Patent Number: 052672875
Section: description

DESCRIPTION OF THE PREFERRED EMBODIMENT FIG. 1 is a plan view of a nuclear fuel assembly 10, in which only the upper end fitting 12 is visible. The upper portion of the fuel assembly 10 is shown in elevation in FIG. 2, immediately below, and spaced from, a horizontally oriented upper core support plate 100. As would be found in a nuclear reactor core. In the reactor core, a plurality of the vertically oriented fuel assemblies are positioned in side by side relationship. After all the fuel assemblies are in place, the upper core support plate 100 is lowered on to all of the assemblies, substantially simultaneously. Each fuel assembly has a multiplicity of nuclear fuel rods 16 which are connected to the base 14 of the upper end fitting 12. The base includes a plurality of holes 18 by which reactor coolant passes into and through the upper end fitting 12, from the flow channels between the fuel rod 16. The upper end fitting in one conventional arrangement as shown in FIGS. 1 and 2, includes four spring packs or members 20, 22, 24, and 26, each of which extends along one respective side of the substantially square end fitting perimeter. In the illustrated embodiment, spring member 20 is situated between corners 50 and 44, spring member 22 between corners 44 and 46, spring member 24 between corners 46 and 48, and spring member 26 between corners 48 and 50. The spring members project upwardly to define the upper limit of the fuel assembly. Each spring member, such as 24 shown in FIG. 2, includes a rigidly supported portion 28, and a free end 30, between which an active portion or surface 32 is adapted to contact and bear against the support plate 102. In other words, as the support plate 100 is lowered onto the fuel assemblies, contact is made between the surface 102 of the plate 100 and the portion 32 of each spring such as 24, whereby the spring members are simultaneously loaded. The support plate 100 is then rigidly secured to the reactor vessel (not shown), such that all the fuel assemblies are resiliently supported during reactor operation. In the embodiment shown in FIGS. 1 and 2, each spring member 24 is cantilevered. Moreover, it is preferred that each spring member 24 include a plurality of nested, cantilevered spring elements including elements 34 and 36. Spring elements 34 and 36 are rigidly connected at one end to spring portion 28 and these are attached near the corner 46 of the end fitting. At their free ends, elements 34 and 36 have openings 38, 40 through which the free end 30 of the main spring element passes substantially vertically. It should be appreciated that as the plate 100 is lowered onto bearing surface 32, and during flexure of the spring member 24 in operation, the interaction between surfaces 32 and 102, and the mutual points of contact among the spring element such as shown at 42, have a sliding e.g., horizontal, component. This friction not only affects the spring rate, but, particularly where vibratory forces on the fuel assembly are manifested at the spring contact surfaces, can give rise to excessive wear and corrosion. This can pose a problem even when inherently corrosion resistant materials are used for the springs and core support plate. For example, it is typical that the elements of spring member 24 are made from Inconel, e.g., Inconel 718, whereas the core support plate 100 is made from stainless steel. Nevertheless, according to the present invention, there is achieved an enhanced lubricity between each spring member such as 24, against the core support plate 100, and preferably, between the spring elements such as 34, 36, and 28, of a given spring member such as 24. This enhanced lubricity is accomplished by coating at least the active, bearing surface 32 of the spring member, with a smooth metallic material. Metal nitrides, particularly ZrN and TiN, are especially effective. Tests on representative samples for the interaction of these nitride coatings on Inconel 718 show significant enhancement of desirable characteristics. TABLE 1 ______________________________________ Properties of Nitride Coated Inconel 718 Property ZrN TiN ______________________________________ Microhardness 2,895 2,575 (Kg/mm.sup.2) (50 g load) Surface Roughness 0.12 0.38 (rms) (um) Coefficient of Friction 0.020 0.035 ______________________________________ The wear rate of Inconel 718 is reduced by a factor of six, and more importantly for the present invention, the frictional force between the coated spring and the stainless steel core plate, is reduced by about a factor of eight. These data are base on coating Inconel 718 samples using a cathodic vacuum arc plasma deposition process as described, for example, in the article "Cathodic Arc Deposition Advances in Coating Technology", P.C. Johnson, Research and Development, February, 1987. It should be appreciated, however, that other coating processes may be employed to achieve the advantages within the scope of the present invention. Although a given process may be more convenient or cost effective than another, the novelty of coating the spring members on the end fittings of nuclear fuel assemblies, to enhance lubricity, is not dependent on the particular process selected. Other coating materials which can provide significant improvement relative to the current practice of using uncoated spring members, include the metal nitrides CrN, HfN, TiAlVN, TaN, and TICN. In addition, other suitable coatings include Cr, TiC, CrC, ZrC, and NiTaB. Although, as a minimum, the bearing surface such as 32 as shown in FIG. 2, is enhanced by means of the coating, alternatively the entire external surface of the spring member 24, or of each of the spring elements constituting the spring member, can be coated. This enhances lubricity at the contact surfaces, and reduces the risk of corrosion at any other spring member surface. FIG. 3 illustrates another upper end fitting embodiment 200, in which two leaf spring members 202, 204, perform the same function as the four cantilever spring members of the embodiment shown in FIGS. 1 and 2. The leaf spring member 202 is rigidly connected near corners 48 and 44, with the apex 206 substantially above corner 46. Similarly, spring member 204 is rigidly connected at its ends near corners 44 and 48, with the apex 214 substantially above the corner (not shown) opposite corner 46. The leaf spring member such as 202,, is preferably formed from two nested leaf spring elements 208, and 210. The bearing surface 206 at the apex is adapted to contact the upper core support plate, deflect when loaded, and experience flexure during operation, in a manner analogous to that described above with respect to spring member 24. The leaf spring embodiment shown in FIG. 3, has a greater contact surface between the respective spring elements 208, and 210, and therefore would benefit significantly from the enhanced lubricity on the full exterior surfaces of each spring element, in accordance with the present invention.