would be vertically offset by a distance of about 1.2 mm. It will be appreciated, therefore, that, unless there is some lifting or elevation of the pellets during their passage along the delivery tube when the rig 3 is in operation, the pellets will not enter the fuel tube 27. To provide air flow through the rig 3, three equi-spaced radially extending holes 31 extend from chamber 23 to the outside surface of pellet guide 7 close to the rearward end thereof. The holes are positioned so that they enter the chamber 23 at a minimum distance of 20 mm from the open end of fuel tube 27. Air lines 33 are engaged with holes 31 and, through appropriate interconnections (not shown), a common air line is connected into a venturi device (not shown). With process air in the air line at a pressure of 6 bar, air in the chamber 23 is withdrawn from rig 7 along air lines 33. As a result, an air flow is produced which travels along the delivery bore 17, into chamber 23 and exits along air lines 33 and through the venturi. When a pellet 19 is inserted into bore 17, it creates a pressure drop within the chamber 23 by reducing the flow area in bore 17. This pressure drop can be controlled by regulating the process air pressure prior to its entry into the venturi. As a column of fuel pellets 19 is drawn towards the open end 29 of fuel tube 27, the pellets achieve a momentum sufficient to jump the gap and enter the fuel tube 27. The reduced air pressure at the open end 29 of the fuel tube 27 allows several of the pellets 19 to be automatically drawn into the bore 17. The remainder of the column is easily pushed in using a very low load. This load increases as the number of pellets within fuel tube 27 increases. Although the fuel tube 27 has an end cap welded onto its opposite end, it has been surprisingly found that fuel pellets 19 will enter the open end of the fuel tube in the above described rig 3. It would seem that the air drawn into the chamber 23 diverges symmetrically around the end of the fuel tube 27, thus creating a pressure drop within that fuel tube. Pellets 19 are transported aerodynamically towards, and into, the open end of the fuel tube 27. If pellets were inserted into the fuel tube under normal atmospheric conditions, the pellets would push against a trapped column of air within the fuel tube. Since the opposition end of the tube is sealed, the air it contains would act as a pneumatic spring, resisting the entry of the pellets. When a pellet 19 enters the fuel tube 27, normal air flow conditions would seem to be re-established, and the momentum of the pellet alone is sufficient to ensure that it enters the tube. Ambient air then refills the fuel tube. Fine adjustment of the fuel pellet/fuel tube concentricity is achieved by the use of three grub screws in the perspex pellet guide 7.