DE2111075C3 - Process for the continuous reduction of gaseous uranium hexafluoride to uranyl fluoride in a fluidized bed reactor - Google Patents

Description

The invention relates to a method for the continuous reduction of gaseous uranium hexafluoride to Uranyl fluoride in a fluidized bed reactor by a pulsating mixture of nitrogen and Water vapor at elevated temperatures and subsequent discharge of the uranyl fluoride particles formed.

One of the intermediate processes was in the manufacture of nuclear fuel from ceramic uranium dioxide in the reduction of gaseous uranium hexafluoride to solid uranium tetrafluoride or uranyl fluoride. This part of the overall production process has so far been carried out in a more or less normal fluidized bed carried out, wherein a seed layer of the reduced solid, such as. B. uranium tetrafluoride or Uranyl fluoride was used and the solids from the uranium hexafluoride were further reduced on the Deposits seed layer particles. In such a process, the deposition on the seed layer leads to Formation of oversized particles of the reduced material, which creates the need to form part of the To enforce the particle layer again in order to maintain a perfect layer turbulence. In addition, once the particles have been converted to uranium dioxide, the product must be of a usable size be crushed. All of these processes add to the cost of producing ceramic nuclear fuel.

The invention is based on the object of an improved method for reducing uranium hexafluoride to create uranyl fluoride, through the uranyl fluoride particles of suitable size in one can be obtained in a single pass.

According to the invention, this object is achieved by dispensing with a customary, initial Seed layer for the formation of fine particles of uranyl fluoride a reaction temperature of 230 to 320 ° C maintained, as well as the water vapor in the mixture of nitrogen and water vapor in im substantial theoretical amount is set, and during the formation of the fine particles of uranium fluoride the impact deflection and the impact frequency of the pulsating gas flow by gradually increasing it the deflection and reduction of the frequency ι can be changed.

In practice, it has proven to be advantageous to apply uranium hexafuoride intermittently to the reaction zone in unison with the intermittent application of the reducing agent,
A mixture of steam and nitrogen is used as the reducing agent.

As a result of the method according to the invention carried out without a customary, initial seed layer, there is no need for a new run and a comminution of the oversized uranyl fluoride particles. In addition, it is possible according to the invention to increase the flow rate to the reduction zone compared to the values previously used, so that the reduction rate is also increased. After the uranium hexafluoride has been reduced by 2u , the solid uranyl fluoride particles are separated from the system gases in that the gas-solid mixture is passed through filters at the exit from the reactor.

An embodiment of the invention is shown in the drawing and will be described in more detail below described.

In the embodiment shown, the reduction / .one is formed by a container 10 which the forn; of a standing hollow cylinder. The jo container is generally inverted with one Provided truncated cone bottom 11, in which the reducing agent is introduced through an inlet pipe 12. The pipe ends coaxially downwards and on the middle Height of the bottom 11 of the container to the reducing agent to be introduced intermittently. The reducing agent, which is a gas mixture of nitrogen and water vapor. is down against the inner surface 14 of the end of the Truncated cone bottom 11 directed to upwards to a to be deflected on the cross section of the container 10 distributed flow.

A pipe 13 passed through the wall of the container 10 is used to introduce the gaseous uranium hexafluoride. The tube 13 is coaxial with the container 10 and with its upwardly directed outlet end Arranged above the truncated cone bottom 11 to the gas on the entire horizontal cross-section of the To distribute reactor vessel.

The top of the container has an or

several filters 15 equipped in a known design, preferably made of Monel metal, to separate the particles formed in the reduction zone from the exhaust gases. The exhaust gases flow through an opening 16 or through openings in the upper bottom 17 of the container 10 for further treatment or for Discharge, whichever is desired.

In the illustrated embodiment, the reactor vessel 10 is for continuous reduction Set up of uranium hexafluoride by the intermittent flow of a reducing agent and with an overflow flue 18 fitted, which is placed in the wall for the removal of particles from the reactor is. Advantageously, the pipe 18 with a shut-off device, such as. B. with a rotary valve 19 provided in order to keep the losses of reducing agent or exhaust gases as low as possible, however allow the removal of reduced solids from the reactor.

