Patent GB-970848-A (GB)
Title: Nuclear reactor subassembly
IPC: G21C3/16
Filed: 1963-09-03
Abstract: 970,848. Nuclear reactors. UNITED STATES ATOMIC ENERGY COMMISSION. Sept. 3, 1963 [Oct. 22, 1962 No.34697/63. Heading G6C. A fuel element-control element subassembly for a nuclear reactor comprises a cluster of parallel elongated fuel elements each containing a plurality of coolant pressure tubes and a control element consisting of a grid of neutron-absorbing material encompassing and interpenetrating the cluster so as to surround each individual fuel element. As shown in Figs. 6 and 7, each fuel element 44 comprises a hexagonal Zircaloy jacket 56 with end caps 57 at the lower end of which is a hollow graphite cup 58 having a ZrO 2  spacer 59 across its open end. At the mid-point of the jacket 56 is another ZrO z  spacer 60 and the volume between the two spacers 59, 60 constitutes a fuel zone 61 which includes an active portion 62 of enriched UO a  and an annular thermal insulation zone 63 of ZrO 2  slabs. The upper half of the jacket 56 is filled with ZrO 2  powder 64. An inlet pipe 48 and an outlet pipe 50 penetrate the upper end cap 57, the inlet pipe 48 terminating in an annular distributing chamber 65 in the ZrO 2  powder 64 and the outlet pipe 50 passing through the chamber 65 to a hemispherical collecting chamber 66. A plurality of inlet pressure tubes 67 extends through the fuel zone 61 from the chamber 65 to a hemispherical return chamber 68 in the graphite cup 58 and surrounds a plurality of outlet pressure tubes 69 extending between the chamber 68 and the chamber 66. Pressure tubes 67, 69 and chambers 65, 66, 68 are fabricated as a bundle and placed in the jacket 56 which is then filled with UO 2  powder. The powder is compacted about the tube bundle by vibration compaction, a density of 90 to 92% of the theoretical density being obtained. The fuel elements 44 are assembled in clusters of seven, each of the seven elements being bolted to a plate 53, these in turn being bolted to a core support grid 30. Each cluster 39 is provided with a header assembly 45 (Fig. 2) which includes an annular inlet header 46, a single header inlet pipe 47, seven fuel element inlet pipes 48 leading from the header 46 to the fuel elements 44, a centrally located outlet header 49, seven fuel element outlet pipes 50 leading from the fuel elements 44 to the outlet header 49, and a single header outlet pipe 51. As shown in Fig. 13, the control element 40 includes seven hexagonal cells 73, each surrounding one of the fuel elements 44 of a cluster and having an upper poison section of stainless steel and a lower follower section of aluminium, the thickness of the materials from which the element is made being sufficient to withstand any conceivable effect caused by the rupture of a pressure tube. Each fuel element 44 is enclosed by either the poison section of the follower section of the control element all the time it is in the reactor. In Fig. 1 is shown a supercriticalpressure, water-cooled nuclear power reactor, incorporating a number of the above-described subassemblies, in which the core 20 is housed within a cylindrical shell 22 having a movable bottom plate 23 and is immersed in a pool of water 28 serving as moderator. A pool of water 32 serving as reflector shield is disposed in a concrete enclosure 24 exterior to the core 20. The coolant water enters the reactor through inlet duct 37A and inlet pipe 47 at 540. 3‹F. and 4,500 psig, the temperature being raised to 805‹F. in a first pass through the reactor and to 1, 050‹F. in a second pass. Following the second pass, the supercritical pressure fluid flows to reheat heat exchangers where heat is transferred to exhaust steam from reheat units of the reactor turbine, its temperature falling to 821‹F. The fluid is returned to the reactor for final heating to 1, 050‹F. before entering the turbine at 3,500 psig. (The critical temperature of water is 705‹F. and the critical pressure 3,193 psig). The control elements 40 are raised and lowered by means of motors 41 operating through cables 42. To discharge a fuel element cluster 39, the associated control element 40 is withdrawn to its uppermost position where handles extend above the core support grid 30 and a retaining bar is inserted through the handles (Fig. 5, not shown) whereupon the cable 42 can be disconnected and the control element 40 rests on the fuel element cluster 39. The coolant pipes 47 and 51 are cut and temporarily plugged as by freezing and the fuel element cluster 39 is then detached from the grid 30, lowered into a transverse conveyer 34 under the core along with the control element 40, and moved to a storage rack (not shown) adjacent to the reactor for cooling. The control element 40 is left in the conveyer 34 ready to receive a new fuel element cluster 39.