Patent GB-1518292-A (GB)
Title: Nuclear reactor fuel sub-assemblies
IPC: G21C3/322
Filed: 1975-05-07
Abstract: 1518292 Fuel elements UNITED KING- DOM ATOMIC ENERGY AUTHORITY 30 April 1976 [7 May 1975] 19171/75 Heading G6C A reactor fuel sub-assembly has in a region which is upstream of the fuel pins with respect to coolant flow, a variable flow restriction means comprising two relatively rotatable coaxial tubular members 1, 2, the outer member 1 forming an extension to the fuel pin-containing wrapper of the sub-assembly and the inner member 2 forming an extension to a spike for engaging a fuel assembly supporting diagrid. The inner member 2 houses a series of spaced discs 5 having apertures 6 for coolant flow. The apertures 6 are arranged in three sectors, Fig. 2, those in each sector being of different diameter to those in the other sectors. The discs 5 are mounted on a central support tube 7 and are separated by spacers 8 to define three discrete axially extending flow passages each including groups of apertures 6. An apertured selector valve seating plate 10 is secured to the lower end of the inner member 2 whilst a selector valve member 13 is secured to the lower end of a control shaft 11. The shaft 11 is carried by a spider 12 secured to the outer member 1 and is keyed to the spider 12 by means (not shown) to prevent relative rotation between shaft and spider whilst allowing relative longitudinal sliding motion. Valve member 13 comprises an aperture in one 120 degree sector only. Coolant flow through the apertured discs 5 is restricted to one of the three flow passages as determined by the position of the aperture in the selector valve member 13. Prior to insertion of the fuel sub-assembly in the reactor core, a passage having a restriction to coolant flow appropriate to the position to be occupied by the sub-assembly is selected by rotating the inner member 2 relative to the outer member 1. Subsequently, after operation of the reactor when the fuel is partly burned-up and/or when its position in the core is to be changed, the selector valve member 13 can be repositioned to increase or decrease the restriction to coolant flow. Precise positioning is achieved by means of a spring-loaded plunger 16 housed in a cavity within a diffuser block 15 engaging one of three equi-spaced indents 17 in the inner surface of the inner member 2. In one reactor construction, the fuel subassemblies are arranged in groups, those of each group being biased to lean towards a central support member of the group. At least one of the support members in the outer region of the core is provided with a socket disposed above the core for engaging the lower end of a subassembly thereby providing a valve adjusting station. A fuel sub-assembly may be lifted from the diagrid by the reactor refuelling machine and transported to the valve adjusting station where the lower end of the sub-assembly is engaged in the socket. The upper end of the sub-assembly can then be rotated relative to the lower end by manipulation of the refuelling machine and the readjusted sub-assembly returned to the diagrid without need of withdrawal from the reactor vault. In an alternative reactor construction, the fuel sub-assemblies are restrained in side-byside abutment by a circumferential restraint girdle. The lower end regions of the inner tubular members and the upper regions of the wrappers are provided with complementarily engageable pads so that in order to vary the coolant flow rate through a selected fuel subassembly, this sub-assembly is raised by the refuelling machine to engage in keying manner the pads on its inner tubular member with the complementary pads on the wrappers of adjacent sub-assemblies and the wrapper of the raised sub-assembly is then rotated relative to the tubular member.