Patent GB-1007206-A (GB)
Title: Nuclear reactor
Inventor: WOODS WILLIAM JOHN
IPC: G21C1/20
Filed: 1963-10-11
Abstract: 1,007,206. Nuclear reactors. UNITED KINGDOM ATOMIC ENERGY AUTHORITY. Sept. 22, 1964 [Oct. 11, 1963], No. 40169/63. Addition to 983,596. Heading G6C. In a liquid-moderated nuclear reactor-in which the moderator acts as a coolant from which vapour is generated in the reactor, a core module comprises a number of elongated fuel elements clustered in parallel spaced relationship around a filler element 30 having no fissile content and being substantially of a moderator material with a neutron slowing-down power less than that of the coolant, a shroud tube 31 encircling the filler element 30 with a clearance into which a hollow neutron-absorbing control element 25, with an open end foremost, is insertable from an open end of the shroud tube 31, the other end of which tube has only a restricted opening for coolant flow. In the module described, each fuel element comprises five fuel rods 13 arranged around a central sheathed fin 14 of a burnable poison inside a tube 12 of a material of low neutron captive cross-section, e.g. a Zr-base alloy such as Zr with 2¢% by weight of Ni, and is suspended by a bar 15 secured to its upper end. This particular module is for a boiling water reactor in which pressurized light water is circulated through the tubes 12 as a primary coolant. The tubes 12 are connected together in pairs by U-bend pipes at their upper ends, one tube in each pair being a riser by virtue of its connection at the lower end to a cylindrical inlet header 19 and the other being a downcomer by virtue of its connection to a bulbous outlet header 20 carried beneath the header 19. A secondary coolant, which is also light water, passes upwardly through the grouped modules constituting the reactor core, externally of the tubes 12 in heat transfer relationship through their walls with the primary coolant and fuel elements, and is thereby caused to boil. The filler element 30 comprises a cylindrical sheath of a Zr-base alloy filled with graphite. A fixed piston on which the control element 25 is slidable is secured to the top of the filler element 30 and a hollow piston rod extending upwardly from the piston is ported adjacent the latter so as to admit secondary coolant at core inlet pressure into the interior of the control element 25 and so maintain it raised out of the core against the bias of two compression springs (Fig. 1A, not shown). The shroud tube 31, also of a Zr-base alloy and having a hexagonal cross-section, is supported at its lower end on a grid 32 and at its upper end by a similar grid 34 which is perforated as at 35. Adjacent its lower end, the shroud tube 31 has a plurality of small orifices 36 whereby secondary coolant enters the shroud tube interior so as to establish a small degree of flow which ensures boiling does not occur in the shroud tube 31 by heating of the coolant due to irradiation. The effect of the shroud tubes 31 is to displace the bulk of the secondary coolant flow to the region of the fuel tubes 12 and thus, for a given supply of coolant, increase the mass flow rate of coolant over the fuel and shifting the onset of voidage (amount of vapour present) in the coolant downstream direction. The voidage in the reactor core is thus considerably reduced.