Patent Number: 050892100
Section: description

Referring to FIG. 1 a reactor R is illustrated having a vessel V, the vessel V having the purpose of containing a nuclear reaction under pressure. A core K of fuel bundles is illustrated. Fuel bundle K includes an underlying group of control rods C which control rods C force cruciformed shaped control rods into and out of the interstices between adjacent fuel bundles (see FIG. 2). The reactor R includes a jet pump J for inducing coolant flow. Specifically, jet pump J induces coolant flow down and into the underlying group of control rods C. Fluid flows upwardly through the core K through a top guide T returning to the top of the reactor. Downward flow is induced by the jet pump J over a shroud SH between the side walls of the vessel V and the core K. Thus a downward fluid flow occurs on the outside and an upward fluid flow on the inside. Produced saturated steam is extracted from the top of the reactor through steam separator S and dryer D. This saturated steam permits the extraction of power through turbines and generators (not shown). Typically the turbines exhaust to a condenser where the spent steam is condensed and made up into reactor feed water for continuous recycling of the coolant. Referring to FIG. 2, the reactor at four side by side fuel bundles is illustrated. A lower core plate 24 forms a barrier. This barrier is horizontal and disposed between the group of control rods C (see FIG. 1; not shown FIG. 2) contained below the fuel bundles and the discrete overlying fuel bundles themselves. Barrier 24 enables water to be forced through the bottom of the fuel bundles into and within the channel around the discrete fuel rods. Each fuel bundle includes a lower tie plate T1 (see FIG. 3). At the upper end, each fuel bundle includes an upper tie plate T2. Extending between the tie plates there is defined a channel 30. It is the function of the channel 30 to confine fluid flow between the tie plates and within the channel so that water moderator can form two discrete functions. First, the water moderator takes the fast high energy neutrons emitted in the fission reaction and causes the neutron to become slow or thermal neutrons. It is the slow or thermal neutrons which cause the continuing nuclear reaction to occur. Second, the upwardly flowing water is turned in part to steam. This steam is used to extract work from the heat of the nuclear reaction by conventional turbine condensers and recycling to the nuclear reactor shown in FIG. 1. Referring further to FIG. 2, a control rod 40 is illustrated. The control rod 40 passes between four fuel bundles. These fuel bundles being denominated 31, 32, 33 and 34. Some attention can now be given to the disposition of the fuel bundles. Typically, and at their lower end, the fuel bundles are each supported on a fuel support casting 50. Fuel support casting 50 extends downwardly into and through holes 26 in the top guide 24. Fuel support casting finds support on the top of the control rod drive housing (not designated). The control rod drive housing in turns passes the weight of the fuel bundles to the bottom of the reactor vessel V (see FIG. 1). The discrete fuel bundles 31-34 are supported at their upper end at a top guide G. Top guide G includes a metallic lattice including cross members 61, 62, 63, and 64. These respective cross members support the fuel bundles 31-34 in vertical upstanding relation. The fuel bundles are spaced apart. This spacing apart forms a cruciform sectioned interstices 70 illustrated between the discrete fuel bundles. This cruciform sectioned interstices 70 has control rod 40 shown placed therein. Control rod 40 is conventional. It typically is inserted through an aperture 26 in core plate 24 from a position of residence in the control rod drive housing (not shown). In controlling the nuclear reactor, the control rod passes upwardly between the respective fuel bundles. Its cruciform section includes a number of flat planar surfaces, 41, 42, 43 and 44. It can be seen that flat planar surface 41 passes in the interstitial area between fuel bundles 33, 34. Planar surface 42 passes between bundles 31, 34. Planar surface 44 passes between bundles 31, 32 and finally planar surfaces 43 passes between bundles 33 and 32. Typically there is only provided one control rod for each group of four fuel bundles. It will further be understood that two flow paths for water are generically present. A first flow path for water is within the channels of the fuel bundles between the tie plates T1 and T2. A second flow path for water is in the so-called core bypass region. This is the region between adjacent fuel bundles. During normal operation of the reactor, the flow path between the tie plates and within the fuel bundle contains steam. This steam is present in higher fractions as the water rises from the bottom of the fuel bundle to the top of the fuel bundle. During start up operation of the reactor, the flow path between the tie plates and within the fuel bundle contains water. This water causes higher neutron moderation. This high neutron moderation requires