Patent Number: 056087683
Section: summary

TECHNICAL FIELD This invention relates generally to nuclear reactors, and particularly to the manner in which certain fuel rods in a fuel bundle are secured to a lower tie plate assembly. BACKGROUND Typical fuel bundle assemblies in boiling water nuclear reactors include a plurality of elongated fuel rods supported between upper and lower tie plates. The fuel rods pass through a plurality of fuel rod spacers which provide intermediate support to retain the elongated rods in spaced relation and to restrain the rods from lateral vibration. Each of the fuel rods comprises an elongated tube containing the fissile fuel (such as uranium or plutonium dioxide) in the form of pellets, particles, powder or the like, sealed in the tube by upper and lower end plugs. Most of the lower end plugs are formed with a taper for registration and support within cavities formed in the lower tie plate, while corresponding upper end plugs are formed with extensions which register with support cavities formed in the upper tie plate. These full length rods are spring biased towards the lower tie plate so as to prevent undesirable axial movement of the rods. It would be desirable to eliminate or reduce the number of parts associated with the upper tie plate while at the same time eliminating concerns about coolant flow impact on the conventional protruding lower tie plate end plugs. Each fuel bundle assembly in current designs also contains several fuel rods which have threaded lower end plugs. These would typically include eight tie rods and as many as fourteen partial length fuel rods (PLR's) which terminate short of the upper tie plate. Fuel bundles with PLR's are disclosed, for example, in commonly owned U.S. Pat. Nos. 5,112,570 and 5,017,332. Fuel bundles including fuel rods serving as tie rods with threaded upper and lower end plugs are disclosed, for example, in commonly owned U.S. Pat. Nos. 4,357,298 and 4,420,458. After irradiation, it has been found that many of the threaded rods are not removable without extreme measures which are time consuming and which can damage the rods beyond repair. The apparent cause of the problem is that the lower end plugs of the rods stick in the lower tie plate as the result of corrosion which occurs on the male threads of the Zircaloy end plugs. It will be appreciated that increasing the number of fuel rods which are threadably connected to the lower tie plate will increase the chances of experiencing fuel rod sticking. Thus, there is a need for a simple and cost effective means for minimizing or eliminating the problem. DISCLOSURE OF THE INVENTION It is one object of this invention to eliminate the biasing springs in the upper tie plate normally used to bias non-threaded full length fuel rods toward the lower tie plate. This objective is achieved by utilizing a threaded end plug with all of the full length fuel rods. This invention has for another object the solution of the fuel rod sticking problem described hereinabove. In the exemplary embodiment, the invention comprises replacing the integral threaded Zircaloy end plug shank with a stainless steel connecting shank which is threaded at opposite ends thereof. The stainless steel shank can thus be threaded into a tapped hole in the Zircaloy end plug body at one end, while the other end is threaded into a hole in the lower tie plate. It is an additional feature of this invention that the opposite ends of the connecting shank can be threaded in opposite directions, i.e., one end is formed with a right-hand thread while the other is formed with a left-hand thread. With this arrangement, when torque is applied which loosens the connecting shank in the tie plate, the shank attachment to the end plug body is tightened. Conversely, when torque is applied which tightens the connecting shank in the tie plate, the shank attachment to the end plug body is loosened. Thus, when removing the fuel rod, the stainless steel connecting shank can either remain in the lower tie plate, or remain in the Zircaloy end plug body and thus be removed with the fuel rod. This capability provides a further option in the event threads are frozen in one of the two threading directions. Thus, in its broader aspects, the present invention relates to an end plug for a fuel rod of a nuclear reactor fuel bundle assembly including upper and lower tie plates, the end plug comprising an upper portion constructed of a first alloy material and including an exterior fuel rod receiving surface and a tapped hole in a lower end thereof; and a removable lower portion constructed of a second alloy material and including upper and lower threaded sections, the upper threaded section receivable within the tapped hole and the lower threaded section receivable within a correspondingly threaded hole in the lower tie plate. In another aspect, the invention relates to a fuel bundle assembly for a nuclear reactor having a plurality of fuel rods including a plurality of full length fuel rods extending between upper and lower tie plates and at least one partial length fuel rod extending between the lower tie plate and a spacer located between the upper and lower tie plates, the improvement comprising an end plug for at least the partial length fuel rod, the end plug secured between the partial length fuel rod and the lower tie plate, the end plug comprising an upper portion constructed of a first alloy material and including an exterior partial length fuel rod receiving surface and a tapped hole in a lower end thereof; and a lower portion constructed of a second alloy material and including upper and lower threaded sections, the upper threaded section receivable within the tapped hole and the lower threaded section receivable within a tapped hole in the lower tie plate. In still another aspect, the invention relates to a method of removing a fuel rod from a fuel rod bundle assembly in a nuclear reactor, wherein the fuel rod bundle assembly includes a plurality of fuel rods extending between upper and lower tie plates, the method comprising the steps of: a) providing an end plug for a lower end of at least one of the fuel rods comprising an upper portion constructed of a first alloy material and including an exterior fuel rod receiving surface and a tapped hole in a lower end thereof; and a lower portion constructed of a second alloy material and including upper and lower threaded sections, the upper threaded section receivable within the tapped hole and the lower threaded section receivable within a correspondingly threaded hole in the lower tie plate; and b) rotating the fuel rod in a first direction to separate the fuel rod and the fuel rod end plug from the tie plate. An optional method step is available if the upper and lower sections of the end plug lower portion are threaded in opposite directions. This construction permits rotating the first rod in a second opposite direction to separate the fuel rod and the upper end plug portion from the lower end plug portion, leaving the latter in the lower tie plate. Other objects and advantages of the present invention will become apparent from the detailed description which follows.