Development of a solution to the debris fretting problem

Abstract

One of the major recognized causes of fuel rod failures is fretting of the clad due to the entrapment of small particles of debris in a fuel rod spacer. Such debris, inadvertently dropped into the primary system during maintenance operations, includes various sizes of particles. Very small particles of the size of a grain of sand pass through the system without harm. Some larger size particles such as large bolts and nuts cannot pass through the fuel assembly inlet (lower tie plate) and thus cause no harm to the fuel rods. Intermediate size particles, however, such as metal cuttings, electrical connectors, metal fittings, pieces of wire, and small nuts and bolts can pass through the holes in the lower tie plate and become trapped between fuel rods in a spacer where hydrautically induced vibrations can cause fretting failure of the fuel rod. A Curved-Blade Tie Plate (FUELGARD™) has been developed which effectively traps the intermediate size debris before it enters the lower spacer. This specially designed tie plate greatly reduces the danger of debris damage with little or no increase in the pressure drop of the coolant flow system.