Patent Number: 051494957
Section: summary

FIELD OF THE INVENTION The present invention relates to a water rod for a nuclear reactor and, in particular, to a tubular rod for enclosing substantially unvoided water in boiling water nuclear reactors. BACKGROUND OF THE INVENTION In a typical boiling-water reactor, fuel is provided in a number of fuel rods. The fuel itself is in the form of cylindrical pellets of enriched uranium. Enrichment is the proportion of fissionable U.sup.235 to the non-fissionable U.sup.238. These pellets are enclosed in a long cylindrical tube and sealed at both ends. The cylindrical tube with the enclosed fuel pellets is known as a "fuel rod." The rods are provided in the reactor in a number of discrete packages, which are called "fuel bundles." Each bundle includes a plurality of rods held between an upper tie plate and a lower tie plate. The tie plates contain seats or apertures for positioning and holding the ends of the fuel rods. Additionally, the tie plates include apertures for permitting a flow of water therethrough in the interstices between the fuel rods. Each fuel bundle is surrounded by a fuel channel. This channel, which is typically square in section, extends from the lower tie plate to the upper tie plate. The channel functions to confine water flow in between the tie plates and around the fuel rods. Typically seven spacers are substantially evenly spaced along the length of the fuel bundle inside the fuel channel. The spacers act to further position the fuel rods along their longitudinal extends. An upper handle portion typically attached to the upper tie plate and a lower nose piece protruding downward from the lower tie plate define the top and bottom of the fuel bundle. The handle and nose piece function to permit ready insertion and removal of the fuel bundles during so-called "reactor outages." Individual fuel rods in a bundle are disposed in a matrix, and are normally arrayed in rows and columns. Typically, some of the rows and columns in the matrix are occupied by tie rods. The tie rods are threaded fuel rods which engage the upper and lower tie plates to provide structural integrity to the fuel bundle. A typical fuel rod is approximately 160 inches in length. In a reactor, a plurality of fuel bundles are positioned in the reactor core. Fuel bundles are positioned between a lower core plate and an overlying top guide. The fuel bundles are supported within the core of the reactor at the elevation of the core plate, and are held in vertical spaced apart relationship at the top guide. Each fuel bundle in the reactor core is typically spaced apart from its neighboring fuel bundles. This spacing establishes a water filled volume in the reactor core known as the core bypass region. Water is maintained in this core bypass region by metering of a small amount of water through the fuel bundle nozzles. The nuclear reaction is controlled by a number of control rods or blades. These are typically in a cruciform shape so that each control blade is adjacent to four fuel bundles. The control rods are inserted into and out of the core bypass region. These control rods contain neutron absorbers such that insertion of the control rods will locally slow or stop the reaction from being critical. During operation of the reactor, water enters the fuel bundle through the lower tie plate. The water rises through the fuel bundles because of heating, and also, where used, from the action of one or more pumps in forcing circulation through the reactor. As the water rises through the fuel bundles and is increasingly heated, during normal operation, it eventually reaches its boiling point. Steam is formed from the boiling water, causing steam voids in the upper portion of the fuel bundle. The water in a boiling-water reactor performs two functions. First, the water carries away heat from the reactor so that it can be converted to useful energy by, for example, a turbine. Second, the water acts as a moderator, i.e., it slows down the "fast" neutrons. Neutrons in a nuclear reaction are present at a variety of energy levels. The neutrons are generally referred to as "fast" neutrons and "slow" (or "thermal") neutrons. Slowing down of the fast neutrons is desirable for at least two reasons. First, the slow neutrons are more reactive in the sense that they maintain the desired chain reaction involving the fission of U.sup.235 atoms. Second, slower neutrons are more easily captured by the control blades than the faster neutrons. Therefore, a moderator, in effect, increases the efficiency of the control blades. As noted, water is a moderator of fast neutrons. However, as water is heated, it becomes less dense and less effective as a moderator. When the water becomes steam, its effectiveness as a moderator decreases drastically, and can be, for some purposes, treated as a negligible moderator. In early fuel bundle designs, all lattice positions in the bundle were occupied by fuel rods. In these early designs, the only space for water in the interior of a fuel bundle was the space between the fuel rods and in the interstitial volume between discrete fuel bundles. Because the space between rods is typically filled with a mixture of water and steam, the moderating effectiveness of this space is less than space between bundles containing "solid" moderator. Accordingly, the most effective moderating water of the reactor was positioned between fuel bundles, i.e., in the core bypass region in the interstices between the bundles, exterior to the fuel channels. In such earlier configurations, the interior fuel rods in any fuel bundle were a large distance away from the large