the establishment of the cold reactivity shut down zone. During all normal operations the core by-pass region is flooded with water. This core by-pass region exerts on those fuel rods adjacent to it a high degree of neutron moderation. Accordingly, higher levels of thermal neutron flux are present in this region. Referring to FIG. 4A a fuel bundle, B1 of the type herein used is illustrated. General observations can be made. First, the bundle includes a rectangular section channel 60 and a central water rod W. Water rod W and the exterior of channel 60 commonly contain water without steam intermixed. Accordingly, these areas of the fuel bundle produce relatively high levels of moderation. Secondly, it can be seen that the respective corners of the fuel bundle 61, 62 and 63 are the locations for the gadolinium containing rods having the burnable absorber gadolinium. Corner 64 is typically adjacent instrument tubing such as that tubing utilized for local power range monitors. Since gadolinium will have an adverse effect on the neutron flux, a quantity measured by the instrument tubing, it is omitted from the bundle corner 64. Typically, the control rod 40 illustrated in FIG. 2 passes with its cruciform shaped intersection adjacent corner 63 of bundle B1. The reader will realize that in the normal operating state of the reactor, the control rod is typically fully withdrawn. As will hereinafter be set forth, the particular design herein disclosed is utilized with a so-called D lattice nuclear reactor. In such a D lattice nuclear reactor, the fuel bundle spacing varies. Typically, and at corner 64, the fuel bundles are relatively closely spaced. Opposite corner 64 and at corner 63, the fuel bundles defined their widest separation. It is into the interstices between the fuel bundle at corner 63 that the control rod 40 shown in FIG. 2 passes. At corner 61, 62 spacing of the fuel bundles apart from one another is intermediate those spacing encountered at corners 63, 64. Remembering that the control rod 40 is normally fully withdrawn, and remembering that the spacial gap adjacent corner 63 is the largest, it can be seen that five rods containing gadolinium and labelled G1 and G3 and G4 are located adjacent corner 63. As distinguished from corner 63, corner 62, 61 includes only three gadolinium rods. These rods being labelled G2, G3 and G4. Finally, corner 64 is without gadolinium. It will further be observed that water rod W occupies approximately 4 lattice position in the 8.times.8 lattice. Accordingly, 49 of the remaining rods have combinations of plutonium mixed with the depleted uranium. Referring to FIG. 4B, the concentrations of plutonium can readily be understood. Fuel rods 5 include plutonium at the level of 5 weight percent. Fuel rods 9 include plutonium at the level of 9 weight percent. Finally, fuel rods 12 include plutonium at 12 weight percent. It will further be seen that 33 of the disclosed rods have plutonium at the level 12 weight percent. It can be seen that at this level, the disclosed bundle design has a relatively high concentration of plutonium. Further, all of the so-called MOx bundle of fuel rods (that is rods 5, 9 and 12) have a depleted uranium enrichment. Specifically, these rods include two-tenths of a weight percent of uranium -235. Preferably, the material is taken from any source of depleted uranium such as enrichment plant tails and the like. It can further be seen that gadolinium is contained in four rod types. In two of these rod types, G1 and G2, the gadolinium is evenly distributed throughout the rod length. This being the case, these particular rods do not contribute significantly to the formation of the cold reactivity shutdown zone. Referring to rods G3 and G4, it can be seen that gadolinium in high weight percents (four weight percent in rod G3 and five weight percent in rod G5) is distributed with large percentiles being within the so-called cold reactivity shutdown zone. The enrichment levels of rod G1 is approximately two weight percent. The enrichment level of rod G2 is 3.95 weight percent. Finally, rod G3 includes 3 weight percent uranium with rod G4 including 3.95 percent uranium -235. Small vertical sections of natural uranium are distributed in the bottom and top 6" of the fuel rods. Referring to FIG. 4C, a bundle average vertical axial profile is set forth. In the vertical axial profile, the shutdown zone of the disclosed bundle is illustrated. The reader will remember that this shutdown zone is imparted solely by the gadolinium rods G3 and G4. Further, the reader will understand that the varying concentrations of plutonium are held to a mere 3 concentrations. Furthermore, the heaviest concentration of plutonium predominates the bundle. It will be understood that practical manufacturing dictates that the gadolinium rods usually be fabricated in a facility separate and apart from those rods including large amounts of plutonium. It can be seen that the disclosed invention only includes 11 gadolinium rods and that these gadolinium rods can be conveniently assembled elsewhere and shipped in the site of fuel bundle