Patent Number: 046876294
Section: summary

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates generally to fuel assemblies for a nuclear reactor and, more particularly, is concerned with fuel rods in a fuel assembly containing annular nuclear fuel pellets having the same U-235 enrichment and different annulus sizes for graduated enrichment loading. 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 nuclear reactors 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. Specifically, in a BWR the fuel assemblies are typically grouped in clusters of four with one control rod associated with each four assemblies. The control rod is insertable within the fuel assemblies for controlling the reactivity of the core. Each such cluster of four fuel assemblies surrounding a control rod is commonly referred to as a fuel cell of the reactor core. A typical BWR fuel assembly in the cluster is ordinarily formed by a N by N array of the elongated fuel rods. The bundle of fuel rods are supported in laterally spaced-apart relation and 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. A hollow water cross extends axially through the outer channel so as to provide an open inner channel for subcooled moderator flow through the fuel assembly and to divide the fuel assembly into four, separate, elongated compartments, each containing a mini-bundle of similar design of the fuel rods. 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 through the water cross and along the fuel rods removing energy from their heated surfaces. Current BWR fuel rod bundle designs make use of solid pellet fuel rods having five to seven U-235 enrichments within a single bundle. These multiple enrichments are needed for radial power shaping within each bundle in order to maintain the fuel rod power peaking acceptably low. However, the requirement of multiple enrichments increases the cost and time required for fabrication. While only a small number of fuel rods in the assembly are involved, approximately one-third of the rods, they require four enrichments to be fabricated. Therefore, just to make these few rods, the UF.sub.6 gas to powder chemical conversion line has to be flushed out four times which is both costly and time-consuming. Market considerations do not justify dedicating a conversion line for each enrichment. The only other alternative, e.g., creating the enrichments needed by blending two or three enrichments, is costly and time-consuming. Consequently, a need exists for a fresh approach to fuel bundle design which will overcome the problems associated with the requirement for multiple fuel enrichment. SUMMARY OF THE INVENTION The present invention provides a fuel bundle composition for a fuel assembly which is designed to satisfy the aforementioned needs. The solution of the invention to the aforementioned problems created by the requirement of solid fuel pellet multiple enrichments is to replace all enrichment grades below the predominant enrichment with fuel rods containing annular fuel pellets. Annular fuel pellets per se are known in the prior art, as represented by U.S. Pat. Nos. (4,493,814), to Beard, Jr. et al (3,215,607), Lackey (4,273,616), Andrews (3,376,201), Bain (3,356,584), Ockert (3,808,099), Ballagny and (3,900,358) Bujas et al. However, none of the annular fuel pellets in the prior art fuel rods have the same enrichment (i.e., the predominant fuel enrichment of the fuel bundle) with preselected different annulus sizes to produce graduated enrichment loading of the fuel bundle. Such graduated enrichment loading by the annular fuel pellets of the invention is generally comparable to the U-235 loading previously provided by the solid fuel pellets multiple enrichments they are replacing but without the problems associated therewith. Thus, by the use of annular fuel pellets having the same enrichment and different annulus sizes, the need for multiple enrichments is eliminated. As a result, the fabrication cost and time required for powder conversion is reduced since only one enrichment is needed. The flushing out of the line after each change of enrichment is also eliminated. Also, the use of annular pellets with graduated annulus sizes permits the fuel assembly design to be customized, if needed, for each utility and each cycle application. The use of multiple enrichments does not permit this customization because of prohibitive costs. Additionally, the use of different annulus sizes to permit one U-235 enrichment to be used for all fuel rods can be extended to the axial direction in single fuel rods. For example, since the core axial power distribution in BWRs is bottom-peaked, the fuel in the top half of the core is under-utilized. That is, for the uranium invested, less energy than desired is produced in the top half vs. bottom half of the core. Thus, it would be advantageous to place the graduated annular pellets of the invention in the top portion of each fuel rod. This increases the energy utilization of the fuel since the kw/kg of uranium is increased. The flattening of the core axial power shape produced by placing the annular pellet fuel in the top portion of each assembly improves the operating margins to the fuel thermal limits by reducing the axial power peaking and maximum hot spot power peaking. The use of annular pellets with graduated annulus sizes in the axial direction can be used as well to provide more shutdown margin in plants that are so limited. A BWR is most limited from a shutdown margin viewpoint at cold conditions. At these conditions, the core axial flux distribution is heavily top-skewed. One means of improving shutdown margin is to reduce the enriched fuel loading in the