volumes of "solid" moderating water. Because of this distance, the most interior positions in the fuel bundle had large ratios of fast to slow neutrons, and were, therefore, less efficient in maintaining those nuclear reactions requiring "slow" or "thermal neutrons". Accordingly, interior rods were typically more enriched to compensate for this lack of efficiency. Such increase in rod enrichment, however, is rather expensive. Therefore, it was previously decided to provide for additional moderating water in interior positions of a fuel bundle. Initially, one or more fuel rods were replaced with a hollow rod (called a "water rod") of equal diameter for flowing water therethrough. The water rod communicated with the lower tie plate and extended through the upper tie plate. The water rod has its own confined water flow path and as a consequence is (like the bypass region) filled with water moderator. A water rod has nuclear and thermal advantages over simply leaving a spaced unoccupied by a fuel rod. By providing a hollow rod, the subcooled water inside is prevented from mixing with the other heated water in the bundle, and is somewhat insulated. The water in the rod, therefore, does not boil as does other water in the bundle. This scheme provided some advantages because of the additional moderator in the interior positions of a fuel bundle. Initially, the water rods were the same size as the fuel rods. Later, attempts were made to provide larger diameter water rods in the fuel bundles, these later water rods exceeding the size of the ordinary fuel rod. These attempts to provide larger water rods involved merely positioning a standard round cross-section pipe, or, in some cases, a square cross-section pipe, in interior positions of a fuel bundle so as to displace one or more fuel rods. No effective attempt was made, however, to depart from spaced-apart, standard round or square tubes, or to systematically analyze the effect of these shapes on reactor efficiency. Since fuel rods when viewed in horizontal section are arranged in rows and columns, it is common to refer to each fuel bundle as occupying a "lattice position". When the water rods were expanded in size, they intruded from one fuel rod position into those fuel rod positions occupied by adjacent fuel rods. As water rod design progressed, a configuration was provided in which a water rod had a circular cross-section with a diameter sufficiently large that it occupied more than one lattice position. In one such design, four lattice positions were sacrificed to accommodate a circular water rod. Water rods have also been developed which have a substantially square cross-sectional configuration, and occupy four or nine such lattice positions. Design Configuration In the development of new water rod design, it has been necessary to bring together certain design considerations. While these considerations have been generally known, I am unaware of their collective use which enables a design as herein disclosed. I therefore set forth the considerations, followed by the design. It will be understood that this assemblage constitutes together my invention. One aspect of water rod design, which has not previously been systematically addressed, is the displacement of fuel rods from their lattice positions. Provision of a water rod necessarily requires reduction of the number of fuel rods in a bundle, and thus results in a reduction of the amount of fuel in a bundle. In spite of the sacrifice of space for fuel, provision of the water rods has been found useful because of the greater overall efficiency obtained when moderating material is positioned interior to a fuel rod bundle. As noted above, because more fuel rods are positioned closer to the moderator, more fuel rods can be provided with lessened enrichment. This reduces fuel costs without sacrificing reactor power. Another factor related to reactor design is the impact on various safety factors. Understanding of this aspect will be promoted by a brief discussion of certain safety factors. Safety requirements provide several constraints on reactor design and operation. It must always be possible to shut down the reactor at any point during its operation. Because a boiling water reactor is most reactive when it is relatively cool, such as during start-up phases, the limiting factor of shut-down ability, is the cold-state reactivity margin. This must always be maintained at least 1% of reactivity. In a boiling water nuclear reactor, fast neutrons induce their own nuclear reactions. In many these fast neutron nuclear reactions, plutonium is produced. Unfortunately, plutonium is more reactive when the reactor is in the cold state. It is thus known that high fluxes of fast neurons can reduce the cold state reactivity margin. In addition to the cold shut-down margin, there is also a hot operating margin. It is desirable that the reactor be operated on a continuing basis near its fuel power. However, the normal continuing operation of a reactor requires that some control rods be positioned in the reactor, even at a full-power state, in order to shape the reaction, i.e., to reduce or eliminate hot spots in the reactor. Accordingly, the reactor must be designed so that the full power reactivity is less than the power that would result if all control rods were withdrawn. This difference in reactivity is known as the "hot excess margin." It is typically desirably about 1%. The reactor reactivity, then, can be viewed as constrained by a "window" of reactivity. It must maintain the cold shut-down margin, and it must also be capable of producing the hot excess margin. This window of operating