DE1244133B - Process for the production of uranium carbides - Google Patents

Description

FEDERAL REPUBLIC OF GERMANY GERMAN wjflS | ^ PATENT OFFICE

EDITORIAL

Int. CL:

COIb

German class: 12 i - 31/30

_C01B -31/30

Number: 1244133

File number: U 11660IV a / 12 i

Filing date: April 23, 1965

Opened on: July 13, 1967

The present invention relates to the production of uranium carbides.

Uranium carbides such as UC and UC ₂ have already been proposed as nuclear fuels, especially for high-temperature and advanced gas-cooled reactors.

From »Ber. d. dtsch. Keramik Ges. ", 40, pp. 153 to 158 (1963), it is known to produce uranium carbide by mixing uranium oxide (UO ₉ ) with carbon, compressing the mixture and" heating it to 1700 ° C, while in German Auslegeschrift 1130 798 is described to carry out the conversion reaction in vacuo.

As from »Ber. d. dtsch. keramischen Ges. «, 40, p. 154, when UO _{2 is used} as a starting material for the production of uranium carbides, difficulties _{arise which are caused by the easy oxidation of UO 2} to a non-stoichiometric material. If a non-stoichiometric material such as UO _{2 + 1 is used} for the reaction with carbon, despite careful calculation of the amount of carbon necessary for the conversion, it can happen that, as a result of further oxidation of the UO _{2 + x} that has taken place in the meantime, even with the greatest care Uranium carbides can be obtained with uncontrollably low carbon contents. It is therefore necessary to carry out all manipulations with the uranium dioxide in a vacuum or under protective gas, e.g. B. Carefully purified argon that contains no more than 10 · 10 ~ ⁶ parts of both oxygen and water. Another disadvantage of using UO ₂ is that it can only be mixed dry with the much lighter carbon, because the difficulty in carrying out a wet mixing process is to find a suitable inert liquid which does not further oxidize _{the UO 2.}

It was therefore the task of a safe and less expensive process for the production of To create uranium carbides, which eliminated the disadvantages of the known method and that in a more convenient way the production of uranium carbides, especially uranium monocarbide, permitted.

An improved process has now been developed for the production of uranium carbides by mixing uranium oxide and carbon, compacting the powder mixture and heating the moldings to a temperature of at least 1300 ° C, which is characterized in that triuranium octoxide (U ₃ O ₈ ) is used as the uranium oxide. begins.

In particular, the invention relates to a

Process for the production of uranium carbides

Applicant:

United Kingdom Atomic Energy Authority,

London

Representative:

Dipl.-Ing. E. Schubert, patent attorney,

Siegen, Eiserner Str. 227

Named as inventor:

Neville Ronald Williams, London

Claimed priority:

Great Britain April 27, 1964 (17,460)

Process for the production of uranium monocarbide from triuranium octoxide, which is characterized in that the heating of the moldings is carried out in a vacuum furnace.

Another preferred feature of the invention is that the powders using Water to be mixed wet; after wet mixing, they are conveniently filtered and placed in air at 100 dried up to 150 ° C.

Finally, it is advantageous that graphite powder is used as the carbon.

The present invention can also be applied to or for the production of carbide mixtures, for example of UC-ZrC, (U, Pu) C and (U ₅ Th) C, by adding a suitable oxide, for example zirconia (ZrO ₂ ), plutonium dioxide or torus earth (ThO ₂ ), to which UgOg-carbon starting mixture can be applied.

Wet mixing of the powders is conventional using a ball mill carried out by water with a small addition of a wetting agent.

If a wet grinding operation were to be carried out when using UO _{2, an inert liquid, e.g.} B. carbon tetrachloride, can be used as a liquid medium in order to exclude possible further oxidation of the oxide caused by the solvent, and the filter cake would have to be dried in a vacuum oven operating at room temperature or below this in order to avoid a possible

709 610/501

Prevent oxidation of its oxide content during drying.

In a comparative experiment, the filter cake, dried after wet milling, was forced or forced through a granulating screen such as a 10-mesh BS screen (approximately 1700 microns). If U ₃ O ₈ graphite powder mixtures were used, then when the filter cake was pressed through, hard granules were obtained which were suitable for the automatic form-filling technique. The granules of U ₃ O ₈ graphite mixture resulted in compression under a pressure of z. B. 1600 kg / cm ^{2 of} defect-free pellets, whereas pellets made from U ₃ O ₈ and carbon tended to be soft and flaky. If, on the other hand, a mixture of UO ₂ and graphite was pressed through, the dried filter cake disintegrated into a fine powder, which could be compressed into flawless pellets, but was not suitable for automatic mold filling.

