Patent Number: 047924292
Section: summary

FIELD OF THE INVENTION This invention relates to nuclear reactor fuel assemblies 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 by means of spacer grids. The reactor coolant flows upwardly from holes in the lower end fitting along the fuel rods, and upwardly through holes in the upper end fitting. When the fuel assembly is loaded in a reactor core, an upper core plate over the fuel assembly reacts against fuel assembly holddown leaf spring assemblies attached by fasteners to the upper end fitting to provide a downward force. This force combines with the fuel assembly weight to prevent fuel assembly liftoff from hydraulic forces during operation of the reactor pumps. BACKGROUND OF THE INVENTION Debris in the circulating coolant which collects or is trapped in fuel rod spacer grids is believed responsible for as many as 30% of known fuel rod failures. Laboratory and in-reactor experience indicate that fuel rod cladding failures can be caused by debris trapped in a grid region which reacts against the fuel rod cladding in a vibratory fashion resulting in rapid wear of the cladding. The size and shape of the debris capable of causing severe damage is quite variable and may include broken fuel assembly fasteners. The invention involves a spring retention cap for a nuclear fuel assembly end fitting leaf spring assembly which is designed to prevent the creation of debris, such as pieces of broken holddown spring retention screws. The cap of the invention also minimizes the loss of holddown force, the chance that a failure of one screw will impart a jacking force which leads to the failure of a second screw, and interference with control rod operation because of rotation of a leaf spring. Moreover, the cap is designed to provide simple installation and removal procedures. The cap also saves fuel reconstitution expense, because it eliminates the requirement of welds for preventing screw rotation. More importantly, the unique cap provides the option of continuing to operate safely, without reconstitution, even if a holddown spring retention screw breaks. SUMMARY OF THE INVENTION The spring retention cap of the invention for a nuclear fuel assembly end fitting or top nozzle having a leaf spring assembly secured by spring retention fasteners is a body with two plane exterior surfaces defining an angled corner. The cap has a base for engaging the end fitting and a leaf spring assembly receiving slot in the base spaced from and extending substantially parallel to the two plane exterior surfaces. An inwardly directed flange is formed at the slot opening by a portion of the base to define a connecting hook-like structure which engages a slot in each of two end fitting exterior surfaces corresponding to the two exterior plane surfaces of the cap. The base has a portion on the opposite side of the slot in the base from the flange which is inward of the hook-like structure defined by the flange. A screw secures the retention cap in position with the ends of the leaf springs and the spring retention screws covered thereby. The distance from the two spaced exterior surfaces of the body to the slot, combined with the cross-section of the screw, are sufficient to provide strength to accommodate a jacking force created by the leaf spring assembly in the event of failure of one of the two leaf spring assembly retention screws. No loading is placed on the other spring retention screw as in some other designs. If the other screw should also fracture, both leaf spring assemblies and retention screws will be captured by the cap and its attachment screw. This will preclude the creation of loose parts. If there are no problems with the spring retention screws, the only function of the cap and the third screw is to protect the spring screws and spring ends from being impacted during fuel handling operations.