Patent Number: 047599120
Section: summary

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates generally to nuclear reactor fuel assemblies and, more particularly, is concerned with a boiling water nuclear reactor (BWR) fuel assembly containing nuclear fuel in a hybrid design for elimination of pellet cladding interaction (PCI) constraints and minimization of PCI failures. 2. Description of the Prior Art Typically, large amounts of energy are released through nuclear fission in a nuclear reactor with the energy being dissipated as heat in the elongated fuel elements or rods of the reactor. The heat is commonly removed by passing a coolant in heat exchange relation to the fuel rods so that the heat can be extracted from the coolant to perform useful work. In a nuclear reactor generally, a plurality of the fuel rods are grouped together to form a fuel assembly. A number of such fuel assemblies are typically arranged in a matrix to form a nuclear reactor core capable of a self-sustained, nuclear fission reaction. The core is submersed in a flowing liquid, such as light water, that serves as the coolant for removing heat from the fuel rods and as a neutron moderator. In a typical boiling water reactor (BWR) fuel assembly, a bundle of fuel rods in a N by N array are subdivided into four separate mini-bundles by a central water cross and each mini-bundle is supported in laterally spaced-apart relation by a plurality of spacers axially spaced apart along its fuel rods. Then, all four mini-bundles of the fuel assembly are encircled by an outer tubular channel having a generally rectangular cross-section. The outer flow channel extends along substantially the entire length of the fuel assembly and interconnects a top nozzle with a bottom nozzle. The bottom nozzle fits into the reactor core support plate and serves as an inlet for coolant flow into the outer channel of the fuel assembly. Coolant enters through the bottom nozzle and thereafter flows along the fuel rods removing energy from their heated surfaces. Such BWR fuel assembly is illustrated and described in U.S. Pat. No. 4,560,532 to Barry et al. Operation of reactors at high power density across the core is desired from the standpoint of operating efficiency. There are, however, practical limits on the power density which can be maintained. These limits are essentially two: namely, fuel and reactor structural material temperature limits, and coolant fluid temperature and pressure limits. One consequence of operating the reactor core so as to approach any of these limits in any region of the core is that the problem of fuel pellet clad interaction (PCI) can arise resulting in possible failure of the clad and chemical reaction of the clad or the fuel with the coolant. This and related problems, as well as some solutions to these problems proposed heretofore, are discussed in detail in U.S. Pat. No. 3,147,191 to Crowther. The need for avoiding PCI related fuel failures has recently prompted BWR fuel manufacturers to increase the number of fuel rods in a fuel assembly, thereby decreasing the power generated per rod. However, in the case of the BWR fuel assembly described above, increasing the 4.times.4 fuel rod array in each fuel mini-bundle to a 5.times.5 array leads to a drastic reduction in the bundle inlet orificing for hydraulic compatibility with existing BWR reload fuel. This leads to unaccepted degradation in the stability performance of the 5.times.5 array. It arises for the resulting fuel due to the presence of increased wetted area from the water cross in comparison to other BWR fuel assemblies having conventional open lattice fuel designs. The wetted area from the water cross refers to the additional flow frictional area due to the cold walls within the fuel assembly. Consequently, a need exists for a technique to improve BWR fuel design so as to avoid the potential PCI constraints and failures, but without, at the same time, producing other unacceptable side effects. SUMMARY OF THE INVENTION The present invention provides features which are designed to satisfy the aforementioned needs. The present invention provides a solution to the PCI problem in the above-described BWR fuel assembly which involves retaining the same 8.times.8 fuel geometry (that is, 4.times.4 fuel in each of the four mini-bundles) but distributing the power generation between the existing sixty-four fuel rods and the radial panels of the water cross. This solution envisions a hybrid fuel design composed of a rod-type fuel, similar to that used heretofore, together with a plate-type fuel on the water cross panels. Such an arrangement provides reduced power generation per each fuel rod and fuel plate and leads to a hydraulically stable fuel bundle, simultaneously minimizing/eliminating the PCI problem. This hybrid approach to fuel design transforms a relative disadvantage (i.e., wetted surface due to unheated water cross panels in the present design) into a sizable advantage for overcoming the PCI-related fuel failure problem. Accordingly, the present invention is set forth in a nuclear fuel assembly which includes an outer hollow tubular flow channel for directing the flow of coolant/moderator fluid through the fuel assembly and a hollow water cross extending through the channel and attached along the interior of the channel so as to divide it into a plurality of separate compartments and provide a central hollow channel for flow of coolant/moderator fluid through the fuel assembly. The present invention is directed to a hybrid fuel design comprising: (a) cladded rod-type nuclear fuel located within the compartments spaced from the outer channel and the water cross; and (b) cladded plate-type nuclear fuel attached on the exterior of the water cross. More particularly, the hybrid fuel design comprises: (a) a plurality of elongated fuel rods located within the compartments between the interior of the outer flow channel and exterior of radially extending members of the water cross; and (b) a plurality of elongated fuel plates attached on the exterior of the water cross members. The fuel plates are generally coextensive in length with the fuel rods, and generally coextensive in width with the water cross members but shorter in length than the members. Still further, each fuel plate includes an inner sheet of nuclear fuel disposed adjacent the exterior of each of the water cross members, and an outer sheet of cladding disposed adjacent the exterior of the inner sheet. The outer sheet of cladding has a periphery attached to the water cross members so as to sealably enclose the inner sheet of nuclear fuel. These and other advantages and attainments of the present invention will become apparent to those skilled in the art upon a reading of the following detailed description when taken in conjunction with the drawings wherein there is shown and described an illustrative embodiment of the invention.