DE1667026B2 - METHOD OF COATING NUCLEAR FUEL PARTICLES - Google Patents

Description

15th

The invention relates to a method for coating nuclear fuel particles with pyrolytic Carbon in a fluidized bed, with the particles at high temperatures with a hydrocarbon gas and optionally brought into contact with a carrier gas.

A method for producing particulate titanium dioxide from titanium tetrachloride is known which at high temperature the titanium tetrachloride is introduced into a device through an injection opening, an exothermic reaction being carried out with carbon monoxide within the device (BE-PS 6 45 378).

It is also a process for making carbon black from liquid or gaseous hydrocarbons in Presence of air and hydrogen-containing gases in chambers known from which the soot along with the exhaust gases is discharged; DT-PS 8 28 241). The aim of this process is to produce a particularly material-friendly Provide a method for cooling the gas flow in the course of the production of soot.

It is also a method of making carbon black by partially oxidizing a liquid hydrocarbon known, wherein hydrocarbon together with preheated steam and with oxygen is deposited in a reaction chamber at high temperature (DT-PS 1 2 29 217). With this the Production of soot with a very large surface area, large adsorption and absorption capacity and high blackness be made possible.

However, the deliberate triggering of a controlled exothermic reaction is disadvantageous in the processes mentioned to achieve certain high temperatures at desired positions of the process apparatus no not possible.

In a method for coating fuel particles with pyrographite layers, it is also known to carry out the coating in a fluidized bed, in which the hydrocarbon is the carrier gas is added (magazine "Atompraxis" 9th issue 7, 1963, p. 262, right column, paragraph 1).

The invention is based on the object of creating a feasible method for coating nuclear fuel particles in the fluidized bed , which method allows nuclear fuel particles to be coated with carbon at a density which goes far beyond what can be achieved with known methods.

This object is achieved in that the gas carbon dioxide, oxygen, air or Water vapor can be added.

The advantages achieved by the invention are in particular that the obtained in a simple manner Carbon layers have so far not been he

have accessible specific gravity.

The introduction of an oxidizing component can advantageously through the injection opening or injection nozzle together with the process gas and the excitation an exothermic zone adjacent to the opening or the nozzle, thereby reducing the risk of Sooting or nozzle clogging is reduced and the fluidized bed temperature is increased at the same time.

Alternatively, the oxidizing constituent can be introduced in a different way, being the The main objective is to develop exothermic heat on a part of the device from the injection port distant, i.e. closer to the actual, substances involved in the process, and the reaction can be controlled so that a required proportion the heat of reaction is supplied.

In addition, an oxidizing component can be injected through the injection port together with the Reaction gases are introduced into the device as well as via another route.

If the reaction gases have a gas that contains a component that is to be pyrolytically deposited on the process material in the high-temperature fluidized bed device or other device, it may be desirable to have both an oxidizing component in the device together with the coating / fluidizing gas and a Inject the oxidizing component into the fluidized bed through a separate opening. Preferably the oxidizing constituent, e.g. B. carbon dioxide, oxygen, air or water vapor in the device together with the reaction gas b / .w. introduced the reaction gases. A high concentration of the coating agent in the reaction gases can then be used and high amounts of precipitation result. The presence of the oxidizing component leads to a lower risk of sooting and enables very high coating temperatures. without clogging the nozzle.

Of course, the surfaces adjacent to the opening, as necessary, suitably adapted to the type of oxidizing component used, so is z. B. in some cases a special nozzle construction around the opening is required to a To prevent backburning. A substance is preferably used as the oxidizing constituent which only becomes such an oxidation constituent at the high temperature reached at the opening.

