DE1244639B - Process for the production of sintered uranium dioxide, in particular fuel material for nuclear reactors - Google Patents

Description

FEDERAL REPUBLIC OF GERMANY

GERMAN

PATENT OFFICE

EDITORIAL

Int. CL:

C04b

German class: 80 b -8 / 13 ^

Number: 1244 639

File number: U12023 VI b / 80 b

Filing date: September 11, 1965

Opened on: July 13, 1967

The invention relates to fuel materials for nuclear reactors.

The main patent 1147 163 describes a method for producing a dense sintered body consisting of uranium dioxide or a mixture of the same with plutonium dioxide, in which a pressed body made of uranium dioxide powder or a mixture of the same with plutonium dioxide powder in an atmosphere of carbon dioxide or a mixture of the same with carbon monoxide a temperature of at least 1300 ⁰ C is heated.

In accordance with a feature of the present invention, a method of making a dense sintered body consisting of a mixed uranium dioxide and plutonia fuel material is characterized in that a compact body of mixed uranium dioxide and plutonium dioxide is heated to a temperature in the range of 1100 to 1200 ⁰ C in an atmosphere consisting of carbon dioxide is heated. Preferably the temperature is in the range from 1100 to 1150 ° C.

Compared to the mechanically mixed oxides used in the examples described in the main patent, the present invention relates primarily to a mixed oxide material which is prepared from a mixture of uranium nitrate and plutonium nitrate solutions, and it is based on the surprising finding that when a compact body made of such a mixed oxide material is heated in a carbon dioxide atmosphere, a slight additional compression of the body above about HOO ⁰ C occurs. The significant reduction in temperature by 200 ⁰ C or more, compared to the temperature of 1400 ⁰ C mentioned in Example 2 of the main patent, is of considerable economic importance, since it is, for. B. reduces both the initial cost and the maintenance problems of the furnaces used for heat treatment.

For a heat treatment period of 5 to 8 hours, the product typically has an oxygen to metal atom ratio in the range 2.05 to 2.09: 1, and it can be a stoichiometric product with an oxygen to metal atom ratio in the range of 2.00 ± 0.01: 1 can be achieved by introducing either carbon monoxide into the carbon dioxide in the preferred volumetric ratio of 10 parts of CO to 1 part of CO ₂ within the range of 1:10 to 100: 1 parts by volume of carbon monoxide to carbon dioxide or that alternatively the carbon dioxide is replaced by dry argon and that in each case the heat treatment

Method of making sintered
Uranium dioxide, in particular fuel material for nuclear reactors

Addendum to the patent: 1147 163

Applicant:

United Kingdom Atomic Energy Authority,

London

Representative:

Dipl.-Ing. E. Schubert, patent attorney,
Siegen, Eiserner Str. 227

Named as inventor:

Albert Graham Adwick,

William Stewart Calcott Reilly, London

Claimed priority:

Great Britain 16 September 1964 (37 787)

ment is continued for a further 2 hours at the same temperature (ie about HOO ⁰ C), followed by cooling at a rate of about 200 to 300 ⁰ C per hour.

It has been observed that the lack of additional significant densification above about 1100 ° C. coincides with increased grain growth and an increased ratio of oxygen to metal atoms. Without limiting the scope of the invention, one possible explanation is that these conditions favor the isolation of pores within the grains or grains, the pores being well removed from the grain edges and attempting to inhibit compaction.
As an example, a method of making a dense sintered body from mixed uranium dioxide and plutonia fuel in accordance with the invention is described below with reference to fast reactor fuel material which typically contains _{10 to 50 mole percent PuO 2.}

A mixture of plutonium and uranium nitrate solutions is reacted with ammonium hydroxide to produce

709 610/481

to give a coprecipitate of Pu (OH) ₄ and ammonium diuranate (ADU). The precipitate or precipitate is filtered, washed, dried and then calcined for 5 hours and in air at 25O ⁰ C. The calcined powder so produced typically has a ratio of oxygen to metal atoms in the range from 2.7 to 2.9: 1 and a surface area on the order of 25 to 50 square meters / g.

