Patent GB-1397723-A (GB)
Title: Guide structure for control elements
IPC: G21C1/08
Filed: 1973-06-28
Abstract: 1397723 Controlling reactors; emerging core cooling systems COMBUSTION ENG Inc 28 June 1973 [28 June 1972] 30898/73 Heading G6C A guide structure for aligning fuel assemblies in a pressurized water reactor core, and for guiding a plurality of control elements for telescoping movement within guide channels in the fuel assemblies, comprises two spaced tube sheets 80, 82 rigidly connected by a plurality of hollow tubes 84 which extend between them and supported by a flange 87 of a support barrel 86 on a lip 52 of the reactor vessel 22, hanging within a core barrel 48 which is itself supported by a flange 50. Each of the hollow tubes 84 is in alignment with a guide tube 32 in a fuel assembly. The primary or shutdown control element assemblies 56, 58 each comprise a plurality of high worth control elements 102 gauged together by means of a yoke 96, 98 resectively connected to a drive rod extension 62 by couplings 100. The assembly 56 is 4-fingered and the assembly 58 is 8-fingered. The drive mechanisms for these assemblies are electromagnetic linear motion drive devices which move the control elements 102 in incremental steps into and out of the reactor core. The fine grain control element assemblies 60 each comprise a single low worth control element (104), (Fig. 6, not shown), completely and individually enshrouded above the reactor core by shrouds or tubing 92 coupled by piping 64 through a single nozzle in the vessel head with an hydraulic actuator which maintains the control elements (104) in either a fully inserted or fully retracted position. The water coolant enters the reactor vessel 22 through inlet nozzles (75), (Fig. 1, not shown), and passes down between the vessel 22 and the core barrel 48, then up through the reactor core into an outlet plenum 74, and finally outwardly through openings 168 in the core barrel 48 and outlet nozzles 76. The outlet plenum 74 is a high cross flow region wherein the coolant is turned to pass through the outlet nozzles 76 and the control elements 102, 104 are shielded from the adverse effects of this high cross flow by the tubes 84 each of which enshrouds one of the control elements. The yokes 96, 98 always remain above the upper tube sheet 82 and are thus never subjected to this high cross flow. Protective shrouds 88, 90 for the gauged control element assemblies 56, 58 are provided above the upper tube sheet 82 and serve to prevent outward bowing of the control elements 102 which might otherwise interfere with the effective insertion of adjacent control element assemblies in the event of an emergency. The fuel assemblies have hollow alignment posts 38 in registry with the upper ends of the guide tubes 32 and these posts 38 enter the lower ends of the tubes 84. The guide structure can be adapted to provide an emergency core cooling system which provides sufficient coolant flow to remove the decay heat should a break occur in the normal coolant system. Emergency core coolant is introduced into an inactive plenum 78 above the upper tube sheet 82 and, as shown in Fig. 10, passes into the outlet plenum 74 via pipes 196 to aid in condensing steam which will then fall back as water into the core 26. Tubes 194 similar to the tubes 84 by-pass the plenum 74 and serve to spray the coolant directly on to the fuel assemblies 28 and their fuel elements 30. Coolant also passes through the tubes 84 and into the guide tubes 32 of the fuel assemblies 28, these latter tubes being provided with holes 200 in the lower region of the core 26 whereby the coolant by-passes the plenum 74 and a substantial portion of the core 26. Steam in the outlet plenum 74 is released into the inactive plenum 78 via pipes 198 where it is condensed by the emergency coolant. A system is also described (Fig. 11, not shown) which by using pressurized emergency coolant can overcome any possible steam blockage occurring in the upper region of the core.