Patent Number: 049960210
Section: claims

1. In a nuclear fuel assembly which includes an upper end fitting and a lower end fitting spaced therefrom and connected thereto by a plurality of elongated guide tubes of one alloy having an open upper end and a closed lower end with spaced fuel element retaining grids mounted on the guide tubes therebetween, the closed lower ends of said guide tubes including a threaded central passageway and the attachment of said guide tubes to said lower end fitting of another alloy being characterized by: an externally threaded bolt with a first end threadably received in said threaded central passageway of the lower end of said guide tube and a head at the other end on the side of the lower end fitting opposite said guide tube;  an interruption in the external threads of said bolt which forms a groove which communicates the interior of the guide tube with the side of the lower end fitting opposite said guide tube and enhances 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.  an externally threaded stainless steel bolt with a first end threadably received in said threaded central passageway of the lower end of said zircaloy guide tube and a head at its other end on the side of the stainless steel lower end fitting opposite said guide tube;  an interruption in the external threads of said stainless steel bolt which forms a groove which communicates the interior of the zircaloy guide tube with the side of the stainless steel lower end fitting opposite said guide tube and enhances its frictional engagement with said threaded central passageway, thereby to hold and attach said zircaloy guide tube and stainless steel lower end fitting firmly together, even through a series of temperature cycles. 2. The attachment of said guide tubes to said lower end fittings of another alloy of claim 1 being characterized further in that said bolt is of substantially the same alloy as said lower end fitting. 3. The attachment of said guide tubes to said lower end fittings of another alloy of claim 1 being characterized further in that said bolt groove is a longitudinal cut made parallel to the bolt axis. 4. The attachment of said guide tubes to said lower end fittings of another alloy of claim 1 being characterized further in that said bolt includes an opening in its head. 5. The attachment of said guide tubes to said lower end fittings of another alloy of claim 1 which is characterized by said other alloy being stainless steel and said one alloy of said guide tubes being zircalloy. 6. The attachment of said guide tubes to said lower end fittings of another alloy of claim 5 further characterized by a zircalloy plug closing the lower end of said guide tubes and defining said threaded central passageway. 7. The attachment of said guide tubes to said lower end fittings of another alloy of claim 6 further characterized by a cup attached between the lower end fitting and the zircalloy guide tube by means of said bolt. 8. The attachment of said guide tubes to said lower end fittings of another alloy of claim 7 further characterized in that said cup is of stainless steel. 9. In a nuclear fuel assembly which includes an upper end fitting and a stainless steel lower end fitting spaced therefrom and connected thereto by a plurality of elongated guide tubes of a zircaloy alloy having an open upper end and a closed lower end with spaced fuel element retaining grids mounted on the guide tubes therebetween, the closed lower ends of said guide tubes including a threaded central passageway and the attachment of said guide tubes to said lower end fitting being characterized by: