Patent Number: 050892100
Section: summary

The present invention relates to nuclear reactors and more specifically to a fuel bundle arrangement for a boiling water nuclear reactor in which so-called mixed oxide fuels including plutonium and uranium are utilized in a nuclear fuel bundle together with a burnable absorber such as gadolinium to optimize the reaction of a nuclear fuel bundle. BACKGROUND OF THE INVENTION 1. Field of the Invention In a boiling water nuclear reactor, a fissile fuel atom, such as U235, PU239, or PU241 absorbs a neutron in its nucleus and undergoes nuclear disintegration. This produces on the average two fissile fragments of lower atomic weight with great kinetic energy and several neutrons, these neutrons also being at high energy. In the boiling water nuclear reactor, the nuclear fuel is in the form of fuel rods, each of which comprises a plurality of scintered pellets contained within an elongate sealed cladding tube or "fuel rod." Groups of such fuel rods are supported between upper and lower tie plates to form separately replaceable fuel assembles or bundles. A sufficient number of such fuel assemblies are arranged in a matrix, approximating a right circular cylinder, to form the nuclear reactor core which is capable of self-sustained fission reaction. The kinetic energy of the fission products is dissipated as heat in the fuel rods. Energy is also deposited in the fuel structure and moderator by the neutrons, gamma rays and other radiation resulting from the fission process. The core is submerged in coolant which removes the heat. Typically such heat removal occurs by the coolant water boiling into steam. From the steam energy is extracted to perform useful work. In a boiling water nuclear reactor the coolant also acts as a neutron moderator. This moderator takes the emitted high energy neutrons and slows down the neutrons to render them thermal in character and hence more likely to be absorbed in the fuel continuing the fission reaction. The commonly used fuel for water cooled and moderated nuclear power reactors comprises uranium dioxide of which from about 0.7 to 5.0% is fissile U235 mixed with fertile U238. During operation of the reactor, some of the fertile U238 is converted to fissile plutonium PU-239 and PU-241. It turns out that the U238 is also fissionable, but only for high energy neutrons. The ratio of fissile materials produced (for example plutonium 239 and plutonium 241) to fissile material destroyed (for example U235, plutonium 239 and plutonium 241) is defined as the "conversion ratio". Fuel bundles are typically replaced at certain "outages". Typically these outages delineate "cycles". During such outages the reactor is opened and remote lifting equipment removes fuel bundles with spent fuel therein and replaces the fuel bundles with those having fresh fissile materials contained within the fuel rods. At each outage, only a portion of the total fuel bundles are removed. This portion is in the order of 25%. STATEMENT OF THE PROBLEMS It is known in the prior art how to reprocess the fuel from used fuel bundles. Typically, plutonium is recovered (for example PU239, PU241). This recovered plutonium is mixed by taking naturally occurring uranium oxides or diffusion plant tails uranium and undergoing a blending process. Typically, the plutoniums and uraniums are converted to oxides. Thereafter, the plutonium oxides and uranium oxides are blended to produce a desired percent by weight of the respective plutonium and uranium compounds. Once the oxides are blended, they are scintered to generate preferably a face centered cubic lattice structure incorporated in fuel pellets. The scintered pellets are placed in sealed zirconium fuel rods. Regarding the matter of blending, these mixed oxide or MOx fuel rods are required to have varying concentrations of Pu. This being the case, it is often necessary to have five or six differing concentrations of plutonium in the varying rods of an individual fuel bundle. It can be appreciated that this sort of recovery, blending and scintering of the plutonium from spent or used fuel bundles to new fuel bundles is difficult. The radioactive gases and elements of the spent fuel rods must be handled, typically remotely. The plutonium itself is toxic, is an alpha emitter and has long half-lives which complicate the removal process. Simply stated, such plutonium recovery, blending and scintering operations must occur in enclosed environments under special processing conditions. Accordingly, the processing and placement of mixed oxide fuels within nuclear fuel rods differs radically from the use of the more conventional uranium oxide compounds. This being the case, the use of so-called mixed oxide fuels (MOx) occurs in isolated processing plants separate and apart from conventional fuel loadings involving uranium oxides and burnable absorbers such as gadolinium. If the reactor is to operate at a steady state power level, the fission inducing neutron population must remain constant. That is, each fission reaction must produce a net of 1 neutron which produces a subsequent fission reaction so that the operation is self-sustaining. The operation is characterized by an effective multiplication factor K.sub.eff which must be at unity for a steady state operation. It is noted that the effective