The use of a compression pressure of 1600 kg / cm ² is not essential, pressures on the order of 160 to 8000 kg / cm ² can be used for the production of defect-free pellets with a diameter up to 15 mm. For the production of larger solids, for example with a diameter of 3.8 cm, low pressures, e.g. 160 to 320 kg / cm ² , it is desirable to avoid damage to the compacts when they are ejected from an unlined mold. When U ₃ O ₈ graphite granules were used, excellent pellets were obtained when an automatic press set at medium compression pressure was used. Although dry mixing of the powders of U ₃ O ₈ and carbon can also be used, the wet mixing technique is preferred because a more homogeneous mixture is achieved more quickly using this latter technique.

When uranium octoxide carbon pellets were placed in a vacuum oven and heated, were three pressure peaks observed. These tips are:

a) below 200 ° C, caused by the general degassing of the pellets,

b) starting at around 400 ° C., probably caused by the reduction of the octoxide to UO ₂ caused by the carbon, with the evolution of carbon dioxide, and

c) starting at about 1300 ° C, caused by the final reduction to uranium carbide below Formation of carbon monoxide.

Heating is preferably continued until no more gas evolution after the third spike is perceptible.

Using the present inventive method, it is possible to use standardized amounts of U ₃ O ₈ and carbon in order to obtain a product with a predetermined carbon content, whereby no analysis of the starting material is necessary. For example, uranium monocarbide pellets can be produced by the process according to the invention with oxygen contents of 0.01 to 0.05 percent by weight and nitrogen contents of 0.015 to 0.02 percent by weight.

Thus, the present inventive method provides means for producing a Uranium carbide product of high purity and also the possibility of regulating the carbon content of the product to a high degree (± 0.05 percent by weight).

The drawing is a graphical representation of the relationship between the molar ratios of U ₃ O ₈ and carbon in the starting mixture and the carbon content of the final product. Using this graph it is possible to quickly _{read off the relative amounts of U 3} O ₈ and carbon required to obtain a product having a predetermined carbon content ranging from 4.2 to 5.2 percent by weight.

The carbon content of the product can therefore be determined exactly by reading from the graph, because the U ₃ O ₈ used as the starting material is a stable compound which, in contrast to UO _2, does not change its oxygen content, whatever it is in the present process according to the invention is treated. This property of the U ₃ O ₈ makes it easy to work in batches, because simple proportions of oxide and graphite can be balanced at the beginning without any corrections that would have to be made, for example, for oxygen fluctuations in the raw material.

The reaction seems to take place in two stages: the reduction of the U ₃ O ₈ to uranium dioxide, then the reduction to carbide. The production of uranium monocarbide can be summarized in the following equation:

U ₃ O ₈ + IOC > 3UC + CO ₂ + 6CO

The equation shows that stoichiometric uranium monocarbide containing 4.8 percent by weight of carbon is obtained from a U ₃ O ₈ carbon mixture in which the molar ratio of U ₃ O ₈ to carbon is 1:10. From the graph, however, it can be seen that with such a molecular ratio in the feed mixture, the monocarbide is substoichiometric and contains only about 4.5 percent by weight of carbon. From the graph, however, it can be seen that in order to obtain a carbide with 4.8 percent by weight of carbon, it is necessary that the molecular ratio U ₃ O ₈ to carbon in the starting mixture must be 1 to about 10.2, and it it was found that, on average, a stoichiometric monocarbide product is obtained with a carbon content of 10.17 mol in the starting mixture. The reason for this deviation from the stoichiometric proportions is assumed that the graphite used contains about 1.5 percent by weight of volatile constituents which do not appear to take part in the reaction with the U ₃ O ₈ . At relatively low temperatures, e.g. B. 1500 ° C, an oxicarbide is formed in full conversion, which is a single-phase material and contains the oxygen in solid solution in the carbide in an amount sufficient to produce a carbon equivalent in the material of 4 to 8 percent by weight to surrender. If the reaction continues to run out at a higher temperature, e.g. B. is carried out above 1800 ° C, no oxicarbide is formed and only traces of oxygen are present in the material.

From the foregoing it can be seen that the graphical representation is only accurate

for the particular type of graphite that is used and that with other graphites which have different volatile content, different ratios of U ₃ O ₈ to carbon are required to give a stoichiometric monocarbide.

It should be noted that for certain uses, the presence of bound Oxygen in the uranium carbide product at levels as high as 0.7 weight percent oxygen can be acceptable; but in other cases, e.g. B. when the carbide melts in an electron beam and is to be subjected to casting processes is the presence of bound oxygen undesirable, and it is then preferred, to have an oxygen content of less than 0.1 percent by weight to have. The combined oxygen content of the product is easy to control by the amount the carbon consumed, which was intended for the complete implementation of the reaction, but if the reaction is not complete, unreacted oxygen will be called uranium dioxide remain, which forms a separate phase. To be roughly certain that a complete Reaction has occurred, if the reaction is carried out in a vacuum oven, it is necessary to ensure that the final pressure in the furnace is less than 1 micron.

