Patent GB-1171383-A (GB)
Title: Nuclear Reactor Fuel Bundle
Inventor: BIGGE WILLIAM BRENDAN
IPC: G21C3/334
Filed: 1967-01-02
Abstract: 1,171,383. Fuel elements. GENERAL ELECTRIC CO. 2 Jan., 1967 [17 Jan., 1966 (2)], No. 170/67. Heading G6C. A fuel bundle comprises a plurality of fuel rods with one or more demountable rods positioned in one or more locations in the fuel bundle along a line of rod positions which intersects the longitudinal axis of the fuel bundle and the extension of which intersects the longitudinal axis of an adjacent control rod. In addition, demountable rods may be located symmetrically on each side of the line. By the positioning of such demountable rods in the fuel bundle, the strength (i.e. neutron absorption effectiveness) of the adjacent control rod and/or the reactivity of the fuel bundle can be adjusted. For example, the strength of the adjacent control-rod is increased by placing an absorber rod on the diagonal of a square fuel bundle in the corner opposite the control-rod. This has the effect of "tilting" the neutron flux pattern about the centre of the fuel bundle, i.e. lowering it adjacent the absorber rod and raising it adjacent the control rod. It has also been found possible to increase the overall reactivity of the fuel bundle and not substantially effect the adjacent control-rod strength by appropriate positioning of demountable rods containing enriched fuel material or, alternatively, to decrease the overall reactivity by appropriate positioning of demountable rods containing either a neutron absorber or a lower enriched fuel than that replaced. The upper tie plate of such a fuel bundle comprises six demountable rod openings (one of which is shown in Fig. 6) each formed by an elongated cylindrical member 103 which includes a slot 104 that extends the entire length of the member 103 and a notch 106 disposed opposite the slot 104 and extending upward a short distance from the bottom of member 103. The demountable rods (Fig. 8) each include an elongated cylindrical region 110 which contains the absorber or fuel material and which has a bottom nose plug 112 for insertion into a fuel rod receptacle of the lower tie plate of the fuel bundle. The upper end plug 120 has a longitudinally extending shank 122 surrounded by a compression spring 132 and terminating in a collar 129. A cylindrical locking sleeve 124 surrounds the shank 122 between the collar 129 and the spring 132. A pin 126 extends radially outward from the lower end of the sleeve 124 and is inserted through the slot 104 when the rod is introduced into the fuel bundle, the rod being locked in position by rotating it until the pin 126 enters the notch 106. A boiling water reactor (Fig. 1, not shown) comprises a plurality of the above-described fuel bundles (20) mounted in groups of four in sockets at the upper ends of control rod guide tubes (24) supported by a bottom grid plate (26). Cruciform control rods (32) extend through the guide tubes (24) and between the fuel bundles (20) of each group. A shroud (34) is mounted coaxially within the pressure vessel (10) to provide a downcomer annulus (36) between the shroud and the vessel wall. Exhaust steam from a turbine (46) is pumped as feedwater to an annular sparger (56) thus mixing it with water from steam separators (40) and steam dryers (42). A recirculation pump (38) transfers water from the lower end of downcomer annulus (36) and returns it to a supply chamber (30) beneath the bottom grid plate (26). Because of this pressure increase, the water flows upwardly from the supply chamber (30) through the fuel bundles (20) and a plenum (27) to the steam separators (40) and steam dryers (42).