Induced nuclear reactions: processes – systems – and elements – Fuel component structure – Plural fuel segments or elements
Reexamination Certificate
1999-04-05
2001-01-30
Jordan, Charles T. (Department: 3641)
Induced nuclear reactions: processes, systems, and elements
Fuel component structure
Plural fuel segments or elements
C376S434000, C376S443000, C376S409000, C376S418000
Reexamination Certificate
active
06181762
ABSTRACT:
TECHNICAL FIELD
The present invention relates to a nuclear fuel bundle for a nuclear reactor and particularly relates to a nuclear fuel bundle for a boiling water reactor having fuel rods with different peak power limits dependent upon lattice location in the bundle.
BACKGROUND OF THE INVENTION
Fuel bundles for nuclear reactors typically include a plurality of nuclear fuel rods extending in generally parallel relation one to the other and arranged in a rectilinear matrix of fuel rods, e.g., 8×8, 9×9, 10×10 arrays, with peripheral or edge fuel rods surrounding interior fuel rods, as well as one or more interior water rods. Characteristic operating conditions of a BWR fuel bundle lattice are very heterogeneous. For example, large quantities of non-voided water (moderator) lie between the fuel rods adjacent lower portions of the bundle, while boiling water (voided) lies adjacent the upper end of the bundle. This results in reduced average water density. The power in any fuel rod is proportional to the low energy neutron density within the fuel rod. When the neutrons are liberated in the fission process, the neutron energy is transferred to the water within the reactor core through elastic and inelastic collisions, resulting in a shift in the neutron energy spectrum towards the low end, with the highest population of low energy neutrons existing near regions of high water density. It has been observed that in regions of the reactor that contain high water density and exhibit large thermal neutron densities, the fuel rods near those regions exhibit relatively high powers. Particularly, it has been observed that fuel rods about the periphery or edge of the fuel bundle typically operate at powers that are substantially, e.g., on the order of 20%, higher than the majority of the interior rods. Interior fuel rods adjacent one or more water rods also exhibit somewhat elevated powers.
Peak power limit for each fuel rod in a nuclear fuel bundle is defined as the maximum power limit at which each rod may operate, i.e., a maximum power output per unit length of fuel rod during steady state operation. Peak power limit is evidenced by a thermomechanical curve that basically identifies the maximum peak power output at which each rod can operate as a function of time. This limiting curve is the same for all fuel rods in the lattice and all fuel rod positions independent of the number of pellets within the fuel rod, their enrichment, column length, fission gas plenum volume and the like. All rods within the fuel bundle, regardless of type, e.g., fuel rods only, rods having a mixture of fuel with poisons such as gadolinium or part-length fuel rods, must operate below the peak power limit. Because the natural power peaking in a BWR fuel bundle is dependent upon the position of the fuel rod relative to the neutron moderator, i.e., water, it is common to have a subset of fuel rods that dictate the maximum power that is achievable in the fuel bundle. Thus, the fuel rods adjacent the periphery or edge of the fuel bundle typically define the maximum power peaking in the BWR fuel bundle. Stated differently, the interior fuel rods typically operate with a greater margin relative to the peak power limit than do the peripheral or edge rods and, in essence, are under-utilized. To offset that, fissile enrichment in these high-power peripheral fuel rods is often depressed, hence increasing the operating margins for the rods while disadvantageously limiting the power output that can be generated from the fuel bundle.
In known prior designs, all of the fuel rods of a bundle have the same peak power limit and all fuel rods of that bundle operate below the peak power limit with different margins. For example, while a majority of fuel rods in a BWR fuel bundle are uranium rods that do not contain poisons, even those rods which do contain poisons such as gadolinium, as well as part-length fuel rods, must operate below the peak power limit. These rods are typically located within the interior of the bundle. Consequently, when the peak power limit is established and the fuel rods are designed to balance power producing capability and fuel bundle weight, the resulting fuel bundle is fundamentally unbalanced because the fuel rod power behavior is very dependent upon the lattice position of the rods within the bundle such that some rods operate near the peak power limit and others have significant margins.
BRIEF SUMMARY OF THE INVENTION
In accordance with the present invention, there is provided a fuel bundle having two or more peak power limits for two or more sets of fuel rods dependent upon lattice location. For example, if the peak power limit for the edge or peripheral rods of the bundle is raised relative to the peak power limit of the interior rods, the edge rods are provided with greater margins, notwithstanding their higher power locations in the lattice. By differentiating between the peak power limits for two or more sets of fuel rods within the same fuel bundle, e.g., a higher peak power limit for edge rods than the peak power limit for the interior rods, bundle power can be increased while optimizing bundle uranium weight, thereby improving operating margin and fuel cycle costs. Thus, the present invention provides a fuel bundle wherein one set of rods, e.g., the edge rods, have a higher peak power limit than the peak power limit of another set of rods, e.g., the interior rods. Further peak power differentiation can also be provided. For example, interior fuel rods adjacent one or more of the water rods may have a higher peak power limit than other interior rods but lower than the increased peak power limit of the edge rods. Peak power limit differentiation within the interior rods, however, is of lower order value than as between the edge and interior rods.
To provide a bundle with differentiated peak power limits optimized for rod lattice position, the peak power limit of a first set of rods, for example, the peripheral or edge rods, is raised. This is manifested in a number of ways. For example, the length of the nuclear fuel column within one or more of the rods forming the peripheral or edge rods can be shortened leaving an increased gas plenum volume at the upper region on top of the fuel rod. Stated differently, the present invention provides for the removal of fuel from the peripheral or edge rods in those regions of low power generation, i.e., near the top or bottom of the fuel rod. For example, a fuel pellet predominantly of natural uranium and therefore of low power generation capacity can be removed at the top of the fuel rod, increasing the gas plenum volume without significantly affecting power output. By increasing the fission gas volume/fuel volume ratio, the power generated from the higher power generating portion of the fuel rod, e.g., approximately 85% of the fuel rod length excluding about the upper and lower 20 inches each of the fuel rod, can be increased, for example, by fuel enrichment in that higher power generating region. Significantly, by increasing the gas plenum/fuel volume ratio in this manner, e.g., by removing fuel from low power regions of the edge fuel rods, the peak power limit of the edge rods can be increased prior to the first fission chain reaction of the fuel bundle in the reactor relative to the peak power limit of the interior rods.
This is distinguished from maintaining the same peak power limit for all rods while flattening the power distribution curve by uniformly reducing fuel in the edge rods to improve margin and hence increase actual power output (while still maintaining all fuel rods below the same initial peak power limit prior to any fission chain reaction). Thus, if the typical fuel rod at the edge has a conventional nuclear fuel column 150 inches long, the length of the fuel column can be shortened, e.g., to approximately 146 inches increasing the available gas plenum volume within the fuel rod. Alternatively, or conjunctively, the fuel pellet density or pellet diameter can be changed. For example, the pellet diameter of the edge rods
Akerlund Sten O.
Elkins Robert B.
Fawcett Russell M.
Jackson Roland O.
Kunz Cary L.
General Electric Company
Jordan Charles T.
Keith Jack
Nixon & Vanderhye
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