multiplication factor K.sub.eff is the neutron reproduction factor of the nuclear reactor considered as a whole. This is to be distinguished from the local or infinite multiplication factor K.sub.inf which defines neutron reproduction of an infinitely large system having throughout the same composition and characteristics as the local region of a reactor core in question. During operation, the fissile fuel is depleted and indeed some of the fission products are themselves neutron absorbers or "poisons". To offset this, the reactor must normally be provided with an initial excess of nuclear fuel which results in initial excess reactivity. This initial excess reactivity requires a control system to maintain the effective multiplication factor at unity. Such maintenance must occur at the beginning of the fuel cycle in the presence of the excess reactivity. The control system also functions to reduce the effective multiplication factor to below unity so that the reactor can be shut down. The control system typically utilizes neutron absorbing material which serves to control the neutron population by nonfission absorption or the capture of neutrons. As the described invention herein relates to a sophisticated manner of loading a fuel bundle, it is necessary to understand both the construction of a typical fuel bundle together with its geometry in relating to a so-called control rod as well as adjacent fuel bundles. Each fuel bundle contains longitudinally extending sealed rods. These rods have the fissile material sealed within them. The material is maintained sealed during the entire life of the fuel bundle. Opening the sealed fuel rods must occur for the processing referred to here. A group of such fuel rods are typically supported between a lower tie plate at the bottom and an upper tie plate at the top. Arrays of 6.times.6, 7.times.7, 8.times.8 and 9.times.9 fuel rods have been utilized. Typically, these arrays define in the indicated number certain so-called "lattice positions" for the fuel rods. It will be understood that portions of these lattice positions can be occupied by other fuel bundle elements. For example, it is common to insert rods for holding water moderator in the center of such fuel bundles to impart to the entirety of the bundle the desired reaction profile for the generation of energy. Typically, the total number of fuel rods has increased with modern fuel bundle design. Presently, lattice positions 8.times.8 and 9.times.9 are utilized. Furthermore, in the 9.times.9 designs, it is now common to have foreshortened fuel rods in some of the lattice positions. Foreshortening of the fuel rods imparts numerous advantages. An example of such advantages is set forth in the patent application entitled Two Phase Pressure Drop Boiling Water Reactor Assembly Design Ser. No. 176,975 filed Apr. 4, 1988. Each fuel bundle includes a surrounding channel for confining water flow between and through the tie plates on the axial length of the individual rods. Water moderator flows in the confining channel from the bottom and out of the confining channel at the top. During its passage in an active reactor, steam is produced in mixture with the passing water. The water moderator is also exterior of the channel. This water typically does not include a high percentage of steam and is contained within what is known as a core bypass zone or region. The core bypass zone or region has water which produces moderation of the fast neutron flux. Fast neutrons rapidly become thermal neutrons capable of initiating continuing nuclear reaction within the core bypass region. The fuel bundles themselves are typically arrayed for controlling their nuclear reaction in groups of four. Typically, four fuel bundles of square cross section are vertically disposed in side by side relation. Each fuel bundle is spaced apart from the remaining fuel bundle so as to define the interstitial core bypass region. Considering four fuel bundles in side by side relation, the bundles will in the interstices between them define a cruciform shape interstitial area. It is into this cruciform shape interstitial area that a complementary cruciform shaped control rod effects penetration for the parasitic absorption of neutrons and ultimate control of the nuclear reactor. Modern control rod design includes many groups of four such fuel bundles. It is common for reactor cores to hold up to 800 such discrete fuel bundles. Each group of four such fuel bundles has a control rod penetrating interstitially of the fuel bundle interfaces to affect the absorption of neutrons and the control of the nuclear reactor. During the lifetime of a fuel bundle, the usual circumstance is that the bundle is not appreciably exposed to the control rods. Consequently, it is the usual case that the thermal neutron flux is relatively high at the fuel bundle corners in comparison to other portions of the fuel bundle. As the fuel bundles are initially supplied with excess reactivity within their fissile materials it is sometimes necessary to incorporate burnable absorbers. Such burnable absorbers act during the beginning of a fuel cycle to absorb neutrons and prevent the excess reactivity which would otherwise be present from preventing control of the nuclear reaction. A burnable absorber is a neutron absorber which is converted by neutron