Patent Number: 048896837
Section: description

DESCRIPTION OF THE PREFERRED EMBODIMENTS In FIG. 1, a removable sub-assembly vehicle 10 having lifting lugs 10a and orientation devices 10b, has two fuel units 11 (only one being shown but see also FIGS. 5 and 6) and two triggerable absorber units 12 (only one being shown). Coolant enters the vehicle at inlet 13, sweeps over the fuel units 11 and diverts over sodium filled bellows 14 (which are associated with the units 12) and discharges at an outlet 15. The units 12 are suspended above dash pots 16. The fissile region of the fuel units 11 is represented by a hatched portion 11a and the absorber region of the absorber units 12 is represented by a hatched portion 12a which is above the region represented by 11a. Coolant flow is indicated by arrows 17. A by-pass coolant flow is represented by arrows 18. This is substantially smaller than the flow represented by arrows 17 and serves to avoid stagnation around the absorber units 12 and also serves to cool the absorber units 12. A triggering mechanism, described in more detail with reference to FIG. 2 below, is identified by the numeral 19. In FIG. 2 there is shown the vehicle 10 which supports a fixed hexagonal shaped plate 20. The plate 20 supports, in its turn, a pair of separately movable rotary wing-shaped plates 21. The plates 21 each have a projection or lip 22 which latches below a rim 23 of absorber units 12. The plates 21 have upstands 24 secured to them and these define cam slots (as mentioned below with FIG. 3). A cam rod 25 is shown between each pair of upstands 24 and this engages with a push rod 26 penetrating apertures 27 in the plate 21. For FIG. 3 there is shown the bellows 14, which has its free end attached to the push rod 26, and the cam rod 25 engaging a cam slot 28 in the plate 24 which is, in turn, secured to a wing-shaped plate 21. The push rod 26 is shown passing through the plate 20. In FIG. 4 a projection 22 on one of the wing-shaped plates 21 is shown latched below the rim 23 of an absorber unit 12. In FIG. 5 the sub-assembly vehicle is shown in plan. This comprises two fuel units 11, two bellows 14 associated with triggerable absorber units and two coolant outlet apertures 50. A partition 51 is also provided. Coolant rises vertically in fuel units 11 and, on discharge at the top of the units, the coolant seeks the outlets 50 by sweeping over the bellows 14 of the absorber units. The coolant flow is indicated by arrows 17. The absorber units 12 lie below the outlets 50. The coolant flow pattern of FIG. 5 is shown in elevation to FIG. 6. The functioning of the apparatus above described can be considered with FIG. 6 to hand. For normal operation (e.g. below 600.degree. C.) sodium heated by a fuel unit 11 flows over a sodium filled bellows 14 as indicated by arrows 17. The flowing sodium heats the sodium in the bellows and causes the bellows to expand. This causes the cam pin 25 to enter the inclined part of cam slot 28. For normal operation FIG. 6 represents the stable operating condition. If the coolant flow 17 becomes overheated the bellows expand further. This drives the pin 25 further along the slot 28. This causes plate 21 to move and the projection 22 comes clear of the rim 23, (FIG. 4) and the absorber unit 12 then falls freely into the vehicle 10 and reduces the reactivity of the fuel unit 11 to set a stable safe condition. These events could typically take place when coolant temperature reaches 700.degree. C. The sodium-filled bellows which is typically 100 mm long, is capable of providing a 2.5 mm deflection for each 100.degree. C. temperature change with a force of 1000 newtons. The unit described above can be re-cocked on removal from its position in the reactor core either by removal to the edge of the core or by removal from the reactor to a shielded manipulation facility. The sub-assembly vehicle above described provides a self-contained unit comprising a control rod, fuel heat source, and control rod trigger fully independent of any external activation and substantially free of distortion problems. In general it will find use as a secondary shut-down device and will provide safety back-up in the event of failure or mobility of primary devices to cope with faults such as loss of coolant pumping. The subassembly vehicle is adaptable in that it can be given a large number of locations in a reactor core and it could accordingly be given alternative positions as burn-up of a core causes reactivity to change. The vehicle is also adaptable in that the fuel to absorber ratio in any one unit can be preselected. For example, it is possible to have any whole number ratio, in a six compartment unit, between 1:5 and 5:1. It is also important to note that the bellows 14 operates unstressed and hence is not subjected to stress recycling risks as the reactor changes temperature. In FIG. 7 there is shown a bellows 70 having a sodium filling 71. The bellows has a top (fixed) end closure 72 and a bottom (free) end closure 73. The closure 72 is held in one part of a structural frame 74 by a screw 75 and the closure 73 is constrained laterally but free to move axially in a cup 76. The cup is supported on a spring trigger 77 (FIG. 8) and it has a stem 78 movable in a guide aperture 79 in another part of the structural frame 74. In FIG. 8 the spring trigger 77 is shown having two divergent curved wall parts 80 and a curved base 81 with a hole 82 to accommodate the stem 78 of the cup 76. The upper edges of the trigger 77 can support control elements 83 by engaging under lips 84 on the rods. This is illustrated in FIG. 7. In FIG. 9 the cup 76 is shown in perspective view. The cup has flats 85 to accommodate movement of the wall parts 80 of the trigger. In FIG. 10 another sub-assembly vehicle 90 is shown . The vehicle 90 consists of a hollow body part 91 in which there are located six units 92. These may be either control units (like elements 83) or fuel units 93, or voids, the combination being selectable. In FIG. 10 four of the units are control units and two are fuel units 93. The control units 83 are latched on the trigger 77 and are supported at a higher level than the fuel units and are, in this way, above the reactor core. In operation, the bellows 10 is situated at a location so as to be responsive to sodium coolant which has flowed upwardly over the fuel units 93 (such flow is indicated by arrows 94). As the coolant temperature rises with the change from zero power in the reactor to operating power so the temperature of the bellows rises and the bellows expands axially. This expansion is unconstrained as the end closure 73 moves freely into the cup 16. Should the temperature of the bellows now rise further (because, for example, there has been an unplanned restraint in the coolant flow) then the closure 73 of the bellows acts on the base of the cup 76 and depresses the cup. At the same time the curved base 81 of the trigger is depressed and this causes the curved wall parts 80 to take up a less divergent orientation until the upper edges of the curved wall parts leave the lips 84 of the control units 83 and the units are free to fall by gravity into the reactor core. The device described above can be designed to use readily available and relatively inexpensive materials compatible with the hostile environment in which the operation has to be performed and of well-tried performance, namely stainless steel and sodium. The temperature at which the trigger releases can be made adjustable by only relatively simple mechanical operations such as inserting a shallower or deeper cup. The trigger is recockable by the simple operation of raising a released control unit 83 until it relatches on the upper edge of the trigger 77. The device described above also has the merit that, apart from the trigger 77, it operates unstressed except at the point of operation and even at that point the stress is very low.