Patent Number: 049960210
Section: summary

FIELD OF THE INVENTION: This invention relates to nuclear reactor fuel assemblies for Pressurized Water Reactors (PWR) and in particular those assemblies which include spaced fuel rod support grids mounted in a reactor core as a unit. The fuel rods are held between an upper end fitting or top nozzle and a lower end fitting or bottom nozzle by means of spacer grids. Guide tubes or thimbles provide the structural integrity between the lower end fitting, the upper end fitting and the spacer grids intermediate the ends of the fuel assembly. BACKGROUND OF THE INVENTION: An attachment of the guide tubes to the lower end fittings is illustrated in U.S. Pat. No. 4,738,820. The guide tubes have plugs in their lower ends to facilitate attachment to the lower end fittings. The guide tubes and plugs are of zircaloy, an alloy of zirconium. The lower end fittings and cups attached thereto to receive the plugged lower ends of the guide tubes are of stainless steel, such as that known as 304. The cup acts as a mounting sleeve for the lowest grid which is also of a stainless steel, such as Inconel 625 or 718. The other grids are of zircaloy. Since the coefficient of expansion of zircaloy and stainless steels such as 304 and Inconel 625 or 718 are significantly different, attachments between zircaloy and stainless steel elements of a structure must accommodate the tendency of the elements to move at relatively different times, rates and magnitudes. Accordingly, attachments of these elements for structures which will maintain their integrity over time are difficult. For example, fuel assemblies may be used in a nuclear reactor, typically, for as many as three cycles of eighteen (18) months each. Many temperature cycles which potentially weaken bottom nozzle to guide tube connections occur during this period of time. SUMMARY OF THE INVENTION: In a fuel assembly having a stainless steel bottom nozzle or lower end fitting to zircaloy guide tube attachment constructed according to the principals of the invention, each zircaloy guide tube has a zircaloy lower end plug. A threaded central passageway in the guide tube plug receives a stainless steel threaded fastener which attaches the plugged tube and a tube receiving stainless steel cup or mounting sleeve to the lower end fitting. The stainless steel faster is an externally threaded bolt having a first end threadably received in the threaded central passageway of the plug in the lower end of the guide tube and a head at its other end in a countersunk hole on the side of the lower end fitting opposite said guide tube. An interruption is provided in the external threads of the bolt which forms a groove communicating the interior of the guide tube with the side of the lower end fitting opposite said guide tube and enhancing its frictional engagement with said threaded central passageway, thereby to hold and attach said guide tube and lower end fitting firmly together, even through a series of temperature cycles. Normally, the plug end receiving stainless steel cup with a fastener opening is secured between the zircaloy plugged guide tube end and the stainless steel lower end fitting. This fully accommodates the temperature expansion differences between zircaloy and stainless steel and provides a better attachment. The groove in the stainless steel fastener, permits lower end fitting fuel assembly reconstitution and provides for coolant flow through the end of the guide tube. These functions are accommodated with the advantage that the longitudinal slot or groove in the threads provides points of increased friction and holding power in the threaded connection and provide "give" for the zircaloy--stainless steel thermal expansion difference. The mean coefficient of thermal expansion.times.10.sup.2 (cm./cm./.degree. C.) in going from 70.degree. F. to 650.degree. C. or 21.1.degree. C. to 343.3.degree. C. for zircaloy-4 and 304 stainless steel are typically as follows: ______________________________________ zircaloy-4: 2.93 in./in./.degree.F. or 5.27 cm./cm./.degree.C. austenitic stainless steel: 9.87 in./in./.degree.F. or 17.77 cm./cm./.degree.C. ______________________________________