Quick-release fastener eases assembly repair

Lynchburg, VA--With a one-piece tabbed sleeve, Douglas (Jeff) Attix created a fastener for a highly specialized task that's sure to find wider application. Attix, an engineer for Framatome Cogema Fuels, (formerly B&W Fuel Co.), developed the fastener to simplify repair of nuclear-fuel assemblies.

The assemblies comprise a precisely spaced grid of 200 or more fuel rods interspersed with guide tubes for instruments and control rods. Zir-caloy guide tubes extend beyond the ends of the fuel rods, and are attached to machined stainless-steel end fittings. The upper end fittings feature springs to hold the rods in place against upward cooling-water flow and provide an attachment for handling the assemblies.

Highly engineered structures, the assemblies withstand the thermal, radiological, and mechanical stresses in a 2,300-psi, 600F, pressurized water reactor. Fuel rods within an assembly sometimes leak due to corrosion or vibration-induced wear.

Replacing a failed rod involves removing the upper end fitting from the guide tubes and lifting out the rod. Conventional fuel assemblies use specialized threaded or bayonet-style connectors between the guide tubes and end fitting. Threaded fasteners create loose parts on disassembly that, if dropped and lost, can cause future damage to the reactor. The machining required for bayonet fastening adds to the cost of the end fittings and fastener. Both require specialized tools to work with remotely.

Attix's patented quick-release fasteners consist of a thin stainless-steel sleeve with two circumferential rows of outwardly protruding tabs. In practice, they are welded to the end of the guide tube with the tabs pointing down. (Alternatively, the sleeve could include a third row of inwardly protruding tabs. Slipped over the end of the tube, those inward tabs would lock into a groove machined there to hold the sleeve in place.) The upper end fitting features appropriately spaced, counterbored holes larger in diameter than the nominal sleeve diameter, but smaller than the apparent diameter of the protruding tabs.

Lowering the end fitting over the guide-tube sleeve compresses the tabs for clearance. As the upper row of tabs passes the counterbored area of the hole, they spring back out, locking the guide tube to the fitting. The lower tabs remain compressed in the non-counterbored section, providing a measure of stability to the tube.

To separate the tube and end fitting requires only a simple cylinder with an OD less than that of the counterbore diameter and an ID equal to the sleeve diameter. Forcing the cylinder down the counterbore over the sleeve recompresses the retaining tabs, allowing the fitting to be lifted off.

Additional details...Contact Douglas J. Attix, Framatome Cogema Fuels, Box 10935, Lynchburg, VA 24506-0935, (804)832-2738.