Patent Number: 054405980
Section: summary

TECHNICAL FIELD The present invention relates to nuclear reactors and particularly to an arrangement and use of neutron absorbing materials such as gadolinia in the fuel rods of the bundle to enhance plutonium utilization in the reactor. BACKGROUND Increasingly, there is interest in the capabilities of nuclear reactors to transform and thereby destruct through reactor burnup large quantities of weapons grade plutonium. It is common, e.g. in boiling water reactors, to employ fissionable material, such as uranium and minor amounts of other fissionable materials, such as plutonium or thorium in the fuel pellets. Additionally, neutron absorbers are frequently used in the nuclear fuel pellets to control the inherent excess reactivity of the fuel in the core to achieve greater efficiency and economy and to prolong the service life of the fuel. For example, in boiling water reactors, uranium, which has initial excessive reactivity, is combined with a depletable neutron absorber, commonly referred to as a burnable poison, such as gadolinium. This initial excessive fuel reactivity is tempered by the introduction of the depletable neutron absorber which progressively expends its capacity to absorb neutrons. Thus, the burnable poison absorbs excess neutrons to level or stabilize the fuel reactivity rate during the period of initial excessive reactivity and then subsequently absorbs neutrons at a decreasing rate approximately commensurate with the diminishing reactivity of the fuel whereby a substantially constant rate of reactivity is maintained. In a typical boiling water reactor, the vast majority of the fuel rods of a fuel bundle comprise fissile uranium material with only a very few of the rods having a combination of the fissile uranium and a burnable poison, such as gadolinium. While plutonium has previously been considered as an alternate fuel for boiling water reactors, as well as a combination of fissile uranium and plutonium with a burnable poison, for example, see U.S. Pat. No. 5,089,210 of common assignee herewith, it has been commonly believed that there is a severe limitation with respect to the quantity of plutonium which may be used in boiling water reactors. Particularly, it has previously been thought that no more than about one-quarter of the fuel rods in a reactor may be loaded with plutonium or mixed oxide fuel without unacceptable operational consequences. Also, reactor designers heretofore believed the presence of plutonium would interfere with the effectiveness of the gadolinium reactivity control. This is indeed the case when gadolinium is loaded into 10 to 20% of the rods, as is typical in reactors of this type. Hence, use of plutonium as a fuel in nuclear reactors has been inhibited by these and other considerations. DISCLOSURE OF THE INVENTION In accordance with the present invention, it has been found that a combination of urania, plutonia and gadolinia may be employed in the fuel rods in a manner to enhance utilization of plutonia, as a fuel for the nuclear reactor, provided the fuel rods are specifically arranged in each fuel bundle in conjunction with an extensive use of burnable poison in the fuel bundle. More particularly, it has been found that if the gadolinium is disposed in the interior fuel rods in combination with fissile uranium and plutonium and the exterior rods forming the perimeter of the fuel bundle are void of gadolinium, plutonium usage as a fuel in the reactor may be enhanced without destabilizing the power level of the reactor. The plutonium is preferably contained in all of the fuel rods of the bundle in various percentages. As well known, the reactivity of a fuel bundle containing enriched uranium and gadolinium over time produces a characteristic reactivity curve, which each fuel bundle must approximate if overall steady-state power levels in the nuclear reactor are to be maintained. By combining the fissile materials of uranium and plutonium with gadolinium, the burnable poison, in a typical fuel bundle arrangement, a characteristically different reactivity curve than the desired curve is produced. It has been found, however, that by arranging the individual fuel rods containing the uranium, plutonium and burnable poison in an interior array of the rods and effecting more extensive use of the burnable poison, the characteristic reactivity curve of a typical enriched uranium fuel bundle with modest use of the burnable poison in largely separated fuel rods, may be approximated enabling steady-state power levels in the reactor. That is, the fuel bundle design containing the plutonium fuel desired to be transformed has reactivity characteristics which closely resemble the reactivity characteristics normally associated with enriched uranium fuel. In a preferred embodiment according to the present invention, there is provided a nuclear fuel bundle comprising a plurality of fuel rods arranged in a generally square array, each of the rods having a predetermined concentration of fissile material with at least a majority of the rods including plutonium, a predetermined number of the rods having a concentration of a material for absorbing neutrons, the predetermined number of rods constituting an interior array thereof, all of which predetermined number of rods lie within a surrounding exterior array of fuel rods of the plurality thereof, the predetermined number of rods in the interior array thereof being in excess of 20% of the total number of the plurality of rods in said fuel bundle. In a further preferred embodiment according to the present invention, there is provided a nuclear fuel bundle comprising a plurality of fuel rods arranged in the bundle, each of the rods having a predetermined concentration of fissile material with at least a majority of the rods including plutonium and a predetermined number of the rods having a concentration of material for absorbing neutrons. The fuel bundle also has a reactivity curve substantially corresponding to the reactivity curve illustrated in FIG. 4c. In a still further preferred embodiment according to the present invention, there is provided in a nuclear reactor core having a plurality of fuel bundles, at least a first bundle of the plurality thereof containing a plurality of fuel rods having a concentration of enriched uranium with certain of the rods thereof having a concentration of a burnable poison, the first bundle having a reactivity curve substantially corresponding to the reactivity curve of FIG. 2c and at least a second bundle of the plurality thereof containing a plurality of fuel rods each having a concentration of enriched uranium and plutonium and certain of the rods of the second bundle forming an interior array thereof having a concentration of burnable poison surrounded by an exterior array of rods void of the burnable poison, the second bundle having a reactivity curve substantially corresponding to the reactivity curve of FIG. 4c. Accordingly, it is a primary object of the present invention to provide, in a nuclear reactor, enhanced usage of plutonium as a reactor fuel.