As shown, the intermittent loading

21 II 075

the reducing agent regulated and controlled by a device which consists of a pressure vessel 20 which has an inlet pipe 21, which the vessel 20 z. B. with separate sources 30 and 31 for the supply of Combines gases or vapors, which are used as reducing agents in the reactor vessel. the The outlet side of the pressure vessel 20 is connected to a pipe 22 provided with a slide, which opens into the pipe 12 which is so set up. that it introduces the reducing agent into the container 10. The valves 23 and 24, which are in the pipes 21 bi.w. 22nd are arranged, are quick-opening and quick-closing valves with control by a clock circuit 25th

In order to achieve the desired thrust direction in the container 10, the valves 23 and 24 are electrical or operated mechanically by a known circuit 25. Usually valves 23 and 24 operated simultaneously so that one opens while the other closes, although recognizable is that a time delay can be provided so that actuation of the valves does not occur simultaneously needs to be done. The choice of operating mode depends to a certain extent on the frequency and the The impact of the surge current to the reduction zone of the reaction vessel 10 starts.

In operation, gaseous uranium hexafluoride is continuous introduced into the reaction vessel 10 through the pipe 13. At the same time there is a surge of steam and nitrogen introduced through tube 12 to contact the uranium hexafluoride gas through container 10 to ascend. The reducing effect of the steam at the preferred temperatures creates fine ones Particles of uranyl fluoride.

The reduction of uranium hexafluoride with steam at a temperature in the range from 230 to 320 ⁰ C, and preferably at 282 to 287 ° C, proceeds quickly when there are essentially theoretical amounts and intensive mixing takes place, as is the case with surge current conditions in the reactor Case is. Nitrogen is mixed with the steam to obtain the volume necessary for the surge flow effect in the reactor. At the start of the process, the initially formed fine particles of uranyl fluoride in the reactor will increase with the introduction of the reductant surge (steam and nitrogen), and will tend to fall in the reactor if the surge continues in its normal interplay. After each collision, further particles form in the reactor, the uranyl fluoride particles formed earlier being slightly enlarged until a visible layer of particles has formed. The depth of this layer is then limited by the removal of particles via the overflow pipe 18. A loss of particles is avoided by the filter 15. The most favorable reduction temperature is created by heating the incoming gas streams, for example by overheating the steam and, if necessary, by external heating of the container 10 25 changed by a gradual increase in Au ^; Blows and an alhiah-like reduction in stumbling frequencies /. Normally, the total volume of flow through the reactor will remain essentially uniform throughout the operating time.

The fine particles produced have the desired size for further processing on the for the Nuclear fuel suitable uranium dioxide.

The further reduction of the fine particles obtained is then carried out in separate containers or Reactors carried out.

Reaction vessel
Container condition

Operating gas quantities -

Reactor temperature

Beginner

Shock deflection

More final
Rash

example 1

- 10.2 cm in diameter

- Empty at the beginning of the
Operating

(No germ layer)
N ₂ - 3.4 mVh
H ₂ O - 20.7 ml
UFb-27.2 kg / h
260 ° C

0.34 bar, frequency
30 cycles per minute

1.04 bar, frequency
17 cycles per minute

The shock frequency and the shock amplitude are changed when the layer is formed, and the final shock ratios are achieved in 50 to 60 minutes. The product withdrawal begins to this point in time. The particle size is essentially less than 10 µm.

Example 2

Container condition

■ 45 operating gas quantities -

Reactor temperature

Beginner

Shock deflection

More final
Shock deflection

Empty at the beginning of the

Operating

(No germ layer)

N? -8.9mVh

H ₂ O- 15.3 mVh

UF ₆ - 20 kg / h

277 to 299 ⁰ C

0.41 bar, frequency
29 cycles per minute

1.18 bar, frequency
17 cycles per minute

The final shock ratios were achieved in 70 to 85 minutes.

1 sheet of drawings

Claims (2)

Patent claims: 1. A method for the continuous reduction of gaseous uranium hexafluoride to uranyl fluoride in a fluidized bed reactor by a pulsating mixture of nitrogen and water vapor at elevated temperatures and then discharging the uranyl fluoride particles formed, characterized in that, dispensing with a customary, initial seed layer to form finer Particles of uranyl fluoride maintain a reaction temperature of 230 to 320 ⁰ C and the water vapor in the mixture of nitrogen and water vapor is set in an essentially theoretical amount and during the formation of the fine particles of uranyl fluoride, the shock deflection and the shock frequency of the pulsating gas flow by gradually increasing the The deflection and the decrease in frequency can be changed. 2. The method according to claim 1, characterized in that a reaction temperature of 282 to 287 ° C is maintained.