DE1183061B - Process for the treatment of powdered uranium dioxide to prevent spontaneous oxidation when stored in air - Google Patents

Description

FEDERAL REPUBLIC OF GERMANY

GERMAN

PATENT OFFICE

EDITORIAL

Boarding school Cl .: C Ol g

German class: 12 η-43/02

Number: 1183 061

File number: C 24606IV a / 12 η

Filing date: July 13, 1961

Opening day: December 10, 1964

The powdered uranium dioxide obtained by reducing the trioxide is strongly pyrophoric and spontaneously oxidizes again in the air; this phenomenon occurs all the more pronounced, the larger the specific surface area of the powder is. As a result of this oxidation there is either a vigorous combustion, in which the uranium compound almost _{comes into the state of U 3} O ₈ , or in the case of a less vigorous conversion a superstoichiometric oxide is formed, which has a black color and in which a Uranium atom has more than 2.3 oxygen atoms.

It is known from US _{Pat. No. 2,906,598 to let the UO 2} particles, which are obtained by high-temperature _{reduction of UO 3 in} order to avoid their spontaneous reoxidation, cool down in a non-oxidizing atmosphere consisting, for example, of helium. and then _{to store this UO 0} in the absence of air. Moreover, according to British patent specification 812791, it is already known _{to bring the UO 2} powder under the protection of an atmosphere which consists of carbonic acid. According to this procedure, the uranium dioxide is also not protected once it is brought into contact with air, especially occasionally from the various manipulations to which it is to be subjected.

A solution to the problem of preventing the spontaneous oxidation of powdered uranium oxide is therefore offered a great practical interest, because in this case they were stabilized in some way Powder, without having to fear burning it, can handle with free access to air in the course of the subsequent treatment steps, e.g. B. in the shaping, the sintering of the Uranium dioxide precedes.

In order to avoid the vigorous reoxidation of the powdered uranium dioxide, a procedure is already in place used, in which a directed reoxidation was carried out. It was through the powder in the cold state passed a nitrogen stream containing 2% oxygen. One material resulted with an average composition in which there were 2.2 oxygen atoms for one uranium atom and which was excellently suited for the sintering process in contrast to the powders, which a strong reoxidation had experienced. But this procedure, which was useful on a laboratory scale, so up to a product quantity of less than 1 kg, it cannot be treated larger Transfer quantities, for the following reasons: the phenomenon of reoxidation is strongly exothermic,

Method of treatment powdery
Uranium dioxide to prevent spontaneous oxidation when stored in air

Applicant:

Commissariat ä! 'Energie Atomique, Paris

Representative:

Dr. phil. W. P. Radt

and Dipl.-Ing. E. E. Finkener,

Patent attorneys, Bochum, Heinrich-König-Str. 12th

Named as inventor:

Yvette Carteret, Paris,

Pierre Chenebault,

Roger Delmas, Grenoble (France)

Claimed priority:

France of July 18, 1960 (833191)

the reaction rate increases with the temperature,

the thermal conductivity of the powdery layer of uranium oxide is very poor.
As a result of these conditions, when working with a dense powder layer of uranium oxide, a predominant oxidation of the surface layers occurs, without it being possible to grasp the inner layers. In addition, the degree of oxidation is very different depending on the size of the individual grains of the powder and also in the interior of a single grain.

It is the aim of the invention to provide a method to come, which is also applicable on an industrial scale, and the special laws To take into account that apply to the adsorption of gas molecules.

The theory of adsorption shows that the following two quantities are essential: the equilibrium pressure ρ of the gas on contact with the oxide and the maximum pressure p _{0 of} the same gas. In the case of a liquid at normal temperature, p ₀ corresponds to the saturation pressure of the vapor at equilibrium with the liquid at the relevant temperature In the case of a gaseous body at ordinary temperature, p ₀ corresponds to the condensing pressure.

409 757/254

It is known that adsorption is generally significant when this ratio ^{p is} close to 1

Po

lies.

