DE1771018C3 - Process for the production of uranium carbide spheroids - Google Patents

Description

nical damage required during the final manufacturing phase is generally less than that Thickness of the layer that is required to remain intact under thermal cycling.

In the case of coated particles which are manufactured according to the process of the main patent, the presence does not exist the gap between the particles and the outer layer is exposed to thermal cycling of the particles without any radial tearing or breaking of the layer due to the larger Thermal contraction of the layer relative to the particles occurs

A coated particle which is produced by the method according to the main patent has a Uranium content, which is determined by three factors. These three factors are the density of the uranium dicarbide core, which by the conversion of the original uranium monocarbide spheroid by reaction with the first layer of carbon is formed, the dimension of the gap that exists between the uranium dicarbide core and the residual layer is present on the spheroid after the conversion, as well as the thickness the layer of the material retaining fission products. The density achieved in the urai dicarbide core which is produced by the transformation of the original uranium monocarbide particle is depends on the density that can be achieved in the original uranium monocarbide spheroid. General uranium monocarbide spheroids cannot have a density greater than 90% of the theoretical density of Uranium monocarbide are produced. An outer layer of fission-retaining material is made in an optimal thickness required for fission product restraint and resistance to mechanical Damage is necessary. At the gap between the uranium dicarbide core and the coating the conversion of the original uranium monocarbide spheroids into uranium dicarbide is present Dimension of the gap to the amount of the initial layer of carbon in a ratio which is required to convert the original uranium monocarbide particles to uranium dicarbide.

The gap that is created is not equivalent to the thickness or layer thickness of the carbon that is used for the transformation of the original uranium monocarbide particles is selected because the conversion of uranium monocarbide to uranium dicarbide results in an increase in volume results. However, it can be said that for a given size of spheroid, the size of the generated The gap, when starting with uranium monocarbide particles, will always be greater than a minimum value is related to the amount of carbon required to make the original monocarbide particles to convert to uranium dicarbide.

For a spheroid of a given diameter, the final diameter, the larger the the uranium concentration in the spheroid is lower. Assuming, regardless of other factors, that the thickness of the outer layer must be at least the optimum for the retention of fission products and Mechanical strength is required, so it is the gap dimension that ultimately determines the uranium concentration determined, which is achieved in spheroids, which have the optimal thickness of the outer layer.

Thus, the process according to the main patent - if one works with uranium monocarbide particles of a given nwsRp - result only in the production of coated spheroids which have gaps which are larger than the minimum size, which is determined by the amount of carbon that is required to form the uranium monocarbide particles to convert to uranium dicarbide. This places a limit on the concentration of uranium in coated spheroids produced by the process of the main patent.
It is the object of the invention, the method according to the

ίο to improve the main patent so that a larger one Uranium concentration can be achieved and that coated spheroids can be made that Have uranium dicarbide cores of higher density than in the process according to the main patent.

This is achieved according to the invention in that the first layer is made of pyrolytic carbon Particles of a composition in the range of UC1.0 until UCi, e is applied and that when applying the second layer of fission products restrained,

ίο refractory material at a higher temperature from the pyrolytic carbon of the first layer and uranium carbide particles of uranium dicarbide of the composition UCi, Bi is formed.

The uranium carbide particles used expediently have a composition of UCi. «To UCi.o.

The first carbon coating is deposited at a temperature in the range from 1000 ⁰ C to 1400 ⁰ C, while the outer layer on the refractory material retaining the fission products is deposited at a temperature

} o temperature above 1450 ° C is deposited.

An initial layer of carbon of greater thickness than for the conversion is advantageous the uranium carbide particles required in uranium dicarbide are deposited on the uranium carbide particles, thereby leaving a residual inner layer of carbon as part of the scale on the particles. the Initial carbon coating can be applied to the uranium carbide particles by pyrolysis of methane or acetylene be deposited.

If one starts with particles of a composition higher than UCi.o, for example a composition UC14 to UCi.b, less carbon is required than with particles of a composition UCi.o to bring about a conversion of the particles to uranium dicarbide. Thus, the initial layer of carbon that is deposited may be less in thickness for particles having a higher carbon content than required for particles of UCi.o, and a narrower gap becomes between the uranium dicarbide core and the outer layer of refractory Material generated when subsequently deposited at a higher temperature. Particles with a composition UCi.4 to UCi * can, if they are produced by reaction sintering of a mixture of UO ₂ and carbon, be produced up to 95% of the theoretical density, while particles of UCi.o only up to a theoretical density of 90% can be produced. The use of particles of a composition from UC1.4 to UC16, which have a theoretical density of 95%, in the present process results in coated spheroids, which uranium dicarbide cores have a higher density and therefore a higher uranium content than can be achieved when using particles of a composition UCi .o and a theoretical density of only 90% is started

The method according to the invention will now be described in more detail using an example.

