Patent Number: 046559914
Section: summary

The present invention relates in general to nuclear fuel assemblies, and more particularly to apparatus for detecting the absence of helical springs on the shanks of fuel rod end plugs. BACKGROUND OF THE INVENTION In certain types of nuclear reactors, the nuclear fuel is contained in fuel rods. The fuel rods are grouped in fuel bundles, within which the fuel rods are equidistantly positioned from each other in a spaced array. The array itself is supported between an upper and a lower tie plate. A number of these fuel bundles are combined to form the nuclear fuel assembly. An arrangement of the type described is shown in U.S. Pat. No. 4,022,661, which is assigned to the assignee of the present invention. As shown, each fuel rod is resiliently supported between the tie plates by virtue of helical springs surrounding the elongated, reduced-diameter shanks of the upper end plugs of the fuel rods. This arrangement allows longitudinal expansion of the individual fuel rods. It also insures that the fuel rods are firmly seated in the lower tie plate to dampen vibration as pressurized liquid coolant flows upward through the fuel bundle to remove heat from the fuel rods. Typically, the lowest coil of each helical spring is of reduced diameter to provide a friction fit on the end plug shank. This inhibits "pop off" of the springs when the upper tie plate is removed. Further, it inhibits the spring from falling off during manipulation of a fuel rod outside the array. Not withstanding these precautions, springs are occasionally lost from the shanks of the end plugs. Poor visibility and other factors may prevent such a condition from being discovered before the upper tie plate is replaced or first installed. Once the fuel bundle is assembled, the upper tie plate obstructs the view of an observer, particularly with respect to those springs which are mounted on end plugs located near the center of the assembly. Thus, any visual inspection for the presence of these springs is precluded without removing the upper tie plate. Since the act of removing the tie plate may itself cause springs to be lost, as well as being a cumbersome and time-consuming procedure, there currently exists no satisfactory way for reliably detecting the absence of helical springs on the shanks of fuel rod end plugs in the assembled fuel bundle. OBJECTS OF THE INVENTION It is a principal object of the present invention to provide new and improved apparatus for detecting the absence of springs on the shanks of fuel rod end plugs, which is not subject to the foregoing disadvantages. Another object of the present invention is to provide a new and improved probe which permits ready and quick inspection of the springs on fuel rods located in an area of a fuel assembly that is inaccessible and blocked to visual inspection. An additional object of the present invention is to provide a new and improved probe which permits the positive detection of springs that are missing from the shanks of the fuel rod end plugs of a fuel assembly, without the necessity of removing parts from the latter. A further object of the present invention is to provide a new and improved probe which positively indicates the location within the fuel bundle of the fuel rod that is missing a spring. Still another object of the present invention is to provide a new and improved probe which locks in place when the absence of a spring is detected and which requires corrective action to be taken before the probe can be withdrawn from the fuel assembly. Yet another object of the present invention is to provide a new and improved probe for detecting the absence of springs on the shanks of fuel rod end plugs which is simple in construction and inexpensive to manufacture. SUMMARY OF THE INVENTION In accordance with the present invention, a probe is provided which comprises an elongate arm having at least one spring-engaging pawl resiliently biased outward from the shaft. When the probe is inserted between two rows of end plugs of the array, or withdrawn, contact with the springs that are present on the end plugs prevents the pawl from assuming its fully extended position urged by the applied resilient bias. If, however, a spring is missing from the shank of an end plug in the array, the pawl is allowed to move outward relative to the arm so as to become lodged between the springless end plug and its immediate neighbor. This action prevents the subsequent withdrawal of the probe from the fuel assembly and requires that the upper tie plate of the fuel assembly be removed. The position of the probe then indicates to the operator where a new spring must be placed on an end plug. The foregoing and other objects of the present invention, together with the features and advantages thereof, will become apparent from the following detailed specification when read with the accompanying drawings in which applicable reference numerals have been carried forward.