Patent Number: 043269222
Section: summary

BACKGROUND OF THE INVENTION 1. Field of the Invention This invention relates to rodded open lattice nuclear fuel assemblies and more particularly provides a fuel assembly having fuel rods of differing diameter. 2. Description of the Prior Art Nuclear reactors typically include a core made up of a plurality of nuclear fuel assemblies disposed within a vessel through which a gaseous or liquid coolant flows. The coolant removes energy in the form of heat as it flows about and through the fuel assemblies. Particularly in pressurized reactors circulating a liquid coolant such as water it is undesirable to reach a boiling condition and, accordingly, reactor cores are designed to prevent a condition commonly referred to as "departure from nucleate boiling" (DNB). If a vapor is permitted to exist along the fuel rods the heat transfer from the heat generating rods to the surrounding coolant is substantially impaired and a potenstial exists for damage to the fuel rods through overheating. Accordingly, DNB related safety criteria impose an upper boundary on the maximum coolant temperature and therefore limit total reactor efficiency. These concerns are further complicated by the desired utilization of mixed oxide or plutonium fuels which are more expensive to fabricate and inherently have a high neutronic capture cross section and a strong moderator temperature coefficient as compared to more commonly used uranium fuels. And, since the coolant becomes progressively hotter as it flows upwardly through the core it therefore progressively changes density, providing progressively less moderation at the upper portion of the core as compared to the bottom. This results in less control and design flexibility. Approaches toward alleviating these limitations, primarily in response to DNB effects, include increasing the water-to-fuel ratio of the core and utilizing full core length fuel rods of smaller cross section which generate less energy per unit length. These approaches, however, are complicated by other factors including excessive fabrication costs and larger cores. Additionally, reactor designs incorporating redundant safety systems which respond to the unlikely event of a rupture of the reactor coolant system causing a loss of coolant, flood the core area with coolant that builds up from the bottom toward the top of the core. Additional flexibility in providing a core design responsive to this condition is advantageous. It is therefore desirable to provide a nuclear fuel assembly which alleviates the temperature and efficiency limitations, and it further is desirable to provide such an assembly which is compatible with mixed-oxide fuel utilization. It is additionally desirable to provide design flexibility to respond to the varying coolant density throughout the core and unlikely coolant discharge occurrences. SUMMARY OF THE INVENTION This invention provides a nuclear fuel assembly which allows higher coolant operating temperatures and enhanced utilization of mixed-oxide fuels. It also provides additional core design flexibility. In one embodiment it provides a vertically oriented assembly of elongated fuel rods of two differing diameters. The upper portion of the assembly includes rods of a smaller diameter than rods of the lower portion of the assembly. Both the smaller upper and the larger lower rods are preferably cylindrical and supported by lattice grid structures which allow axial rod expansion. The grids form cells about each rod for lateral support, and the lowermost grid of the upper rods is preferably directly fixed to the uppermost grid of the lower rods. The grids are also joined through tubular guide thimbles which extend through both the upper and lower rod arrays and through which neutron absorbing control rods can be inserted and withdrawn from the assembly to adjust the neutron flux. A slight space is provided between the upper and lower fuel rods to allow, among other advantageous functions, axial rod expansion without interference among the upper and lower fuel rods. The assembly is advantageously adaptable to use of mixed-oxide fuel in the bottom rods and uranium fuel in the upper rods. In this configuration the space between the upper and lower rods tends to alleviate the sharp power peak which could otherwise occur at the interface of the plutonium and uranium fuels. In a typical reactor the coolant flows upwardly through the core and locating the smaller diameter rods in the upper portion of the assembly produces a lower generated energy per unit length of the fuel rod in the area where the coolant is the hottest. This allows a higher coolant dicharge temperature from the core and a more efficient system. The relative cross sectional size of the individual upper and lower fuel rods can also be varied as dictated by the designer to respond to the change in moderation with coolant temperature change. Further, a plenum is typically provided in the upper portion of most fuel rods to allow accumulation of gaseous fission products released during reactor operation. Because of the lower generation rate in the upper fuel rods, this upper plenum can be of reduced size in comparison to previous fuel rods. A plenum is also provided for the more energetic lower rods; however, it is located at the bottom of the lower rods to reduce flux peaking at the center of the core. Additionally, neutron absorbing materials can be positioned at the top of the lower fuel rods and at the bottom of the upper fuel rods to limit moderation in the area between the two rod arrays.