Patent Number: 051475987
Section: description

DETAILED DESCRIPTION OF THE INVENTION In the following description, like references characters designate like or corresponding parts throughout the several views Also in the following description, it is to be understood that such terms as "forward", "rearward", "left", "right", "upwardly", "downwardly", and the like, are words of convenience and are not to be construed as limiting terms. In General Referring now to the drawings, and particularly to FIGS. 1 and 2, there is shown a pressurized water nuclear reactor (PWR), being generally designated by the numeral 10. The PWR 10 includes a reactor pressure vessel 12 which houses a nuclear reactor core 14 composed of a plurality of elongated fuel assemblies 16. The relatively few fuel assemblies 16 shown in FIG. 1 is for purposes of simplicity only. In reality, as schematically illustrated in FIG. 2, the core 14 is composed of a great number of fuel assemblies. Spaced radially inwardly from the reactor vessel 12 is a generally cylindrical core barrel 18 and within the barrel 18 is a former and baffle system, hereinafter called a baffle structure 20, which permits transition from the cylindrical barrel 18 to a squared off periphery of the reactor core 14 formed by the plurality of fuel assemblies 16 being arrayed therein. The, baffle structure 20 surrounds the fuel assemblies 16 of the reactor core 14. Typically, the baffle structure 20 is made of plates 22 joined together by bolts (not shown). The reactor core 14 and the baffle structure 20 are disposed between upper and lower core plates 24, 26 which, in turn, are supported by the core barrel 18. The upper end of the reactor pressure vessel 12 is hermetically sealed by a removable closure head 28 upon which are mounted a plurality of control rod drive mechanisms 30. Again, for simplicity, only a few of the many control rod drive mechanisms 30 are shown. Each drive mechanism 30 selectively positions a rod cluster control mechanism 32 above and within some of the fuel assemblies 16. A nuclear fission process carried out in the fuel assemblies 16 of the reactor core 14 produces heat which is removed during operation of the PWR 10 by circulating a coolant fluid, such as light water with soluble boron, through the core 14. More specifically, the coolant fluid is typically pumped into the reactor pressure vessel 12 through a plurality of inlet nozzles 34 (only one of which is shown in FIG. 1). The coolant fluid passes downward through an annular region 36 defined between the reactor vessel 12 and core barrel 18 (and a thermal shield 38 on the core barrel) until it reaches the bottom of the reactor vessel 12 where it turns 180 degrees prior to flowing up through the lower core plate 26 and then up through the reactor core 14. On flowing upwardly through the fuel assemblies 16 of the reactor core 14, the coolant fluid is heated to reactor operating temperatures by the transfer of heat energy from the fuel assemblies 16 to the fluid. The hot coolant fluid then exits the reactor vessel 12 through a plurality of outlet nozzles 40 (only one being shown in FIG. 1) extending through the core barrel 18. Thus, heat energy which the fuel assemblies 16 impart to the coolant fluid is carried off by the fluid from the pressure vessel 12. Due to the existence of holes (not shown) in the core barrel 18, coolant fluid is also present between the barrel 18 and baffle structure 20 and at a higher pressure than within the core 14. However, the baffle structure 20 together with the core barrel 18 do separate the coolant fluid from the fuel assemblies 16 as the fluid flows downwardly through the annular region 36 between the reactor vessel 12 and core barrel 18. As briefly mentioned above, the reactor core 14 is composed of a large number of elongated fuel assemblies 16. Turning to FIG. 3, each fuel assembly 16, being of the type used in the PWR 10, basically includes a lower end structure or bottom nozzle 42 which supports the assembly on the lower core plate 26 and a number of longitudinally extending guide tubes or thimbles 44 which project upwardly from the bottom nozzle 42. The assembly 16 further includes a plurality of transverse support grids 46 axially spaced along the lengths of the guide thimbles 44 and attached thereto The grids 46 transversely space and support a plurality of fuel rods 48 in an organized array thereof. Also, the assembly 16 has an instrumentation tube 50 located in the center thereof and an upper end structure or top nozzle 52 attached to the upper ends of the guide thimbles 44. With such an arrangement of parts, the fuel assembly 16 forms an integral unit capable of being conveniently handled without damaging the assembly parts. As seen in FIGS. 3 and 4, each of the fuel rods 48 of the fuel assembly 16 has an identical construction insofar as each includes an elongated hollow cladding tube 54 with a top end plug 56 and a bottom end plug 58 attached to and sealing opposite ends of the tube 54 defining a sealed chamber 60 therein. A plurality of nuclear fuel pellets 62 are placed in an end-to-end abutting arrangement or stack within the chamber 60 and biased against the bottom end plug 58 by the action of a spring 64 placed in the chamber 60 between the top of the pellet stack and the top end plug 56. Prior Art Inteoral Fuel Burnable Absorber Rods In the operation of a PWR it is desirable to prolong the life of the reactor core 14 as long as feasible to better utilize the uranium fuel and thereby reduce fuel costs. To attain this objective, it is common practice to provide an excess of reactivity initially in the reactor core 14 and, at the same time, provide means to maintain the reactivity relatively constant over its lifetime. As mentioned earlier, one prior art approach to achieving these objectives is to use fuel rods which are referred to as integral fuel burnable absorber (IFBA) rods, one being shown in FIG. 4. Such IFBA rods are provided in the prior art VANTAGE 5 nuclear fuel assembly manufactured and marketed by the assignee herein. The IFBA rod is a fuel rod 48 which has some fuel pellets 62 containing a burnable absorber or poison material. Specifically, end-to-end arrangements, or strings, of fuel pellets 62A containing no poison material are provided at upper and lower end sections of the fuel pellet stack of the fuel rod 48 and a string of the fuel pellets 62B with the poison material is provided at the middle section of the stack. As seen in FIGS. 5 and 6, each fuel pellet 62A containing no burnable absorber is in the shape of a solid right cylindrical body of nuclear fuel or fissionable material, such as enriched uranium dioxide. As seen in FIGS. 7 and 8, each fuel pellet 62B containing burnable absorber is composed of a solid right cylindrical body 66 serving as a substrate of the nuclear fuel or fissionable material, such as enriched uranium dioxide, and a thin cylindrical circumferential coating 68 on the exterior continuous outer surface 70 of the body 66. The coating 68 is preferably zirconium diboride (ZrB.sub.2), in which the boron-10 isotope is an effective neutron absorber; alternatively the coated fuel pellets 62B can be composed of a burnable absorber or poison material, such as gadolinia, mixed integrally with the enriched uranium fuel. The zirconium provides the cohesive matrix for holding the boron together to prevent fragmentation of the coating as the burnable absorber is burned up. Composite Fuel Burnable Absorber Arrangement of the Invention As described earlier, one problem in the case of the above-described IFBA rods with using the same burnable absorber, zirconium diboride, for controlling both power peaking and moderator temperature coefficient is that a large number of IFBA rods have to be employed, resulting in a higher residual penalty. The present invention avoid the drawback of IFBA rods by using two different absorber materials in a composite nuclear fuel and burnable absorber rod which has the same construction as the IFBA rod 48 except for the composition of the burnable absorber coated fuel pellets 62B. In the composite rod, two burnable absorber materials are used: one material, namely a boron-bearing material such as zirconium diboride, is tailored primarily for controlling power peaking; and the other material, namely an erbium-bearing material such as erbium oxide, is tailored primarily for controlling moderator temperature coefficient. The result is a significant reduction in the number of composite fuel and burnable absorber rods that need to be used, and consequently a reduction in the residual penalty without any loss in peaking factor or moderator temperature coefficient control. Boron in the zirconium diboride coated on the nuclear fuel is the preferred material for power peaking control in view of its advantages of no moderator displacement and very low residual penalty. Erbium coated on or mixed in the nuclear fuel is the preferred material for moderator temperature coefficient control. Erbium has a large resonance around 0.5 ev, which leads to a strong negative contribution to moderator temperature coefficient. As the spectrum hardens due