Patent Number: 053176138
Section: description

DESCRIPTION OF THE PREFERRED EMBODIMENT Referring to FIG. 1, a reactor vessel V is shown in section for a forced circulation boiling water reactor. The reactor contains a core C with a plenum P therebelow. A plurality of control rods R extending through plenum P penetrate on a selected basis to control reactive output of the core C in its fission reaction. The vessel V includes a circuitous flow path which includes upward flow through core C into steam separators S and steam dryers D. Liquid rejected by the separators S passes downwardly between the core and the outside wall of the vessel in the direction of vector 14 and into the plenum P. At plenum P, the water again circulates up through the core. This reactor is a forced circulation reactor including pumps 16-19, which each include propellers 20 for drawing the water downwardly along the sides of the core, and into plenum P. At plenum P, released water passes upwardly through the core, repeating in a cycle of endless circulation, the flow path here described. Typically, feed water is introduced at an inlet 22 and disbursed through spargers 24. Likewise, steam for energy extraction is discharged from outlets 26 with extraction occurring at conventional turbines and condensers (not shown). Discharge from the condensers occurs to feed water makeup apparatus (also not shown) with return at apertures 22 of the reactor. The bottom of the core includes a boundary known as core plate 29. Core plate 29 provides lateral support for fuel support pieces to form a hydraulic boundary at the bottom or entrance to core C. The fuel support pieces rest upon the control rod guide tubes 27. Fuel assemblies 31 are positioned in turn on the fuel support pieces. Specifically, the fuel assembly lower tie plate 30 rests upon the fuel support pieces embedded within the core plate 29. The lower tie plate is further described by referring to FIG. 2, showing a cut away cross sectional view. Two types of apertures may be seen in the lower tie plate 30. A first set of apertures 32 communicates to the interior of each fuel channel, one of which, as described below, surrounds each individual fuel rod in the fuel assembly. Moderating coolant entering through the aperture 32 cools the fuel rods, and is eventually generated into steam for the extraction of energy. Additionally, a second set of apertures 34 is illustrated. Apertures 34 flood the so-called core bypass region in between the fuel channels. This bypass region provides a surrounding moderator to each of the respective fuel rods. Referring to FIGS. 3A and 3B, the configuration of the core can be better understood. Referring first to FIG. 3B, each fuel rod includes fuel cladding 40, surrounding a column of fuel pellets 42. A gap 41 typically exists between the pellets and the clad for the purpose of collecting and distributing released fission product gases. As is common in the art, the columns are sealed at their respective ends 44 and constitute discrete pressure vessels containing fuel pellets and collected fission product gases interior of the reactor. As a novel feature of this invention, each column is surrounded by its own discrete channel 50. Channels 50 are communicated at their lower end 52 to each of the apertures 32 in the lower tie plate 30. It will be understood that there will be two flow volumes interior of the core. The first flow volume 60 is defined in the upright concentric annulus about the fuel rods 40. It is in this region that the coolant is heated by the fuel rod. Secondly, there is a bypass region 64 between the respective channels. It is into this region that liquid moderator from the lower tie plate extends the entire distance between the respective lower tie plate and the top of the core with proper sizing of holes 34, as illustrated at the top portion of FIG. 3A. The fuel rod 40 is displaced from the fuel channel 50 by a spacer 80. Although various spacer designs are possible, a novel and exemplary design is illustrated in FIG. 4. Rings 80 and 84 form tight ferrule bushings between the channel wall 50 and fuel rod 40, respectively. Angular disposed vanes 86 maintain a fixed distance between the fuel channel 50 and fuel rod 40. Spacers are periodically provided along the axial extent of the fuel channels as shown in FIG. 3A. The new design contains additional structural material in the channels 50 over present conventional BWR fuel designs. This additional material has the advantage of adding considerable rigidity to the contained fuel rod 40. Unfortunately, this channel design absorbs neutrons. Since each fuel rod has its own channel, more neutrons will be lost in channel absorption than in conventional fuel designs. This absorption of neutrons at the channel walls can be partially mitigated. It is to be noted that by optimizing fuel rod cladding and fuel rod channel dimensions, half of the channel material can be compensated for by elimination of the relatively thick fuel bundle channel of current designs. Accordingly, representative dimensions compared to a conventional fuel design which can be utilized are contained in the following table: ______________________________________ This Invention Conventional ______________________________________ Number of fuel rods 91 64 Fuel rod geometry Triangular Pitch Square Pitch Cladding Outer Diameter 8.0 mm 12.3 mm Cladding Thickness 0.64 mm 0.76 mm Number of Channels 91 1 Channel Geometry Cylindrical Tube Square Box Channel Inside Span 15 mm 134 mm Channel Thickness 0.64 