DE1939610A1 - Process for the production of a nuclear fuel from sintered uranium dioxide - Google Patents

Description

"Process for the production of a nuclear fuel made of sintered uranium dioxide "

The Eri'indu!) "; Relates to a novel process for the production of a sintered Ui-andioxyds, which is used as nuclear fuel in nuclear reactors. Here uran (iio-cycl at a temperature that is not higher than about L" ) OO 0 sintered in a chemically controlled gas atmosphere until the desired density is reached. The u-asatüiosphUi'e üiindesteus consists of two gases, which give a Sauerstoiipartialdruclc in equilibrium sufficient hold un ü.ie [Jrandioxydzuiianaeusetzung in Sinterteuiperatu-r at eJiieiü oxygen / Uranverhältiiis of at least 2.005 üii.

009808/1522

Patent Attorneys DipL-Ing. Martin Licht, Dipl.-Wirtsch.-Ing. Axel Hansmann, Dipl.-Phys. Sebastian Herrmann

8 MÖNCHEN 2, THERESI ENSTRASSE 33 · Telephone ι 281202 · Telegramtn-Adrei * «: Llpafli / MOnchtn Bayer. Vereinsbank Munich, branch. Oskar-von-Miller-Ring, account no. 882495 <Postscheck-Account ι Munich No. 1633 97

Oppenau office: PATENTANWALT DR. REINHDtD SCHMIDT

1939810

It is known that uranium dioxide, which is used as a nuclear fuel in the nuclear reactors for civil use, is an exception to the law of certain proportions because ¹¹ UO ₀ "actually indicates a single stable phase involved in the composition of UO. "Can vary until UO _{0 OK} . For practical reasons, the industry has chosen a ratio of 2.00, which can consistently be produced in commercial quantities. The ^ reactors are therefore set up for fuels which have an ^ O / U ratio of preferably 2.00, but are suitable up to a ratio of 2.01, and have a density of about ^ h ^ι , Ό of the theoretical density.

In the conventional manufacturing processes for such fuels, a sintering process is used in which an incandescent body made of uranium dioxide or compact UOp powder is burned, which have an O / U ratio of approximately 2.1p. It is fired at temperatures of 1650 to 1800 C in a hydrogen atmosphere until r.ian contains a final sintered product that has an O / U ratio of about 2.00 and the desired density. in such a method, however, a special electrically overall r-heated oven is required, whose Erhaltu & ng is very costly.

It has been suggested to use the uranium dioxide in a steaming atmosphere to sinter to bypass special high temperature devices for the hydrogen combustion process. By using steam, the amount of sinter can be increased significantly. However, this method has the disadvantage that the sintered product produced has an O / U ratio of approximately 2.20. The advantages of the steam used are compared to the process with hydrogen lost because that achieved with the steam process

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Sintered product must be further treated "to reduce the oxygen content to be changed according to the reactor specifications.

The invention was therefore based on the object of a novel sintering process for the production of uranium dioxide nuclear fuels to develop that avoids the disadvantages of conventional methods ·.

The invention is based on the knowledge that a sintered product can be produced from uranium dioxide, which the Corresponds to reactor specifications both in terms of density and in terms of the desired O / U ratio, by burning pellets from uranium dioxide powder at high temperatures hundreds of degrees lower than the temperatures used in conventional processes will.

In the method according to the invention, temperatures used below 1500 ° C and obtained satisfactory Results at temperatures down to 10000 ° C, without the procedural time taking too long and the O / U ratio of the uranium dioxide is increased during the sintering process held at a level of at least 2.005.

It could also be determined, that the 0 / U ratio Rrandioxyd can easily be stopped at the beginning of the firing process and roaring throughout the sintering period by means of control and regulation of the oxygen partial pressure one> ίχεο "ιιιηκ ans gases, among which the Sintered uranium dioxide compacts are maintained * can.

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It is actually only a small but critical amount of oxygen in the furnace atosphere necessary for this Target, and this amount can easily be adjusted by the ratio of one gas to the other, such as from carbon monoxide to carbon dioxide, and by continuously passing the gas mixture through through the sintering chamber.

