DE1471195A1 - Process for the production of a solid atomic fuel - Google Patents

Description

Process for the production of a solid atomic

fuel

The invention relates to a method for producing a solid atomic fuel, in particular which is coated with pyrolytic graphite.

Nuclear fission is a process in which the nuclei of heavy elements, i.e. a solid atomic fuel, be split into two parts by a neutron, whereby the neutron acts as a projectile, so to speak, A neutron has no charge and can easily penetrate the core of a heavy element. The presence of such a particle in the nucleus

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an instability of the heavy element, which breaks down into two fission products. Be in this process in addition, further neutrons are released, which in turn cause the fission of further nuclei and thus lead to a chain reaction. The total mass of the cores of the fission products is always less than that of the original split particle and the loss of mass is converted into kinetic energy of the fission products, which in turn generates and converts heat. This heat can be removed continuously by using a coolant is passed through the core of the reactor, which in turn can generate electrical energy. Viewed as a whole, the fission reaction is one of the most effective energy sources known to date »

Solid nuclear fuels such as uranium carbide, dicarbide and oxide are most effective in the fission processes. However, the applicability of many solid fuels of this type is in many cases impaired by fission products, which arise during the cleavage reaction as described above. These fission products have both high radioactivity as well as high kinetic energy. They are able to work in a number of materials penetrate and change their crystal structure, strength and other properties »In this way

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The presence of fission products can often affect the construction of the reactor in the vicinity of the cores ". These fission products also contaminate often the coolant circulating through the fuel elements of the reactor core ... The coolant - if it Becomes radioactive - can contaminate the heat exchangers and turbines of the power generation system. in the in general, it is less expensive to change the radioactive elements than it is to decontaminate the system. This procedure brings with it inadequacies, losses and costs. In any case, the Radioactivity of the system, which is introduced by the fission products, falls below certain limits, so that the operating personnel will be harmed immediately or not in the future.-It is not known that excessive radioactive radiation causes extensive damage to the skin, blood, lymphatic tissue and bone marrow of the Body leads. All these difficulties that arise with the use of many solid nuclear fuels, are a result of the fact that the fission products of the fission reaction are highly radioactive.

An object of the invention is a method for producing a solid nuclear fuel with a Coating that originally prevents the radioactive fission products from spreading on the fuel itself limited space occupied

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U71195

It has been found that - when using a solid nuclear fuel such as uranium carbide, uranium dicarbide and oxide an even coating of pyrolytic graphite - the products of the splitting process within of the clay surrounding the coating. In such a case, you can be the overdone Particles as fuel without any of the expected drawbacks due to the solid fuel application use. The reason for this lies in the fact that the pyrolytic graphite coating has increased Temperature has a high strength and is corrosion-resistant and gas-impermeable. Will be the solid Fuel applied in the cracking process will become smaller and within the pyrolytic graphite coating form a cavity. At the same time, the fuel becomes fission products in the form of poisonous gases develop which occupy the free space within the coating. These gaseous fission products can do not escape from the coating as it is impermeable to gas. You can also put the coating through chemical attack does not soften as it is corrosion resistant. Finally, the toxic gases can Do not burst the coating even if it is pressurized up to 70 atm. (1000 psi) as the pyrolytic graphite has high strength even at high temperatures. Thus, the fission products

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In this way , the radioactive products cannot contaminate the plant or the operating personnel, and in many cases many of the restrictions placed on the design of a reactor can be removed or at least reduced to some extent.

The method of the invention consists in coating a solid nuclear fuel with pyrolytic Graphite and is characterized by the fact that the solid nuclear fuel is in the form of particles of uranium carbide, dicarbide and oxide with hydrocarbon vapors Temperatures between about 1700 and 3000 ° 0 is brought together, with on the surface of each particle a uniform coating of pyrolytic graphite is deposited.

This process uses a vapor phase technique, wherein pyrolytic graphite is applied to a hot surface by thermal decomposition of a hydrocarbon gas being knocked down. This method is explained in more detail using the accompanying figures.