mm 2.0 mm ______________________________________ It will be seen that the resulting core and fuel rod configuration includes a more homogenous cross section. Consequently, all of the contained fuel rods 42 will approach their respective thermal limits with uniformity. As a consequence, the power density of the reactor can be increased. A summary of the advantages realized by this design includes advantage realized in maximum linear heat generation rate, minimum critical power ratio, and stability of the reactor at certain less-than-rated coolant flow rates. Maximum Linear Heat Generation Rate More uniformly distributed core bypass flow volume 64 (of the liquid water phase in a BWR application) provides for flatter axial power distributions and lower power mismatch between fuel rods at any given axial level ("local peaking" reduction) due to the relatively more homogeneous mixture of materials throughout the extent of the core. This in turn results in smaller deviations of the maximum linear heat generation rate (MLHGR) from average value during operations, permitting higher power densities while still satisfying the MLHGR criterion. Use of the triangular pitch also yields a higher packing density of fuel rods within the core relative to that attained with a square pitch. Relative to the current square pitch, this means lower power per fuel rod for a given common power level, providing additional MLHGR relief and opportunity for increasing the power density. Minimum Critical Power Ratio Reductions in rod-to-rod power mismatch also has a beneficial effect for the minimum critical power ratio (MCPR). All rods uniformly approach this critical power ratio limitation. Further benefits are provided by the capability to selectively orifice each discrete fuel rod at its discrete channel to match flow rates to power production rates and to promote flow turbulence near the fuel rod clad boundary. As a result, the bundle critical power increases and margin is provided to increase power density. Selective orificing can be provided by adjusting the angular deflection of the fixed vanes 86 shown in FIG. 4. This changes the coolant flow resistance or pressure drop characteristics of the individual fuel channel, forcing a particular distribution of inlet flow. Another means to selectively orifice is show in FIG. 6. Referring to FIG. 6, the bottom section 52 of two channels 50 are illustrated. It can be seen that the bottom sections 52 can be provided with different size apertures 72, 74. These differing size apertures throttle the inflow of the moderator coolant and permit varying flow rates to occur through the bundles of this invention. Stability Stability is not necessarily a limitation for increasing power density for the forced circulation plant illustrated in FIG. 1. In such forced circulation reactors, power-flow map exclusion regions can be defined and appropriate controls and instrumentation installed to avoid operations, even for upset conditions, in an operating region which may potentially result in fuel channel flow instability. Some designs, for example, selectively insert control rods automatically when flow is reduced below a certain setpoint to limit the core power level and avoid unstable power-flow regions. Nonetheless, this invention provides capability to improve stability from the viewpoint of root cause, which is insufficient coolant flow and high pressure drop in the upper two phase region of the fuel bundles. It can be seen that through selective orificing, higher flows can be promoted in fuel rod channels which would otherwise have lower margins for stable operation. Spacer Construction Referring to FIG. 4, the construction of a spacer utilized with this invention can be understood. Spacer 80 consists of two concentric ferrules 82, 84 separated by vanes 86 through which coolant flow passes. The inner ferrule 82 fits over the fuel rod clad 40, while the outer ferrule 82 fits against the fuel rod channel 50 at the inner wall. Spacers 80 are positioned at the entrance, exit and intermediate positions of the fuel rod channel as necessary to satisfy mechanical requirements to limit fuel rod and channel displacements. Spacers can also be positioned as appropriate to promote turbulent flow and improved heat transfer characteristics. Vanes 86 are pitched in a propeller-like array. The pitch of vanes 86 may impart a spiral curvature to upwardly flowing fluid. This spiral curvature imparted to the fluid can promote additional flow turbulence. Additionally, the pitch of the spacer vanes 86 can provide an orificing effect for matching inlet flow to the discrete fuel rod power production. As can be seen, vane 86 in FIG. 4 has imparted to fluid flowing by vane 86 a spiral path. Referring to FIG. 5, it can be seen that individual fuel channels 50 each with contained fuel rods 40 and spacers 80 are bundled together using external bundle wrappers 100 (straps). The bottom and top of the fuel bundle are secured with lower tie plate 30 and upper tie plate 31, respectively. Selected channels extend through the tie plates and are provided with end threads for the purpose of bolting the bundle to form a secured assembly. As seen in FIG. 5, ninety one fuel rods 40 are each fastened in an hexagonal configuration with their respective surrounding channels 50. It is contemplated that this group of ninety one fuel rods 40 and fuel channels 50 would be handle as a unitary group, much in the same manner that many fuel rods contained within more conventional fuel bundles are now handled.