The invention relates to a process for the production of sintered uranium dioxide-Iernbrennstoffkörper, which is characterized in ^ that UO ,, - powder is pressed to overall Grünkörporn and is the Grüulcörper at a Terapei'atur lower than about 1300 ⁰ C, to which desired density while maintaining the body's oxygen to uranium ratio at at least about 2.00p.

By the method according to the invention is a uranium dioxide sintered product in a furnace that is at a relatively low temperature is maintained, without the need for an additional lirennstufe is necessary that in the conventional sintering process with a steam atmosphere is necessary. Thus the after- k parts of conventional driving avoided. That after The uranium dioxide sintered product produced by the method of the invention meets the requirements of the nuclear reactor specifications.

The method according to the invention / No "offsetting" disadvantages accompanied. V. 'e'terhin. a sintered product is obtained in at least the same yield and in at least the same quality as the conventional ones Procedures and no subsequent treatment stages are necessary.

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In the method according to the invention, uranium dioxide is at a temperature below 1500 ° C in a chemically controlled Gas atmosphere sintered to the desired density, the gas atmosphere consisting of at least two gases in one predetermined relationship exists. The gases in equilibrium result in a sufficient oxygen partial pressure, around the uranium dioxide composition at the sintering temperature on a U / U ratio of at least 2.005 hold.

When the uranium dioxide is sintered, the particles do not become melted, but are subject to diffusion in the solid state, i.e. the ionic movement of the oxygen ions and uranium ions are sufficient at the sintering temperature for diffusion to take place and the particles to the desired one Growing closer together. However, this happens much more slowly with the urcinioneii than with the oxygen ions, whereby the sintering rate or the sintering speed is reduced and high sintering temperatures are necessary will. The increased sintering speed according to the present method can be explained on the basis of the movement the ions of the uranium dioxide lattice network. One suspects that the oxygen present in the furnace atmosphere builds up on the oxygen ion framework, thereby increasing the number the voids or the lattice gaps in the oxygen lattice is decreased and the number of voids in the uranium lattice is increased. This increase the concentration of the Empty spaces in the uranium lattice increase the diffusion rate of the uranium ions and this also increases the sintering speed .

The in. The present S internal method according to the invention The uranium dioxide particles used can have a surface area in the range of about 2 to 12 square meters per gram.

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With such particles, the sintering process can be carried out over an advantageous period of time and at advantageous temperatures will. Particles with a smaller surface area can also be used, but in this case it is a higher sintering temperature is necessary, while particles with a larger surface area due to their high chemical Responsiveness result in difficulty in treatment.

Before sintering, the powder is pressed into a mold which has the necessary mechanical strength for the subsequent treatment and which, after sintering, has the desired size and corresponds to the reactor specifications. The pressing can be carried out with conventional devices, such as, for example, steel pressure molds. For most uses, the powder is compressed into small spheres. A pressure of 1410 to 3570 kg / cm is generally used to produce spheres or pellets with a density of about 50 % of the theoretical value. A higher pressure does not produce pellets with a higher density, since the particles cannot be compressed any further. If too low a pressure is used

t , the pellets do not have the required mechanical strength.

In the present method according to the invention, a sintering furnace with a controlled atmosphere is used, so that the desired gas atmosphere is present during sintering. Before sintering, the furnace is nit a inert gas, such as nitrogen, is flushed through to remove entrained air, which affects the O / U ratio could adversely affect ivenn the furnace is not equipped with an introduction device, such as a gas lock, through which the uranium

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can put dioxide in the oven without affecting the atmosphere, can remove the uranium dioxide before cleaning with the inert gas can be added to the furnace.

