Patent Number: 052456439
Section: summary

This invention relates to a fuel bundle for a boiling water nuclear reactor having so-called part length fuel rods. More specifically, a fuel bundle design is disclosed in which part length fuel rods have top filled water regions or containers overlying the end of the part length fuel rods for the improvement of the nuclear performance of the fuel design. BACKGROUND OF THE INVENTION Fuel bundle designs for boiling water nuclear reactors are known. Such fuel designs are fabricated in a standard fashion including a lower tie plate for supporting an upstanding matrix of fuel rods in side-by-side relation and permitting the inflow of water coolant into the fuel bundle. Most of the fuel rods of such a fuel bundle extend from the supporting lower tie plate to an upper tie plate. This upper tie plate serves to maintain the fuel rods in upstanding side-by-side relation and to permit the exit of water and generated steam from the fuel bundle. The fuel bundle is typically surrounded by a fuel bundle channel, which channel surrounds the lower tie plate, extends upwardly around the fuel rod matrix, and surrounds the upper tie plate. This fuel bundle channel isolates the flow path through the fuel bundle so that water and steam generated in the interior of the fuel bundle are separate from the so-called core bypass region surrounding the fuel bundle. This core bypass region contains water moderator and occupies generally cruciform shaped volumes between the fuel rods into which control rods can penetrate for the absorption of thermal neutrons for the control of the nuclear reaction. In operation of the boiling water nuclear reactor fuel bundles, liquid moderator--water--is introduced at the bottom of the fuel bundle through the lower tie plate. The water passes upwardly interior of the fuel bundle and performs two major functions. First, it moderates so-called fast or energetic neutrons produced in the nuclear reaction to slow or thermal neutrons need to continue the nuclear reaction. Secondly, the water moderator generates steam which is utilized for the generation of power. It will be understood that the fuel rods interior of the fuel bundles are long slender sealed tubes containing fissionable material and are flexible. If such fuel rods were to be unrestrained, they would vibrate and even come into abrading contact with one another during the generation of steam. To restrain this tendency as well as maintain the fuel rods in their designed side-by-side spacing for efficient nuclear operation, so-called spacers are utilized. These spacers are placed at selected vertical intervals within the fuel bundle. Usually, seven evenly distributed fuel rod spacers are utilized in a fuel bundle having an overall length in the order of 160 inches. These spacers surround each individual fuel rod maintaining the precise designed spacing of the fuel rods along the entire length of the fuel bundle. It is standard practice to improve the performance of boiling water fuel assemblies by introducing special water regions which distribute controlled amounts of water liquid moderator within the fuel assembly lattice of fuel rods. This is often accomplished by the use of hollow rods ("water rods") or other generally vertically aligned parallel flow conduits, through which substantially single phase water flows. Typically, a small amount of water is bypassed from the lower tie plate region through these water regions--more often referred to as water rods--and finally discharged out the top of the fuel bundle. These controlled water regions are particularly effective in the upper portions of the fuel assembly where neutron moderation is normally reduced by the steam which displaces the liquid water for a large fraction of the coolant flow area. However, since the introduction of such water regions occupies space that would otherwise contain more uranium fuel, the net performance benefits of the water regions are a trade-off between the positive effects of improved neutron moderation and the negative effects of decreased uranium fuel content. As a consequence, careful studies are required to establish optimum shapes and numbers of these water regions in any particular boiling water reactor fuel bundle design. It is further standard practice for these water regions to extend upward from the bottom of the fuel assembly at the lower tie plate. This is done so that flow holes can be allowed for entry of subcooled water to the water regions from the lower tie plate at the bottom of the fuel assembly. This direct entry of subcooled water to the water regions is used in order to avoid the entry of steam into the water regions and to avoid unknown neutron moderation conditions that would result if both water and vapor were present within the water regions. Having a direct flow of subcooled water to the water regions avoids or minimizes subsequent steam formation from neutronic heating at higher elevations in the fuel bundle within the water regions. However, extending a water region from the bottom of a fuel assembly also has a detrimental effect on the fuel assembly performance. The adverse effect results from the removal of uranium fuel in the lower region where the adverse effect resulting from the removal of fuel rods is not compensated by large benefits from increased neutron moderation. Thus, this adverse performance in the lower portion of the fuel assembly limits the overall effectiveness achieved through addition of water regions to the boiling water fuel assemblies. So-called part length rods have been introduced into this standard fuel bundle construction. These part length fuel rods extend from the lower tie plate only partially the distance to the upper tie plate. The fuel rods typically terminate underlying the upper tie plate so as to define an unoccupied vertical interval within the fuel bundle starting at the top of the part length fuel rod and extending to the upper tie plate. These part length fuel rods have many advantages, which advantages are summarized in Dix et al. U.S. patent Ser. No. 07,176,975 entitled TWO-PHASE PRESSURE DROP REDUCTION BWR ASSEMBLY DESIGN now issued as U.S. Pat. No. 5,112,570 on May 12, 1992. SUMMARY OF THE INVENTION Part length water regions are located above part length fuel rods in boiling water nuclear reactor fuel bundles. The part length water regions include discrete containers having water entry ports at the top of the part length water regions for capturing water from the passing liquid-vapor stream, and vent ports for permitting internally generated steam from gamma ray and neutronic heating to escape the water region. The advantages of improved neutron moderation in the upper portion of the fuel assembly are present while the uranium fuel removal requirements from the more reactive lower portion of the fuel bundle are minimized. This design concept allows for greater flexibility in the length, shape, location, and attachment of water regions, since no connection to the region of the lower tie plate is required. In addition, the top filling of the water regions also eliminates the parasitic loss of flow from the active fuel region that occurs with standard bottom entry water region designs. While this design has inherent advantages by eliminating any connection to the fuel assembly inlet, it also has the inherent disadvantage of not providing liquid subcooling to minimize vapor formation within the upper top filled water region. This internal vapor formation reduces the neutron moderation improvement of the water region. Fortunately, the fraction of liquid displaced by vapor formation is small and easily predictable for the low velocity counter-current flow conditions that exist within these water region designs. In the typical design herein set forth, vapor will typically occupy less than 20% of the water region moderator volume. As a consequence, fuel assembly designs using these top filled water regions must balance these additional trade-offs to optimize overall performance. In the disclosed openings within the top filled water regions, accommodation is required to vent vapor produced by nuclear particle heating as well as to enable downward flow into the water regions of replacement liquid. The large density of water relative to the surrounding vapor flow in the upper two phase region of the fuel bundle produces a countercurrent flow condition with downward liquid flow into the water region. However, to assure that sufficient replacement liquid enters the water region, it is advantageous to have devices which deflect liquid into the top openings of the water region. Such devices can act on liquid which flows as a film on the exterior surface of the water region. Flow openings can also be placed adjacent to existing upper tie plate or top fuel rod spacers, and thereby use the normally occurring flow diversions at those locations to deflect liquid into the top of the water regions.