Patent Number: 061817627
Section: description

DETAILED DESCRIPTION OF THE INVENTION Referring now to FIG. 1, there is illustrated a fuel bundle B having fuel rods arranged according to the present invention. Bundle B includes an outer channel C surrounding a plurality of fuel rods 10 extending generally parallel one to the other between upper and lower tie plates U and L, respectively, and in a generally rectilinear matrix of fuel rods as illustrated in FIG. 2. The rods 10 are maintained laterally spaced from one another by a plurality of spacers S vertically spaced one from the other along the length of the fuel rods within the channel C. The fuel bundle B illustrated in FIG. 1 is conventional in all respects except that the fuel rods are arranged to have different peak power limits in the bundle as set forth below. Referring now to FIG. 2, there is illustrated an array of fuel rods, i.e., in this instance, a 10.times.10 array, surrounded by the fuel channel C. The fuel rods 10 are arranged in orthogonally related rows and also surround one or more water rods, two water rods 12 being illustrated. The fuel bundle B is arranged in one quadrant of a control rod 14 as is conventional, it being appreciated that a fuel bundle is arranged in each of the other quadrants of the control rod and that the bundles constitute part of a large number of bundles disposed in a nuclear reactor core. In accordance with a preferred embodiment of the present invention, the fuel bundle B has differential peak power limits for the fuel rods. Preferably, the peak power limit for the fuel rods comprising the periphery or edge of the bundle have a higher peak power limit than the peak power limit of the fuel rods interior of the peripheral rods. The peripheral or edge fuel rods are identified in FIG. 2 at 10p, while the interior rods are identified at 10i. By increasing the peak power limit of the peripheral or edge fuel rods which typically operate at higher power levels, higher bundle power producing capability is achieved. As well known to those of skill in this art, the maximum or peak power limit is the maximum power output per unit length of fuel rod for steady state operation. Each and every fuel rod in the fuel bundle must operate at or below the peak power limit. Because of the heterogeneous nature of the bundle lattice in a boiling water reactor, the fuel rods of the fuel bundle will have different margins between their actual power output and the peak power limit. It has been observed that the fuel rods near the regions of high water density and large thermal neutron density exhibit relatively higher power output than other fuel rods adjacent regions of less water density and smaller neutron densities. Typically, the fuel rods on the edge or periphery, i.e., rods 10p of the fuel bundle B, typically operate at output powers higher than the majority of the interior rods, although fuel rods adjacent the water rods 12 also exhibit higher power outputs than other interior fuel rods. In a conventional nuclear fuel bundle, all of the fuel rods have the same power limit, beyond which they may not operate. Given that all of the fuel rods of that bundle operate at different power outputs, any power increase in the entire bundle, while operating the nuclear reactor, will decrease the margins until one of the fuel rods operates at the peak power limit. Because the edge or peripheral fuel rods 10p normally operate at power outputs higher than the power outputs of the interior rods, one or more of the edge or peripheral rods will approach and obtain the peak power output before the remaining interior rods. Once that limit is reached, it will be appreciated that there remain significant power output operating margins in a number of the other, primarily the interior fuel rods, which to that extent are under-utilized. By increasing the peak power limit for the rods which would otherwise first approach or obtain the lower peak power limit common to all rods of a conventional fuel bundle, i.e., edge rods, in comparison with the peak power limit of those operating with greater margins, i.e., interior rods, it will be appreciated that the power output of the entire bundle can be increased during operation of the nuclear reactor. This is because the power output of the peripheral or edge fuel rods may be increased to decrease their margin and approach the higher peak power limit. To accomplish this, i.e., to enable operation of the edge or peripheral fuel rods at or below a higher peak power limit than the peak power limit of the interior rods with an overall increase in bundle power output, a decrease in the magnitude of the nuclear fuel is provided in each of the edge or peripheral rods in comparison with the magnitude of the nuclear fuel provided the interior rods. This can be accomplished in a number of different ways. For example, the length of the column of nuclear fuel in each of the edge or peripheral rods may be shortened with respect to the length of the column of nuclear fuel for each of the interior rods thereby increasing the fission gas volume of the fuel rod, principally plenums 15a and 15b relative to the fuel volume as evident from a comparison of FIGS. 3A and 3B. It will be appreciated that typically the nuclear fuel in the fuel rods is contained in pellets 13 (FIGS. 3A and 3B) stacked one on top of the other within the nuclear fuel rod 10. By reducing the length of the stack of pellets by omitting one or more pellets or forming pellets shorter in length, the length of the nuclear fuel column in the peripheral or edge fuel rods 10p, as illustrated in FIG. 3B, can be shortened thereby increasing the ratio of plenum/fuel within the fuel rod. As seen in FIG. 3B in comparison with the prior art of FIG. 3A, the plenums 15a and 15b are increased in volume at the top and bottom, respectively, of the fuel rod. That is, fuel is reduced in the