Induced nuclear reactions: processes – systems – and elements – Fuel component structure – Plural fuel segments or elements
Reexamination Certificate
2001-06-18
2003-07-29
Carone, Michael J. (Department: 3641)
Induced nuclear reactions: processes, systems, and elements
Fuel component structure
Plural fuel segments or elements
C376S428000, C376S438000, C376S439000, C376S440000, C376S442000, C376S443000, C376S444000, C376S447000, C376S449000
Reexamination Certificate
active
06600800
ABSTRACT:
BACKGROUND OF THE INVENTION
Field of the Invention
The invention relates to a fuel assembly, in particular for a boiling water nuclear reactor, having a fuel assembly channel which is open at the top and bottom as cladding around a multiplicity of fuel rods which are supported with respect to one another and on the fuel assembly channel by spacers.
For fuel assemblies for boiling water nuclear reactors, it is necessary to be at a sufficiently great distance from what is known as the maximum power for transition to boiling. In this context, the maximum power for transition to boiling is the power at which the formation of steam in the fuel assembly does not yet cause the quantity of heat dissipated to the coolant to be reduced. If the maximum power for transition to boiling is exceeded, a film or even a layer of steam forms on the surface of fuel rods contained in the fuel assembly, and this represents a resistance to heat transfer. Since the quantity of heat generated in the fuel rod is then temporarily no longer completely dissipated, the temperature of the fuel rod rises, until a new thermal equilibrium is established. This may lead to the fuel rod overheating and therefore to thermal overloading of a fuel rod cladding tube. Overheating of this nature must be avoided at all costs, since it would shorten the service life of the fuel rod and therefore of the fuel assembly.
It is known from European Patent EP 0 517 750 B1 to increase the cooling of the fuel rods by segregating water droplets and steam. In this case, the water droplets are diverted, by vanes on the top side of the spacers, out of the center of a sub-channel for a coolant, which is formed by four fuel rods, onto the surface of the fuel rods, while the steam continues to flow upwards in the center of the sub-channel.
However, vanes of this type cannot in fact be disposed in the steam region, (i.e. the upper region of the boiling water fuel assembly, where a considerable part of the cooling water is already present in the form of steam, i.e. the “two-phase region”) without having an adverse effect on the hydraulic stability of the flow of coolant. This is because the spacers increase the pressure loss in the two-phase region. In addition to an undesirable reduction in the water throughput as a result of a pressure loss being caused by a fuel assembly with additional internal fittings of this type, the increase in volume which occurs during evaporation prevents the coolant from flowing out. If these negative effects are to be reduced or eliminated altogether, the pressure loss in the two-phase region has to be reduced. Such a reduction can be achieved, inter alia, by enlarging the cross section of flow. The cross section of flow can be enlarged by providing empty positions in the fuel rod grid (see EP 0 517 728 B1). For these reasons, hitherto part-length fuel rods have been used, while vanes of this type have not been deployed. Moreover, the resultant empty positions in the upper fuel assembly region contribute to improving the shutdown performance.
Although shortening some rods reduces the fuel volume in the upper region of the fuel element, it does significantly improve the moderator/fuel ratio and the fuel that is enclosed in the long fuel rods burns more successfully.
It is known that in this case the maximum power for transition to boiling, which is determined by the remaining long rods, decreases. It is stated in Japanese Patent Application JP 1-176986 A (1989) that the enlarged cross section of flow and the correspondingly lower flow resistance leads to a velocity profile of the coolant according to which the film of water flows down the fuel rod surface more slowly and therefore evaporates more quickly, i.e. already contributes to a lower heating power for transition to boiling. Therefore, it is proposed in this document to increase the flow resistance of the spacers which lie above the ends of the part-length rods. This can be achieved by thicker grid webs of the spacers or by protrusions (for example bent-off bottom edges of the grid webs) which project into the space that lies above the shorter fuel rods.
For the same reasons, U.S. Pat. No. 5,229,068 proposes that the upper spacers be constructed from higher webs or for the distances between the spacers to be reduced. Displacement bodies or helically twisted sheet-metal strips above the part-length fuel rods are also intended to completely or partially compensate for the reduction in the flow resistance, so as to restore the original pressure conditions. Above all, it is recommended for the number of spacers in the upper part of the fuel assembly to be increased, i.e. for the distance between the spacers to be reduced continuously or in steps. The greater flow resistance is regarded as a precondition for achieving a higher power for transition to boiling.
However, the hydraulic conditions which are required for sufficient cooling can considerably impair a greater flow resistance and therefore prevent the use of a fuel assembly of this type even if a higher power for transition to boiling were to be possible.
Even other experiments have failed to show a clear relationship between the power for transition to boiling and the flow resistance. Rather, measures are required which enable the remaining long fuel rods, the power of which is high on account of the favorable moderator/fuel ratio, to be effectively cooled without the pressure loss being increased excessively. This is the object of the invention.
SUMMARY OF THE INVENTION
It is accordingly an object of the invention to provide a fuel element for a boiling water nuclear reactor which overcomes the above-mentioned disadvantages of the prior art methods of this general type, in which fuel rods are effectively cooled without a pressure loss being increased excessively.
With the foregoing and other objects in view there is provided, in accordance with the invention, a fuel assembly for a boiling water nuclear reactor. The fuel assembly contains a multiplicity of fuel rods having top ends, bottom ends and a fuel assembly channel having a top, a bottom, and openings formed therein at the top and the bottom, The fuel assembly channel encloses the fuel rods. Spacers are connected to the fuel assembly channel and support the fuel rods with respect to one another and on the fuel assembly channel. The spacers are divided into a lower group having identical vertical distances from one another and, an upper group having vertical distances which at least differ from the identical vertical distances of the lower group. A mean distance between the spacers in the upper group is smaller than a mean distance between the spacers in the lower group. Some of the fuel rods are shorter fuel rods being shorter than others of the fuel rods and the shorter fuel rods end below at least two of the spacers. At least some of the spacers of the upper group have an upper side and vanes disposed on the upper side. The vanes are bent obliquely into a flow of a coolant flowing upward between the fuel rods and imparting a turbulence to the flow of the coolant, and none of the vanes projecting into a space which lies above the shorter fuel rods. A lower rod-holding plate is disposed in the fuel assembly channel such that the bottom ends of the fuel rod lie practically at a same height as the lower rod-holding plate.
The invention therefore provides for, in the region of the boiling water fuel assembly which lies above the shorter fuel rods, vanes to be disposed on the top edges of the spacer webs and the number of spacers to be increased (i.e. the distance between the spacers to be reduced) compared to the number of spacers which would result if the spacers were to be disposed at the same distance as in the lower region of the fuel assembly. The vanes are bent obliquely into the flow of coolant flowing along the long fuel rods, in such a manner that they impart turbulence to the flow of coolant, the turbulence guiding the liquid water onto the fuel rods under centrifugal force and keeping the steam away from the surface. This leads to a t
Bender Dieter
Bender Otmar
Lippert Hans-Joachim
Übelhack Walter
Carone Michael J.
Framatome ANP GmbH
Greenberg Laurence A.
Locher Ralph E.
Stemer Werner H.
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