Patent Number: 042736161
Section: summary

BACKGROUND OF THE INVENTION This invention relates generally to nuclear reactors, and more particularly to nuclear fuel rods for water-moderated commercial reactors. Current practice in water reactors is to use fuel rods having Zircaloy cladding encapsulating a column of uranium dioxide (UO.sub.2) pellets in fuel assemblies where the fuel rods are spaced apart and supported by a plurality of grids. Typically, several of these grids are positioned axially along the full length of the fuel rods, which is typically 10 to 15 feet. The conventional UO.sub.2 pellet is a solid cylinder with a variety of end shapes, usually dished and chamfered. Recently, commercial nuclear power plant owners have found it advantageous to burn the nuclear fuel in longer cycles to improve the economics in view of the inability to reprocess spent fuel. One fuel pellet design which is known to permit extended burnup is in the form of a hollow cylinder. When stacked in columns of the type required for commercial water reactors, these hollow pellets form a long and continuous central passage within the fuel rod. During operation, the fuel pellets can crack, forming pellet debris which can enter the passageway and accumulate at the bottom of the fuel rod. If this debris deposit is sufficiently large, the redistribution of the UO.sub.2 in the fuel rod could significantly affect the power distribution generated by the fuel rod. This redistribution is not desirable and may in some instances limit the operating level of the reactor. SUMMARY OF THE INVENTION The present invention solves this problem associated with using hollow fuel pellets in modern water reactors by locating spacer plugs at selected elevations along the pellet column to trap pellet debris that may fall through the passage. This prevents accumulation of debris at the bottom of the column, and reduces the shift in power distribution of the fuel rod. The plugs are preferably located at the same elevations as the fuel assembly grids. Because the grids are typically parasitic thermal neutron absorbers, the grid locations usually operate at a depressed power level when compared to areas away from the grids. Thus, the perturbation in the physics characteristics of the fuel rod caused by the presence of the plug is minimized. In another embodiment, the plugs are made of graphite, which is a good neutron moderator. The addition of moderator helps restore some of the thermal neutron flux which has been depressed by the grids.