low power regions adjacent the top and bottom of the edge rods as compared with the interior rods while the magnitude of fuel in the high power regions intermediate the length of the fuel rod remains the same. Another way of reducing the magnitude of the nuclear fuel in the edge or peripheral fuel rods in comparison with the interior rods is to reduce the diameter of the fuel pellets. Thus, in FIG. 4A, a conventional fuel pellet having a very small gap a between the interior wall of the fuel rod 10 and the exterior wall surface of the pellet 13 is illustrated. In accordance with the present invention, as illustrated in FIG. 4B, the diameter of the fuel pellets 13 within the edge or peripheral fuel rods 10p can be reduced whereby the gap b between the interior wall surface of the fuel rod 10p and the exterior wall of the fuel pellet 13 is increased, hence reducing the magnitude of nuclear fuel in the rods and hence increasing the gas plenum/fuel ratio. Additionally, the density of the nuclear fuel can be diminished or decreased in the edge or peripheral rods. It will therefore be appreciated that by reducing the magnitude of the nuclear fuel in the lattice positions which typically exhibit higher power outputs, i.e., the edge or peripheral lattice positions, the power output of those fuel rods occupying the edge or peripheral positions, is reduced. However, during operation of the nuclear reactor, the power output of the bundle can be increased beyond the power output otherwise available from a bundle having a conventional single peak power limit because power peaking occurs in the edge rods which, in accordance with the present invention, have increased peak power limits to afford greater margin. The capability for operating the nuclear reactor at higher power outputs is significantly more important than effecting a minor reduction in the magnitude of the nuclear fuel available in the edge or peripheral rods, particularly in the low power output regions of the rods, i.e., the top and bottom. It is, of course, important that the mass of nuclear fuel within the bundle be maintained at the highest level possible to produce the highest energy output. The minor reduction in the magnitude of nuclear fuel in the edge or peripheral rods is small in comparison with the overall fuel mass within the bundle and is preferably taken from one or more low power output regions. Power output is therefore not generally affected by reducing the fuel in the low power regions of the edge rods but beneficially the gas plenum volume is increased, enabling higher power outputs by increasing enrichment in the edge rods and operating interior rods with decreased margin vis-a-vis the lower peak power limit. With differential peak power limits, there is thus the significant benefit of adding increased power output capability during nuclear reactor operation. Consequently, a significant aspect of the present invention resides in the edge or peripheral rods having the higher peak power limit in those higher power locations in the bundle lattice and comparatively greater magnitudes of nuclear fuel in comparison with the interior rods which have a comparatively lower peak power limit in bundle lattice locations with naturally lower power outputs. It will therefore be appreciated that increased fissile loadings or fuel enrichments can be provided the outer or peripheral rods of the fuel bundle. Thus, additional fissile content can be placed in the edge or peripheral fuel rods and they can be operated at higher power levels, even though there is a lesser magnitude of nuclear fuel in the edge or peripheral rods. The reduction in the magnitude of the nuclear fuel is small relative to the increased power output capability available due to the differential peak power limits and the increased enrichment. It will also be appreciated that other peak power limit differentiations may be provided within the fuel bundle dependent on lattice position within the bundle. For example, the fuel rods adjacent the one or more water rods of the bundle have been observed to operate at powers higher than other interior rods although at reduced powers relative to the edge rods. Thus, the foregoing described principles may apply to those rods adjacent the one or more water rods within the bundle and hence their peak power limit may be raised similarly by reducing the nuclear fuel content within those rods as aforedescribed. Further differentiations may also be provided dependent upon lattice position. To summarize, the present invention provides peak power limit differentiation among various fuel rods in the fuel bundle dependent upon lattice position. To provide high value differentiation, a higher peak power limit is provided for the edge or peripheral fuel rods which typically have higher power outputs than the interior rods as compared with the peak power limit of the interior rods which results in enhanced bundle power output capability. This is manifested by decreasing the magnitude of nuclear fuel in the edge or peripheral rods while raising their peak power limit, thereby increasing the margin and enabling increased overall power output for that bundle. The edge or peripheral rods can also have increased enrichment to obtain enhanced power output capability. Other lesser value peak power limit differentiations within the fuel bundle may also be provided. Of course, the peak power limits are determined prior to the first fission chain reaction of the nuclear fuel bundle in the nuclear reactor. While the invention has been described in connection with what is presently considered to be the most practical and preferred embodiment, it is to be understood that the invention is not to be limited to the disclosed embodiment, but on the contrary, is intended to cover various modifications and equivalent arrangements included within the spirit and scope of the appended claims.