Patent Number: 047327308
Section: summary

BACKGROUND OF THE INVENTION 1. Field of Invention The present invention relates broadly to nuclear reactors wherein steam is generated by heat from nuclear fuel rods arranged in fuel assemblies. Periodically, these fuel rods must be replaced. In doing so the used fuel rods are removed from spent fuel assemblies. The present invention is particularly directed to a device to be used during derodding of spent fuel assemblies if a fuel rod becomes stuck in a partially withdrawn position. 2. Description of the Related Art During the operation of a nuclear reactor, the fuel assemblies, which include cylindrical fuel rods, periodically must be renewed. This is accomplished by replacing the rods in spent fuel assemblies. Initially, the spent fuel assemblies must be derodded and on occasion a fuel rod may become stuck in a partially withdrawn position. Continued handling of the fuel assembly requires severing of the stuck fuel rod. Severing of the fuel rod using standard hydraulic shears, bolt cutters, or pipe cutters results in the gross release of radioactive debris, which is, of course, highly undesirable. SUMMARY OF THE INVENTION Accordingly, it is a primary object of the present invention to prevent the gross release of radioactive debris upon the severing of a spent fuel rod. It is a further object of the present invention to accomplish the substantial closure of the severed ends of a spent fuel rod. Another object of the present invention is to provide for the closure of the severed ends of a spent fuel rod through the use of caps or capping sleeves. Other objects and further scope of applicability of the present invention will become apparent from the detailed description given hereinafter. However, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from this detailed description. In accordance with the present invention, a stuck fuel rod capping sleeve is provided in order to permit severing of a fuel rod without the gross release of radioactive material. The stuck fuel rod capping sleeve is a bi-metallic cylindrical device made up of an inner sleeve and an outer sleeve. The inner sleeve is made of a low work hardening highly ductile material (e.g., Inconel 600). The outer sleeve is made of a moderately ductile material (e.g., 304 stainless steel). Severing of the fuel rod is performed by using a bolt cutter device. Upon initial pressure being applied by the bolt cutter device, the bi-metallic sleeve provides strength to permit fracturing of the ceramic fuel without severing of the fuel rod cladding. Continued application of pressure by the bolt cutter device results in separation of the fractured surfaces and a closure of the rod ends. During this stage of cutting, ductile flow of the inner sleeve results in a barrier that restricts the release of radioactive fuel material. The final application of pressure by the bolt cutter device fractures the outer sleeve and results in a separation of the upper and lower portions of the fuel rod. Although the capping sleeve does not hermetically seal the fuel rod, it does prevent the gross release of radioactive fuel material. The stuck fuel rod capping sleeve of the present invention has been tested on unirradiated LWBR fuel rod tubing and simulated (aluminum oxide) fuel pellets. Cutting forces of approximately 20,000 lbs. were needed to sever a 0.30 inch diameter fuel rod. In the tests, end gaps as small as approximately 0.0005 inch were achieved, thus demonstrating substantial closure of the severed ends of the fuel rod. Tests wherein a single Inconel capping sleeve was utilized have also been performed. Although comparable end closures were obtained, in many cases severing of the sleeve was not obtained. In a second embodiment of the stuck fuel rod capping sleeve, each end of the inner sleeve is tapered and split in the region beyond the outer sleeve. Lower and upper locking sleeves are threaded onto threaded portions at each end of the outer sleeve prior to placement of the stuck fuel rod capping sleeve over the stuck fuel rod. Following severing of the stuck fuel rod capping sleeve and the fuel rod, the locking sleeves are advanced to seal the shank of the fuel rod to the inner sleeve and caps are threaded onto the locking sleeves to seal the severed end of the fuel rod. In a third embodiment of the stuck fuel rod capping sleeve, a single Inconel capping sleeve with a recess is used to seal the rod. The recess is packed with an underwater epoxy prior to placing the sleeve on the rod. The epoxy reduces the amount of fission product release by providing a seal between the rod and sleeve. The epoxy also locks the capping sleeve on the fuel rod.