The invention is based on the discovery that significant adsorption can also be achieved can, especially in the case of an increased adsorption temperature, if the ratios - significant

Pa

be below 1; however, the value 0.05 seems to represent a lower limit, so that the rule is applicable

0.05 < ? < 1. Po

The new adsorption treatment of uranium dioxide has the considerable advantage that it is the specific Surface of the powder protected in this way is not reduced.

The degree of protection against reoxidation depends on the nature of the inhibitor used and has one or the other of the following effects:

1. Reoxidation is practically zero; this is the case when using carbon tetrachloride. The uranium oxide is then obtained with a stoichiometric composition. The inhibitor can be removed in the course of the later sintering,

2. The reoxidation is delayed and takes place very slowly in the air without any appreciable Heating the oxide; this is the case when using steam.

The implementation of the new adsorption process

The method according to the invention consists in that each grain of the powdery oxide of The action of at least one gaseous or vaporous inhibitor is exposed, with as vaporous Inhibitor water vapor, carbon tetrachloride can be used in extremely different ways, volatile alcohols, volatile ketones of lower follow. You are free to choose and you can do it yourself rigen molecular weight or volatile hydrocarbons - the special conditions of the individual case in particular aromatic hydrocarbons fit, with the special type of implementation such as benzene and toluene, and as a gaseous inhibitor in itself no influence whatsoever on the carbonic acid discussed above or ammonia can be used, has characteristic magnitudes of adsorption. and that when the inhibitor acts, such one can be used to carry out the invention

Select pressure and temperature conditions adhered to the following methods: that the ratio of the equilibrium pressure of the gaseous inhibitor to the maximum Pressure of the inhibitor is between 0.05 and 1 30, with the proviso that a at least a monomolecular layer of the vapor or gaseous inhibitor is formed on each powder grain will.

When using a steam one works with an equilibrium pressure which corresponds to the steam tension of the substance used at the reaction temperature; this means that ρ = p ₀ and therefore

- ^p - = 1

Pt

In contrast to this, when using a gas with a; Working equilibrium pressure, which corresponds to the atmospheric, and the various values of ¹ P _{9 of} the substances used, all of which are above 1, generally lead to ^ values below 1, usually to those in the

Range between 0.05 and 0.2.

In all cases and especially when you have one

Steam used, d. H. either water vapor or 50 shape and sinter.

that of an organic substance, one can look at the following five examples

temperature, which makes the practical implementation very easy.

However, in the case of a gas, and especially carbon dioxide, it can happen that the

Ratio - when working at normal pressure

Po

tet, does not meet the above conditions, ie is even less than 0.02. To get the right value, you * a temperature working then at 60 380 ¹ ^ C carried out calcination of ammonium · below 0 ⁰ C, was recovered as below diuranate is reduced, and is indeed explained in more detail.

The adsorption reactions with the substances listed above take place in relative terms a short time, namely within a period of time between a few minutes and several hours and its duration depends on the working conditions and the quantities processed.

The powdered uranium oxide can be converted into an organic liquid (carbon tetrachloride etc.) immerse and evaporate the excess liquid;

the organic liquid can be evaporated and passed over the powder where it is • precipitates;

you can the powder with an atmosphere of a pure gas acting as an inhibitor in Bring touch;

a reducing or inert gas (hydrogen, oxygen, Argon, etc.), in which the inhibitor liquid is suspended as a mist; you can pass a reducing or inert gas through the bed of powder, which with is saturated with the vapor of the inhibitor. · Regardless of which of the above After treatments have been applied, the uranium dioxide can be handled in the air without it that special precautions must be taken, and according to normal procedures

Leadership of the new process to prevent spontaneous oxidation of powdered uranium dioxide closer described.

example 1

The treatment is carried out by a mixture of gas, namely hydrogen and vaporous carbon tetrachloride. Uranium trioxide produced by the

either by means of hydrogen or by means of cleaved Ammonia according to one of the known processes. ··· '' After complete cooling under hydrogen or cracked ammonia, the stoichiometric uranium dioxide act hydrogen, which is loaded with carbon tetrachloride. ■

The action lasts for 2 hours, the uranium dioxide can then be treated in air. Its specific surface is between 10 and 15 mVg.

