Patent Number: 040381331
Section: description

DETAILED DESCRIPTION OF THE INVENTION In the first example of the invention shown by FIGS. 1 through 6, a small section of the concrete pressure vessel 1 of a gas-cooled fast breeder reactor is shown, this supporting the upper end of one of the fuel assemblies vertically elongated casings or cans 2 containing a bundle of fuel rods 3 positioned in the usual manner, and having an upper end 2a and extending downwardly therefrom and having an open lower end 2b extending below the bottom ends of the fuel rods 3. This lower end of the casing is normally termed the nozzle, because the gas coolant flows downwardly therethrough at relatively high velocity. Although not shown in the case of this first example, a plurality of the fuel assemblies are grouped together adjacent to each other, the casings being of hexagonal shape so that they can nest close together in groups, as indicated by FIG. 7 showing a second example of the invention. The casing 2 has a vertical tubular suspension rod 3 with a supported connection above the casing via the pressure vessel top 1 and a lower end with a releasable screw-threaded connected 3a to the casing's upper end 2a, so that the casing is suspended thereby. The suspension rod customarily is tensioned not only by the weight of the fuel assembly, but also because it is required to hold the assembly with its upper end 2a tightly within one of the openings in the usual grid plate 4 and down through which the gas coolant flows into the upper end of the assembly's casing, the details of the construction involved not being shown because it can be conventional. The casing 2 can, of course, be lowered from the grid plate opening upon release of the upward force applied through the suspension tube 3. The instrumentation tube 5 is shown with its lower end 5a terminating within the bottom end or nozzle 2b of the casing, this instrumentation tube 5 having an upper end 5b which independently of the connection of the suspension tube 3 with the pressure vessel, is fixed above the casing 2 by projecting upwardly through the pressure vessel top into the tubular pressure containment 6 where the instrumentation tube's top end has an enlargement or collar supporting that upper end by resting on the top wall 6a of the pressure containment tube 6. Although not shown, normally a thermocouple would be inthe bottom end 5a of the instrumentation tube with its necessary electrical lines extending upwardly to the top end of the instrumentation tube above the wall 6a, from which the lines would extend to external instrumentation. When servicing is required, the instrumentation tube 5, being slidable throughout its length relative to the other parts, can be slid upwardly and removed from the pressure vessel 1, without disturbing the fuel assembly or its casing's suspension rod 3. On the other hand, during core servicing, by release of the casing's upper end from the suspension rod, the fuel element can be lowered while, if desired, with the instrumentation tube remaining unmoved. The present invention provides a secondary or emergency suspension system for the casing 2 in which the fuel rods and all the other necessary parts of the fuel assembly are carried, by taking advantage of the above facts concerning the typical construction of the components of a gas-cooled reactor core. As previously noted, the bottom end of the casing 2 is formed by the nozzle 2b. The lower part of the hexagonal portion or can proper merges into these nozzles, this being done in the usual fashion in the present case but, and this is noteworthy, the nozzle connection is formed by abutting parts 7. In other words, the nozzle can direct an upward force into the casing extending above it. The top end of the nozzle has a cross bar 8 extending across it and including a central collar 9 which is internally threaded and into which an externally threaded vertically bored mounting 10 is screwed, the mounting 10 depending from the cross bar 8. The mounting 10 has a bottom shank 10a which is counterbored and internally threaded. To hold the instrumentation tube 5 concentrically with respect to the casing's bottom end or nozzle 2b down through which the gas coolant flows, a tube guide 11 has an externally threaded shank which is screwed into the internally threaded counterbore of the mounting 10. The instrumentation tube 5 is vertically slidable in this tube guide 11 but is normally held by the tube guide concentrically within the nozzle. According to the present invention, this tube guide is made with an annular recess 11a which mounts four symmetrically arranged latches 12 which swing inwardly to engage an annular recess 5c formed in the bottom end of the instrumentation tube 5, this annular recess serving as a common latch catch for all four of the latches. To effect this mounting, the upper ends of the latches, in each instance, form inwardly extending hooks 12a which form pivot points by rocking in the recess 11a, the latches depending from these upper ends and forming inwardly extending latch surfaces 12b which, upon downward movement of the fuel assembly if accidentally released, engage the annular shoulder formed by the bottom end of the annular recess 5c, the latches' upper ends 12a then abutting an external flange formed by the bottom surface of the tube guide 11 defined by the recess 11a. Under these conditions, the latches function as four vertical struts or columns transmitting the weight of the fuel assembly from the shoulder of the tube guide 11, formed by its annular recess 11a, downwardly through the latches and to the latch surfaces 12b to the shoulder formed by the recess 5c in the instrumentation tube's bottom