1. Field of the Invention
This invention relates generally to a nuclear fuel assembly spacer grid for use in fuel assemblies utilizing a hexagonal cross-section. More specifically, this invention relates to a grid which features a design and a fabrication technique which enables a low cost and a potentially low pressure drop alternative to the existing designs.
2. Description of the Related Art
Six-sided fuel assemblies are used in various reactor core designs, especially in Europe and Asia. One possible fabrication technique resides in brazing individual hexagon-shaped tubular components together to form the overall structure. The fuel rod contact features are created prior to final assembly by piercing and forming portions of selected tube walls, such that these portions project inwardly toward the rod. In some designs, certain of the hexagonal components are eliminated in special assembly locations which will later contain cylindrical members, or thimbles, to guide control rods and form the longitudinal structure of the fuel assembly.
Drawbacks to this type of design reside in the fact that many individual components are required to form the grid, and that various fabrication problems associated with the brazing process, most notably its incompatibility with low neutron capture zirconium alloys, are encountered. Moreover, the double wall thickness which inherently occurs between each pair of fuel rods induces a relatively high pressure drop.