Patent Number: 054405980
Section: description

BEST MODE FOR CARRYING OUT THE INVENTION Reference will now be made in detail to a present preferred embodiment of the invention, an example of which is illustrated in the accompanying drawings. Referring now to FIG. 1, there is illustrated a core C of a typical boiling water nuclear reactor, including four side-by-side fuel bundles B disposed in a square array, the four bundles being illustrated at 12, 14, 16 and 18. The fuel bundles are supported in a manner well known to those skilled in this art and description of their support is not believed necessary. As illustrated, each fuel bundle includes a plurality of discrete fuel rods 20 containing nuclear fuel pellets and a water rod 22, whereby the reactor is supplied with fissionable material. Suffice to say that the self-sustaining fission reaction produces fission products, the kinetic energy of which is dissipated as heat in the fuel rods with the heat being removed by coolant water surrounding the rods and bundles boiling into steam from which useful work is extracted. Referring to FIGS. 2a-2c of the prior art, FIG. 2 illustrates a typical fuel bundle design using enriched uranium as fuel. As illustrated in FIG. 2a, each fuel rod 24 in the lattice of rods is provided a number indicating its fuel loading. In FIG. 2b, the enriched uranium and burnable poison, e.g. gadolinium, loadings of each rod are illustrated. For example, the fuel rod denoted 3 in FIG. 2b indicates a fuel rod containing 3.6% enriched uranium, the balance of the rod being formed substantially of uranium. Also, all rods designated 3 in the bundle B of FIG. 2a have these constituents. The rod numbered 7, however, contains 3.95% enriched uranium and 5.00% burnable poison, e.g., gadolinium. As noted in FIG. 2a, the rods numbered 7 having the combined enriched uranium and gadolinium concentration are few in number and are arranged geometrically symmetrically within the interior of the fuel bundle. Thus, only eight of the fuel rods of the sixty rods illustrated in a typical fuel bundle are loaded with the burnable poison gadolinium. In FIG. 2c, there is illustrated a typical burnup reactivity curve showing the reactivity (K-.infin.) as a function of fuel exposure (burnup). Burnup is a measure of the energy produced by the fuel during its useful lifetime. Note that the fuel reactivity rises sharply from a value near 1 at the beginning of its life, to a value of about 1.15 at a predetermined time corresponding to about 8 GWd/MT on the part of the curve designated a. This initial reactivity then declines in a nearly linear fashion to the end of its useful lifetime as indicated on the part of the curve designated b. It is important to maintain this characteristic curve for each fuel bundle as it permits fresh fuel bundles with increasing reactivity to offset the declining reactivity of the older fuel bundles. The reactivity balance enables the reactor to operate for extended periods at a relatively stable steady rate. In FIGS. 3a-3c, there is illustrated a fuel bundle incorporating plutonium as fuel. Similarly as in the preceding drawing figures, enriched uranium, plutonia and gadolinia concentrations are shown for each rod. For example, rod numbered 3 includes 0.2% enriched uranium and 3.00% plutonium as the fissile material, the balance of the rod comprising substantially uranium. Gadolinia is not provided the rods numbered 3 in the lattice of FIG. 3a. Rod numbered 6, as illustrated in FIG. 3b, contains 0.20% enriched uranium, 5.00% plutonium and 3.00% gadolinia. As illustrated in FIG. 3a, the gadolinium-loaded fuel rods numbered 6 are few in number and lie within the fuel rod lattice, eight such rods being shown lying symmetrically within the interior of the fuel rod bundle B. A characteristic reactivity curve for these fuel rods is illustrated in FIG. 3c. Note that the reactivity curve is considerably different in shape, including slope, than the reactivity curve for the enriched uranium fuel rods of FIG. 2a. Turning now to FIGS. 4a-4c, illustrating a fuel bundle design according to the present invention, it will be appreciated that the same number of control rods in an 8.times.8 array as in the prior two fuel bundles is illustrated. FIG. 4b, as in the corresponding figures of the prior art, illustrates the constituents of the fuel rods. For example, fuel rods numbered 5, 6 and 7 have a gadolinia concentration of 1.00% in combination with various percentages of enriched uranium and plutonium. Rods numbered 1, 2, 3 and 4 are void of gadolinia. As illustrated in FIG. 4a, the gadolinia-loaded rods are arranged in an interior array and are wholly surrounded by an exterior array of fuel rods numbered 1, 2, 3 and 4 void of gadolinium. In FIG. 4c, there is illustrated a burnup reactivity curve for the fuel bundle of FIG. 4a having the constituents identified in FIG. 4b. Note the rise in the fuel reactivity from startup to a peak indicated at a' and the nearly linear decline from the peak indicated at b'. In accordance with the present invention, it has been found that the number of fuel rods containing the burnable poison, e.g. gadolinium, when combined with plutonium in an interior array of fuel rods, should be in excess of 20% of the total number of fuel rods in the fuel bundle in order to produce the characteristic reactivity curve illustrated in FIG. 4c. Note the substantial similarity in shape including slope between the reactivity curve of FIG. 4c and the reactivity curve of FIG. 2c, as well as the substantial dissimilarity between the reactivity curve of FIG. 4c and that of FIG. 3c. As indicated previously, it is important that the fuel bundle of the present invention, which results in enhanced use of plutonium as the fuel as in FIG. 4a, has a reactivity curve substantially corresponding in shape including slope to the reactivity curve of the bundles employing enriched uranium and gadolinium, as illustrated in FIGS. 2a-2c. These two reactivity curves of FIGS. 2c and 4c also substantially correspond in reactivity values. Thus, in FIG. 4 a, each rod of the interior array of 32 fuel rods has a combination of enriched uranium, plutonia and gadolinia, hence enhancing plutonium usage, while the exterior array of 28 fuel rods is void of the burnable poison gadolinia and has a combination of only plutonium and enriched uranium. Lesser number of interior fuel rods containing the plutonium, enriched uranium and gadolinium can be used with the reactivity curve remaining substantially as illustrated. Consequently, the number of fuel rods containing the gadolinium lying in the interior array of fuel rods in the bundle should number in a range of 20% to 60% of the total number of rods in the fuel bundle. Also, each rod in the fuel rod bundle preferably has a percentage concentration of plutonium as one of its constituents and in excess of the percentage constituent of any other fissile materials in the rods. In this manner, the reactor fuel contains enhanced quantities of plutonium as compared with the quantities of plutonium previously thought possible as part of the fuel for reactors of this type. By substituting a fuel bundle of the type illustrated in FIGS. 4a and 4b for a conventional fuel bundle of the type illustrated in FIGS. 2a and 2b in a nuclear reactor core, enhanced plutonium usage is obtained. This is made possible because of the substantial correspondence of the reactivity curves of these two different types of fuel bundles as illustrated by a comparison of FIGS. 2c and 4c. By substituting over time the bundles of FIGS. 4a and 4b for those of FIGS. 2a and 2c in the core, the nuclear reactor core may be operated and controlled similarly as if employing fuel bundles of the type illustrated in FIGS. 2a and 2b. While the invention has been described with respect to what is presently regarded as the most practical embodiments thereof, it will be understood by those of ordinary skill in the art that various alterations and modifications may be made which nevertheless remain within the scope of the invention as defined by the claims which follow.