Induced nuclear reactions: processes – systems – and elements – Fuel component structure – Plural fuel segments or elements
Reexamination Certificate
2000-09-25
2003-07-29
Carone, Michael J. (Department: 3641)
Induced nuclear reactions: processes, systems, and elements
Fuel component structure
Plural fuel segments or elements
C376S434000, C376S438000, C376S349000
Reexamination Certificate
active
06600799
ABSTRACT:
BACKGROUND OF THE INVENTION
Field of the Invention
The invention relates to a fuel element having fuel rods disposed substantially parallel to a fuel-element axis and substantially perpendicularly to a polygonal internal cross section perpendicular to the fuel-element axis. In this case, in a first region, a ratio of the free area of the internal cross section to the area through which the fuel rods pass is smaller than in a second region.
To improve the thermohydraulics and a neutron economy in a fuel element, it is known that a distance between the fuel rods—in particular in relation to a radius of the encasing tube—can be set variable, but so as to be constant for a fuel element (see European Patent EP 0 373 418 B1). This serves in particular to reduce the pressure loss, improve the moderation ratio and reduce the neutron absorption by the structure material. In this case, it is advantageous if a situation which is as homogeneous as possible with regard to the thermohydraulics and the neutron economy can be achieved over the cross section of the reactor core and in particular over the cross section of one fuel element.
However, this is not the case as a rule, but rather the thermohydraulic and neutron-economy properties of the reactor core have considerable inhomogeneity and asymmetry.
Such inhomogeneity or asymmetry may occur, for example, in a pressurized-water fuel element due to the configuration of control rods, instrumentation tubes or other tubes without fuel. In a boiling-water-reactor fuel element, the configuration of the control rods between adjacent fuel elements may, for example, be the cause of an anisotropic neutron flow. Inhomgeneity and asymmetry may also occur in light-water reactors and/or light-water-reactor fuel elements, for example, due to a distribution of differently enriched fuel rods, and/or fuel rods provide neutron absorption, over the cross section of the fuel element.
To remove this or similar inhomogeneity, various measures which take into account the cause of inhomogeneity are known. For example, it is known according to U.S. Pat. No. 5,094,805, in a pressurized-water-reactor fuel element, to make the distance between a control rod and a fuel rod adjacent to the control rod smaller than a distance between two adjacent fuel rods.
Such a measure normally results in an essentially centrosymmetrical distribution of the fuel rods, which brings about the removal of inhomogeneity caused by a centrosymmetrical distribution of the control rods.
A similar approach is specified in Published, European Patent Application EP 0 196 655 A1 for a pressurized-water-reactor fuel element, in which the cross section of the fuel rod adjacent a control rod is reduced and thus the fuel quantity in the vicinity of a control rod is reduced. The distance of the fuel rod from an adjacent control rod increases as a result. Here, too, there is an essentially centrosymmetrical size and/or distance distribution of the fuel rods for removing essentially centrosymmetrical inhomogeneity (e.g. see FIG. 6 in EP 0 196 655).
An essentially centrosymmetrical fuel and/or encasing-tube size distribution is also provided for a boiling-water-reactor fuel element (see FIG. 5 in EP 0 196 655). In this case, there is a slight deviation merely at a marginal edge of such a boiling-water-reactor fuel element in order to compensate for the inhomogeneity which is caused by an adjacent control rod in the center of a group of four of such boiling-water-reactor fuel elements.
The hitherto known measures for removing inhomogeneities in the thermohydraulics and/or the neutron economy therefore relate essentially to centrosymmetrical distributions of fuel-rod distances and/or sizes, at most with locally limited deviations from the centrosymmetry which serve to remove highly localized effects—for example at the margin of a boiling-water-reactor fuel element due to a control rod.
However, inhomogeneity of the thermohydraulics and neutron economy of a ligh-water-reactor fuel element cannot always be removed in a satisfactory manner with an essentially centrosymmetrical or highly localized variation in a fuel-rod cross section or a distance between adjacent fuel rods. This is because the spatial range of a measure usually cannot be locally restricted in its effects and is in addition possibly different for the thermohydraulics and neutron economy.
The reason for this is that, if such a measure is taken, because, for example, the neutron flow is to be influenced in a certain sense via the volumetric ratio of moderator to fuel, the flow resistance and the flow distribution, via the cross sections of flow available for the coolant, will at the same time also be influenced to an extent which is possibly not optimal. Increased moderation in a region of the fuel element can lead there to a higher output and to greater formation of steam, for the rapid removal of which a higher flow velocity than can be achieved by an increase in the fuel-rod distance would be desirable.
SUMMARY OF THE INVENTION
It is accordingly an object of the invention to provide a fuel element with fuel rods for a boiling water reactor that overcomes the above-mentioned disadvantages of the prior art devices of this general type.
With the foregoing and other objects in view there is provided, in accordance with the invention, a fuel element including a fuel element body having a fuel element axis, a polygonal internal cross section perpendicular to the fuel-element axis, a first region, and a second region; and fuel rods having outer surfaces disposed substantially parallel to the fuel-element axis and substantially perpendicularly to the polygonal internal cross section in the fuel element body. In the first region, a ratio of a free area of the polygonal internal cross section to an area through which the fuel rods pass in the first region is smaller than in the second region. The first region forms a first corner of the polygonal internal cross section and the second region forms second corners of the polygonal internal cross section. And a distance between the outer surfaces of in each case two adjacent ones of the fuel rods increases monotonically in one direction starting from the first corner of the fuel element body.
The object of the invention is to specify a light-water-reactor fuel element that has both improved thermohydraulics and an improved neutron economy.
The object is achieved according to the invention by a fuel element of the type mentioned at the beginning in that the first region forms a first corner of the internal cross section, and other corners of the internal cross section are formed by the second region. In addition, the distance between the outer surfaces of in each case two adjacent fuel rods increases montonically—in one direction starting from the first corner of the fuel element. This is the case in particular along a diagonal and/or a side of the internal cross section.
In this case, a fuel rod is disposed virtually perpendicularly to the polygonal internal cross section and passes through the latter with the fuel-rod cross-sectional area. There is thus a free area, accessible to the cooling water, of the internal cross section and an area of the internal cross section through which fuel rods pass.
The invention is based on the knowledge that, to achieve the object, it is favorable to effect a redistribution of the coolant flow and moderator flow in a light-water-reactor fuel element of the type mentioned at the beginning from the first region toward the second region. The object is achieved by a reduction in the free area in the first region and by the configuration of the first region in the first corner of the internal cross section in the fuel element mentioned at the beginning. This is favorable in the case of a coolant flow that is virtually a two-phase flow, that is a flow having a high steam portion (e.g. up to about 40% to 50%). In the case of such a two-phase flow, as occurs in particular in a boiling-water-reactor fuel element, the steam in particular will escape into the second region, since
Framatome ANP GmbH
Greenberg Laurence A.
Locher Ralph E.
Matz Daniel
Stemer Werner H.
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