Patent Number: 047708476
Section: summary

BACKGROUND OF THE INVENTION Fuel assemblies in certain boiling water nuclear reactors contain fuel rods and water rods. Typically, cladding containers of both types of rods are made of zirconium alloys such as Zircaloy-2 and Zircaloy-4. The fuel rods contain fuel material, such as uranium dioxide pellets whereas the water rods have water flowing through them. Under the conditions prevailing in an operating nuclear reactor, zirconium alloys exhibit a certain amount of irradiation growth. The amount of irradiation growth depends on many factors, including the alloy material, the temperature, neutron energy, and the neutron flux. Mechanical interaction between fuel pellets and the cladding container introduces a growth component in the fuel rods not present in water rods. This introduces an irradiation growth differential between fuel rods and water rods. As the radiation exposure increases, pellet-cladding mechanical interaction (PCMI) increases and as a result the difference in irradiation growth between fuel rods and water rods increases. In such a nuclear fuel assembly the fuel rods are positioned lengthwise between upper and lower tie plates in which the ends of the fuel rods are secured, for example by means of end plugs having studs fitted into holes in the tie plates. Water rods are positioned lengthwise between the tie plates and are likewise secured by means such as end plugs with studs fitted into corresponding holes in the tie plate. There is typically an axial irradiation growth differential among fuel rods and water rods. A small axial growth differential is accommodated, for example, by incorporation of expansion springs between the upper tie plate and the upper end of the fuel rod cladding. The expansion springs are disposed around a portion of the end plug studs. The distance between the upper and lower tie plates increases according to the axial irradiation growth of the fuel rods. There is concern that the difference in axial irradiation growth between fuel rods and water rods may become too great to be accommodated by the expansion springs and may cause one or more end plug studs of water rods to release from the upper or lower tie plate. SUMMARY OF THE INVENTION In accordance with the invention there is a process for providing different irradiation growth rates between first and second cladding tubes made of anisotropic metals such as zirconium alloys. The process comprises regulating the crystallographic structure of each cladding tube wherein low irradiation growth occurs when the crystalline structure of the metal is in a low energy state or configuration and larger irradiation growth occurs when crystalline structure of the metal is in a higher energy state. A preferred means for regulating the crystalline structure of the anisotropic metal in each cladding tube comprises tailoring the tube fabrication schedule to achieve the desired crystallographic structure. More specifically, the process comprises forming a first cladding tube of an anisotropic metal, preferably a zirconium alloy by a first fabrication schedule. The final dimensions are achieved by a selected cold-work reduction, which imparts a high energy state to the crystalline structure of the metal. The final cold-work reduction is followed by heat treating the first cladding tube at a selected temperature and for a selected time to allow the metal to at least partialy recrystallize to a lower energy state. A second cladding tube of the same metal as the first cladding tube is formed by a second fabrication schedule which includes a selected cold-work reduction to achieve given final dimensions which may be approximately the same as the first cladding tube. The second cladding tube may then be heat treated at a selected temperature and for a selected time sufficient to induce less recrystallization than the first cladding tube to thereby retain a higher energy state of the crystalline structure than that of the first tube. The second cladding tube will display greater irradiation growth than the first cladding tube due to its higher energy state. The energy state of the crystallographic structure of the metal is thus regulated by controlling the size of the final cold-work reduction in the tube fabrication schedule, the temperature of the heat treatment following the final cold-work reduction and the length of the heat treatment. A preferred embodiment of this invention comprises controlling the irradiation growth of fuel rods and water rods in a water cooled reactor. Proper tailoring of the cladding tube fabrication schedules enables water rods, which typically undergo less irradiation growth than fuel rods under operating conditions of a boiling water reactor, to exhibit substantially equivalent irradiation growth as fuel rods.