Patent Number: 039403114
Section: summary

BACKGROUND OF THE INVENTION This invention relates generally to nuclear reactors and, particularly, to reactors having fluid pressure operated control rod mechanisms. The core of a modern pressurized water cooled reactor of 1000 MWE output contains approximately 40,000 individual fuel rods, each having two welded end plugs. The total length of the rods is nearly 100 miles. With the high number of welds and large clad surface area, there is a probability that one or more fuel rods will develop defects during operation even though the highest grade of quality control is maintained during the manufacture of the rods. The defects probably will be in the form of pin holes or cracks in welds or cladding material. In any case, it permits escape of some of the fission products into the coolant stream and causes a rise in radioactivity in the entire coolant system. (The most abundant of the fission products are Kr88, Rb88, I131, I133, Xe133, Xe135 and Cs137). A certain amount of fission product leakage can be tolerated without causing too much of a problem, since the level of radioactivity can be limited by continuous removal of the fission products with available systems. For example, the Xenon gases are removed by gas stripping techniques in the volume control tank and the gas decay tank, while the others are removed in the demineralizers. However, if the leakage of fission products into the coolant exceeds the capacity of these systems, the general level of radioactivity gradually increases until it exceeds permissible limits and it becomes necessary to shut down the reactor or, at least, to continue operation at reduced power. After the reactor is shut down, the task of locating and removing the defective fuel assembly still remains. In prior nuclear power plants, means are available for determining during operation only that a leaky fuel assembly is present in the core and, at best, in which quadrant it is located. Pin-pointing of the fuel assembly requires removal of all fuel assemblies in turn to a special inspection chamber for testing. Since the production of fission products ceases at shutdown and the possibility exists that a crack may seal itself when the temperature is reduced, the detection of a defective fuel assembly becomes a very difficult and intricate task which may require a month or more of reactor shutdown time. Therefore, it is desirable to provide a system for determining the exact location of a defective fuel assembly during normal reactor operation to permit speedy removal of the assembly before conditions become intolerable, or preferably during a scheduled shutdown. The latter becomes a real possibility if a "Rapid Refueling" system is adapted since scheduled refuelings take place at much shorter intervals than with conventional reactors. Also, it is desirable to simplify the structure of the upper internals of a reactor to facilitate testing for a defective fuel assembly. SUMMARY OF THE INVENTION In accordance with the present invention, a nuclear reactor having fluid pressure operated control rod drive mechanisms is provided with completely enclosed guide tubes for the control rods and their drive shafts. The guide tubes are mounted inside support tubes extending between the upper core plate and the upper support plate of the reactor internals. The control rod drive shafts enter the reactor vessel head through adapters having the drive mechanisms mounted exteriorly of the vessel. The control rod mechanism fluid pressure system is utilized for failed fuel rod detection by obtaining coolant samples from all fuel assemblies provided with control rods. A defect in any of these fuel assemblies, which constitute 35 to 40 percent of the total number, can be located directly and without difficulty. A sample of the coolant from a selected fuel assembly is caused to flow directly to the fluid pressure mechanism through an isolated guide tube and associated adapter tube. The coolant passes through the mechanism and then to a radiation monitor. A sealing arrangement is provided between the lower end of the adapter tube and the top of the guide tube assembly to insure that sampling water is not permitted to mix with water above the upper support plate before arriving at the adapter tube. Defects in other fuel assemblies can be located by indirect methods. By suppressing the power output of a tested, non-defective fuel assembly of the above group by temporarily inserting all control rods, coolant from adjoining assemblies is caused to mix with coolant from the tested assembly in sufficient quantities to determine if any of these has developed a defect. Pinpointing of a defective assembly is them accomplished by testing in the immediate neighborhood.