Patent Number: 040594840
Section: summary

BACKGROUND OF THE INVENTION In an attempt to meet licensing requirements of the Nuclear Regulatory Commission, NRC, (formerly the U.S.A.E.C.), fuel designers have had to increase the number of rods per assembly, thereby reducing the LHGR (linear heat generation rates). Along with this change a small size fuel rod is required and a larger number of control rod guide tubes per assembly e.g., a switch from 15 .times. 15 to a 17 .times. 17 fuel array keeping the same external dimensions, with the number of control rod guide tubes increasing from 16 per assembly to 20 or 24. The number and location of control rod guide tubes in the lower LHGR design is not compatible with the number and location of the control rod guide tubes of the higher LHGR design now in use. This change in fuel assembly design would normally require a redesign and retrofitting of the control rod clusters as well as the control rod guides in the upper core structure which is separate from the control rods or the fuel, upon switching from the 15 .times. 15 design to the 17 .times. 17 design in an existing reactor. This retrofitting is costly and entails repair of a radioactive upper structure measuring approximately 15 ft. in diameter. Such repair must be performed at the reactor site and is expected to lead to reactor shutdown over periods of several weeks. The subject invention is intended to enable continued use of existing control rods and control rod guide structures while reducing both maximum and average linear heat generation rates of a bundle as required by licensing criteria (10CFR50, Appendix K). Prior art patents, specifically considered in connection with the present invention include the following U.S. Pat. Nos.: 2,848,404 -- Treshow; 2,961,393 -- Monson; 3,036,964 -- Horning; 3,049,487 -- Harrer et al; 3,060,111 -- Sherman et al; 3,206,369 -- Fortesque et al; 3,211,621 -- Kreegan; 3,147,191 -- Crowder; 3,733,252 -- Georges et al and 3,745,069 -- Sofer et al. While these patents disclose the use of fuel rods of the same size containing different types of nuclear fuel, none disclose an arrangement wherein there is provided a hybrid fuel assembly having different size fuel rods, and more particularly wherein a basic fuel array with islands of fuel rod clusters is used. SUMMARY OF THE INVENTION In accordance with the present invention, there is provided a fuel assembly including island arrays of fuel rod clusters of a first large size disposed within a matrix of fuel having fuel rods of a second small size, in the vicinity of the control rod guide tubes of the fuel assembly. The islands replace a predetermined number of the smaller size fuel rods, which enables retention of the control rod geometry normally associated with a fuel rod array comprising fuel rods of the size of the rods situated in the islands. This arrangement reduces the linear fuel rod heat rate, which reduces the loss of coolant accident (LOCA) temperature to substantially below the maximum fuel rod cladding temperature 2200.degree. F. (required NRC criteria) during LOCA. At this heat generation rate pellet melting along its centerline is avoided under those conditions where excessive power is generated by the assembly. The use of such a hybrid fuel assembly configuration has particular utility for reload fuel for the standard reactor wherein all fuel rods are of the same size and is particularly suitable for pressurized water reactor assemblies (e.g. the Babcock & Wilcox supplied power reactor at Oconee, N.C.) which normally contain about sixteen control rods per assembly. By using the reload arrangement which contains larger size fuel rods, it is possible to retain the control rod guide tube locations in the same position as they would be in a normal fuel assembly. The rod to rod pitch for the large size fuel rods and the actual size of these rods may have to be modified (i.e., increased) from those used in the uniform array, e.g. 15 .times. 15. According to the present invention, the water coolant normally employed in the pressurized water reactor is distributed in the region of the control rods more uniformly when the hybrid fuel arrangement is used than in a fuel assembly having fuel rods of only one size (i.e., uniform) throughout including the region adjacent to the control rods, because the larger hydraulic diameters toward the center of the bundle tend to balance the flow channeling that takes place in the space between bundles. The present invention, when used together with an appropriate fuel enrichment distribution, enables a reduction of the maximum linear heat generation rate (LHGR), by approximately 20 to 25 percent, or in other words, reducing the maximum LHGR from 17.5 KW/ft. to about 12.5 KW/ft. for smaller rods and to 14 KW/ft. for larger rods. The advantages, nature, construction and arrangement and operation of the present invention will be apparent upon consideration of the illustrated embodiments now to be described in detail in connection with the accompanying drawings.