constraints is referred to as the "hot-to-cold swing." Additional moderator in the bundle improves the above-described cold shut-down margin. This is at least partly because more moderating water produces a higher ratio of thermal neutrons. Thermal neutrons are not as efficient in producing plutonium. Therefore, more water generally results in less creation of plutonium. Plutonium is known to increase cold reactivity. Thus, more water, in general, will desirably resulting less cold reactivity. More water in the fuel bundle also improves the hot excess margin. This is because a larger amount of water increases reactivity by providing more thermal neutrons. Because the increase of water in the fuel bundles helps with both the cold shut-down margin and the hot excess margin, it provides a bigger hot-to-cold swing. In addition to safety considerations, another factor is the life span of a fuel load. Reactivity generally decreases as the particular fuel load ages. Thus, the cold shut-down margin must be within safety requirements when the fuel load is new and most reactive. This places an upper limit on the reactivity of a new fuel load. As the fuel load ages, reactivity drops to the point where refueling becomes necessary. Refueling is an enormously expensive proposition, and any extension of the amount of time between fuel loads is greatly beneficial. Thus, if the rate at which the reactivity drops, as a function of aging of the fuel load, can be lessened, it will take more time for reactivity to drop to the point where refueling becomes necessary. One way of decreasing the rate of this drop in reactivity is to add gadolinium oxide or other "burnable absorbers" to the fuel. These burnable absorbers capture thermal neutrons and inhibit the nuclear reaction; because of this property of inhibiting the nuclear reaction, they are sometimes called "poisons." Such poisons act to initially decrease reactivity of fuel in the discrete fuel rods. However, because these burnable absorbers are depleted or "burned up" as the reactor ages, it decreases reactivity of a new fuel load more than it decreases reactivity of an aging fuel load. In this way, the rate at which reactivity decreases with aging is reduced. However, these burnable absorbers have detrimental effects as well. During the aging stages of a fuel load, there is still some amount of residual burnable absorber, usually gadolinium, which reduces reactivity at a time when such a reduction is not desired. Therefore, it is generally preferable to reduce new-load reactivity without using (or using a reduced amount of) gadolinium. Another factor which is important in reactor design is the existence of non-nucleate boiling. Instabilities leading to non-nucleate boiling can include both thermal-hydraulic oscillations and coupled nuclear-thermal-hydraulic oscillations. These oscillations are manifested when the two phased pressure drop, particularly in the upper portions of the fuel bundle, becomes too high compared with the single phase pressure drop. The resulting fuel coolant flow has an oscillatory component superimposed on the normal steady state flow. The above-described hydraulic oscillation may be augmented by the dynamic nuclear-thermal feedback process. As steam voids are created, nuclear reactivity is reduced, since steam is a poor moderator, compared with liquid water. Thus, a negative feedback system can occur whereby the nuclear reaction creates heat which, in turn, creates steam voids. The steam voids then reduce the reactivity, because of poor moderation, leading to a reduction of the heat transferred out of the fuel and an increase in the water-to-steam ratio. The increase in water-to-steam ratio results in increased reactivity, thus beginning the cycle again. Under severe circumstances, this oscillatory behavior of the fuel surface hot flux and the coolant flow rate may result in a non-nucleate boiling process, resulting in a local increase in the fuel cladding temperature. Thus, hydrodynamic oscillations are undesirable. Some aspects of water rod design are elucidated by a brief discussion of the history of bundle designs. The designs of fuel bundles has shown a progression in the number of fuel rods in a bundle. Early bundles were formed with a 7.times.7 array of fuel rods, thus having 49 lattice positions. Fuel bundles having an 8.times.8 array of fuel rods were next produced. Most recently, fuel bundles having an array of 9.times.9 fuel rods have been produced. The physical size and cross-sectional area of the fuel bundles have not increased; rather, the progression has been to a larger number of smaller-diameter fuel rods in the bundle. The heat generated by smaller fuel rods is more quickly conveyed to the surrounding water. This increased rate of thermal transfer causes an increase in the tendency for nuclear-thermal augmented hydrodynamic oscillations. Water rods are useful in controlling such oscillations. More "solid" moderator is available to reduce the sensitivity of the nuclear fission rate to changes in the in-channel moderator density. Therefore, the tendency for a hydrodynamic oscillating is reduced. Although the provision and increase in size of water rods have lead to some desirable results, there are also undesirable effects of larger water rods. First, larger water rods displace more nuclear fuel so that the total heat-generating capacity of the reactor is affected. Second, larger water rods have a larger bundle pressure drop, i.e., difference in water pressure between the bottom tie plate and the top tie plate increases. This increase in pressure drop has been found to be associated in an increased tendency