GB844765A - Improvements in or relating to nuclear reactors - Google Patents

Abstract

844,765. Nuclear reactors ; handling materials in radioactive environment. GENERAL ELECTRIC CO. Ltd. Sept. 26, 1957, No. 29608/56. Class 39(4). In a reactor having within the pressure vessel a graphite core provided with channels containing fuel elements cooled by the flow of CO 2 , a travelling auxiliary vessel is provided capable of attachment to the pressure vessel at any one of a number of positions so that the interior conditions of the two vessels may be matched and communication made therebetween through a passage, there being housed within the auxiliary vessel a plurality of separate servicing mechanisms for carrying out a number of different service operations upon the reactor. The invention is particularly intended for use with a reactor having vertical fuel channels refuelled from beneath the pressure vessel as disclosed in Specifications 789,022 and 844,764, the auxiliary vessel being situated above the top of the reactor and serving primarily for the removal and replacement of control rods. A shown, Fig. 2, the auxiliary vessel 1 is movable along a transverse gantry 7 itself movable along rails 8 so that its axis may be aligned with any one of a number of standpipes 3 extending through the concrete shielding 2 and a gastight connection established between the vessel and a pipe. The vessel 1 contains six servicing mechanisms I ... VI, Fig. 4, arranged adjacent its inner periphery, when inoperative, but movable by drive units 13 towards the axis of the vessel for a servicing operation, as shown for mechanism VI in Figs. 5, 6. Mechanism VI serves for withdrawal or replacement of control rods and comprises a lead control rod shield 28 movable by gravity away from the wall of vessel 1 on a parallel link motion 25, 26 when permitted by cables running on lifting drums 31 driven by one of the units 13. Also movable with the link motion 25, 26 is a pair of grabs 33, 39 separately controlled by further units 13 through lifting cables 34, 38. A control rod 49, Fig. 7A, normally occupies a channel 48 in the reactor core 46 adjacent a fuel channel 47 and is surrounded by a distance tube 52 resting on the core and serving to prevent the coolant gas flow from knocking the rod against its channel. The rod suspension cable 51 extends through a boron shield 45, heat resistant shell 44 and the pressure vessel 43 to a motor-driven winch drum 54 located within the top of a standpipe 3. The top 57 of the standpipe is detachable by a grab head 59 which is engaged by the grab 33 after the link motion 25, 26 has been moved into the Fig. 5 position, lifting of the top 57 also causing removal of the drum 54, its motor 56 and a concrete plug 53 and these parts are raised together to the position shown in Fig. 5. The link motion is next moved to the Fig. 6 position and a pulley 64 is actuated by a piston and cylinder arrangement 65 to move the cable 51 slightly off the axis of the vessel 1 so that the grab 39 may be lowered through the shield 28 to engage the cable in the region of the pulley, the grab then being retracted to withdraw the rod 49 into the shield as shown in Fig. 6. A travelling gantry 15 serves to remove the cap 18 of the vessel 1 and handle the control rods removed from the core. Thus after each removal of the cap 18 the vessel 1 must be repressurised with CO 2 before another servicing operation can be performed. Mechanism II is identical with mechanism VI so that an old control rod may be removed and a new one inserted in a single sequence of operations. Mechanism I comprises telescopic tubes, Figs. 10, 11 (not shown), which may carry a ramming attachment, Fig. 12B (not shown), for ramming the top of the uppermost fuel element in a channel 47 ; alternatively, a television camera may be carried, Fig. 12C (not shown). Mechanism III, Figs. 8, 9 (not shown), serves for the recovery of a control rod which has become detached from its cable 51 and includes an electrically or gas operated grab, Fig. 7C (not shown), suspended on a cable and adapted to grip the rod. Mechanism IV, which is similar to I, serves for the insertion of a thermocouple fuel element in a channel 47, the thermocouple lead extending to the top of the standpipe 3, Fig. 13 (not shown), this lead being cut by shears lowered by the mechanism when it is desired to discharge the thermocouple fuel element through the base of the core 46. Mechanism V is similar to mechanism III and serves for the removal of a distance tube 52 by a grab, Fig. 7B (not shown), this tube needing to be removed prior to the operation of mechanisms I, III, IV. Mechanism I may also comprise a grab which may be lowered right through the core 46 to retrieve a fuel element lodged in the refuelling apparatus of the above-mentioned specifications. The Provisional Specification describes a slightly modified arrangement with reference to the Figures.