Patent Number: 040381331
Section: summary

Prior art, gas-cooled fast breeder reactors commonly include a concrete pressure vessel containing a reactor core formed by a plurality of laterally adjacent fuel assemblies, each assembly comprising a vertically elongated casing containing a bundle of fuel rods, the casing having an upper end and extending downwardly therefrom and having an open lower end extending below the fuel rods. Each casing has a vertical tubular suspension rod having an upper end with a supported connection with the concrete pressure vessel top and a lower end with a connection to the casing's upper end so that the casing is suspended thereby. At least one of the connections of the tubular suspension rod is made releasable so that any one of the fuel assemblies can be lowered from the core for servicing of the core. These vertical tubular suspension rods form the primary suspensions for the fuel assemblies. If any one of these suspensions fail so as to accidentally release an assembly during operation of the reactor, the assembly can drop from the core. These primary suspensions are designed to provide a redundancy of safety, but a failure by one or more of the suspensions is at least hypothetically possible. During operation of the reactor the flow of gas coolant is downwardly through the fuel element casings and it is desirable to determine the operating temperatures of each of the fuel assemblies individually. Therefore, each fuel element has an instrumentation tube having an upper end releasably fixed above the casing, this upper end normally projecting upwardly through the top of the concrete pressure vessel within an external pressure tube providing for the passage of electrical lines in a pressure-tight manner to instrumentation external of the reactor pressure vessel. In each instance, this tube suspends from its upper end downwardly and slidingly through the suspension rod and casing to terminate with a lower end below the fuel rods and in the casing's lower end where, for example, the tube contains a thermocouple from which electrical lines extend upwardly through the instrumentation tube to the outside of the pressure vessel. The instrumentation tube is, in each instance, made so it can be removed upwardly by sliding upwardly through the fuel assembly casing and the tubular suspension rod suspension by which fuel assembly is suspended. The upper ends of the instrumentation tubes are fixedly supported by connections entirely separate from the connections by which the upper ends of the tubular suspension rods are fixed, although the upper ends of the instrumentation tubes must be releasable so that the tubes can be drawn upwardly, to assure against the hypothetical possibility that one or more of the instrumentation tubes might be inadvertently released for falling during operation of the reactor. The connections supporting the top ends of the instrumentation tubes are made with a redundancy of security or resistance to inadvertent release, and the instrumentation tubes are themselves made of metal and with dimensions providing a redundancy of tensile strength. SUMMARY OF THE INVENTION The present invention provides a secondary emergency suspension for each of the fuel elements without substantially altering the above described prior art construction, thereby eliminating any need for redesigning or reengineering that time-proven construction. In addition, this secondary or emergency suspension is provided by simple parts which are very inexpensive relative to other possible expedients. Keeping in mind that in the case of an accidentally released fuel assembly, both its instrumentation tube and all or at least most of the adjacent fuel assemblies can reasonably be expected to remain securely suspended and form fixed parts relative to the released assembly, this invention provides a simple latch means for normally latching each casing of each of the assemblies, to one of such parts that can be expected to remain securely suspended, thus providing for each of the fuel elements, a secondary or emergency suspension without the use of any extra parts other than those required for the latch means. With each assembly casing latched or locked to either its instrumentation tube or one or all of the casings of the assemblies adjacent to that assembly, the problem is presented of unlatching the latch means when it is desired to intentionally release the assembly for downward movement during core sevicing. At that time, the concrete pressure vessel in which the core is positioned, is designed to permit access upwardly to the bottoms of the fuel elements. With this in mind, the present invention provides each of the latch means with a latch release means on the inside of the casing, together with a tool that is insertable upwardly via the casing's open lower end, in each instance, for actuating the latch release means to release the latch means. The same tool can be used for one assembly after another or several of the tool means could be used simultaneously to release the latch means as to a group of the fuel assemblies. The tool means is, of course, used when the reactor is shut down and it is necessary to lower one or more of the fuel assemblies downwardly for core servicing operations. Even though the latch means is in simple form and does not apply vertically directed force to any of the parts, in the manner of a clamp means, and even though the latch means involves some looseness with respect to the interlatched parts, any looseness involved can be made so minor in the vertical direction as to catch an accidentally released fuel assembly almost immediately and before it has achieved any downward velocity sufficient to acquire destructive momentum with respect to the fixed parts to which it is latched. The above factors mean that if each assembly is latched to its instrumentation tube, the latter is able to reliably prevent the assembly from falling, or if it is latched to the casings of adjacent assemblies, their normal primary suspension means are well able to carry the weight of the released assembly. Correspondingly, the latch means may be made in the form of relatively small latches which do not interfere with the downward coolant gas flow through the core to an appreciable degree, while permitting the latch means to be provided by latches of simple design and which, therefore, do not add greatly to the overall cost of the core construction. The necessary tool means for releasing the latches is of simple construction and is, of course, capable of being used repeatedly.