for hydraulic oscillations. Third, water is known to act not only to slow down neutrons, but also to absorb thermal neutrons. Thus, when too large an amount of water is provided, the water excessively absorbs thermal (as well as fast) neutrons and decreases reactivity of the reactor. Previous approaches to the problem of configuring a water rod for inclusion within a fuel bundle have generally been empirical in nature. No effective general procedure for analyzing or designing characteristics of water rods has been available. Accordingly, previous designs have largely been confined to conventional tube shapes, such as substantially circular or square cross-sectional tubes. Some of the problems associated with providing water rods, such as foregoing or sacrificing lattice positions for fuel, have been known. Because there was no general method of analysis, however, the relative benefits and problems of additional moderator were not systematically taken into account in the design. Further, practical considerations, such as the manufacturing feasibility of constructing various rod shapes and the methods for connecting the water rods and fuel rods to each other with the desired spacing, placed additional constrains on the types of water rods previously provided. Accordingly, as noted, previous water rods have typically included only spaced-apart circular or square tubular shapes. SUMMARY OF THE INVENTION The present invention includes the provision of a new design parameter for water rods which, in general terms, is a measure of how well the sacrificed fuel rod positions are utilized. This new parameter has been termed "water rod efficiency." The water rod efficiency includes consideration of three factors: 1) the cross-sectional area of the interior of the water rod; 2) the number of lattice positions which are sacrificed or displaced; and 3) the cross-sectional area of a single lattice position. The water rod efficiency is then calculated as the cross-sectional area of the water rod divided by the area of a single lattice position, relative to the number of sacrificed lattice positions. Water rod designs are provided which are efficient in terms of space utilization and, in particular, which have a water rod efficiency greater than about 0.6, preferably greater than about 0.7. The water rods occupy a number of lattice positions which have been found to be selectable to produce the desired amount of moderation, and yet to avoid too large a decrease in active flow area and number of fuel rods. By providing a water rod with increased efficiency, several advantages are produced. In general terms, these advantages relate to efficiency because they provide the benefits of water rods, but with a decreased need to sacrifice potential fuel rod positions. Efficient provision of a larger volume of moderator improves the cold margin by providing more moderator closer to fuel rods. The disclosed designs improve the hot margin by increasing hot reactivity, since larger amounts of moderator are present. Therefore, the hot-to-cold swing is improved. Accordingly, the amount of gadolinium oxide can be reduced to reduce gadolinium residual. Efficient provision of larger amounts of moderator also increases the water-to-steam ratio in the two-phase portion of the reactor. When this ratio is improved, the tendency for instabilities is reduced. Such a reduction in the instability tendency at least partially offsets the increase in pressure drop associated with larger water rods. By providing more fuel rod positions which are adjacent to a water rod, a lager number of fuel rods are in positions of high worth (i.e., close to moderator). Therefore, less enriched and less expensive fuel can be used without sacrificing reactivity. By providing a larger number of low-enrichment fuel rods in a bundle, a more even thermal distribution can be produced, reducing rod-to-rod and bundle-to-bundle peaking. A water rod with larger cross-sectional area concentration in one part of the fuel bundle has been found to be preferable to the same cross-sectional area provided by a plurality of spaced-apart, smaller rods. The efficient provision of larger amounts of moderator reduces the tendency for hydraulic and nuclear-thermal hydrodynamic instabilities. This, in turn, permits use of high-exposure fuel rods, such as a 9.times.9 array of potential fuel rod positions. This advantage is further beneficial because a larger number of lattice positions affords greater flexibility for placement of water rods. By providing a new design parameter for use in designing water rods, candidate water-rod shapes can be efficiently screened, and proposed designs can be selected on an objective, efficiency basis. Particular water rod cross-sectional shapes, which can be feasible and economically produced, are provided. The preferred configurations include a "FIG. 8" shape having two adjacent circular portions and a cross-sectional, "peanut" shape which has two substantially triangular rounded-cornered portions separated by a constricted portion. Other designs include a substantially "rectangular" cross-sectional design and a "cruciform" design having four concave portions separating four lobes. The particular designs are of feasible construction, and produce the desired efficiency and range of moderation. A method for analyzing water rod characteristics is provided, which includes the determination of a defined water rod efficiency and substantial occupation of a number of lattice positions in a predetermined range. A device and method for positioning and connecting the water rod with respect to the fuel rods and spacers is also provided.