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.
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