assembly. We have here shown in the preferred embodiment a fuel bundle whose initial reactivity profile has been tailored to define a so-called "cold reactivity shut down zone". The reader will appreciate that this is the preferred embodiment. It will be understood that this invention may be practiced on a less than preferred basis by the location of gadolinium rods at the corner locations without tailoring of the cold reactivity profile to create the "cold reactivity shut down zones". Having set forth, the physical construction of a fuel bundle, two graphic illustrations can further show the advantage of this fuel design. Referring to FIG. 5A, it can be seen that the plot of an ordinary fuel bundle against the improved bundle of this disclosure illustrates improved reactivity. Specifically by plotting infinite reactivity against exposure in gigawatt days per short ton it can be seen that virtually at all times during the life of the fuel bundle that the reactivity remains higher. Specifically, and remembering that the bundles of this design will be distributed throughout the core, it can be seen that the improved reactivity will be imparted to the remainder of the core. This being the case, less enrichment over all of the core will be required. Referring to FIG. 5B, the reader will understand that the physics of the plutonium combinations herein utilized will inevitably cause higher peaking. Peaking is that phenomena of local heating which local heating limits the overall power of a fuel bundle to avoid local damage to any part of a nuclear fuel rod within the fuel bundle. It has been found however, that the peaking with exposure although higher does not significantly exceed the peaking at the beginning and end of a typical fuel bundle cycle. This being the case, an acceptable compromise of this parameter occurs. Having set forth the construction of a fuel bundle on an 8.times.8 array, referring to FIG. 6A, 6B and 6C, the construction of a fuel bundle on a 9.times.9 array is illustrated. Referring to FIG. 6A, a fuel bundle B2 including first and second water rods, W1 and W2 is illustrated. These respective water rods displace seven lattice positions leaving 84 positions to be filled. So-called partial length rods are utilized in this invention. Specifically, these partial length rods denominated P5 and P12 extend 5/8 the full height of the fuel bundle. That is to say assuming that the fuel rods include 11' 8" of nuclear fuel, partial length rods extend 8'6" from the bottom of the fuel rod to and towards the top. These partial length rods include respectively 5% plutonium for rods P5 and 12% plutonium for rods P12. Both include a mixture of uranium 235 in the range of 0.2 weight percent. It can also be seen that each of the partial length rods is located in the second row removed from the channel wall of the bundle B2 has been found that such a location of the partial length rods imparts maximum advantage to the design herein disclosed. The cold reactivity shutdown zone (see FIG. 6C) is imparted solely by the axial distribution of rods G3 and the 3 weight percent of gadolinium there specified. Specifically, such rods have a uniform 3% uranium enrichment. Within the shutdown zone, (see FIG. 6C) gadolinium in the amount of 3 weight percent is utilized. It can be seen that there is one rod G3 in the respective fuel bundle corners 61, 62 and 63. A weighted gadolinium rod has been eliminated from fuel bundle corner 64. This is because fuel bundle corner 64 is typically immediately adjacent instrumentation such as the local power range monitor. Adjacent the corners, there can be seen a second gadolinium rod G2. Gadolinium rod G2 has two weight percent gadolinium distributed only to the shut down zone. Finally, and again near the corners a uranium rod U1 without any plutonium intermixed is specified. These uranium rods are again adjacent the corner lattice position. It thus can be seen that the disclosed fuel design includes only three classes of gadolinium rods and one conventional uranium rod containing 3.6 weight percent uranium. The remaining rods all contain plutonium. Specifically, rod P5 includes 5.0 weight percent plutonium. Rod P9 contains 9.0 weight percent plutonium, rod P12 includes 12 weight percent plutonium. These respective weight percents of plutonium are intermixed with depleted uranium sources on the order of 2/10ths of a percent. Referring to FIG. 6C, the bundle average vertical profile of the fuel bundle can be seen. In the case of the preferred embodiment here illustrated, gadolinium has been tailored in its vertical profile to create the so-called "cold reactivity shut down zones". As hereinbefore set forth, embodiments without these cold reactivity shut down zones, while not preferred, are within the scope of this invention. In the following claims, the term "adjacent the corners" will be used. The reader will understand by using this term we intend to include the corner location, the two rods adjacent the corner, these latter two rods being adjacent the channel. Thus, this definition can include twelve lattice positions within a fuel bundle.