In the following examples, the present invention is illustrated with particular reference to manufacture of uranium monocarbide.

example 1

A batch of 300 g U ₃ O ₈ (area 4m ² / g) and 43.55 g graphite (area 65m ² / g and 99.17 percent by weight C, remainder ash) was weighed out and resulted in a mixture of the composition U ₃ O ₈ + 10.09 ° C. This mixture was placed in a porcelain ball mill containing clay balls, 400 ml of water and a few drops of wetting agent. The mill was closed and set in rotation for 90 minutes at a speed of 120 pm. The contents of the mill were then poured through a coarse sieve to hold the balls back into a vacuum filter funnel lined with two filter papers. In order to limit the possibility of segregation or segregation to a minimum, the pulp adhering to the clay balls retained on the sieve and the material remaining on the walls of the mill were not washed onto the filter papers. The filtered mixture was predried on the filter paper at room temperature and drying was completed in an electric oven operating at 150 ° C.

The dried filter cake was passed through a 20-mesh sieve (BSS) [about 780 microns] and the resulting granules fed to a hydraulic press on which pellets 7.6 mm in diameter and 7.6 mm in length were applied using a pressure of 1600 kg / cm ² were produced.

A 150 g batch of the pellets was placed in a graphite crucible lined with tungsten, the was then placed in a vacuum oven. The furnace chamber could be through a diffusion pump evacuated with two upstream rotary pumps. The diffusion pump could take off from the rest shutdown of the system. The oven was then raised to a pressure of about 1 micron evacuated, during which time there is considerable degassing of the pellets.

The crucible was heated slowly until, after about 10 minutes, the pressure dropped from its maximum to 0.5 microns. This pressure drop indicated the near completion of degassing of air trapped in the pellets. The temperature was then subsequently increased to 900 ° C., with a pressure increase to about 1 millimeter and a final pressure drop to less than 4 microns occurring within 11 minutes. The temperature was now increased further to 1800 ° C, during which time a large amount of gas was released as soon as the temperature rose above 1300 ° C. This release of gas caused the pressure to rise to over 1 millimeter, which made it necessary to switch off the diffusion pump from the pump system. After 25 minutes the pressure dropped to 400 microns and the diffusion pump was turned back on in the system. The temperature was maintained for an additional 90 minutes, after which time a vacuum of 0.05 microns had been reached. The oven was then allowed to cool to room temperature. The crucible was then removed under an argon atmosphere high purity (less than 10 x 10 ~ ⁶ parts of oxygen and under ΙΟ-ΙΟ ^"6 parts water vapor) from the furnace and the pellets stored under an argon atmosphere.

A metallographic examination of the pellets (density 9.5 g / cm ³ ) showed that they consisted of UC with free uranium at the grain boundaries. Analysis of the pellets showed a carbon content of 4.62 percent by weight and an oxygen content of 0.33 percent by weight. These pellets were then crushed, compacted and sintered into a sintered product with a density of approximately 13 g / cm ³ .

Example 2

A 50 g sample of _{the mixture consisting of U 3} O ₈ and graphite having the same composition as the mixture of Example 1 (ie U ₃ O ₈ + 10.09 C) was placed in a molybdenum reaction tube furnace. The sample was then heated to 1600 ° C. under a flowing atmosphere of purified argon to give a uranium carbide product having a carbon content of 4.62 weight percent and an oxygen content of 0.26 weight percent. The material was crushed, compacted and sintered in argon to give a sintered product with a density of 13 g / cm ³ and a carbon content of 4.6 percent by weight.

A comparison of Examples 1 and 2 shows that similar carbidic products by reaction of the Mixture can be achieved in a vacuum or in an inert atmosphere.

Example 3

800 g of a mixture with the molecular ratio UjO ₈ + 10.15 C was heated to about 1700 ° C for a period of 3½ hours, after which time the pressure had dropped to 0.05 microns. The end product was uranium oxycarbide containing 4.72 weight percent carbon and 0.11 weight percent oxygen (carbon equivalent 4.80 weight percent). It can be observed that the reaction proceeds faster at this temperature.

The reaction proceeds at 1500 ° C. The product obtained was similar to that obtained at 1500 ° C was obtained.

Example 4

The reaction was carried out at 1800 ° C and the product consisted of a carbide phase with minor Quantities of uranium. This latter was in insufficient amount ip so that none liquid sintered phase and thereby grain growth could be induced. No pyrophoric property could be observed in this material. The carbon content was similar to that in the samples obtained at the low temperatures were included, but the oxygen levels fluctuated considerable, even within the same pellet. That

Mixture consisted of U ₃ O ₈ + 9.89 C and 120 g of this was heated for 3 hours giving a final pressure of 0.05 microns. The product contained 4.38 weight percent carbon and the oxygen content ranged on the order of 0.12 to 0.56 weight percent giving a carbon equivalent content in the range 4.5 to 4.8 weight percent.

