Nuclear fuel spacer grid assemblies serve to maintain the appropriate spacing between fuel rods in reactor cores, preventing rod vibration and providing support. Alumina’s mechanical strength, high-temperature resistance and thermal shock resistance make it an ideal material for the small tubes that serve as rod spacers. One advanced ceramic manufacturer recently sought to improve its grinding process, as the consistency and reliability of these alumina rod spacers are of the utmost importance.
“The length of the tubes is critical to the efficiency of the fuel rod,” says John Bannayan, president of precision grinding solutions provider Glebar Co. “The components need to be processed at a high production rate with the length verified for all parts. The biggest challenge was how to handle this delicate ceramic material, as it is susceptible to chipping.”
Finding a Solution
The ceramic manufacturer turned to Glebar’s DD-7 double-disc grinder, which was configured with servo-driven spindle slides with 0.1-micron linear glass scale feedback to the motion controller. All slide components are mounted on a solid granite bed for stability and vibration dampening. The final result is a seemingly simple machine design, where a vibratory bowl feeder feeds the components to a conveyor that loads and clamps them on a fixture mounted on a precision motorized slide. The part is shuttled between the grinding wheels, which grind the material off and qualify the length. As the part is removed from the grinding wheels, two probes measure the length of the part and compensate for size variation as the part is ejected onto an exit conveyor.
Selecting the appropriate grinding wheel composition is crucial for achieving the alumina spacer’s perpendicularity of better than 5 microns. Abrasives are designed to efficiently remove material while exposing the grit that is performing the removal to a sharper layer underneath, maintaining the cutting efficiency of the abrasive over time. In this application, diamond wheels were used to grind the alumina, with a lead-in shape allowing the component to be gently introduced to the abrasive section of the wheel, progressively removing the material in a controlled yet efficient fashion.