top third to half high flux and, therefore, high importance region of the core. For example, the average U-235 enrichment can be decreased by approximately 0.1 w/o U-235 in the top third of the core and increased by 0.05 w/o U-235 in the bottom two-thirds of the core to preserve the same overall core average U-235 loading. The decrease of the U-235 enrichment in the top third of the core could be realized by using annular pellets of larger annulus sizes than those in the bottom two-thirds. The net result would be a 0.5 to 1.0% delta-k improvement in shutdown margin with this design. Another benefit of the use of annular pellets with graduated annulus sizes relates to the adverse effects of operating the fuel with control blades inserted adjacent to that fuel. In BWR reactors, a certain number of control rods must be inserted in the core to hold down the excess reactivity of the fuel which is installed to offset the loss of fuel reactivity with fuel depletion. The insertion of a control blade next to a BWR fuel assembly dramatically tilts the pin-wise power distribution within the bundle away from the control blade. That portion of the fuel closest to the control blade therefore does not deplete nearly as fast as the rest of the fuel in the bundle. The result is that, once the blade is withdrawn, the underdepleted fuel is too reactive and therefore produces too much power. This represents a substantial loss of available margin to the fuel operating limit on peaking factor and power density. The problem is further aggravated by the fact that the introduction of a control blade greatly shifts the neutron flux energy spectrum to higher energies. This has the consequence that more fissile plutonium is produced. Again, once the control blade is withdrawn, the higher fissile plutonium inventory in the fuel causes higher power production than desired and loss of operating margin to the fuel operating limits. To minimize these adverse effects of control blade insertion (commonly referred to as control rod history effects) on the power peaking of the fuel, the control blades must be regularly rotated to new positions throughout the cycle. Currently, these control blade rotations (commonly known as rod exchanges) must be done at substantially-reduced reactor power and the plant slowly returned to full power thereafter. This represents a very substantial loss of operating capacity factor for the plant. The use of annular pellets with graduated annulus hole sizes markedly improve the above-described control blade condition. In the proposed design, the largest annulus size pellets are placed on the outermost rows of the assembly. It is precisely those fuel rods that are most affected by control blade insertion. By using annular pellets as opposed to solid pellets and by placing the largest annulus sizes in that region of the fuel assembly closest to the control blade, the fuel rods with the least amount of uranium loading are placed closest to the blade. Since these fuel rods have less uranium, there is less conversion to plutonium possible which reduces the power peaking experienced from the plutonium production. The reduced uranium loading of these fuel rods near the control blade also offsets the lower power production of these rods, due to the control blade presence. The result is that the depletion rate of the fuel measured in units of power produced per unit of uranium loaded is not reduced nearly as much for those rods adjacent to the control blade when annular pellets are used. This again helps to minimize any power peaking increase on those fuel rods once the control blade is withdrawn. Thus, the use of annular pellets with graduated annulus sizes helps to provide greater operating margin to the fuel thermal limits, to reduce the frequency of control blade rotations that are needed and, by so doing, improve the operating capacity factor of the plant. Accordingly, the present invention is directed to a fuel rod for a nuclear reactor fuel assembly, which includes: (a) a cladding tube; and (b) a plurality of fuel pellets being contained in the tube and composed of fissile material having a single enrichment. At least some of the fuel pellets have an annular configuration, with certain ones of the annular fuel pellets having an annulus of a first size and others thereof having an annulus of a second size different from the first size. Such different annulus sizes of the annular fuel pellets allow graduation of axial enrichment loading of the fuel rod. The fissile material is uranium dioxide having a single U-235 enrichment. Also, the present invention relates to a nuclear fuel assembly having a multiplicity of fuel rods, including: (a) a plurality of fuel pellets contained in each of the fuel rods and being composed of fissile material having a single enrichment; (b) each of the fuel pellets in a majority of the fuel rods having a solid configuration; and (c) each of the fuel pellets in a minority of the fuel rods having an annular configuration. Also, each of the annular fuel pellets of some of the fuel rods has an annulus of a first size. Each of the annular fuel pellets of other of the fuel rods has an annulus of a second size. The provision of the single enrichment in combination with different annulus sizes allows graduation of enrichment loading between the solid and annular fuel pellets of the respective majority and minority of fuel rods. The fissile material is uranium dioxide having a single U-235 enrichment, and the value of the single enrichment is at the level of the maximum enrichment loading of the fuel assembly. These and other advantanges 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.