Patent Number: 052672875
Section: summary

BACKGROUND OF THE INVENTION This invention relates to nuclear reactor fuel assemblies and in particular to assemblies which are mounted side by side in a reactor core. The fuel rods of each assembly are held by spacer grids between an upper end fitting or top nozzle and a lower end fitting or bottom nozzle. 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 spring members on 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. The holddown spring members are exposed to the high pressure and temperature of the circulating coolant. They experience flexure while accommodating relative movement between the fuel assemblies and the support plate, and they experience some friction at the active surface which is in contact with the core support plate or when stacked with each other. This friction can have two adverse consequences. First, the spring coefficient can be affected if the point of contact of the spring active surface against the support plate or each other, cannot adjust as the spring flexes. Secondly, and perhaps more importantly, friction can produce wear of the upper core plate and spring which can affect the available spring force and the integrity of the spring member itself. SUMMARY OF THE INVENTION It is, accordingly, an object of the present invention to increase the lubricity of the contact surfaces between the spring members on the end fittings of nuclear fuel assemblies, and the core support plates in a nuclear reactor. According to one embodiment, the invention is directed to a method for fabricating an end fitting for a nuclear fuel assembly, wherein the spring members of the end fitting are coated, at least on the active external surface which contact the core support plate, with a lubricity-enhancing material. Suitable materials include a variety of nitrides, and Cr, TiC, CrC, ZrC, and NiTaB. In the apparatus embodiment, the invention is directed to a nuclear reactor having a substantially horizontally oriented core support plate and a plurality of nuclear fuel assemblies each having at least one spring member bearing against the support plate, wherein the spring members include a metallic coating to reduce friction at the bearing surface of the spring member against the support plate. Preferably, the support plate is stainless steel, the spring member is Inconel, and the coating on the spring member, is one of ZrN or TiN. The spring members in some types of conventional upper end fittings, are composed of a plurality of nested, cantilevered spring elements. Preferably, the coating is applied not only to the upper spring element, which contacts the support plate, but also to the other spring elements which are in contact with each other and the upper spring element. This reduces friction and wear within the spring member, thereby also maintaining a predictable spring rate and avoiding excessive wear and corrosion.