The sintering of the uranium dioxide is carried out in a specific gas atmosphere. The initially present stoichiometric O / U ratio of the uranium dioxide used can vary, for example in the range from 1.7 to 2.25. Ks is possible because the stoehiometric ratio of the uranium dioxide is set in the furnace atmosphere during the sintering process. The Ofenatmospliüre consists of a predetermined ratio of at least two gases, which enter in equilibrium oxygen in an amount sufficient to provide the uranium dioxide during the ¹ -Sintems desired O / U ratio to maintain. A preferably used gas atmosphere consists of a mixture of carbon monoxide and carbon dioxide, a mixture of hydrogen and steam, and a mixture of these four components, which, if desired, can be obtained from a burned mixture of air and methane (natural gas).

In the process after the invention, you let yourself flow equilibrium gas mixture through the furnace. This makes the furnace atmosphere continuous replenished so that the oxygen partial di-uclc is not significantly changed during the sintering.

The specific partial oxygen pressure that occurs during the The sintering process must be maintained depends on the O / U ratio desired during the sintering and on the sintering temperature. It can be determined from the following equations - in which the ratio of χ given in UOp against the partial pressure of the oxygen is.

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^iog 10 ^P 0 ₂ = A + B log ₁₀ x + C (log ₁₀ x) ² (l)

Λ = 33> 6O2 + 40.842 x 10 " ⁵ T - 12.615 χ 10 T ² (ta) B = 9.983 + 18.021 χ 10" ³ T - 5.387 x'lO " ⁶ T ² (lb) C = 3.366 + 5.011 χ 10 "- ¹⁷ T - 1.478 χ 10" ⁶ T ² (lc)

where P _n = partial pressure of oxygen in the atmosphere T = sintering temperature in degrees C.

The equations (l) to (lc) are based on values as follows References were taken from:

PA Kroger, "Search for a Defect Model for UO ₂ ",

Z. for Physilc Chemistry 49, 178-197, 1966,

K. Hageniark and M. Broli, "Equilibrium Oxygen Pressures over the Kon-Stoichioraetric Uranium Oxides UO ₂ " and U-Og at Higher Temperatures "J, Inorg.

Nucl. Chem., 28, 2837-50 (1966).

The ratio of the specific gas mixture can be determined from the partial pressure of the O _{2 from the free energy equation}

Δ F = -RT In Kp ■ (2)

The free formation energies 4, ^F can be found in the literature.

For the reaction 2 CO + 0 _r . - ^ 2C0 _o / P _nn j. applies AF = -135.100 + 41.5OT = -RT in j γ

C0

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-135,100 + hl. 5OT = -RT In ^ -) + RT In _p (2b)

V co ^p co ₂ \ ²

log P _n = log I ^ - ^ 1 - 135.100 + 41.50 (2c)

4.56T h.

log P ₀ = log 1 - ^ - 4 - ^ u ₊ 9.10 (2d)

^U 2 I CO ' ^L

P ₀₀
The ratio of 2_ is determined by taking the known

^P C0

Values of the sintering temperature for T in degrees Kelvin and the known partial pressure of 0 _o for P _{n are} replaced.

The gas mixture used in the method according to the invention provides a practical technique for establishing the desired partial pressure of oxygen in the furnace atmosphere. This Oxygen partial pressure is very low. Becomes a C0p / C0 gas atmosphere used, for example, the following reaction takes place in the oven:

0 _o + 2C0

Such a gas system is well buffered because the carbon dioxide has the capacity to give off oxygen and the carbon monoxide can consume hydrogen without the partial pressure of the oxygen in the system being significantly changed. Should oxygen be consumed during the process to change the O / U ratio of the uranium dioxide, then the COp / CO ratio of the gases would not be changed significantly. If, for example, a ratio of 1 5 1 is used for COp / CO, then the partial pressure of the oxygen at 1400 ^° C. can be calculated from equation (2d)

-8th
and is about 10 atmospheres. Such a par-

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tialdruek corresponds to about 0.01 parts of oxygen per 1 million parts. Since there is no practical method for Measuring 0.01 parts of oxygen for every 1 million gas parts in the furnace there can be a mixture of oxygen and a inert gas cannot be used in the present process. However, it is very easy to create a mixture of CO / CO " set in a ratio of 1: 1. Also when a mixture of oxygen and an inert gas is used the oxygen can be quickly depleted by the uranium dioxide during the equilibrium gas mixture is well buffered according to the present method

j and a substantially constant partial pressure of oxygen

f results.

