Patent Number: 051494914
Section: description

DESCRIPTION OF THE PREFERRED EMBODIMENTS In accordance with the present invention, a boiling-water reactor 100 comprises a vessel 102, a core 104, a chimney 106, a steam separator 108, and a dryer 110. Control rod drive housings 112 extend through the bottom of vessel 102 and support control rod guide tubes 113. Control rod guide tubes 113 extend to the bottom of core 104 so that control blades therein can be inserted into and retracted from core 104 to control its power output. Water flows, as indicated by arrows 114, into core 104 from below. This subcooled water is boiled within core 104 to yield a water/steam mixture which rises through core 104 and chimney 106, as indicated by an arrow 115. Steam separator 108 helps separate steam from water, and the released steam exits through a steam exit 116 near the top of vessel 102. Before exiting, any remaining water entrained in the steam is removed by dryer 110. Water is returned down a peripheral downcomer 118 by the force of the driving steam head provided by chimney 106. Feedwater enters vessel 102 through a feedwater inlet nozzle 120 and a feedwater sparger 122 to replenish and to help cool the recirculating water in downcomer 118. Core 104 is bounded from below by a core support plate 124, along with associated orificed support stubs 126, and bounded from above by a top guide 128. These structures support and aid in the installation of fuel bundles 130 that constitute core 104. Fuel bundles 130 are arranged in a two-dimensional array, as shown in FIG. 2. Spaces are left between groups of four fuel bundles for control rods 232 with cruciform cross sections to move vertically to regulate power output. Fuel bundles 130 are divided into three groups, a group of relatively fresh bundles 234, a group of bundles 236 at mid-life, and a group of bundles 238 near the end of their useful lifetime. Orificed support stubs 126 are likewise divided into three groups, small-orificed stubs 242 (small circles in FIG. 2), large-orificed stubs 244 (large circles in FIG. 2), and peripheral stubs 246 (at locations marked "P"), which also have small orifices. Fresh fuel bundles 234 are disposed over small-orificed stubs 242, mid-life bundles 236 are disposed over large-orificed stubs 244, and late-life fuel bundles 238 are disposed over peripheral stubs 246. The locations of fresh bundles 234 are radially between the more central locations of the mid-life bundles 236 and the more peripheral late-life bundles 238. Small-orificed stubs 242 and peripheral stubs 246 define 1" apertures through core support plate 124, while large-orificed stubs 244 define 2" apertures through core support plate 124. The fuel arrangement of FIG. 2 results from the fuel management method 300 illustrated in FIG. 3. Fresh fuel bundles are inserted (at step 301) into core locations defined by small-orificed stubs. A first reactor cycle, including operation and shutdown, is implemented (at step 302). During this first reactor cycle, the restricted coolant flow established by the small orifices result is a fast neutron spectrum, limiting fissioning and promoting the conversion of fertile U238 to fissile Pu241. The conversion/fission ratio is between about 0.7 to 0.8, compared to a 0.5 to 0.6 in a conventional fuel bundle arrangement. (A ratio in excess of 1.0 would characterize a breeder reactor which produces more fissile fuel than it uses.) This substantially increases the fertile fuel available at mid-life. During the present or a subsequent refueling cycle, the subject bundle, now at mid-life, is inserted (at step 303) into a more central location over a large-orificed stub. A second reactor operation cycle is implemented (at step 304). During this second cycle, the large-orificed stub permits a relatively fast coolant flow through the bundle, resulting in a smaller steam void, more moderation, and a more thermal neutron spectrum. As a result, fissioning is promoted at the expense of conversion. During this cycle, the conversion ratio is about 0.3-0.4. Thus, fewer actinide products are being generated, while those resulting from the first cycle are given at least the entire second cycle for burnup. While the present invention provides for disposal of a fuel bundle (at step 307) after this second cycle, as indicated by branch 310, a late-life bundle can be inserted into a peripheral location (at step 305). During a subsequent third cycle and shutdown (at step 306), the late-life bundle makes a modest contribution to core reactivity and shields core externals from the more intense radiation near the center of the core. Eventually, the spent fuel bundle is removed and disposed of (at step 307). In the illustrated embodiment, the different sized apertures through the core support plate were defined using stubs with different sizes of orifices. Alternatively, the apertures could be defined in the core support plate itself. Alternatively, an attachment could narrow the otherwise large orifice of a stub to provide smaller-orificed stubs. In this vein, the orifice constriction can be built into the bundles, and either adjusted or removed during a refueling operation. While the illustrated embodiment used only two orifice sizes, more gradations are employed in other embodiments. Further, coolant flow rates can be varied by location in ways not relying on flow apertures. While the illustrated embodiment shows core locations arranged in three bands in radial succession, many alternative arrangements are provided for. More bands can be used to provide a succession of steps toward locations with larger orifices and/or locations otherwise provided with faster coolant flow rates. Bands can alternate to smooth the reactivity profile along a core radius. For example, a small-orificed band can be inserted between two large-orificed bands to soften a thermal peak near the core center. Furthermore, orifice-define groups of locations need not be defined by bands at all. Checkerboard arrangements, random arrangements, and many alternatives are also provided for. Wherever a design analysis indicates excess reactivity, a small-orificed location can be exchanged for a large-orificed location. Fuel bundles can progress cycle by cycle. Alternatively, some fuel bundles can remain in place or be transferred within orifice size subgroup during a refueling cycle. The present invention provides for some bundles following patterns not in accordance with FIG. 3, as long as some fuel bundles progress from small-orificed locations to large-orificed locations. It should be noted that the present invention is compatible with many other fuel management strategies including bundle inversion to improve axial burnup uniformity. These and other modifications to and variations upon the described embodiments are provided for by the present invention, the scope of which is limited only by the following claims.