to the increase in water temperature and the reduction in the moderator density, the harder spectrum leads to a larger resonance absorption. The contribution to moderator temperature coefficient is strong enough that even a low concentration of erbium in selected fuel rods, would eliminate the need for additional zirconium diboride or other burnable absorber materials for moderator temperature coefficient control. The low concentration of erbium, coupled with its good depletion characteristics, would lead to low residual penalty. Thus, erbium would control the moderator temperature coefficient directly through resonance absorption. Without it, the coefficient will have to be controlled indirectly through reduction of soluble boron in water by absorptions in an increased number of IFBA rods. At the same time, a smaller number of IFBA rods will be used to control the power peaking. The combination of zirconium diboride and erbium, used in the same cycle, takes advantage of the strength of both of these absorbers in the most appropriate manner. Turning now to FIGS. 9-12, there are illustrated the various embodiments of the two burnable absorber materials, boron-bearing material and erbium-bearing material, incorporated with nuclear fuel for use in the reactor core 14, in accordance with the principles of the present invention. Preferably, the fuel 72 has a substrate 74 of fissionable material, such as enriched uranium dioxide, configured as a cylindrical body or pellet having a continuous outer cylindrical circumferential surface 76. In the first embodiment shown in FIG. 9, the fuel 72A has the erbium-bearing burnable absorber material, such as erbium oxide and represented by the dashed lines, interspersed or mixed with the substrate 74A of fissionable material. The boron-bearing burnable absorber material, zirconium diboride, is provided in the form of an outer coating 78A on the outer surface 76 of the substrate 74 of the fuel 72A. In the second embodiment shown in FIG. 10, the fuel 72B has an outer coating 78B in the form of a sputtered mixture of erbium oxide and zirconium diboride. In the third embodiment shown in FIG. 11, the fuel 72C has an outer coating 78C composed of two coating layers 80 and 82, the inner layer 80 being zirconium diboride and the outer layer 82 being erbium boride. In the fourth embodiment shown in FIG. 12, the fuel 72D has an outer coating 78D also composed of two coating layers 80 and 82. However, now the inner layer 80 is erbium oxide and the outer layer 82 is zirconium diboride. Various methods of applying the coatings can be used. Examples of different methods which can be used are disclosed in above-cited U.S. Pat. No. 3,427,222, the disclosure of which is incorporated herein by reference. Referring to FIG. 2, there is shown one exemplary embodiment of an arrangement in the nuclear reactor core 14 of first and second groups of fuel rods, in accordance with the present invention, for controlling power peaking and moderator temperature coefficient factors. For purposes of brevity, in FIG. 2 the locations of fuel rods of the first group are identified by the letter "x", whereas the locations of fuel rods of the second group are identified by the letter "o". It will be noted that the first and second groups of fuel rods are illustrated in separate fuel assemblies 16. However, it should be understood that fuel rods from both groups can be contained in the same fuel assemblies. The fuel rods in the first group at locations "x" contain fissionable material but are free of any burnable absorber material, whereas the fuel rods of the second group at locations "o" contain both fissionable material and the two burnable absorber materials. As described above, the two burnable absorber materials can be provided as separate coatings or a mixture. Preferably, the one burnable absorber material is the erbium-bearing material such as erbium oxide and the other is the boron-bearing material such as zirconium diboride. Alternatively, the erbium-bearing material can be interspersed or mixed with the fissionable material. The fissionable material preferably contains enriched uranium dioxide. It is thought that the present invention and many of its attendant advantages will be understood from the foregoing description and it will be apparent that various changes may be made in the form, construction and arrangement thereof without departing from the spirit and scope of the invention or sacrificing all of its material advantages, the form hereinbefore described being merely a preferred or exemplary embodiment thereof.