Urancarbidsphäroide be prepared in a known manner In the present case, the relative amounts used of uranium dioxide and carbon chosen so that particles of a desired composition in the range of UCi are generated _{from 0} to UCI. ₆

The particles are sieved to a size between, for example, 400 and 500 microns Sorted out diameter. The coating of the particles with pyrolytic carbon is now carried out in a vortex reactor, with high-purity argon is used as fluidizing gas. A hydrocarbon gas, such as methane, is used for the separation of the pyrolytic carbon coatings used The methane is mixed with the argon fluidizing gas stream

An initial coat of pyrolytic carbon fabric is applied to the Urancarbidteilchen at a deposition temperature ranging from 1000 ° C to 1400 ° C, for example at 1300 ⁰ C, applied is an initial carbon coating of at least a sufficient thickness to react later with the Urancarbidteilchen and these To be able to convert to uranium dicarbide (UCiss) is applied to the uranium carbide particles
For example, in the case of particles, a

ίο Composition UCi.o, UCu, UCi.4, and UCi.b, which have a density of 90% of the theoretical density and an applied initial layer of carbon, density 1.5 g / cm ³ , the following minimum thicknesses of an initial carbon coating required for conversion to UCi.85.

Table I.

Particle Particle through (Micron) Initial over / ugs- together knife thickness that is needed establishment, through to be a divider in the chemical Uranium dicarbide (UCtB '') Formula selected to convert presses (b) (in microns) (a) 500 (away) UC10 400 500 19.75 25.25 UC 1.2 400 500 15.2 18.75 UCi.4 400 500 10.3 12.5 UCi.b 400 500 6.0 7.0 UC 1.85 400 0 0

The particle with the composition UQ.85 is to Included for comparison purposes.

An outer coating of cleavage products restrained refractory material, such as pyrolytic carbon or pyrolytically deposited silicon is now deposited at a temperature above 1450 ⁰ C, for example in the range of 175O ° C to 2000 ⁰ C, onto the particles. During the deposition of the outer layer of refractory material retaining fission products, the initial layer of carbon reacts with the particles to form uranium dicarbide (UC ^ es), creating a crevice or void under the coating around the uranium dicarbide particles.

The following Table II shows the size of the gap for the particles of the composition UC1.0. UCu, UC1.4 and UCi, 6 in Table 1 above, provided that a carbon thickness just sufficient for the conversion of the particles to uranium dicarbide is applied to the particles and an outer coating of fission product-retaining material in a thickness of 60 microns
Particle is applied.

finally to the

The size of the gap created depends on the degree of sintering that occurs in the particles when they are converted into Uranium dicarbide is going on. However, the gap between maximum and minimum values arises as follows Base:

(a) No reaction sintering takes place; H. there is no percentage change in the theoretical Density of the particle when converting the lower carbide to dicarbide,

⁴ ^ (b) complete reaction sintering takes place, ie the particle is converted into 100% dense uranium dicarbide.

Some sintering and densification is likely to occur when the outer layer - in particular over 1500 ° C - is applied, and it is closed assume that the size of the gap in the following table is approaching or near the maximum value achieved.

Table 11 Particle Gap dimension (microns) (b) ηΐίΐλ. Particle Gap dimension (microns) (b) max. Urangclialt Particle diameter 14th diameter 18th a 500- together 11.8 14.4 Micron part Settlement (through 9.5 11.5 chens with the chemical 6.85 8.5 coating Formula off (Micron) (a) min. 0 (Micron) (a) min. 0 g / cm ¹ pressed) 400 6.8 500 8.8 4.83 UC 1.0 400 4.7 500 5.35 4.94 UCu 400 2.05 500 2.4 " 4.98 UC M 400 0.8 500 1 5.00 UC 1.0 400 0 500 0 5.02 ι ir. «

From Table II it can be seen that the uranium content of the coated particles increases as the carbon content of the original particle increases. The numbers relating to particles of the composition UC |, ₈ 5 are included for comparison purposes, but are of no practical importance because particles of the composition UC135 cannot be represented to 90% of the theoretical density, and in any case, with such particles no gap between the core and the outer layer, so that the outer layer can fail in the event of thermal cycling. The benefit of the increased uranium content levels off between particles of composition UCm and UCi, 6 and this is the preferred range of compositions.

