Patent Number: 055662170
Section: summary

TECHNICAL FIELD The present invention relates to a spacer for nuclear fuel rods and particularly to a reduced height spacer having minimum spacer material with consequent minimum impact on fuel bundle performance. BACKGROUND In nuclear reactors, for example, a boiling water reactor, nuclear fuel rods are grouped together in an open-ended tubular flow channel, typically referred to as a fuel assembly or bundle. A plurality of fuel assemblies are positioned in the reactor core in a matrix and a coolant/moderator flows upwardly about the fuel rods for generating steam. Fuel rods are supported between upper and lower tie plates in side-by-side parallel arrays. Spacers are employed at predetermined elevations along the fuel bundle to restrain the fuel rods from bowing or vibrating during reactor operation. Typical spacers often include a plurality of ferrules arranged in side-by-side relation and secured, for example, by welding to one another to form the support matrix of the spacer for the nuclear fuel rods. Generally, each ferrule includes circumferentially spaced protuberances and a spring assembly along an opposite side of the ferrule from the protuberances for centering and biasing each fuel rod against the protuberances, thereby maintaining the fuel rods in fixed relation one to the other across the spacer. Generally, the role of a spacer in a fuel bundle is to position the fuel rods for peak performance and to protect the fuel rod assembly during possible loading events, such as handling and shipping. The spacer itself, however, constitutes an obstacle to bundle performance in that its cross-section interferes with the flow of water/moderator through the bundle. An ideal spacer would have minimal impact on bundle performance (thermal hydraulics, critical power), while still restraining the rods in their intended positions and protecting them. Consequently, an optimum spacer should have as little cross-section as possible, use a minimum amount of material and simultaneously meet structural requirements for positioning and protecting the fuel rods. DISCLOSURE OF THE INVENTION According to the present invention, there is provided a spacer which employs ferrules to capture and retain the nuclear fuel rods in the bundle in the intended array. Ferrules are employed because of their excellent structural integrity as compared with other possible cross-sectional shapes. By shortening the height of the ferrule in accordance with the present invention, the structural integrity of the ferrule is retained, while simultaneously, the magnitude of the material employed in the spacer is greatly reduced. A separate spring, also having a reduced quantity of material, is provided for each ferrule and along a side thereof opposite a pair of stops formed along the interior ferrule surface. The spring constitutes a leaf spring extending above and below the opposite edges of the ferrule and having upper and lower contact points for engaging the fuel rod and biasing it against the stops. A central portion of the spring extends in a central opening of the ferrule and projects in an opposite direction toward an adjacent ferrule. Thus, the adjacent ferrule, which is welded to the one ferrule, provides a reaction force for the spring so that the spring biases the fuel rod in the one ferrule against its stops. That is, the spring preloads the fuel rod in one ferrule by engaging it at locations above and below the opposite edges of the one ferrule and engaging the adjacent ferrule to provide a reaction force for the spring. This ferrule configuration thus meets the requirements for a spacer having minimum material and cross-section and the necessary structural requirements. In a preferred embodiment according to the present invention, there is provided a subassembly for a spacer useful in a nuclear fuel bundle for maintaining a matrix of a plurality of nuclear fuel rods passing through the spacer in spaced-apart relation, comprising first and second ferrules lying adjacent one another for receiving respective nuclear fuel rods, each ferrule having a pair of fuel rod contacting points along one side of the ferrule for abutting a fuel rod within the ferrule and an opening along a side of the ferrule opposite the one side, a spring including a spring body lying in a plane having opposite end portions projecting to one side of the plane, a central portion between the end portions projecting to the opposite side of the plane and openings on opposite sides of the central portion between the central portion and the end portions, the spring being disposed between the ferrules with the central portion in the central opening of the first ferrule and bearing against the second ferrule between the contacting points, portions of the first ferrule on opposite sides of the opening therethrough extending in respective openings of the spring, the end portions extending beyond opposite upper and lower edges of the first ferrule for bearing directly against a fuel rod passing through the first ferrule and biasing the fuel rod against the contacting points along the one side of the first ferrule. In a further preferred embodiment according to the present invention, there is provided a spacer for maintaining a matrix of nuclear fuel rods in spaced-apart relation between upper and lower tie plates, the spacer assembly comprising a matrix of adjacent ferrules for receiving the fuel rods in the spacer, each ferrule having a pair of fuel rod contacting points along one side thereof for abutting a fuel rod within the ferrule and having an opening along a side of the ferrule opposite the one side, a plurality of springs, each spring including a spring body lying in a plane having opposite end portions projecting to one side of the plane, a central portion between the end portions projecting to the opposite side of the plane and openings on opposite sides of the central portion and between the central portion and the end portions, each spring being disposed between an adjacent pair of the ferrules with the central portion in the central opening of one of the adjacent ferrules and portions of the one ferrule on opposite sides of the opening therethrough extending in respective openings of the spring. Accordingly, it is a primary object of the present invention to provide a novel and improved spacer for the nuclear fuel rods of a nuclear fuel bundle, affording improved performance by shortening the height of the spacer and minimizing its cross-section, hence reducing the spacer material while simultaneously providing the necessary structural integrity to maintain the fuel rods in position.