Induced nuclear reactions: processes – systems – and elements – Fuel component structure – Plural fuel segments or elements
Reexamination Certificate
1999-02-10
2001-05-01
Carone, Michael J. (Department: 3641)
Induced nuclear reactions: processes, systems, and elements
Fuel component structure
Plural fuel segments or elements
C376S414000, C376S442000, C376S448000, C376S462000, C376S305000
Reexamination Certificate
active
06226342
ABSTRACT:
TECHNICAL FIELD
The present invention relates to components of a light-water nuclear reactor. The invention specifically relates to a ceramic device to retain fuel rods therein.
BACKGROUND OF THE INVENTION
A fuel assembly in a boiling-water nuclear reactor comprises a long tubular container. The container is often made with a rectangular or square cross section and is open at both ends to make possible a continuous flow of coolant through the fuel assembly. The fuel assembly comprises a large number of equally long tubular fuel rods, arranged in parallel in a defined typically symmetrical, pattern. The fuel rods are retained at the top by a top tie plate and at the bottom by a bottom tie plate. To allow optimum coolant optimum flow along the fuel rods, it is important to keep these at a distance from each other and prevent them from bending or vibrating when the reactor is in operation. If the flow of coolant around a fuel rod is prevented, so-called dryout may occur on the surface of the fuel rod, which may result in damage of the fuel rod. To secure the flow of coolant along the fuel rods, a plurality of spacers are distributed longitudinally along the fuel assembly; however, each new spacer contributes to the pressure drop across the fuel assembly. To minimize the risk of dryout to the fuel assembly the flow of coolant is maintained at a fixed margin, the dryout margin, designed to exceed the coolant flow at which dryout occurs under the relevant conditions.
A fuel assembly for a pressurized-water nuclear reactor has, in principle, the same construction as a fuel assembly for a boiling-water nuclear reactor except that the fuel rods are not enclosed by any tubular container and that their number is greater.
Normally, a fuel assembly comprises components for retaining the elongated elements, such as spacers, top tie plate and bottom tie plate. These components are normally made of metallic materials, preferably of zirconium alloys or so-called superalloys based on nickel. The resilient clamping force which the retaining components apply to the fuel rods decreases, relaxes, during operation of the reactor as a consequence of the metallic material mechanically degrading, under the conditions prevailing in the fuel assembly during operation. In addition, corrosion, erosion and abrasion damage arise under conditions which prevail in the fuel assembly during operation of the nuclear reactor. The degradation of the retaining components, as well as the occurrence of abrasion, erosion and corrosion damage, may be predicted. When dimensioning these components, the degradation is taken into consideration by oversizing the thickness of the material of the components. This oversizing of the thickness of the material of the components results in an increase of the pressure drop across the fuel assembly.
The object of the present invention is therefore to provide a device for retaining elongated elements such as fuel rods in a fuel assembly for a light-water nuclear reactor. The device makes possible an increased operating margin, that is, a margin with respect to dryout as a consequence of too low a coolant flow such that the components included in the device exhibit both improved mechanical properties, including a reduced relaxation of the clamping force which is applied to the elongated elements, and an improved resistance to damage caused by corrosion, erosion and/or abrasion.
SUMMARY OF THE INVENTIONS
According to one aspect of the present invention, a fuel assembly comprising a device for retaining elongated elements, such as fuel rods, is provided. The retaining element of this invention retains its mechanical rigidity under the conditions of heat, fluid flow and other environmental conditions that occur during the operation of a nuclear reactor.
A fuel assembly according to the invention comprises components, which are completely or partially made of a ceramic material, such as zirconium dioxide. Preferably, a two-phase zirconium dioxide is used, based on material where the high-temperature shape of the zirconium dioxide has been stabilized by the addition of a stabilizing dopant, such as an oxide of magnesium, calcium, yttrium, or a mixture of two or more of these oxides, that is, a so-called partially stabilized zirconium dioxide.
These components are preferably formed in such a way that the ceramic part of a component constitutes a mechanically self-supporting structure.
The good mechanical properties and good resistance against erosion, corrosion and abrasion of the ceramic parts of the components which are included in a device according to the invention are ensured by manufacturing ceramic bodies with low porosity and with few and controlled defects by pressing and sintering, starting from a ceramic powder. Especially advantageous is the internal stress state which exists in a ceramic body of a partially stabilized zirconium dioxide.
One advantage of the enhanced resistance to corrosion, erosion or abrasion of the components included in the device is that the contamination of the reactor water by radioactive corrosion products is prevented. The radioactive corrosion products are thus prevented from being deposited in the circulation system for the reactor water and from there emitting radioactive radiation, against which the personnel have to protect themselves in connection with service work in this circulation system.
According to the invention, forming at least one mechanically supporting part of the components, included in a fuel assembly, for retaining the elongated elements such as spacers, bottom tie plate and top tie plate in a ceramic material, the thickness of the material of this component may be considerably reduced. In addition, the risks of degradation of the mechanical properties of the components as a result of creeping and/or as a result of damage caused by erosion, corrosion and/or abrasion are minimized. The reduced thickness of the material also results in a reduction of the pressure drop across a fuel assembly which may be utilized for arranging more spacers in the fuel assembly. This results in a stabilization of the fuel assembly and the flow of coolant through the same, thus attaining the object of the invention, that is, to increase the dryout margin. By increasing the dryout margin, the total cost of the fuel cycle is reduced.
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Micko Anders
Rudling Peter
Wolfram Dietmar
ABB Atom AB
Carone Michael J.
Connolly Bove & Lodge & Hutz LLP
Mun Kyongtack K.
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