Patent Number: 051165678
Section: summary

BACKGROUND OF THE INVENTION This invention relates to nuclear reactors and, more particularly, to fuel arrangements in a reactor core. A major objective of the present invention is to provide for more thorough fuel burnups to enhance fuel utilization and minimize active waste products. Fission reactors rely on fissioning of fissile atoms such as uranium isotopes (U233, U235) and plutonium isotopes (Pu239, Pu241). Upon absorption of a neutron, a fissile atom can disintegrate, yielding atoms of lower atomic weight and high kinetic energy along with several high-energy neutrons. The kinetic energy of the fission products is quickly dissipated as heat, which is the primary energy product of nuclear reactors. Some of the neutrons released during disintegration can be absorbed by other fissile atoms, causing a chain reaction of disintegration and heat generation. The fissile atoms in nuclear reactors are arranged so that the chain reaction can be self-sustaining. To facilitate handling, fissile fuel is typically maintained in modular units. These units can be bundles of vertically extending fuel rods. Each rod has a cladding which encloses a stack of fissile fuel pellets. Generally, each rod includes a space or "plenum" for accumulating gaseous byproducts of fission reactions which might otherwise unacceptably pressurize the rod and lead to its rupture. The bundles are arranged in a two-dimensional array in the reactor. Neutron-absorbing control rods are inserted between or within fuel bundles to control the reactivity of the core. The reactivity of the core can be adjusted by incremental insertions and withdrawals of the control rods. Both economic and safety considerations favor improved fuel utilization, which can mean less frequent refuelings and less exposure to radiation from a reactor interior. In addition, improved fuel utilization generally implies more complete fuel "burnups". A major obstacle to obtaining long fuel element lifetimes and complete fuel burnups is the inhomogeneities of the neutron flux throughout the core. For example, fuel bundles near the center of the core are surrounded by other fuel elements. Accordingly, the neutron flux at these central fuel bundles exceeds the neutron flux at peripheral fuel bundles which have one or more sides facing away from the rest of the fuel elements. Therefore, peripheral fuel bundles tend to burn up more slowly than do the more central fuel bundles. The problem of flux density variations with radial core position has been addressed by repositioning fuel bundles between central and peripheral positions. This results in extended fuel bundle lifetimes at the expense of additional refueling operations. Variations in neutron flux density occur in the axial direction as well as the radial direction. For example, fuel near the top or bottom of a fuel bundle is subjected to less neutron flux than is fuel located midway up a fuel bundle. These axial variations are not effectively addressed by radial redistribution of fuel elements. In addition to the variations in neutron flux density, variations in spectral distribution affect burnup. For example, in a boiling-water reactor (BWR), neutrons released during fissioning move too quickly and have too high an energy to readily induce the further fissioning required to sustain a chain reaction. These high energy neutrons are known as "fast" neutrons. Slower neutrons, referred to as "thermal neutrons", most readily induce fission. In BWRs, thermal neutrons are formerly fast neutrons that have been slowed primarily through collisions with hydrogen atoms in the water used as the heat transfer medium. Between the energy levels of thermal and fast neutrons are "epi-thermal" neutrons. Epithermal neutrons exceed the desired energy for inducing fission but promote resonance absorption by many actinide series isotopes, converting some "fertile" isotopes to "fissile" (fissionable) isotopes. For example, epithermal neutrons are effective at converting fertile U238 to fissile Pu239. Within a core, the percentages of thermal, epithermal and fast neutrons vary over the axial extent of the core. Axial variations in neutron spectra are caused in part by variations in the density or void fraction of the water flowing up the core. In a boiling-water reactor (BWR), water entering the bottom of a core is essentially completely in the liquid phase. Water flowing up through the core boils so most of the volume of water exiting the top of the core is in the vapor phase, i.e., steam. Steam is less effective than liquid water as a neutron moderator due to the lower density of the vapor phase. Therefore, from the point of view of neutron moderation, core volumes occupied by steam are considered "voids"; the amount of steam at any spatial region in the core can be characterized by a "void fraction". Within a fuel bundle, the void fraction can vary from about zero at the base to about 0.7 near the top. Continuing the example for the BWR, near the bottom of a fuel bundle, neutron generation and density