In another method, an initial layer of pyrolytic carbon can be of larger size Thickness than that required alone for converting the particles to uranium dicarbide applied to the particles will. When applying an outer coating of fission products, retardant refractory Material at the higher temperature can contain a certain proportion of the initial carbon coating in accordance with, for example, the thickness of the carbon required for the transformation of the particles in uranium dicarbide, as indicated in Table I, is required, and corresponding column arise as indicated in Table II. However, a certain proportion of the nickel layer is considered to be carbon Part of the coating of the particles is left behind, leaving a layer within the outer coating Fission products retaining refractory material is formed, which through the gap from the core of the Particle is separated. For example, if for particles such as those listed in Table II, the coating is off Fission product-retaining material plus residual carbon layer with a thickness of 60 microns is provided, the same uranium content is achieved in the particles, which of course requires application a thinner outer layer of fission product-retaining refractory material on top of the particles, to achieve the same final total coating thickness. The presence of the residual inner layer made of carbon in the coated particles forms a barrier against the escape of fission products, generated in the core during irradiation. Therefore, the residual carbon layer should have a thickness of about 25 to 50 microns and to provide a total coating thickness of 60 microns, must be an outer coating of fission product-retaining refractory material of appropriate thickness in the range of 35 to 10 microns can be applied to the particles.

To a desired «: thickness of residual carbon ir

To achieve plating becomes an initial coals

A layer of fabric of a thickness equal to the amount of voi Corresponds to carbon, which is necessary for the conversion of de

Particle in uranium dicarbide is required, plus di <

required residual thickness applied to the particles

For example, in Table I is the required Dicki of carbon necessary for the conversion of 400 micron particles of the composition UCi ^ bi: UCi, 6 is needed, ranging from 6 to 15 microi specified; therefore the thickness is the initial egg Carbon layer used for the manufacture of be

layered particles is required, which coating « with a residual carbon layer in the range of 25 to 50 microns, 31 to 40 microns (around the minimum 25 micron residual carbon layer) and 56 to 65 microns (around the maximum 50 micron To produce residual carbon layer).

Since the outer layer is resistant to mechanical damage, an essential Reduction in the thickness of the outer layer au: fission products retaining material - around the to achieve the same total composite layer thickness - be undesirable.

For particles with an original diameter of 400 to 500 microns, as in Tables I and II above indicated, however, which residual carbon layers, for example in a thickness in the range from 25 to 50 microns, for example, it may be desirable to have the outer layer of fission products retaining material has a minimum thickness of 50 microns, which is a total coating thickness in Range of 75 to 100 microns compared to that Coating thickness of 60 microns used in the examples of Tables I and II where the coating only refractory material retaining fission products - without the presence of a residual carbon layer - which naturally means that the coated particle is a larger one

Final diameter - with a resulting reduction in uranium content - has

The following Table III gives details of such coated particles with an original 500 microns in diameter, which have coatings that have a residual carbon layer of 25 microns thick and an outer coating of fission product-retaining material in one 50 microns thick (i.e. 75 microns total coating thickness).

Table HI

Surface composition (through the chemical formula atis pressed)

Gap dimension below thickness of the initial thickness of the initial thickness of the remaining Urangehali des the assumption that carbon layer, carbon layer, duaJen carbon-coated

the one at the transition layer in the end particle

complete sintering which is separated from methane from the particle

Conversion to UC135 (density 13 g / cm ³ ) takes place

Jung of a particle in valid Oberzug UCijs is used

(Micron)

(Micron)

(Micron)

(Micron)

UCu
UCm
UCu.