absorption into a material of lesser neutron absorbing capability. A well known burnable absorber is gadolinium. The odd isotopes (GD-155 and GD-157) have very high capture cross sections for thermal neutrons. The burnable absorbers available for use also have an undesirable effect. Specifically, and during the end of the fuel bundle cycle, the residual burnable absorbers decrease the efficiency of the fuel bundle. Its operation would be far better if the burnable absorbers were not present, or at least maintained at an absolute minimum. For example, if gadolinium is used as a burnable absorber, the high cross section isotopes (GD-155 and GD-157) deplete rapidly. Unfortunately, these elements are converted into elements which contain reduced neutron capture cross sections but nevertheless detract measurably from the efficiency of the fuel bundle. For example, in gadolinium, the produced isotopes (GD-154, GD-156 and GD-158) still continue to absorb neutrons and detract from the overall efficiency of the fuel bundle. As is well known, burnable absorbers, such as gadolinium operate in a self-shielding mode when present at sufficient concentration. That is, upon exposure to neutron flux, the neutron absorptions occurs essentially at the outer surface of the absorbers so that the volume of absorber shrinks radially at a rate that depends upon the concentration of the absorber. It is additionally known that plutonium, especially fissile PU239 and PU241 have high neutron absorption cross sections relative to uranium. If burnable absorbers such as gadolinium are utilized in combination with fissile plutonium, the gadolinium itself can be shielded from neutrons by the plutonium. Hence to use the control feature of the burnable absorber, much larger concentrations of gadolinium must be utilized where fissile plutonium is present. There is, related to the present fuel design, a further complication. During operation, the percentage of steam voids within the fuel bundles increases to and towards the top of the reactor. These steam voids lead to decreasing moderation in the top regions of the reactor because water is present in lesser quantities. Thus, there results a power distribution that is skewed towards the lower regions of the fuel bundles forming the reactor core. It is a known practice to compensate for this by distributing burnable absorber in an axially inhomogeneous manner. A number of fuel rods are provided with burnable absorber having a distribution skewed towards the axial region of hot operating maximum reactivity. A typical configuration is shown in U.S. Pat. No. 3,799,839. However, the situation is very different in the cold shutdown state. More particularly, in the cold state, the top of an irradiated boiling water reactor core is more reactive than the bottom. This greater reactivity occurs because during normal operation there is greater plutonium production and less U235 destruction in the reactor top. Specifically, a greater population of fast neutrons is present at the top of the reactor. These fast neutrons create a greater conversion ratio and smaller burnup in the fuel rods. In the cold shutdown condition, the steam voids in the upper part of the core are eliminated because little, if any, steam is present in the moderator. This makes the top of the core more reactive than the bottom in the cold shutdown condition. Typical licensing standards require a 0.38% reactivity shutdown margin (K.sub.eff less than 0.9962) with any one control rod stuck out of the core. To provide margin for prediction uncertainties, a design basis of 1% predicted shutdown margin (K.sub.eff less than 0.99), to be provided by the control rods and the burnable absorber is typically required and used. Some reactors have fuel assemblies requiring special designs directed to their so-called "cold reactivity." In such prior art fuel assemblies, the burnable absorber is asymmetrically distributed to allow cold shutdown margins to be met with minimum penalty to operating efficiency. The assembly includes a component of fissile material distributed over the axial extent of the fuel bundle. Mixed within fissile materials a component of neutron absorbing material is added. This neutron absorbing material has an axial distribution characterized by an enhancement in a relatively short axial zone known as the "cold shutdown control zone". This cold shutdown control zone corresponds to at least a portion of the axial region of cold shutdown maximum reactivity. The axial distribution of the component of neutron absorbing material is typically characterized by an additional enhancement in an axial zone known as the "hot operating" control zone. The component neutron absorbing material is conventionally incorporated into at least some of the fuel rods. This enhancement in the cold shutdown zone may be provided at least in part by one or more fuel rods having absorber only in the cold shutdown control zone. This enhancement of the neutron absorbing material in the cold shutdown control zone may be additionally supplemented by reduced fuel enrichment in the cold shutdown control zone. It should be further understood that it is desired to keep the distribution of gadolinium within a fuel bundle to an absolute minimum. Gadolinium, in addition to absorbing neutrons, reduces the thermal conductivity