Patent GB-928394-A (GB)
Title: Improvements in or relating to fuel element assemblies
Inventor: EMERSON DAVID COLIN
IPC: G21C3/14
Filed: 1959-10-21
Abstract: 928,394. Nuclear fuel elements. UNITED KINGDOM ATOMIC ENERGY AUTHORITY. Oct. 14, 1960 [Oct. 21, 1959], No. 35661/59. Class 39 (4). A nuclear fuel assembly 1 comprises a tubular coolant-conducting housing 2 along the length of which at spaced intervals are disposed transverse grid members 11 formed by intersecting webs 10 disposed edge-wise on to the coolant flow to provide a lattice of aligned apertures, sheathed fuel elements 3 extending through the apertures with clearance and being supported from the top grid member 11 by support members 7, 8, 9 and 33 (Fig. 4), the dimensions of these support members being such as to permit coolant flow through the top grid member, and lateral projections 13 extending from the sides of the webs 10 to locate the fuel elements in the apertures. Each fuel element 3 comprises a stack of hollow cylinders 26 reinforced by a heat-insulating filling 27 and enclosed in a thin sheath 28. Insulating discs 31 at each end of the stack space end caps 32 from the cylinders 26. Preferably, the cylinders 26 are of UO 2 , the fillings 27 and discs 31 of MgO and the end caps 32 and sheath 28 of stainless steel. The coolant flows in the downward direction and each fuel element 3 is provided with a streamlined cap 38 to smooth out the coolant flow with a minimum of turbulence. The upper end cap 32 of the central fuel element has three equi-spaced projections 33 which bear upon the upper edges of the central webs 10 of the upper grid 11. The upper end caps 32 of the other fuel elements which are arranged in three concentric groups 4, 5 and 6 have shoulder portions 34 (Fig. 1) which engage with and are welded to apertures 35 in their respective support plates 7, 8 and 9. By means of these support plates and the projections 33, all the fuel elements 3 are suspended from the upper edges of the webs 10 of the upper grid 11. A number of these fuel assemblies 1 are connected end-to-end as shown in Fig. 6 by means of external splines 18 on a ring 17 at the lower end of each housing 2 being inserted into recesses 16 in internal splines 15 on a ring 14 at the upper end of each housing 2. This arrangement allows a measure of articulation between the housings 2 which is advantageous when loading the assemblies into reactors in which the fuel element channels have become " bowed." The upper fuel assembly 1 is connected to a support 19 in a similar manner by means of splines 15, 21 (Fig. 1), the upper end of the support 19 being formed with equi-spaced lugs 22. When the interconnected fuel assemblies 1 are inserted into a reactor fuel channel, the lugs 22 rest upon an annular support in the upper end of the channel. The spacing of the fuel elements 3 is such that the coolant free-flow cross-section per fuel element increases progressively from the centre of the lattice to the outer ring of elements, the neutron capture rate (and thus the heat generation rate) of the fuel elements 3 increasing in a similar manner and the transfer of heat therefore being substantially uniform across the lattice.