Patent Number: 051436906
Section: description

DESCRIPTION OF THE PREFERRED EMBODIMENTS In accordance with the present invention, a natural-circulation 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 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 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 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 assemblies 130 that constitute core 104. Fuel assemblies 130 are arranged in a two-dimensional array, as shown in FIG. 2. Spaces are left between groups of four fuel assemblies for control rods 232 with cruciform cross sections to move vertically to regulate power output. As schematically indicated in FIG. 3, each fuel bundle 130 comprises a channel wall 310 and a fuel bundle 301. Fuel bundle 301 comprises a top tie plate 306, a bottom tie plate 308, and fuel rods 302. Channel wall 310 defines a coolant channel 304 through which coolant for rods 302 can flow. Fuel rods 302 are arranged in a 15.times.15 array within channel 304 of bundle 130. Channel 304 has a square cross section, which defines the square cross section of fuel assembly 130 (as shown in FIG. 2). Not all positions within the array are filled with fuel rods 302. Some positions are left open to provide additional coolant throughput and to optimize the fission characteristics of bundle 301. Channel wall 310 extends between tie plates 306 and 308. wall 310 defines a channel for coolant to flow through. Tie plates 306 and 308 maintain fuel rods 302 in their respective array positions, yet allow for vertical expansion, which occurs as temperatures rise. Tie plates 306 and 308 have respective ridges 316 and 318. These ridges are designed to seat securely on support stub 126. Thus, assembly 130 can be supported in either an inverted orientation (in which case ridge 316 engages support stub 126) or an uninverted orientation (in which case, ridge 318 engages support stub 126, as shown). Each tie plate 306 and 308 includes respective pairs of radial holes 326 and 328, for admitting prongs of a refueling tool 330, shown about to engage holes 326 in FIG. 3. Refueling tool 330 aids in the insertion, removal, movement and inversion of fuel assembly 130. Refueling tool 330 is inserted from above while reactor 100 is shut down and the top of vessel 102 is removed. Thus, both tie plates 306 and 308 provide for both seating on support stub 126 and for manipulation by tool 300. This achieves a measure of symmetry required for the inversions of the present invention. The symmetry corresponding to bundle inversion is a 180.degree. rotational symmetry about a line perpendicular to the vertical axis of bundle 130. It is generally desirable to have a greater pressure drop at the downstream end of a fuel bundle than at the upstream end. To this end, prior art fuel bundles including a constrictive orifice at their base. However, if built into the fuel bundle, such an orifice would not meet the symmetry requirement. Accordingly, support stub 126 defines the required constriction 332, which remains in place however fuel bundle 130 is oriented. Despite the smoothing effects of inversion, different positions within a fuel bundle 301 are exposed to different conditions. The fuel near the top of the bundle 301 undergoes a different history than does the fuel at the bottom. Accordingly, fuel 140 in rods 302 is distributed non-uniformly. For example, more fertile fuel can be located near the top of bundle 301. Fertile fuel at the bottom of bundle 301 is not converted during a first cycle, and does not contribute significantly to power during the second cycle. Bottom fertile fuel is converted during the second cycle and is available during an optional third cycle. However, since further cycles are not anticipated, it is not desirable to generate too much fissile actinide products, since those that do not burn up wind up as long-term radioactive waste. Mechanically, each fuel rod 302 has a top plenum 342 and a bottom plenum 344 on both sides of fuel 346. These plenums 342 and 344 are designed to accommodate gaseous fission products that escape the fuel. Plenums are incorporated at both ends to provide symmetry in the fuel position for fuel bundle 130. Top plenum 342 houses a spring 348 that helps compact fuel 346 while permitting thermal expansion. Bottom plenum 346 houses a ventilated pedestal 350 that keeps fuel 346 out. Pedestal 350 also opposes the force of spring 348 to maintain compression of fuel 346. The use of a spring in one plenum and a pedestal in the opposing plenum, and the non-uniform distribution of fuel 356 are examples of acceptable asymmetry in fuel assembly 130. In accordance with the present invention, a fuel management method 400 for reactor 100 involves inverting fuel assemblies between operational cycles. Preparatory steps include assembly of fuel rods and of fuel assembly. During a refueling operation, a fuel assembly is inserted, at step 401, top side up into core 104. Once refueling is complete, reactor 100 is operated, at step 402, for a first cycle. After this cycle and any additional refueling operations and operating cycles not involving inversion of this specific fuel assembly, the reactor is shut down. During the next refueling operation, the fuel assembly is inverted, at step 403, and replaced in its original position in the core. The present invention also provides for changing the array position of the fuel assembly in accordance with other refueling strategy considerations. A second operational cycle is implemented, at step 404, followed by a respective refueling shutdown. At this point, there are two major alternatives provided by the present invention. The fuel assembly can be disposed of, branch 410, or it can be inverted again and reinstalled, branch 411. In the latter case, the second inversion, at step 405, restores the original orientation of the fuel assembly. One purpose of this reversion is the burn up of actinide fissile material generated during the second operating cycle. After this inversion, a third operating cycle and shutdown, step 406, is implemented. During a following refueling operation, the fuel assembly is removed and disposed of, step 407. A new fuel assembly is added to the core in its place. Where the reversion is not implemented, steps 405 and 406 are skipped and step 407 follows step 404. The branches of method 400 can be practiced in the alternative or together with respect to different assemblies in the same core. The election of two versus three cycles depends on core position of a fuel assembly and the distribution of fuel in the bundle. A major consideration is the amount of fissile fuel generated during the second operating cycle. 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.