of the fuel rods and increases fission gas release. Consequently the gadolinium containing rods are frequently the most limiting rods in the fuel assembly. Thus, and because of these limiting rods, the entire fuel bundle must be downrated in power with a corresponding adverse effect on local power distributions. The amount of power downrating that is required depends upon the required gadolinium concentration. This required gadolinium concentration sometimes becomes a serious problem in extended burnup fuel designs and/or high energy cycle designs where increased gadolinium concentrations are required in order to provide adequate cold shutdown margins. Unfortunately, the relatively dense 9.times.9 arrays utilized with modern reactors are examples of fuel bundles in which the excess gadolinium can produce problems. The reader must appreciate at this juncture that the above recited background includes a summary of only pertinent operating and shutdown constraints. These pertinent constraints have been summarized so that the following optimized fuel bundle design can be understood. SUMMARY OF THE INVENTION A fuel bundle design incorporating oxides of recovered plutonium mixed with uranium (MOx) which can maximize the content of plutonium and minimize the number of different MOx pellet concentration types. In a boiling water nuclear reactor, a fuel bundle is loaded with MOx containing rods at all locations save and except rods at the corners or adjacent to the corners of the bundle. Fuel rods adjacent the corners are provided which preferably do not contain MOx and are instead uranium gadolinium rods. The disclosed uranium gadolinium rods can have their gadolinium asymmetrically loaded so as to impart axially of the fuel bundle the desired cold reactivity shutdown zones. As a consequence, a fuel bundle design is disclosed which can maximize the use of recovered fissile plutonium from previous fuel cycles, minimize the number of different MOx concentration type and enable a reduction of the amount of uranium enrichment required. At the same time, the desired axial shaping for the so-called cold reactivity shutdown zones can be accomplished independent of the MOx rods and more importantly without the mixture of MOx fuels and gadolinium in the same fuel rods. Further, by the placement of the gadolinium uranium rods at the periphery of the bundle near the permanent water gaps with their high thermal neutron flux, the maximum worth of the gadolinium is achieved. Shielding of the gadolinium by the high neutron cross section plutonium is minimized. With the disclosed design, uses of the burnable absorber gadolinium is reduced to a minimum resulting in an improved overall fuel bundle energy output. Other Objects, Features and Advantages An object of this invention is to disclose a fuel bundle design in which high levels of plutonium per bundle are utilized. An advantage of the disclosed design is that it limits the number of discrete plutonium concentrations that are required in each MOx fuel bundle. In the preferred embodiment herein disclosed only three variant concentrations of plutonium are required. This simplifies the blending, mixing and individual fuel rod assembly by limiting the number of discrete plutonium mixtures that are used with the disclosed design. A further object of this invention is to realize maximum worth of the gadolinium that is utilized with the disclosed fuel bundle. In accordance with this aspect of this invention the gadolinium is placed in corner locations. In these corner locations, the gadolinium sees relatively high neutron flux. At the same time, the gadolinium is not shielded by the relatively high cross section of plutonium used as the fissile material in the bundle. An advantage of this aspect of the invention is that the gadolinium is disposed where it easily accommodates excess reactivity during the first part of the fuel cycle. A further advantage of this feature is that when the gadolinium is expended, typically after the first quarter of the end reactor life of the fuel bundle, the remaining gadolinium presents a minimal inefficiency to the fuel bundle. Yet another advantage of this invention is that the gadolinium placed within fuel rods only at the corner of the fuel bundle can be utilized for the purpose of imparting the cold reactivity shut down zone to the fuel bundle. No varying of the plutonium concentration of the so-called MOx rods is required. Yet another object of this invention is to disclose a MOx fuel bundle which has a higher reactivity profile during its full life cycle within a reactor. This higher reactivity is imparted to the whole core with the advantage that the higher reactivity helps maintain a fissile reaction. An advantage of this aspect of the invention is that with the disclosed fuel bundles dispersed throughout the core, the requirement for enrichment in neighboring fuel bundles is reduced. A further advantage of this disclosed design is that it can be utilized with the dense array of modern fuel bundle designs. For example, two embodiments here utilized are 8.times.8 and 9.times.9 fuel rod arrays. Yet another advantage of this invention is that the design can accommodate partial length rods. Typically, the partial length rods, being placed inside the bundle, are the MOx rods in the disclosed design.