Self rupturing gas moderator rod for a nuclear reactor

Abstract

An arrangement for controlling the moderation of a nuclear reactor which includes multiple fuel assemblies each of which includes top and bottom nozzles with fuel rods and control rod guide thimbles located therebetween. Certain ones of said assemblies include at least one tube initially filled with a gas. One end of the tube rests on the lower nozzle upper surface while the upper end terminates at a point just below the lower plate in the upper nozzle. A spike immovably fixed in the lower plate of the upper nozzle is directed downwardly toward the end of the tube. As the tube expands or grows during operation in a reactor, and at a point where the criticality factor may proceed to less than one, the expanded tube will be pierced by the spike and thus permit the escape of gas from the tube which is then replaced by water coolant circulating through the reactor. The additional water represented by the filled tube will act to slow down fast neutrons to a thermal neutron level and thus provide a greater amount of neutron effective in carrying forward the fissioning process.

I claim: .[.1. A nuclear reactor comprising: 
     
        when it is pierced by said spike..]. .Iadd.5.  A nuclear reactor comprising: a plurality of fuel assemblies, each of said assemblies having top and bottom nozzles with fuel rods and control rod guide tubes interposed between, said top nozzles having lower adaptor plates, and said bottom nozzles having upper surfaces;   at least one gas-filled tube in a portion of said assemblies, said gas-filled tube having a bottom end and a top end, said gas-filled tube extending substantially the full length of said assembly and having said bottom end resting on said upper surface of said bottom nozzle, and said gas-filled tube comprising material which will axially expand during reactor operation;   a pointed piercing device immovably mounted in said lower adaptor plate of said top nozzle and having its point directed downwardly toward said top end of said gas-filled tube, the point on said piercing device and said top end of said gas-filled tube being spaced a distance equal to the amount said gas-filled tube will expand during operation in a reactor, minus a distance sufficient for said piercing device to pierce said top end of said gas-filled tube and permit the gas therein to be displaced by   
     
     
        water coolant circulating through the reactor. .Iaddend. .Iadd.6.  The combination according to claim 5 wherein said gas-filled tube is initially charged with a gas sufficient to prevent collapse thereof when exposed to the coolant pressure in the reactor. .Iaddend. .Iadd.7. The combination according to claim 5 wherein said top end of said gas-filled tube comprises a plug, and said plug has an upper section sufficiently thin to be pierced by said piercing device as said gas-filled tube expands in length. .Iaddend. .Iadd.8. The combination according to claim 7 wherein said piercing device has longitudinal grooves extending from approximately the tip of said point to where the full diameter of said piercing device commences to permit the escape of gas and ingressive coolant into said gas-filled tube when it is pierced by said piercing device. .Iaddend. .Iadd.9. The combination according to claim 5 wherein said gas-filled tube comprises Zircaloy. .Iaddend.