■ In the following examples, unless otherwise stated

i give the following procedure:

The COo / CO ratio in each experiment is set so that that there is a partial oxygen pressure that is sufficient the O / U ratio of the uranium dioxide during the sintering process and to keep the sintering temperature used at 2.005.

The COg / CO ratio was calculated from equation (2), using a partial pressure of oxygen calculated from the equation (1) or from a graph Illustration based on equation (1).

Commercially available CO 2 and CO gases were used. The gases ^were} gäeitet over calcium chloride and into the furnace via conventional capillary and Thermoelementetrömungsmesser

• were provided. The gases were from the knives in the

led, which is approximately to the desired gas ratio

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Sintering furnace passed to an oxygen sensor and then to a Blaseiiablausventil that the entire system under a water pressure of about 5 cm hMt. In this way revealed the oxygen sensor has a continuous registration the oxygen activity of the sintering furnace atmosphere and was capable of changes due to furnace emissions and the delivery or uptake of oxygen by the To determine the sample.

There was an alumina tube furnace is used, a length of about 50 cm and a diameter of approximately 3 8 cm _f possessed. The stove was wrapped in platinum and heated electrically.

The flow of the C0 ₂ / C0 gas mixture through the furnace was 150 ml / min.

The O / U ratio of the uranium dioxide was determined by oxidation of a weighed amount to U_Og up to a constant weight at 750 ° C in air. The increase in weight gives the amount of oxygen absorbed and, if the molecular weight of UO ₂ and UOg is known the value of χ was calculated. The sintered uranium dioxide was analyzed in the same way, except that the sample was crushed into particles prior to oxidation which could pass through a sieve with a sieve opening of 2.0 mm (-10 mesh) .

The density of the sintered TJ rand i oxy ds was increased by displacement in CGI »measured in g / cc.

The ■ uranium powder vatrde isostatically pressed at 750 kg / cm, Granulated to a particle size that passes through a sieve with a sieve opening of 0.084 mm (-20 mesh),

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Wrapped in a glass jar for 5 minutes and then pressed at 2460 kg / cm in a 9.525 ram printing forme, the walls and plunger surfaces of which were lubricated with steric acid. The green density of the pressed final pellets was 47 io of the theoretical value.

The test samples were carried out in platinum boats, which were heated to the desired sintering temperature for one hour and then the furnace was cooled.

EXAMPLE I

Uranium dioxide powder with a surface area of about 9 »5 square meters per gram and an O / U ratio of 2.19 was isostatically reduced to a green density of 4? ^c jo of the theoretical value /reprst.

Two pellets were placed in an oven which was then purged with nitrogen for about 10 minutes to remove entrained air. A mixture of carbon dioxide and carbon monoxide in a ratio of 4.13 ί 3 was then passed through the furnace, which then wants to be heated to a temperature of 1300 C.
maintain lane.

of 1300 ⁰ C was heated. This temperature was two hours

The sintered pellets were then allowed to reach room temperature cool in the oven atmosphere. The oven was then purged with nitrogen and the pellets removed. the Pellets had an average density of 95.9% des theoretical value and an O / U ratio of 2.013.

The following experiments, given in the table, were carried out in the same way as described in Example I

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guided. In the test series Λ to F, the pellets were made from the same uranium dioxide powder as in
Example I described. In the test series G to J
the uranium dioxide powder had an O / U ratio of 2.182 and a surface area of 3.62 square meters per gram For each time period the sample consisted of two pellets and the values given in the table are averages for two pellets.

