Patent GB-1240871-A (GB)
Title: Reactor core with removable core elements
IPC: G21C1/12
Filed: 1970-03-05
Abstract: 1,240,871. Reactors. UNITED STATES ATOMIC ENERGY COMMISSION. 5 March, 1970 [4 April, 1969], No. 10602/70. Heading G6C. In the top layer of a reactor core formed from core elements stacked in columns, the core elements are interlocked by means including loose, shared keys extending between keyways in pairs of adjacent core elements, these keys being readily slideable from the keyway in one of a pair of adjacent core elements with removal of the other of the pair. In a reactor core refuelled by an " off-line " operation, a combination of loose keys 23 and fixed keys 26 is used, as in the embodiment shown in Figs. 4 and 8, the reactor being shut down and a group of core elements in the top layer removed prior to removing core elements from the next lower layer. The topmost core element 13 in each column is formed of metal clad graphite blocks 28 of hexagonal cross-section. The metal cladding 27 covers the sides and tops of the graphite blocks 28, there being a graphite to graphite interface with the next lower core elements. The reflector core elements are formed of solid graphite blocks while the fuel-bearing core elements have coated carbide particles packed in vertical bores in the graphite blocks. Each topmost core element 13 has a central opening 29 for receiving the grapple of a refuelling machine, a metal tube extending from this opening through a central bore in the graphite block 28 to an annular bottom plate. The lower, unclad core elements likewise have grapple-receiving openings. The topmost core elements 13 are of the plenum kind, a hollow chamber 39 extending between the top cladding wall 33 and the upper face 41 of the graphite block 28. Cooling gas flows into the chamber 39 through an opening 45 in the cladding 27 and then down through a series of passageways 44 in the block 28 which are aligned with similar passageways in the lower core element blocks. The core elements in the lower layers are interlocked to each other and to the top layer core elements by means of locating dowels 18 projecting across the interfaces into sockets in the lower surfaces of the adjacent core elements. Six of the fuel-bearing core elements are grouped around a control rodbearing core element, the arcuate edges 47 of the openings 45 forming a circular-shaped opening for admitting cooling gas into one refuelling region. Each refuelling region of seven elements is supported on one support block (Fig. 1, not shown) having a central coolant passage to receive cooling gas from the converging coolant passageways in the lower reflector core element immediately above it. The keyways 24 for the loose shared keys 23 are formed in vertical bars 69 welded inside the side walls of the metal cladding 27, and are of T-section. The keys 23 are of I-section and so dimensioned as to slide in pairs of adjacent keyways 24. The throat walls 71 of the keyways 24 are bevelled outwardly at their upper ends to facilitate entry of a key 23, the lower end of which is tapered with a rounded tip. The keys 23 are detained within the keyways 24 by detent mechanism each having a spring-biased plunger with a rounded nose 89 for projecting into a narrow detent groove 74. When a key 23 is being lifted with one of its core elements, the lower ends of the flanges of its I-section may bottom on the lower ends of the keyway 24 in the rising core element to overcome any resistance to the key sliding upwardly in the keyway of the adjacent stationary core element. When the rising key brings its detent noses 89 into engagement with the top end of the detent groove 74 in the stationary core element, the noses are cammed back until the key is lifted clear of the stationary core element. The loose shared keys 23 are on the three sides of the core elements facing outwards from a refuelling region and connecting with core elements in other refuelling regions, two of the remaining three sides having fixed keys 26 of rectangular section with a lower pointed nose 93 for insertion into an enlarged flared slot 95 of a keyway 96 having a similar rectangular section, and the third of the remaining three sides having a keyway 96. In a reactor core refuelled by an " on-line " operation, only loose keys 23 are used (Fig. 17, not shown) and each one of a vertical column of core elements is removed before the removal of another top layer core element in another column.