Fig. 1 is a front view of the reactor used in the process, Fig. 2 is the cross section of this Reactor.

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A reactor such as shown in Fig. 1 is charged with solid particles of nuclear fuel.

Pig's reactor. 1 consists essentially of a hollow graphite cylinder 11 with two connected Parts 12 and 13. These parts are designed to be separated to make it easier to mist up the reactor can be. The inner part of the hollow cylinder has longitudinal ribs that extend over the entire length of the Cylinder body 11 extend. The cylinder ends are drawn in so that they form the pipe parts 14 and 15, which act as waves. The parts 14 and 15 are rotatably mounted in slots that are in the vertical Walls 16 and 17 of the graphite support 19 are located. This position allows the reactor to be rotated its longitudinal axis. Tube 15 serves as an inlet and is provided with a retractable injector 21 for the hydrocarbon gas provided, while tube 14 is provided with an insertable plug 22. The one on the cylinder 11 The subsequent pipe part of 14 is equipped with exhaust gas nozzles 25. An induction coil 26 is around the Cylinder wrapped and used to generate heat. Plug 22 is releasably connected to a shaft, which to a variable speed motor 24 belongs.

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The reactor is placed under argon until the desired pressure is reached, whereupon the cylinder is rotated about its longitudinal axis at a suitable speed. The temperature of the system is then increased from room temperature up to 1700-3000 ⁰ C. After the pressure and temperature have been set, a hydrocarbon gas such as methane, ethane, propane or benzene is introduced through feed line 21. The cylinder is rotated throughout the process to ensure that all particles are evenly exposed to the hydrocarbon atmosphere. A rotation of the cylinder is also necessary in order to achieve a short contact time between two surfaces of the individual particles. This is necessary to avoid sintering together of the particles during the deposition of pyrolytic graphite. Sintering is the formation of bridges made of graphite from one particle to another. The outer surface of the individual particles should now have a uniform protective coating made of pyrolytic graphite; if this is the case, they are suitable for use in cleavage processes.

The rate of deposition, or the rate at which the pyrolytic graphite builds up

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the surface of individual particles of solid nuclear fuel is deposited depends on the Temperature, pressure and the amount and velocity of gas within the system. All of these factors are explained in more detail below. It is further stated that the maximum deposition rate also depends on the period of time during which the individual particles are with each other rotating reactor. It has been found that the deposition rate is around 20 / rev each Should be hour if the contact time is approximately 0.5 seconds. This contact time is obtained when the reactor is rotated at 30 rpm. Is the deposition rate larger, bridging is already taking place under these working conditions. Bridging is the bond between the pyrolytic graphite coating of the individual particles. In all In cases where the contact time is reduced, the deposition rate can be increased without the risk of bridging increase something. Determination of the maximum deposition rate and the given working conditions can be carried out by any specialist. In many cases, an average coating thickness between 30 and 200 / U are found to be sufficient; in individual cases the average is Coating thickness preferably 50 to 125 / u, in particular 80 / u.

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The coating thickness depends on the material to be coated and the conditions of the envisaged Splitting process

The inventive method is carried out at a temperature at which a gaseous hydrocarbon decomposes to form pyrolytic graphite and not pyrolytic carbon. A coating of pyrolytic carbon compared with that of graphite shows low strength, density and high gas permeability. A pyrolytic carbon coating resembles an onion skin and cannot withstand the high temperatures that occur during fission processes. The inventive method in which pyrolytic graphite onto particles of solid nuclear fuel is deposited, can be carried out to 3000 ⁰ C within the temperature range of about 1700s. If the process is carried out at a temperature below 170O ⁰ C, there is the coating of amorphous carbon. Working above approx. 3000 ⁰ C is probably impractical. The rate of reaction also increases with increasing temperature. This is because the hydrocarbon gas is much more effectively decomposed to form pyrolytic graphite at the higher temperature within the above range. The procedure is feasible between

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approx. 1700 and 3000 ⁰ C, but is preferably operated at 2050 to 225O ⁰ C. It has been found that the process is highly effective in particular at approximately 215o C and ^0, possess high strength and high density, the coatings were deposited at these temperatures. “Deposition temperature” is understood to mean the temperature at which the gas (or gases) is (are) split with the formation of a precipitate of pyrolytic graphite on a suitable surface.

