Induced nuclear reactions: processes – systems – and elements – Fuel component structure – Plural fuel segments or elements
Patent
1997-12-03
1999-10-12
Wasil, Daniel D.
Induced nuclear reactions: processes, systems, and elements
Fuel component structure
Plural fuel segments or elements
G21C 3356
Patent
active
059664193
DESCRIPTION:
BRIEF SUMMARY
FIELD OF THE INVENTION
The invention relates to a fuel assembly for a nuclear reactor, the framework of which includes structural spacer grids and additional grids for mixing reactor coolant, which consists of interlaced metal straps defining cells which are preferably arranged in a lattice with square repeat units.
The fuel assemblies of nuclear reactors, in particular of light water nuclear reactors, generally consist of a bundle of fuel rods, i.e., tubes made of a material which is a weak neutron absorber, filled with pellets of fissile material. The fuel rod bundle constituting a fuel assembly is held by a rigid framework including spacer grids spaced along the length of the bundle, longitudinal guide tubes introduced and fixed inside some cells of the spacer grids, and end nozzles which close the framework and hold the rods at each of the ends of the fuel assembly. The structural spacer grids, which consist of interlaced straps defining cells arranged in a lattice generally having square repeat units, transversely hold the fuel rods in the bundle, the fuel rods passing through the interior of the cells of the grids. In this way, the rods have a regular arrangement in the transverse directions, which is set by the grids. Some of the cells are occupied by guide tubes which are connected at their ends to the closure nozzles of the assembly. Each cell of a spacer grid receives either a fuel rod or a guide tube.
In order to hold the fuel rods transversely and/or axially inside the cells of the spacer grids, it is necessary to exert transverse forces on these rods, and for this purpose to provide holding devices, supported by the walls of the cells of the grid and extending into the cell.
Each of the cells has a size such that a large clearance is provided between the rod and the walls of the cell. This clearance is necessary to allow flow of the coolant and to facilitate mounting of the rods in the assembly.
The devices for holding the rods inside the cells of the grid should hold the rods in the cells to an extent which is sufficient to avoid excessively large displacements and deformations of the rods, as well as exaggerated flexing under the effect of the deformations caused by heat and irradiation.
It is furthermore necessary to avoid exerting excessively large clamping forces on the tubular cladding of the rod, risking causing the cladding to flex. It is also necessary to avoid a fastening method which risks giving rise to exaggerated wear of the cladding of the rod.
BACKGROUND OF THE INVENTION
Devices for holding the fuel rods in the cells of spacer grids have therefore been proposed which provide an exceptional compromise between these seemingly contradictory requirements. For making the structural grids and the elements for holding the fuel rods, it is furthermore necessary to use materials which have satisfactory mechanical properties at the operating temperature of the nuclear reactor and have sufficient stability under the conditions prevailing in the nuclear reactor core, in the presence of the cooling fluid of the reactor and the irradiation.
It has, for example, been proposed to make the spacer grids of an alloy with high yield strength, such as a nickel alloy, springs and rigid stops for holding the rods being obtained by cutting and pushing in some portions of the straps constituting the grid. Such a solution has the drawback of introducing a relatively large volume of neutron-absorbing material into the assembly.
It has also been proposed to make the straps constituting the cells of the grid from a material which is a weak neutron absorber, such as a zirconium alloy, the elements for holding the fuel rods being attached to these zirconium alloy straps. The holding elements may consist of attached springs made of a highly elastic alloy and of rigid stops formed on the straps constituting the various walls of the cells of the grid, in such a way that in each cell the springs are on walls opposite the walls on which the rigid stops are formed. The springs may consist, for example, of a le
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patent: 5732116 (1998-03-01), Petit
Patent Abstracts of Japan, vol. 014, No. 581 (P-1147), Dec. 26, 1990 and JP-A-02 251789 (Nuclear Fuel Ind. Ltd.) Oct. 9, 1990.
Cogema
Wasil Daniel D.
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