In a special case, i. H. in the pyrolytic deposition of carbon on particles in one Fluidized bed, the carbon-containing gas is used as the oxidizing constituent, preferably CO2 or so-called Coating gas added. This has the additional advantage that it is useful when coating Particles that contain oxides increase the partial pressure of CO and, in this way, the formation of carbides suppressed.

example 1

A quantity of 200 g of refractory carbide particles, each about 4-500 microns in diameter, was placed in a fluidized bed furnace. The amount was swirled in a gas mixture containing argon, methane and, as an oxidizing constituent, carbon dioxide in a volume ratio of 4: 5: 1, for 15 minutes. During this time the temperature of the furnace was increased to 2150 ⁰ C, and the partial pressure of the methane was adjusted by adjusting the

Gas flows increased with the final gas composition being 95% methane and 5% carbon dioxide. The coating process began and carbon from the methane was deposited on the particles. The process was continued under these conditions for I ³ A hours; after this time the particles were 133 microns thick. The weight of the lot showed an increase of 196 g. The pyrolytically deposited carbon layer had a high density.

Example II

227 g of heat-resistant carbide particles, each with a nominal diameter of 500 μ, were placed in a fluidized bed furnace. The particles were at 2300 ⁰ C in a gas mixture of argon, methane, and - from carbon dioxide in a volume ratio of 4 - as Oxydierungsbesiandteil: whirled 1: 6.

After a reaction time of 120 minutes the particles removed. Upon examination, it was found that the applied carbon layer was a possessed high density.

Example 111

40 g of particles of a refractory oxide material, each having a nominal diameter of 400 μ were charged in a fluidized bed furnace, the temperature was elevated already at 1500 ⁰ C. The amount was whirled up in a gas mixture of argon, methane and carbon dioxide in a volume ratio of 3: 6: 1. Later, the temperature of the furnace was increased to 2100 ⁰ C. After this increase in temperature, the oven was allowed to run for 20 minutes; to test the effectiveness of the carbon dioxide oxidizing component, the supply of this component was then stopped. The nozzle clogged with deposited carbon within two minutes.

Example IV

22 g of a refractory oxide was placed in a fluidized bed furnace at a temperature of 1500 ° C brought in. After coating with a first coating of pyrolytically deposited carbon at 1500 ° C, the particles were whirled up in a stream of an argon-methane gas mixture, the Methane was passed through water in the form of bubbles before entering the furnace. Introducing the with Saturated gas vapor as the oxidizing component was added for 90 minutes at an oven temperature continued from 2100 "C and then interrupted.

After cooling, the particles were removed from the oven. Examination of the particles showed that, first, a dense layer of pyrolytically deposited carbon had deposited, that second, no appreciable conversion of the oxide to carbide had taken place; and that third when examining the device, no significant deposit was found in the area of the injection opening became.

Example V

A lot of particles of (Th, U) O2 were similarly placed in a fluidized bed furnace at 1500 ° C introduced and whirled up in a stream of a gas mixture containing argon, methane and carbon dioxide in the Volume ratio of 3: 4: 1 contained.

The temperature of the fluidized bed was increased at 1850 ⁰ C. After 15 minutes, the flow rates were changed so that the volume ratio A: CH4: CO2 was 3: 4: 2. The increased proportion of CO2, which is used to remove the soot formed from the ChU, enables the layer to be observed.

After one hour, the particles were removed from the oven and examined. It was found that they had a dense layer of carbon and were still in the oxide form.

Example Vl

A quantity of 20 g of heat-resistant particles was introduced into a ^{fluidized bed vessel heated to 1800 °} C. and whirled up in a gas mixture of methane and oxygen in a volume ratio of 7: 1 for 20 minutes. The particles were then removed and the nozzle of the fluidized bed furnace examined. No damage was found on the nozzle. There was a dense layer of pyrolytically deposited carbon on top of the particles.

Although in the examples described above, the oxidizing component through the injection opening together was introduced with the process gas, this component can also be introduced in other ways will. The methods described above can be used in conjunction with others as desired Coating processes are used.

Claims (1)

Claim: Process for coating nuclear fuel particles with pyrolytic carbon in one Fluidized bed, the particles at high temperatures with a hydrocarbon gas and optionally a carrier gas are brought into contact, characterized in that the gas carbon dioxide, oxygen, air or Water vapor can be added.