Patent Number: 048184799
Section: summary

BACKGROUND OF THE INVENTION This invention relates to liquid metal cooled fast flux nuclear reactor core assemblies and, more particularly, to a nuclear reactor spacer grid formed by the interconnection of grid strips and having a hydraulic spring and to a ductless core component employing this spacer grid. Cores designed for use in liquid metal cooled fast nuclear reactors typically include closely packed hexagonal fuel assemblies whereby the core approaches the configuration of a right circular cylinder. Typical fuel assemblies include a plurality of fuel rods of circular cross section bundled in a triangular array within a full length thin wall duct. The fuel rods are supported at one end, allowing free axial expansion, and laterally supported along their lengths by wire wraps about the rods and/or by egg-crate type grid structures positioned at selected locations along the assembly length within the duct. Such full-length ducted assemblies limit coolant cross flow among adjacent fuel assemblies, increase the amount of metal in the core, thereby lessening nuclear efficiency, and increase the pressure drop through the core which further decreases efficiency. These adverse characteristics can be avoided by use of open-lattice fuel assemblies which eliminate the full length duct in favor of positioning grid structures at selected axial positions, as well known in the generally square fuel assemblies of light water reactors having fuel rods positioned in a square array. One form of ductless fuel assembly is described in U.S. Pat. No. 4,285,771 issued to Robert E. Downs on Aug. 25, 1981 and assigned to Westinghouse Electric Corporation. The fuel assembly described therein includes coextending fuel rods of circular cross section arranged in a triangular rod array with the bundle of rods approaching the configuration of a regular hexagon. At each corner, one or more rods is removed and there is located a strong back, generally coextensive with the fuel rods, which is the axial support for the assembly. The outer periphery of the strong back is also the area of contact among adjacent assemblies. While the strong back structures collectively contain less metal than the thin walled duct which they replace, the amount of metal in the strong backs remain significant. Also each strong back occupies a position which could be occupied by one or more rods containing nuclear material. The egg-crate type grid structures currently employed for laterally supporting rods containing nuclear material within a nuclear core assembly for a liquid metal cooled reactor are formed by the interconnection of grid strip members. However, the method currently used to interconnect the grid strip members to form a spacer grid having cell members therewithin results in approximately half of the walls of each cell member being formed by portions of two grid strip members. The variations in thickness of the grid strip members must be within a certain tolerance. In the walls of the cell members which are made of portions of two grid strip members, the combined variations in grid strip thickness of the two grid strip member portions which make up the walls must be within the same tolerance as the portion of the grid strip members forming single thickness walls. What is needed is a method of manufacturing a grid structure which will overcome this tolerance problem and provide a laterally strong grid structure which does not overly restrict flow of coolant through the core component. Within each cell member formed by the interconnection of grid strip members in the grid structure, a nuclear material containing rod must be laterally supported. This lateral support generally takes the form of hardstop members disposed on two of the walls of the cell member and a spring member disposed on the third wall forcing the rod against the hardstop members. The force of the spring is derived from bending of the metal. During operation of the nuclear reactor, exposure to radiation and high temperatures can result in spring relaxation. This lessening of the biasing force of the spring can permit vibration of the rod within the cell member, causing damage to the rod. What is needed is a spring for providing lateral support for a rod in a nuclear reactor spacer grid which does not have a lessening bias force after continual exposure to radiation and high temperatures. It is thus desirable to provide an open lattice core component for a liquid cooled, fast neutron flux reactor so as to achieve, among other advantages, coolant cross flow among core components. It is further desirable to provide a grid structure for use in this core component which provides sufficient lateral constraint for the nuclear material containing rods, does not overly restrict coolant flow through the core component, and which has cell member walls formed from a portion of only one grid strip member to avoid tolerance problems. It is also desirable to provide within each cell member of this grid structure a spring for laterally constraining the nuclear material containing rod which is not subject to spring relaxation by prolonged exposure to radiation and high temperatures. SUMMARY OF THE INVENTION This invention provides a ductless core component for use in a liquid cooled, fast neutron flux reactor core for axially and laterally constraining a plurality of rods containing nuclear material comprising an upper support plate member, an upper spacer grid member, a plurality of intermediate spacer grid members, a lower spacer grid member, a lower support plate member, a nozzle rigidly engaged to the lower support plate member, a plurality of orifice plates disposed between the lower support plate member and the nozzle, a plurality of support rods passing through the intermediate spacer grid members and fixedly attached to the upper support plate member and the lower support plate member, the support rods being provided with passage axially therethrough for passing a flow of liquid coolant, and a plurality of spacer sleeves disposed about each of the support rods between the upper spacer grid member and the uppermost of the intermediate spacer grid members, between each of the intermediate spacer grid members and the intermediate spacer grid member immediately beneath it, and between the lowermost of the intermediate spacer grid members and the lower spacer grid member for maintaining a predetermined spacing between the grid members between which the spacer sleeves are disposed. The invention further provides in combination with a nuclear reactor spacer grid for use in a nuclear reactor core assembly to laterally constrain elongated rods containing nuclear material passing through the spacer grid, which nuclear reactor spacer grid includes grid strip members, rigid cell members formed by the interconnection of the grid strip members and having central passages for passing axially therethrough the elongated rods and for directing therethrough a flow of coolant, which is provided by a reactor coolant pump, in a generally axially upward direction and into contact with the elongated rods, hardstop means disposed in the cell members on two of the grid strip members forming the cell members proximate the elongated pin, and spring means disposed within the cell member on the balance of the grid strip members forming the cell members and having a face in contact with the elongated rod, the improvement wherein the spring means is provided with an aperture proximate the bottom of the spring means opening into channel means between the spring means and the grid strip member upon which the spring means is disposed for directing a portion of the coolant flow passing upwardly through the cell members to flow through the aperture means and the channel means and into contact with the spring means opposite the face in contact with the rod to provide the spring means with a lateral hydraulic biasing force for continually urging the elongated rod against the hardstop means, the strength of the hydraulic biasing force being dependent on the rate of flow of coolant through the reactor core assembly. The invention also provides a nuclear reactor spacer grid for use in a nuclear reactor core assembly to laterally constrain elongated rods containing nuclear material, which nuclear reactor spacer grid comprises a plurality of top grid strip members provided with first slot means along the bottom edges thereof, a plurality of middle grid strip members provided with second slot means along the top edges for engaging the first slot means of the top grid strip members when the top grid strip members and the middle grid strip members are interconnected, the middle grid strip members are further provided with third slot means along the bottom edges thereof, a plurality of bottom grid strip members provided with fourth slot means along the top edges for engaging the third slot means when the middle grid strip members and the bottom grid strip members are interconnected, the interconnection of the top grid strip members, the middle grid strip members, and the bottom grid strip members, forming a plurality of cell members with each elongated pin passing through and being laterally constrained with one of the cell members, and an outer grid strip member extending about the periphery of the spacer grid and engaging the lateral edges of the top grid strip members, the middle grid strip members, and the bottom grid strip members.