Abstract: An improved grid structure is provided for a nuclear reactor fuel assembly whose sole purpose is to promote a crossflow mixing of the coolant through the fuel assembly rather than the support of the fuel rods. The grid straps form an egg-crate configuration creating cell openings for receiving the fuel rods. Associated with each cell opening is at least one coolant flow mixing vane that projects over the cell opening and four dimple protrusions which are open to the flow of coolant therethrough. The dimple protrusions extend into the cell opening to a further extent than the mixing vane so as to prevent damaging impact of the fuel rod with a vane upon transverse movement of the fuel rod across the cell opening. In reducing the pressure drop of the coolant flow through the assembly, the height of the inner grid straps is substantially less than the height of the outer border strap.

Abstract: Method of making a burnable neutron absorber of boron carbide in a matrix of aluminum oxide. In the practice of this method, boron-carbide particles are coated with a pore former and then they are mixed in a slurry with aluminum oxide powder. When the slurry is subsequently dried and sintered, the pore former is burned out and a body is formed including the boron-carbide particles in a matrix of aluminum oxide with substantially all boron-carbide particles in the body surrounded by a void.