Electric heating – Microwave heating – Field modification
Reexamination Certificate
2002-06-26
2003-09-09
Van, Quang T. (Department: 3742)
Electric heating
Microwave heating
Field modification
C219S756000
Reexamination Certificate
active
06617558
ABSTRACT:
BACKGROUND OF THE INVENTION
Field of the Invention
The invention relates to a furnace for microwave sintering of nuclear fuel.
The industrial use of microwave furnaces is currently restricted to the drying of bodies or materials, sterilizing (for example of foodstuffs), the polymerizing of rubber, curing of plastics and similar processes which take place at moderate temperatures. The ceramics industry is interested in the use of microwaves for sintering, but that has previously been restricted to virtually only a laboratory scale. That is because although past experience shows that shorter sintering periods are adequate, higher temperatures are allegedly necessary (causing greater wear on the furnaces) and altogether higher energy losses occur, although better material properties (for example a finer grain in the ceramic structure) can possibly be achieved. However, until now no products with a satisfactory quality have been achieved at all with microwaves.
However, in International Application No. PCT/EP97/04513, which is not a prior art publication, there is a description of a process by which green compacts that are pressed from unsintered nuclear fuel are sintered to form finished sintered nuclear-fuel compacts. In that case, not only the shape of the sintered bodies and their density but also the mechanical/chemical properties meet the requirements for use in nuclear reactors. In that case, over the same period of time, lower temperatures are necessary than in conventional processes, with the result that maintenance is simplified and that wear and energy losses are reduced. However, the configuration described therein, which is constructed on an empirical basis, is difficult to optimize. The aimed-for homogeneous temperature distribution in the fuel, low temperature losses and low thermal stressing of the furnace parts are difficult to achieve and not always reproducible.
The special characteristic of ceramic nuclear fuel is that it “couples” adequately well to the microwaves, i.e. it can absorb energy from the microwave field, without being electrically conductive at low temperatures. However, at higher temperatures the electrical conductivity increases and the fuel increasingly behaves like a metal. Local overheating, arcs and distortions of the microwave field therefore occur (for example, an already well-sintered, conductive region may hinder the microwaves from penetrating into neighboring regions of the fuel). That results in irregularly sintered, partially melted and deformed pellets. Therefore, the aim is to achieve the most homogeneous possible distribution of the energy and temperature without highly pronounced local maxima.
According to that older proposal, the microwaves are generated by a magnetron or a similar electrical component (for example a klystron) and passed through a waveguide into a furnace chamber (working chamber), which is constructed as a resonator, i.e. it is shielded on all sides by microwave-reflecting (metallic) walls. In that case, the magnetron is regarded as the sole source of the microwave field, the nuclear energy is regarded as just a sink of the field and the waveguides with the resonance chamber are regarded merely as a lossy transmission of the microwaves. It is intended for the geometry of the resonance chamber and of the waveguides to be empirically chosen in such a way that the heat losses are minimized. In other words, as much energy as possible is taken from the field by the nuclear fuel. In addition, by changing the position of the microwaves at the working chamber, the most uniform possible temperature distribution is set in the fuel. In order to provide the necessary power, a plurality of magnetrons are respectively provided over a waveguide, which has one end that merges with its full cross section into the resonance chamber. The individual magnetrons are individually controlled, in order to bring about the most homogeneous possible temperature distribution by superposing the wave fields generated by them.
Uniform quality is achieved in that case only by pushing the material to be sintered through a ceramic tube which has sintering gas flowing through it and extends transversely through the entire resonance chamber. With the unavoidable local inhomogeneities of the wave field and the temperature distribution, all of the regions of the fuel are then subject to the same local conditions, so that ultimately all samples of the fuel should have the same prehistory with regard to the temperatures they have undergone. A precondition therefor is that the microwave field does not undergo any pronounced fluctuations over time. With regard to the temperatures, sintering times, sintering atmospheres and advantageous devices provided for the sintering (for example gas locks for introducing the fuel into the tube through which the sintering gas flows) and further details of a sintering system with microwaves, that document contains a wealth of proposals which can also be applied to the present invention. The content of that document therefore also constitutes part of the content of the present application, with which the radiating of the microwaves into the working chamber (resonance chamber) is improved.
SUMMARY OF THE INVENTION
It is accordingly an object of the invention to provide a microwave furnace for the sintering of microwave fuel of a quality required for use in a reactor, which overcomes the hereinafore-mentioned disadvantages of the heretofore-known methods and devices of this general type.
With the foregoing and other objects in view there is provided, in accordance with the invention, a method for treating nuclear fuel in a microwave furnace, which comprises feeding microwaves from a microwave radiator into an antenna cavity; feeding the microwaves from the antenna cavity through a plurality of narrow connecting openings into a resonance chamber; and introducing nuclear fuel into the resonance chamber.
With the objects of the invention in view, there is also provided a microwave furnace for treating nuclear fuel at temperatures of between 20 and 2000° C. and an average temperature of between 1200 and 1800° C., comprising a resonance chamber shielded on all sides by walls reflecting microwaves; a gassing and degassing system associated with the resonance chamber; at least one holder for nuclear fuel in the resonance chamber; an access for introducing nuclear fuel into and removing nuclear fuel from the resonance chamber; an antenna cavity shielded on all sides by walls reflecting microwaves; a separating wall separating the antenna cavity from the resonance chamber, the separating wall having at least one narrow opening formed therein providing an interconnection between the antenna cavity and the resonance chamber; and at least one microwave radiator disposed outside the resonance chamber and feeding into the antenna cavity.
With the objects of the invention in view, there is additionally provided a microwave furnace for producing sintered nuclear fuel compacts by sintering molded green compacts of nuclear fuel in a sintering gas at average temperatures of between 1200 and 1800° C., comprising an elongate resonance chamber shielded on all sides by walls reflecting microwaves, the resonance chamber having a longitudinal side and a longitudinal direction; a gassing and degassing system associated with the resonance chamber; at least one elongate holder associated with the resonance chamber for holding green compacts; an access associated with the resonance chamber for introduction and removal of green compacts; an elongate antenna cavity shielded on all sides by walls reflecting microwaves; a separating wall separating the antenna cavity from the resonance chamber; the antenna cavity connected to the resonance chamber by a plurality of slots mutually offset in the longitudinal direction of the resonance chamber; at least one waveguide on the longitudinal side of the resonance chamber, the waveguide having an open end leading into the antenna cavity and an opposite closed end; and a microwave radiator disposed at the closed
Dörr Wolfgang
Gerdes Thorsten
Gradel Gerhard
Schmitt Bruno
Willert-Porada Monika
Framatome ANP GmbH
Greenberg Laurence A.
Locher Ralph E.
Stemer Werner H.
Van Quang T.
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