Importance

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Abstract

A grid for use in nuclear fuel assemblies is made up of double grid members which are deformed so as to provide channels for the coolant. At least the upper portions of these channels are inclined relative to the axes of the fuel rods and are disposed in such a manner as to cause circulation of fluid about each individual rod and within the assembly. In one embodiment the grid is positioned between the positioning grids and serves primarily as a coolant deflector. In another embodiment so constructed as to serve the positioning grid, as well as a flow deflector, the channels are straight in their lower portions i.e. parallel to the axes of the fuel rods. In their upper portions they curve to provide the desired deflection of the cooling fluid and the straight sections have a slight bow outward, relative to the plane of the grid member of which they are a part. This outwardly bowed portion engages the fuel rod and acts as a positioning spring. The grid members are cut away adjacent the straight sections to give the necessary flexibility. This design is particularly useful in pressurized water reactors. In grids intended for use in boiling water reactors, where the lateral loading is less severe, the straight portion can be omitted. The two plates may be welded together at their intersections, at their upper edges only, while the lower edges are left free. In this case, the flexing of the grid members themselves serves to provide the spring forces necessary to position the fuel rods. In order to give improved flexibility, and proper fit with the rods when loaded, the surfaces of the channels which contact the fuel rods are given a twisted or helical formation.

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First Claim