ίο Examples 5 and 6

U ₃ O ₈ and carbon mixtures were reacted at a temperature in the range from 1850 to 1900 ° C. The products contained small amounts of oxygen and were not pyrophoric. The results are summarized in Table I.

Panel I.

example

weight
G

Moic /

Moles of U ₃ O ₈

Heating time
in hours

Final pressure in microns present
Phases

Composition, percent by weight

Carbon equivalent

carbon

oxygen

1000
750

10.15
10.08

4V ₂

0.05
0.05

UC + U

UC + U

4.73
4.65

0.03
0.05

4.75
4.69

Examples 7 and 8

The mixtures were reacted at 2000 ° C. The results are summarized in Table II.

Plate II

example

weight
G

Mole/

Moles of U ₃ O ₈

Heating time
in hours

Final pressure in microns present
Phases

Composition, percent by weight

Carbon equivalent

carbon

oxygen

250
250

10.14
10.14

I ¹ / *
6th

0.05
0.03

UC + U UC

4.73
4.83

0.01
0.01

4.73
4.83

It can be seen that with prolonged heating time an essentially stoichiometric product is obtained is obtained. This is obviously due to the volatilization of the uranium, which takes place in a Increase in the carbon content in the product of Example 8 compared to that of Example 7, affects. It can also be seen that complete reaction occurs quickly (Example 7) and that the final oxygen content is very low.

From Examples 3 to 8 it can be seen that the carbon content of the material is essentially is independent of the reaction temperature, but that at higher temperatures the products only contain small amounts of oxygen, while at low temperatures the product is appreciable May contain amounts of oxygen that are dissolved in the carbidic phase. It should also be noted that the carbon ratio in the starting material is slightly less than 10.17 moles in all cases which - as the graphic representation shows - are necessary to obtain a stoichiometric Product.

Example 9

The process was applied to the manufacture of large uranium carbide pellets suitable for electron beam melting and casting. The pellets weighed approximately 50 grams and had a composition of 4.65 percent by weight carbon and less than 0.1 percent by weight oxygen. This composition would take into account the losses of uranium by evaporation during fabrication and provide an end product with a slight excess or slightly more than 4.8 weight percent carbon equivalent. The required ratio of octoxide to carbon was U ₃ O ₈ + 10.08 C, and the mixture composition, taking into account the ash content, was 1 kg U ₃ O ₈ + 144.2 g graphite.

The powders were wet ground, nitrided, dried and granulated through a sieve. The pellets were prepared by weighing 64 g of the mixture into a mold and ^{compacted at a pressure of 160 km / cm 2 to give} a pellet 3.5 cm in diameter and 3.2 cm in length and with a density of 2.1 g / cm ³ .

Five pellets were placed in the furnace and the pressure before heating started was reduced to below 1 micron. The heating rate was slow to avoid damage to the large pellets. Therefore, 20 minutes passed before the pressure rose to more than 500 microns (caused by the evolution of carbon dioxide). The degassing occurred in 85 minutes and at the end of this period the temperature was 1200 ° C and the pressure less than 30 microns. The temperature was increased at a rate of 7 ° C per minute at 1900 ⁰ C and maintained at 1900-1920 ° C for a period of 3 hours ¹ A, before the heating was stopped.

The resulting pellets measured approximately 2.3 cm in diameter by 12.7 mm in height and had a bulk density of 10 g / cm ³ . Analysis of the product showed a carbon content of 4.65 percent by weight and less than 0.07 percent by weight oxygen.

From the example it can be seen that uranium carbide products in a predetermined composition easily obtained by the present invention without undue precautionary measures must be applied when mixing, filtering, granulating and compacting the material.

Claims (5)

Patent claims: 1. A process for the production of uranium carbides by mixing uranium oxide and carbon, compressing the powder mixture and heating the shaped bodies to a temperature of at least 1300 ° C, characterized in that the uranium oxide used is triuranium octoxide (U ₃ O ₈ ). 10 2. A method for the production of uranium monocarbide according to claim 1, characterized in that that the heating is carried out in a vacuum furnace. 3. The method according to any one of claims 1 and 2, characterized in that the powder wet mixed using water. 4. The method according to claim 3, characterized in that the powders after wet mixing filtered and dried in air at 100 to 150 ° C. 5. The method according to any one of claims 1 to 4, characterized in that as carbon Graphite powder is used. Considered publications: German Auslegeschrift No. 1130 798;
Ber. d. German ceramic Ges., 40, p. 153
to 158 (1963). 1 sheet of drawings 709 610/501 7.67 © Bundesdruckerei Berlin