Patent Number: 054250714
Section: description

Referring to FIG. 1, cylindrical fuel pellets 1 which are made of a mixed oxide fuel comprising uranium dioxide and plutonium dioxide are formed into rows within a containment area 2 which is bounded by radiation shielding walls 3. The fuel pellets are subsequently loaded into a fuel pin cladding tube 4 located outside the containment area to form a nuclear fuel pin. Within the containment area 2 the pellets 1 are arranged in end-to-end relationship in a trough provided in a supporting tray 5. The tray 5 has a number of parallel troughs in which several rows of fuel pellets are formed. From the tray 5 a row of pellets 1 is pushed onto a weighing channel 6 by fingers 7 associated with a transfer mechanism 8. A feed stop 9 restrains the leading pellet until a stack containing the required number of pellets is formed. When the stack is of the required length, as determined by weighing equipment associated with the weighing channel, it is forwarded to the vibratory table 10. This may be achieved by vibrating the weighing channel 6. Outside the containment area 2 a cladding tube 4, fitted with a bottom end plug 11, is positioned in alignment with te pellet stack. Pressed on the end of the cladding tube is a disposable sleeve 12, preferably made of a plastics material, which is slidably located in a resilient sphincter seal 13. The sphincter seal 13 is attached to a mounting plate 14 which is resiliently connected to the containment shielding wall 3 by a diaphragm 15. The sleeve 12 extends through a central opening 16 in the mounting plate with an end surface 17 located against a retractable stop plate 18. The sphincter seal is provided with several rubber rings 19 each of which is divided into sectors. As the sleeve 12 is passed through the centre of the seal the rings 19 are deformed but maintain positive sealing contact by pressing against the sleeve. Clamps 20 are operated to lightly hold the cladding tube 4 in position and an end stop 21 is moved over the bottom end plug 11 to prevent longitudinal movement of the tube. In operation, the vibratory table 10 and the cladding tube 4 are subjected to vibration by connection to a bi-modal vibrator (not shown). This causes the stack of pellets to migrate along a guide tube 22 integrally formed with the vibratory table 10. The end of the tube 22 is in the form of a semi-circular passage 23 which guides the pelets through the sleeve 12 and introduces them into the cladding tube 4. Advantageously, the guide tube 22 is made of a material which is compatible with the fuel pellets, suitable examples being stainless steel or a zirconium alloy. The pellets migrate along the cladding tube 4 until the leading pellet encounters the bottom end plug 11. The arrangement described for inserting the fuel pellets into the cladding tube is by way of example only and other systems incorporating, for example, soft handling surfaces or pelelt inserting rams, can also be used. After retractiing and parking the vibratory table 10 and guide tube 22, a disposable plug carrier 24 (see FIG. 2c) is used to insert a top end plug 25 and plenum spring 26 into the end of the cladding tube 4. Preferably the plug carrier 24 is made of a plastics material. Reaction plates 27 are operated to bring them behind the sleeve 12, the clamps 20 are released and the end stop 21 is removed. The cladding tube 4 is withdrawn while the sleeve 12 and plug carrier 24 are restrained by the reaction plates so that they remain trapped in the sphincter seal 13. The withdrawn cladding tube 4 passes through a girth welder 28 which makes a circumferential weld to join the top end plug 25 to the cladding tube. Further withdrawal of the cladding tube brings the top end plug 25 into a helium filling and welding device 29 which injects helium into the tube through a fine hole 30 (see FIG. 2c) in the plug and then fills the hole with weld material. The pellet loading sequence will now be described with particular reference to FIGS. 2a, 2b, 2c, 2d and 2e. FIG. 2a shows a cladding tube 4 fitted with a bottom end plug 11 at one end and with a disposable sleeve 12 mounted on the other end. At one end the sleeve has a reduced diameter seating 31 which seats as a press fit on the end of the cladding tube 4. An internal end face 32 of the seating 31 coincides with an end surface 33 of the cladding tube. Thus the external end surface of the cladding tube 4 is covered by the seating 31 and is thereby protected from radioactive contamination by the environment in the containment area 2. As the cladding tube 4 moves in the direction of arrow A the sleeve 12 engages a sleeve 12a, enclosing a plug carrier 24a, which have been retained in the sphincter seal 13 following the preceding pellet loading procedure. With the stop plate 18 in a retracted position, the new sleeve 12 pushes the sleeve and plug carrier assembly out of the sphincter seal 13 into the containment area 2 (FIG. 2b). Since the new sleeve 12 enters the sphincter seal 13 before the previously used sleeve and plug carrier assembly is removed from the seal, leakage of radioactive substances from the containment area is prevented. The stop plate 18 is then lowered to provide a location for end surface 17 of te sleeve 12. FIG. 2c shows the semi-circular end passage 23 of guide tube 22 extending into the sleeve 12. Upon vibration of the vibratory table 10 ad cladding tube 4, as previously described, the pellets 1 migrate along the semi-circular passage 23 and then down the cladding tube until the leading pellet encounters the bottom end plug 11. When the stack of pellets has been loaded into the cladding tube 4 the table 10 and guide tube 22 are retracted. The top end plug 25 together with a plenum spring 26 are then pressed into the end of the cladding tube 4 using the disposable plug carrier 24. The plug carrier is of cylindrical shape and ahs an outer diameter such that it is a sliding fit within the sleeve 12. A blind hole 35, drilled along the longitudinal axis of the plug carrier 24, has an increased diameter recess 36 at its forward end. The depth of the recess is sufficient to completely receive a head portion 37 of the top plug 25. It is particularly important to ensure that the end surface 38 and peripheral surface 39 of the head portion 37 are completely covered so as to protect them from possible contamination. At the leading end of the top plug 25 is a shoulder 40 which provides a seating for the plenum spring 26. After retracting and parking the vibratory table 10 and guide tube 22 a remotely controlled manipulator, not shown, provided in the containment area, may be used to push the plug carrier 24 through the sleeve 12 and to press fit the top plug 25 into the end of the cladding tube 4. During installation of the top plug the end stop 21 prevents axial movement of the cladding tube. The plenum spring 26 serves to restrain the fuel pellets during subsequent handling and transportation of the fuel pin so that they do not become damaged. As seen in FIG. 2d, the reaction plates 27 are moved inwardly to locate behind the sleeve 12 while clamps 20 are released and the end stop 21 is retracted clear of the bottom plug 11. As the fuel pin is withdrawn, the sleeve 12 and plug carrier 24 remain located in the sphincter seal 13 (FIG. 2e). The fuel pin then passes through the girth welder 28 for end plug welding and the helium filling and welding device 29, as previously described, to complete the fuel pin assembly. The sequence of operations is then repeated for the next fuel pin. It will be seen that during the pellet loading operation none of the external surfaces of the cladding tube or top end plug is exposed to the radioactive environment existing in the containment area. This eliminates the need to subject the fuel pin to a costly decontamination process.