GB1186162A - Improvements in or relating to Nuclear Reactor Fuel Elements. - Google Patents

Abstract

1,186,162. Fuel elements. UNITED KING- DOM ATOMIC ENERGY AUTHORITY. 9 May, 1967 [26 April, 1966], No. 18150/66. Heading G6C. A fuel element comprises a cluster of fuel rods with a support grid carrying the fuel rods and at least one spacer grid, the cluster being disposed within a sleeve of structural moderating material such as graphite, the support grid and the or each spacer grid each being secured to the sleeve by an annular peripheral framework of the grid fitting in an annular recess in the interior wall of the sleeve, releasable locking means being provided retaining the annular framework in the annular recess whereby longitudinal movement of the grid in either direction in the sleeve is prevented. As shown in Fig. 1, a cluster-type fuel element consists of a structural sleeve 1 of graphite housing a plurality of fuel rods 2 supported at their lower ends in a support grid 3 and spaced at their upper ends and intermediately by spacer grids 4, 5. Each grid consists of a plurality of cells fabricated from stainless steel strip bent into arcuate edge-on form and providing webs serving to join the cells. In the case of the support grid 3, the peripheral webs 14 and additional radial supports 15 are secured at their outer ends to an annular frame 17 which has twelve equiangularly spaced upstanding lugs 18 and a short annular skirt 19 extending below the level of the lower edges of the cells 13. A lower end, non-ribbed portion of each fuel rod sheath 6 engages within the respective cell 13, a bottom circumferential flange 20 of the sheath 6 engaging the upper edge of the cell 13. A split ring 21 engages with a recess 22 in each sheath 6, the upper edge of the ring 21 engaging the lower edge of the cell 13, and is retained in position by a spring ring 23. The cells 28 of the spacer grids 4, 5 each have four equi-angularly spaced inward projections 29 which contact the fuel rods 2 at the reactor operating temperature. The top spacer grid 4 is secured to the sleeve 1 by disposing in an annular recess 33 formed in the interior surface of this sleeve a member 34 channel-section in profile but having an annular lip 35 at its upper edge and an annular lip 36 at its lower edge. Keys 42 on the underside of the member 34 engage corresponding keyways 43 in the sleeve 1 and prevent rotation of the member 34. The upper wall and lip 35 of the member 34 are interrupted by a slot 40. The skirt 32 of the frame 26 of the grid 4 is placed within the annular lip 36 and the grid 4 turned until one of its twelve upstanding lugs 31 registers with the slot 40. The lug is then deformed outwardly until its upper edge lies within the member 34. This procedure is repeated for each of the lugs 31 and in the final position of the grid 4, none of the lugs 31 is in register with slot 40. This position is retained by a pin 44 (shown in relation to grid 3). The support grid 3 and the intermediate spacer grid 5 are secured to the sleeve 1 in a similar manner. Each fuel rod 2 consists of a stainless steel sheath 6 enclosing a stack of cylindrical fissile pellets 7, each having a recess 8 at each end forming voids for accommodating gaseous fission products and also for reducing distortion and cracking due to differential expansion as a result of a radial temperature gradient. A pellet 12 of insulating material, e.g. alumina, is interposed between each end cap 10 and the end pellet 7 of the stack.