This powder is then reduced (in order to obtain a ratio of oxygen to metal atoms in the order of magnitude of 2.00 to 2.05: 1) at 800 to 900 ^° C. for 5 hours in an atmosphere consisting of 1 part carbon monoxide and 10 Parts of carbon dioxide, the temperature used being governed by the surface area value of the calcined powder. For example, for a surface area of 25 m2 / g of a calcined powder, the surface area of the reduced powder is typically 6 m2 / g, and the tap density is in the range from 2.3 to 3.0 g / cm ³ .

While the use of a carbon monoxide-carbon dioxide atmosphere is preferred for the reduction of the calcined powder, it is also possible (in a manner known per se) to use hydrogen, typically for a period of 5 hours at 700 ^° C. It has been found that it is then desirable to include an additional process step in which the surface area and tap density are controlled to be within the preferred ranges of values of 2.0 to 3.1 g / cm ³ for tap density and accordingly 7.0 to 2.0 qm / g for the surface area. This additional stage is advantageously carried out in an argon atmosphere. In this manner yielded a mixed oxide powder having 15 mole percent of PuO ₂ and having a surface area of 21 m² / g and a tapped density of 1.6 g / cm ^3, when it was heated in argon for 5 hours at 650 ⁰ C, a surface area value of 5 qm / g and a tap density value of 2.9 g / cm ³ . For the same period of heating in hydrogen at a temperature of 1050 ⁰ C was required to obtain respective Oberflächenbereichs- and Abstichdichtenwerte. The additional step of setting the surface area and tapping density can alternatively also be carried out in a vacuum. There was thus obtained a mixed oxide powder having a surface area of 33 m² / g and a tapped density of 1.6 g / cm ³ upon heating to 700 ⁰ C for 5 hours in a vacuum of less than 1 micron of mercury a sinterable product powder of the surface area of 4 sq / g and the tap density of 2.67 g / cm ³ .

The reduced powder material is then milled in a ball mill for about 2 hours to break up agglomerates, and the milled powder is then mixed with a binder such as that known as "Cranko" (polybutyl methacrylate in dibutyl phthalate) and adjusted to -22 + 72 British standard sieve, granulated. After drying, the granules are compressed at 20 to 40 tsi (about 2800 to 5600 kg / cm ² ) to form compact bodies in the form of tablets or pellets having a "green" density of typically 5.5 g / cm ³ for a powder tap density of 2.0 g / cm ³ and of 6.4 g / cm ³ for a powder tap density of 3.1 g / cm ³ .

The pellets are then sintered by heating in an oven to 1100 ^° C. at a rate of 100 to 200 ^° C. per hour in an atmosphere that consists of carbon dioxide, and kept at this temperature in the CO ₂ atmosphere for 5 to 8 hours . Carbon monoxide is then added to the atmosphere, in a preferred volumetric ratio of 10 parts carbon monoxide to 1 part carbon dioxide, and the pellets are ^{heated for a further 2 hours at 1100 °} C., followed by cooling in the same atmosphere at a rate of 200 to 300 ° C

ίο follows every hour.

Instead of the incorporation of carbon monoxide and carbon dioxide can be replaced with dry argon (which is sent via windings or uranium -schlangen at 600 ⁰ C prior to entry into the furnace) and the pellets can be heated ⁰ C for a further 2 hours at HOO followed by cooling in the same argon atmosphere at a rate of 200 to 300 ° C per hour.

The sintered pellets typically have a sintered density greater than 95% of theoretical and a ratio of oxygen to metal atoms of 2.00 ± 0.01: 1.

The sintering step described above also serves to satisfactorily remove the binder, which is used to granulate the reduced powder, the sintered pellets having a carbon content in the range of 20 to 50 parts per million (ppm).

The following examples are given:

(I)

A mixed oxide fuel material which contains 15 mole percent PuO ₂ and in which the uranium content of the UO _{2 is} naturally enriched was reduced in hydrogen, heated in argon and, as described above, pressed to form pellets which were heated at HOO ⁰ C were sintered for 3 hours in carbon dioxide, and cooled in a mixture of 10 parts by volume to 1 part CO CO _2, wherein the heating and cooling rate was 200 ⁰ C per hour. The sintered density was 10.64 g / cm ³ , which corresponds to 96.5% of the theoretical density (11.02 g / cm ³ ). After sintering in CO ₂ and before cooling in the CO — CO ₂ mixture, the oxygen-metal ratio was 2.09 ± 0.01: 1, and after cooling in the CO-CO ₂ mixture, the oxygen-metal ratio was -Ratio 2.00 ± 0.01: 1.