are relatively low, but the percentage of thermal neutrons is high because of the moderation provided by the low void fraction water at that level. Higher up, neutron density reaches its maximum, while void fraction continues to climb. Thus, the density of thermal neutrons peaks somewhere near the lower-middle level of the bundle. Above this level, neutron density remains roughly stable while the percentages of epithermal and fast neutrons increase. Near the top of the bundle, neutron density decreases across the spectrum since there are no neutrons being generated just above the top of the bundle. The inhomogeneities induced by this spectral distribution can cause a variety of related problems. Focusing on the upper-middle section, problems of inadequate burnup and increased production of high-level waste are of concern. Since the upper-middle section has a relatively low percentage of thermal neutrons, a higher concentration of fissile fuel is sometimes used to support a chain reaction. If the fuel bundle has a uniform fuel distribution, this section could fall below criticality (the level required to sustain a chain reaction) before the other bundle sections. The fuel bundle would have to be replaced long before the fissile fuel in all sections of the bundle were depleted, wasting fuel. The problem with waste disposal is further aggravated at this upper-middle section since the relatively high level of epithermal neutrons results in increased production of actinide-series elements such as neptunium, plutonium, americium, and curium, which end up as high level-waste. One method of dealing with axial spectral variations is using a control rod. For the BWR, control rods typically extend into the core from below and contain neutron-absorbing material which robs the adjacent fuel of thermal neutrons which would otherwise be available for fissioning. Thus, control rods can be used to modify the distribution of thermal neutrons over axial position to achieve more complete burnups. However, control rods provide only a gross level of control over spectral density. More precise compensation for spectral variations can be implemented using enrichment variation and burnable poisons. Enrichment variation using, for example, U235 enriched uranium, can be used near the top of a fuel bundle to partially compensate for a localized lack of thermal neutrons. Similarly, burnable poisons such as gadolinium oxide (Gd.sub.2 O.sub.3), can balance the exposure of bundle sections receiving a high thermal neutron flux. Over time, the burnable poisons are converted to isotopes which are not poisons so that more thermal neutrons become available for fissioning as the amount of fissile material decreases. In this way, fissioning can remain more constant over time in a section of the fuel bundle. By varying the amount of enrichment and burnable poisons by axial position along a bundle, longer and more complete burnups can be achieved. In addition, the enrichment and poison profiles can be varied by radial position to compensate for radial variations in thermal neutron density. Nonetheless, taken together, the use of control rods, radial positional exchange of bundles, selective enrichment and distribution of burnable poisons still leave problems with axial variations in burn rates and neutron spectra. Furthermore, none of these employed methods effectively address the problem of the high level of fissile material produced and left in the upper-middle sections of the bundle due to the high level of epithermal neutrons and the low level of thermal neutrons. What is needed is a system that deals more effectively with axial spectral variations in neutron flux so that higher fuel burnups are provided and the so that high-level waste is minimized. SUMMARY OF THE INVENTION In accordance with the present invention, a nuclear reactor with a recirculating heat transfer fluid has a bi-level core which provides enhanced flexibility in fuel arrangement. The bi-level core includes two sets of fuel units, one set arranged on a first level, the other set arranged on a second level. Preferably, fuel units of the second level are arranged in vertical alignment with fuel units of the first level. This permits a fuel unit of the first level to be accessed by removing only the adjacent fuel unit of the second level. During refueling operations, fuel units can be shifted from one level to the other, providing additional flexibility in arranging units at various stages of burnup. Preferably, fuel units of the first level are inverted relative to the fuel units of the second level. The inversion provides for placing plenum sections of fuel rods in different levels away from each other so that the plenums do not introduce a discontinuity in neutron generation. In the context of a boiling-water reactor, fuel bundles are arranged into upper and lower matrices. The fuel bundles share a common form factor so that each fuel bundle can be placed in any position in either matrix. During refueling operations net transfers are as follows: spent bundles are removed