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Sample series

co ₉ / co

Sintering temp. ⁰ C

1 H.

Sintering time 2 hours

4 hours

8 hours

16 hours

Uranium dioxide; O / U 2.19 and surface area of 9 l / 2 m / gm

A B G D

255/1

ο
co

00

'op co

cn

G H I J

8.7 / 1 3.7 / 1 1.6 / 1

900

1000

27.9 / 1 1100 9.0 / 1 1200 4.13 / 1 1500 1.60 / 1 1400

1100 1200 1300 1400

O / U 2.028 D. 95.5 #

O / U 2.011 D. 95.5

O / U 2.195 D. 93.8 %

O / U 2.119 D. 96.6 5i.

O / U 2.006 D. 95.2 ^c / o

O / U 2.052 D. 96.9 ^c / o

O / U 2.007 D. 95.8%

0/1 '2.008 D. 95.7 Io

O / U 2.010 D. 95.9 ^c / o

2.00 <) D. 95.2>

Uranium dioxide: O / U 2.182 and surface area of 5.62 ηΤ / g;

in

O / U 2.040

D. 85 ^c / o

Ο / ϋ 2.010

D. 86.5 °, e

O / U 2.010 D. 91 ⁰ Jo

O / U 2.007 D. 94%

O / U 2.009 D. 92.6 <? Ο

0 / U 2.007 D. 94.5 1ο

O / U 2/16 D.88.6 %

Ο / ϋ 2.008 D. 93.75%

O / U 2.007 D. 94.9%

o / u 2.005

D. 96.4 %

O / U 2.006

D. 94.55%;

O / U 2.008, D. 95.5 Ä '

r t I *. ■ * f

• · fit

tv « 1 ι ι t t

ta et «* f» 4

f «ff 9 9 f«

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The table shows that the & after the present i) Process of the invention obtained good results after different sintering periods and at different temperatures will. From the above description it can be seen that according to the "method of the present invention, advantageous materials can be produced, which are particularly well suited for the nuclear reactors currently in use. These Materials are manufactured at temperatures at least two hundred degrees below those currently in use of conventional manufacturing processes that use a hydrogen atmosphere.

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Claims (8)

Munich ,, the h. August I969 GENERAL ELECTRIC COMPANY ScheneCtady 5, N.Y., your mark Our mark River Road I / Lü V. St. A. Patent application? "Process for the production of a nuclear fuel from sintered uranium dioxide ' ¹ PATENTANS PRUCIIE fl A method for producing sintered uranium dioxide nuclear fuel _{bodies, characterized in that UO 2} powder is pressed into green bodies and the green bodies are sintered to the desired density at a temperature lower than about 15ΟΟ C, while the oxygen-uranium -Ratio of the body is kept hot at least about 2.005. 2. The method according to claim 1, characterized in that that the oxygen / f-uranium ratio is set and is maintained during the sintering stage by the Oxygen partial pressure: the sintering atmosphere is regulated. 3. The method according to claim 1, characterized in that that the sintering step is carried out in an atmosphere of mixed gases which in equilibrium an oxygen fpartial pressure will be issued, which is in a position to 0 09808/1522 Patent attorneys Dipl.-Ing. Martin Licht, Dipl.-Wirtsch.-Ing. Axel Hansmann, Dipl.-Phys. Sebastian Herrmann 8 MÖNCHEN 2, THERES I ENSTRASSE 33 · Telephone: 281202 -Telegram address: Lipatli / Mönchen Bayer. Vereinsbank Munich, branch. Oskar-von-Miller-Ring, toilet no. 882495 Postal check account: Munich No. 163397 Oppenauerßuro: PATENT ADVOCATE DR. REINHOLD SCHMIDT 1? 193961Q · "" Nl ■ " imagined the oxygen-uranium ratio in the uranium dioxide solids establish and maintain. 4. The method according to claim 3, characterized in that the gases in equilibrium of carbon monoxide and
Carbon dioxide. 5. The method according to claim 3, characterized in that the gases in equilibrium of hydrogen and steam stand 6. The method according to claim J, characterized in that the gases consist of combusted air and methane in equilibrium. 7. The method according to claim 3> characterized in that the uranium dioxide powder has a surface in the range of about 2 to 12 square meters per gram. 009808/1522