A mixed oxide fuel powder containing 15 mole percent of PuO ₂ and in which the uranium content of the UO enriched ₂ was reduced in a gas mixture of 9 parts by volume of CO to 1 part of CO ₂ at 500 ⁰ C for 5 hours and then had a tapped density of 2.97 g / cm ³ , a surface area of 4 m2 / g and an oxygen to metal ratio of 2.00: 1. The powder was granulated at 10 v / w% "cranko" and ^{pressed at 20 tsi (about 2800 kg / cm a} ) to form compact bodies in the form of pellets weighing approximately 1 g and a nominal diameter of 0.2 inches (about 5 mm). The “green” density of the pellets was 6.56 g / cm ³ . The pellets were sintered at 1200 ⁰ C in an atmosphere consisting of carbon dioxide, wherein the heating rate was 100 to 12O ⁰ C per hour. The carbon dioxide was then replaced by dry argon, and the temperature of 1200 ⁰ C was increased for a further 2 hours.

right, followed by cooling at a rate of 100 to 120 ⁰ C per hour. The sintered pellets had a density of 10.75 g / cm ³ (corresponding to 97.5% of the theoretical density) and an atomic ratio of oxygen to metal of 2.01: 1.

(III)

Green pellets, which were prepared as in Example (II), were ^{sintered in carbon dioxide at 115O 0} C for 5 hours. Carbon monoxide was then introduced in a volumetric ratio of 10 parts of CO to 1 part of CO ₂ , and heating was continued for 2 hours, followed by cooling. The sintered pellets had an oxygen-to-metal ratio of 2.00 ± 0.01: 1 and a sintered density of 10.55 g / cm ³ , which corresponds to 95.8% of the theoretical density.

The method described above, for example, results in addition to the aforementioned advantage of the possible Reduction of process costs also the possibility of calcining, reducing, binder removal and to carry out sintering steps in hydrogen-free atmospheres, in which way the the risk of fire that is present when using hydrogen is avoided. In line with the diminished Process costs are made possible by the modified temperature range according to the present invention the use of "nichrome" or "kanthaie coils or snakes" in electrically heated ones Sintering furnace, while the higher temperatures of the process according to the main patent the use of coils or coils made of materials such as platinum or platinum-rhodium alloys (which are expensive are) or require molybdenum, which requires protection against oxidation and thus the furnace construction complicates.

The invention also relates to modifications of the embodiment outlined in claim 1 and refers above all to all features of the invention, which individually - or in combination - are disclosed throughout the specification.

Claims (6)

Patent claims: 1. Process for the production of a dense sintered, from uranium dioxide or a mixture the same body consisting of plutonium dioxide, with a pressed body made of uranium dioxide powder or a mixture of the same with plutonia powder in an atmosphere of carbon dioxide or a mixture of the same with carbon monoxide, according to patent 1147163, characterized in that the body is at a temperature in the range of 1100 is heated up to 1200 ° C. 2. The method according to claim 1, characterized in that after heating in one of Carbon dioxide then puts the body in an atmosphere composed of a mixture of carbon dioxide and carbon monoxide existing atmosphere is heated. 3. The method according to claim 2, characterized in that the volumetric ratio of Carbon monoxide to carbon dioxide ranges from 1:10 to 100: 1. 4. The method according to claim 1, characterized in that after heating in one of Carbon dioxide then puts the body in an atmosphere made up of dry argon Atmosphere is heated. 5. The method according to any one of claims 1 to 4, characterized in that the pressed body is prepared from a powder, which surface area and tap density values in the range of 2.0 to 3.1 g / cm ³ for the tap density and accordingly 7, 0 to 2.0 qm / g for the surface area. 6. The method of claim 5, characterized in that the surface area and tap density values by heating the powder in one of a mixture of carbon monoxide and carbon dioxide existing atmosphere can be achieved. 709 610/481 7. 67