The pressure in the process according to the invention has some relation to temperature. The process can be carried out at a pressure of approx. 1 to 200 mm Hg carry out. A change in pressure affects the rate of deposition of the pyrolytic graphite. Amounts to if the pressure in the process is below 1 mm Hg, the deposition rate drops rapidly. Increases however, if the pressure is above approx. 200 mm Hg, the precipitate consists primarily of soot. Definitely the reaction rate increases with increasing pressure until a limit value of the pressure is reached and the deposition of soot begins. The inventive The method can therefore be used at pressures between approx. 1 to 200 mm Hg, preferably 2 to 21 mm Hg, where

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a particularly high quality is achieved. The procedure is very effective for one Pressure of approx. 20 mm Hg, since above this pressure a slight deposition of soot begins. It it is also noted that a 10-fold increase in deposition rate is achieved when the working pressure is increased from 1 mm to 20 mm Hg.

The hydrocarbon gases which can be used for the process according to the invention include any carbon-containing gas which allows the deposition of pyrolytic graphite on a corresponding surface at suitable decomposition temperatures, for example methane, ethane, propane or benzene. The amount of hydrocarbon gas used depends on the temperature and pressure within the system. The gas velocity is also dependent on the amount of gas. In each pail it could be found that the reaction speed increases with increasing gas quantity or speed

The process according to the invention is illustrated by the following examples.

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example 1

30 g of urandicarbide in the form of small balls was introduced into the above reactor. The grain size of the particles was 177 to 250 / u. Argon was filled into the reactor at a pressure of approx. 20 mm Hg and the cylinder was rotated at −30 rpm. The temperature of the system was increased to 1700 ^° C. and methane was introduced at a rate of 0.2 l / min. After 4 hours, the hydrocarbon feed was turned off and the system was allowed to cool to room temperature. The average thickness of the graphite coatings on each particle was approximately 80 / rev.

In the above example, the average size of the individual fuel particles was 177 to 250 M, but the size of the applicable particles is not limited, but can vary within wide ranges, depending on the size of the reactor, the treatment conditions and the use,

Example 2

The measures of Example 1 were repeated with the exception that the temperature was 215O ⁰ O and the pressure was 2 mm Hg. The separation rate was approx. 20 / uA

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The coating of solid fuel particles offers various advantages, for example a nuclear fuel coated with pyrolytic graphite allows working temperatures of the reactor of up to 2500 ⁰ O.

The inventive method can with regard to the substances used, the operating conditions and the construction of the device vary widely.

Claims

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Claims (6)

-H- 1A-23 643 14711S5 claims 1. Atomic fuel particle, characterized in that ^ each particle with a Is provided with a pyrolytic graphite coating. 2. Atomic fuel particles according to claim 1, characterized characterized in that the atomic fuel particles are uranium carbide, dicarbide or oxide contain" 3. A method for producing solid atomic fuel particles according to claim 1 or 2, characterized in that the particles are ^{exposed to a hydrocarbon gas at a temperature between about 1700 and 3000 0} G and a pressure below about 200 mm Hg. 4. The method according to claim 1, characterized in that a temperature between 2050 and 225O ⁰ O, preferably 215O ⁰ C, is maintained. 5. The method according to claim 1 or 2, characterized in g e that a pressure of about 2 to 21 mm Hg is maintained. -15-80990 2/0788 - 15 - 1A-23 643 6. The method naoh claim 1 to 3, characterized in that the solid nuclear fuel used is uranium-containing material, preferably uranium carbide, dioarbide or oxide, ¹ T. method naoh claim 4, characterized in that the nuclear fuel substance with a grain size between 177 and 250 / u used 809902/0788