Patent Number: 042241073
Section: summary

This invention relates to nuclear reactor fuel assemblies each constituted by a cluster of parallel fuel pins of substantial length and small cross-sectional area each containing a stack of nuclear fuel pellets within a metal can. This fuel cluster is associated along the length of the fuel pins with spacer grids placed at uniform intervals so as to permit suitable bracing of the fuel pins with respect to each other while preventing vibrations of these latter and deformation of the initial geometry of the cluster under the action of the stresses to which the fuel assembly is subjected during operation within the reactor core. In a fuel assembly of this type which is usually intended to be employed in a light-water reactor, the invention is more particularly applicable to an improvement in the construction of the spacer grids of the fuel-pin cluster. It is known that a spacer grid of the type mentioned above is usually constituted by an open structure formed by two sets of sheet metal members which are parallel to each other but oriented in two perpendicular directions in each set. Said sheet metal members are intended to interengage in slits formed lengthwise in these latter and thus constitute a series of compartments having a generally square cross-section through which the fuel pins are intended pass in a direction parallel to the plane of said sheet metal members. A number of different solutions have already been proposed in order to ensure correct positioning of said fuel pins within each compartment of the spacer grid and especially in order to ensure suitable stress distribution on the fuel-pin cans at the level of each spacer grid. In particular,* the sheet metal members of the grid are provided with wide openings separated by narrow corrugated strips forming springs and projecting either inwardly or outwardly from the compartments so as to provide, in conjunction with other corrugated strips or with bosses formed in the solid portions of the sheet metal members, a series of bearing points for each fuel pin at the location corresponding to the passage of said pin through each compartment. Each fuel pin is thus maintained in two different directions at four points spaced at uniform intervals at its periphery and also at a number of points in the longitudinal direction by means of as many spacer grids as the total number provided for the fuel assembly. By making use of narrow corrugated strips for separating open portions having appreciable dimensions, the coolant fluid which circulates in contact with the fuel assembly ensures efficacious cooling of the fuel pins; furthermore, the shape and arrangement of the corrugated strips and of the bearing bosses makes it possible to ensure efficacious absorption of vibrations and expansions during operation. FNT * in the U.S. Pat. No. 3,674,687 of June 20, 1969 In a practical design of this type, the corrugated strips which form springs must maintain the fuel pins with a sufficient bearing force against the fixed bosses of the sheet metal members, especially in order to prevent wear as a result of impact of the fuel-pin cans against said bosses under the action of vibrations produced by the flow of the coolant fluid. On the other hand, the clamping action thus produced on the fuel pins must not be too powerful in order to prevent collapse of the fuel cans. However, the springs constituted by the corrugated strips must provide adequate compensation for manufacturing tolerances allowed on the diameters of the fuel pins and on the dimensions of the spacer grid compartments. In consequence, the curve representing the force applied by the spring formed by each corrugated strip as a function of its deflection must be as flat as possible within the operating range corresponding to a bearing pressure which varies only to a slight extent in respect of a range of deflection which can on the contrary vary to an appreciable extent; it is therefore found necessary to increase the flexibility of the spring without thereby permitting the possibility of modifying the spacing of the sheet metal members of the grid. The present invention is directed to an improvement which is made in spacer grids of the type referred-to in the foregoing and which meets this requirement. To this end, the invention is characterized in that the sheet metal members are joined to the corrugated strips by means of a zone which has lower mechanical strength and increased flexibility and increases the deflection due to increased flexibility of each strip in respect of the same applied force.