end, the instrumentation tube 5 then carrying the weight of the assembly, in tension. Radially outward movement of the four latches is prevented by a vertically slidable locking ring 13 from which two wings 13a diametrically extend in radial directions with their outer ends spaced inwardly from the inside of the casing's nozzle, this locking ring 13 normally resting on radially outwardly extending shoulder elements 12c which extend from the latches in each instance, the latches having wedge surfaces 12d above these shoulders 12c and which are engaged by the locking ring 13 under normal conditions. In this way the four latch members are pressed radially inwardly so long as the locking ring 13 rests on the shoulder 12c and wedge inwardly the wedging surfaces 12d of the latches. The midportions of the latches form inwardly extending shoulders 12e which form abutments, while the bottom end of the tube guide 11 forms an opposing abutment, and a ring 14 is positioned slidably on the instrumentation tube 5 between these two mutually opposing abutment surfaces. In the event of an accidental release of the casing and theoretically possible buckling of the upstanding portions of the latches, the locking ring 13 holds the latches firmly inwardly while the ring 14 is engaged between the opposing abutment surfaces so as to carry the weight of the fuel assembly, even in the event of such partial failure of the latches. Although the latch arrangement described may be formed by relatively loose parts, in the event the suspension of the fuel assembly should fail, the possible falling distance of the fuel assembly is extremely short and its downward motion is restrained before the assembly can gain an unmanageable downward velocity. To effect unlatching when either the instrumentation tube is to be lifted or the fuel assembly is to be lowered, the tools shown by FIG. 5 and 6 may be used, FIG. 6 showing only the upper end of the tool but it being understood that the tube is long enough to be used through the vessel bottom when the reactor is shut down. This tool comprises an outer tube 15 having vertical slots 15a positioned to provide clearance for the four latches, these latches being flat parts as indicated by FIG. 5. Using this tool, as shown by FIG. 3, the outer tube 15 can be inserted through the open bottom end of the casing's nozzle, to engage and lift the vertically slidable locking ring 13 so that the four latches are unlocked and can be swung radially outwardly. The tool comprises also a relatively slidable inner tube 16 having a top end chamfered inwardly, the latches having outwardly flared lower ends 12f for engagement by the inwardly chamfered end of the inner tube 16 as it is slid upwardly while the outer tube 15 holds the locking ring 13 upwardly, the latches being then swung to their unlocking positions as shown by FIG. 4. When the latches are thus unlocked, intentional lowering of the fuel element can be effected, the tool being correspondingly lowered while holding the latches unlocked. Also, of course, the instrumentation tube 5 can be lifted free of the fuel assembly. In the event no tool is available, and other forms of tools are possible, the wings 13a of the ring 13 permit almost any kind of tool to be used to push the locking ring upwardly, and with the latches thus unlocked, the use of other tools can be used to move these latches to their unlatched position. When the fuel assembly is returned to its position shown by FIG. 1, the tool can be used to hold the latches unlatched during reinsertion of the instrumentation tube 5, this ordinarily being drawn upwardly during intentional removal of the fuel assembly. In the second example of the invention, the casings of the adjacent or adjoining fuel assemblies of the group of fuel assemblies shown by FIG. 7, are used as a secondary suspension means. A reactor core may comprise a substantially larger number of assemblies, only seven being shown in FIG. 7. Having reference to FIGS. 8 and 9, it can be seen that the latches substantially correspond to those shown in the case of the first example. The difference is that in this second example the portions 12a' of the latches, rock in openings 12e' formed in each of the flat sides of the hexagonal shape, while the locking ring 13' rests on the inside surfaces of all of the latches by way of their inwardly extending shoulders 12c', while the latching surfaces 12b' engage in recesses formed in the rings 17 which encircle the nozzles and, in part, provide the abutting surfaces 7 previously referred to. With this somewhat reversed arrangement, there are two latches adjacent to each other as to each of the opposing flat sides of the fuel assembly group forming the core, as can be seen from FIG. 7. Therefore, as to any one of the assemblies, the latches of all of the assemblies surrounding that assembly, must be released when the one assembly is to be removed, and this is shown by FIG. 9 in the case of three of the assemblies. In this case the tool has the inner sliding tube 16' slotted for clearance of the latches, and it is the outer sliding tool 15' which has the chamfered ends, the chamfer being inwardly in this case to engage the depending latch lever portions which function as latch release means. FIG. 9 shows the action with the latches released. The latch catches formed by the recesses in the ring 17 may be subject to wear, but these rings can be made of suitable hard material. In this second example, the instrumentation tube is not used as the secondary suspension, but it could, or course be used for its normal purpose to determine the temperature of the gas coolant flowing downwardly through the nozzle of the assembly, as to each assembly.