The product processed in this way can be shaped by the customary methods and sintered to a density of 10.5 ^{in a hydrogen atmosphere at a temperature of 135O 0 C.}

Example 2

IO

^{A powdery uranium dioxide with a specific surface area of 13 m 2} / g obtained in the same way as in the previous example in a rotary kiln is swept by a stream of nitrogen which is laden with vapors of ethyl alcohol (saturation temperature 25 ^° C.). After one hour of treatment, the powder can be exposed to air without the risk of reoxidation.

After 5 hours of sintering under hydrogen at 1400 ° C., a density of 10.5 is obtained.

Example 3

The uranium dioxide produced in a continuous furnace is put into vessels containing about 60 kg of powder take up, brought under hydrogen. 11 carbon tetrachloride is introduced into each vessel, which contains the Oxide partially moistened. After 2 hours of treatment, the powder emptied from the vessels can be used expose it to air without risk.

Example 4

In this case, the treatment takes place by means of steam or rather by a mixture of gas and water vapor. The powdery uranium dioxide was obtained by treating the uranium trioxide in a furnace through which ammonia split in countercurrent was passed, containing 15 to 18 mg Water vapor per liter was loaded.

When leaving the furnace in the cooling zone, the water vapor is adsorbed on the powder.

The goods leaving the furnace can be exposed to the air and processed in any way. Its specific surface is between 10 and 15m ² / g. The reoxidation takes place slowly without increasing the temperature.

Example 5

In this case, the treatment takes place with a gas, namely with carbonic acid. It should be noted that that when the adsorption is carried out at ordinary temperature, only the grains in one size under 2 mm would be protected against reoxidation, while in the larger grains a lively one Reoxidation would set in. In this case the relationship lies

ρ lat

55

Po

56 at

near 0.02. This low value means that one is according to the general laws of adsorption does not obtain effective adsorption on grains of all different grain sizes.

To increase the ratio -, ρ can be left at the value of 1 at and p _{0 can be made} smaller, which can be achieved by lowering the temperature.

The uranium dioxide obtained, as in the other examples, by reducing the trioxide in an oven is collected in a vessel and the vessel is immersed in a cold bath at -45 ° C. Then one introduces carbonic acid. This is how it works

one with a ratio - close to 0.15.

Pt

Under these working conditions, all grain sizes of the uranium dioxide are effectively protected. As in The powder obtained in this way can also be used in the other exemplary embodiments without special protective measures be processed into sintered bodies of high density.

Claims (6)

Patent claims: 1. Process for the treatment of powdered uranium dioxide to prevent spontaneous oxidation when stored in air, characterized in that the powder is exposed to the action of at least one gaseous or vaporous inhibitor, wherein as a vaporous inhibitor water vapor, carbon tetrachloride, volatile alcohols, volatile Low molecular weight ketones or volatile hydrocarbons and as gaseous Inhibitor carbonic acid or ammonia can be used, and that when exposed to the Inhibitors such pressure and temperature conditions are maintained that the ratio the equilibrium pressure of the gaseous inhibitor to the maximum pressure of the inhibitor is between 0.05 and 1, with the proviso that the adsorption is at least monomolecular Layer of vapor or gaseous inhibitor is formed on each powder grain. 2. The method according to claim 1, characterized in that the uranium oxide powder in a liquid that consists of one or more volatile inhibitors is immersed, then that Powder removed from the liquid and the excess liquid retained by the powder evaporates. 3. The method according to claim 1, characterized in that that one evaporates the inhibitor and lets it precipitate on the powder. 4. The method according to claim 1, characterized in that that a non-oxidizing gas is passed through a bed of powder in which the inhibitor as the fog is suspended. 5. The method according to claim 1, characterized in that one bed of the Powder conducts a non-oxidizing gas that is saturated with the vapor of the inhibitor. 6. The method according to claim 1, characterized in that the powder during a Hour is subjected to the action of a stream of nitrogen with ethyl alcohol vapors is loaded. References considered: British Patent No. 812,791; U.S. Patent No. 2,906598. 409 757/254 12.64 © Bundesdruckerei Berlin