from the lower matrix, partially spent bundles from the upper matrix are inserted into the lower matrix, and fresh bundles are inserted into the upper matrix. This fuel bundle "flow" is an average flow and does not exclude the possibilities that some elements are retired from the upper matrix, some fresh fuel bundles are inserted into the lower matrix, and that some partially spent fuel bundles are transferred from the lower matrix to the upper matrix. The fuel bundles can contain multiple fuel rods. Each fuel rod can include a plenum at one end where gaseous fission byproducts can accumulate. The plenum ends are preferably directed away from the interface between the upper and lower matrices. In other words, the plenums are up in the upper matrix and down in the lower matrix. Otherwise, at least one plenum would be positioned between the fuel in the same rod and the fuel in the corresponding rod in the other matrix. This would introduce discontinuities in neutron generation and temperature. Separation of fuel in the upper and lower matrices is minimized by inverting the fuel bundles when they are moved from one matrix to the other. Moreover, channel and core stability are enhanced using this inverted fuel bundle arrangement. Stable thermal hydraulic operation, that is, the propensity to damp stochastic disturbances in flow and void fraction, is promoted more effectively where there is a liquid water phase adjacent to the fuel rod plenums than where there is a combination of liquid and vapor phases. Relative to one-level cores in which all plenums are near the top, the present invention provides greater stability since at least part of the plenum volume is at the core entrance where there are no steam voids and the overall two-phase flow pressure drop is reduced. Due to heating by the core, the void fraction of the water increases at higher levels so that the steam void fraction is greater at the level of the upper matrix than it is at the level of the lower matrix. Accordingly, neutron moderation is more effective at the lower level than at the upper level. Because of the difference in moderation, fuel bundles in the upper matrix are subjected to a harder neutron spectrum than are the fuel bundles in the lower matrix. The harder neutron spectrum can be taken advantage of by the fresher fuel bundles in the upper matrix. The harder neutron spectrum contains a higher percentage of fast and epithermal neutrons, while the thermal neutron spectrum contains a higher percentage of slower thermal neutrons. Thermal neutrons are more effective than faster neutrons at causing fission. The faster neutrons are more likely to be subjected to resonance absorption, which is likely to result in a non-fissioning neutron absorption. Non-fissioning neutron absorption results in isotopic enhancement. In other words, the hard neutron spectrum breeds fissile fuel from fertile material. The primary reaction is the absorption of a fast neutron by fertile U238 to yield fissile Pu239. Neutron absorption by Pu239 can result in fission or in the formation of the next plutonium isotope, fertile Pu240. Neutron absorption by fertile Pu240 results in a fissile Pu241 isotope. The net effect of the hard neutron spectrum is production of additional fissile material as the original fissile material is partially spent. Thus, the relatively hard neutron spectrum of the upper fuel matrix can be used to breed fissile fuel, enhancing the operational lifetime of a fuel bundle. The harder neutron spectrum in the upper matrix is less effective in inducing fission. This is not a problem where relatively fresh fuel bundles in the upper matrix contain relatively high concentrations of fissile fuel, typically, U235. As the U235 is depleted faster than additional fissile fuel is created, the hard neutron spectrum would eventually be unable to support a chain reaction. Prior to this point, the no-longer-fresh fuel bundle can be transferred from the upper matrix to the lower matrix, which is exposed to a more thermal neutron spectrum. Since thermal neutrons are most effective at inducing fission, fuel in the lower matrix can be more fully utilized. This provides advantages in fuel economics as well as waste disposal. Since the fuel in the lower matrix is subjected to a thermal spectrum, there is less resonance absorption, resulting in less high-level waste. In addition, the thermal neutron spectrum at the lower matrix is less prone to breed additional fissile material. Thus, isotopic enhancement, which might otherwise contribute to higher levels of radioactivity in the spent fuel elements, is minimized by the soft neutron spectrum of the lower matrix. In summary, the present invention provides for enhanced fuel arrangement flexibility which can take advantage of axial neutron spectral shifts through the core. As a result, fuel lifetimes are increased and the quantity of high-level nuclear waste is minimized. These and other features and advantages of the present invention are apparent in the following description with references to the drawings below.