14.4

11.5

8.5

43.75

37.5

32

If the initial carbon layer is applied by pyrolysis of acetylene instead of methane, the deposited carbon is of a lower density, 18.75
12 pounds
7th

25th 25th 25th

432 434

& h, the density of carbon deposited by pyrolysis median is in 1.5 g / cm ³ , while the density of carbon deposited by pyrolysis of

709612/71

¥ 947

Acetylene is applied, 1.0 to 1.2 g / cm ³ . If acetylene is used to deposit an initial carbon layer of sufficient thickness to leave a residual inner layer of carbon in the coating after the particles have been converted to UCi.85, this residual carbon layer will be of a lower density than that produced by a initial carbon layer remains, which has been deposited by pyrolysis of methane. The lower density carbon layer deposited from acetylene provides a more effective barrier against fission products than a higher density carbon layer deposited from methane. However, since an initial carbon layer deposited by pyrolysis of acetylene has a low density, for example 1 g / cm ³ , a greater thickness of carbon must be used in the initial Table IV

A layer for converting the particles into UQ ₁ I can be provided as is the case when the initial carbon layer is deposited from methane.

Table IV shows the comparison between the thickness "of the initial layer is required to Teilchei having a diameter of 500 microns which is give NEN composition convert (a) when de; the initial carbon coating is applied by pyrolysis of methane, and (b) when the initial carbon coating is applied by pyrolysis of acetylene. In the case of the deposition of the initial carbon layer from the acetylene, a greater carbon thickness would be required for the conversion, and this is of course reflected in the size of the gap created.

Particle composition (a)
(expressed by the chemical formula Initial Carbon) layer thickness necessary for the order

conversion is required if it is separated from methane (carbon density 1.5 g / cm ³ ) (microns) (a)
Gap dimension

UC 1.2

UCu

UC 1.6

18.75 12.5
7th

(Micron)

14.4

11.5

(b)

Initial carbon layer thickness required for the conversion if it is deposited from acetylene (carbon density I.Og / cm ')
(Micron)

(b) spa

gap dimension

(Micron)

In the production of uranium carbide particles, particles of a mixture of uranium dioxide and carbon are brought to a desired size and then reacted in a two-temperature process in a vacuum and sintered. At the lower temperature, for example at 1400 ° C, UO ₂ is reduced by carbon so as to form uranium carbide, and when the reaction is complete, sintering at the higher temperature, 1600 ⁰ C to 170O ⁰ C is effected, the carbothermal reduction Takes place according to the chemical equation

27
18th
10

23.5 16.5 11.9

UO ₂ + 3c

= UC + 2 CO.

If the carbon-UO ₂ ratio in the original mixture is that given by the above equation, then the end product is essentially uranium monocarbide. If the carbon content is then at a higher value, a two-phase product is generally formed, which has uranium monocarbide and uranium dicarbide phases, the proportions being dependent on the original carbon concentration. Finally, the end product consists entirely of dicarbide, although the carbon content of this phase is variable and always lower than that defined by the formula UC ₂ In general, in preparations made by reducing UO ₂ , the carbon content can be given by the formula UC135 be expressed . If the CO evolution is not kept at a steady rate and some sintering at the reaction temperature can take place, which slows down the reduction reaction, then it is possible to also bring the sesquicarbide phase (U ₂ Cj) ZUT nucleation . It is characteristic of the above process that when UCip is the intended end product, it is difficult to completely remove all oxygen because of the lack of ideal homogeneity of the mixture and because of the difficulty in removing all of the CO beforehand sintering is effected. Thus contains the final product, although single-phase, zw.schen 0.1 weight percent and 0.2 weighting- sprozent oxygen, and the final sintered? No / V ^a U ^{U emen Maxim} Alwert of 12.3 g / cm ³ that is carbon, ^"he i ° ^r " ^ical density, limited if the

.0 SÄ ³¹ ' ^{So far we increased} d. that the end product _{can be expressed by the formula UC U} , the oxygen content drops to 0.05 percent by weight, and with a final composition i ^ Jr. ^M *, ^{mm Sauer} "° ffgehalte of 50 ppm RedSctΙΓΛ ^addition," eight of these complete ΪΤ ^'he -?! ^Reakti ° nstemperatur product iff ^{rerrelat ver density mö "Iich} · Ζ""example ^{Dih% ei}

50 high

55

60

65 bfs bePnr * ^{fm d} : ^tDkh * ^des
, Η- ä · *? ^{lative densities}

^rehearse

product «rapidly

**** «60% and

The accordingly

“A strong one

Conditions, and

^ ^{M TeÜchen a} theoretical "" Z ^ anuneasetzungsbereicb of ^ ^s ^^, even higher 7 "? * ^Wefden" "k ^m <*""* Tables ^{4 Λβ} ^ pull * ⁵ ^ ⁸ ^ ^Te3dlen that the theoretical density ^Ä Z C

^of

* < ^{95% of the} theoretical ^ feölä besdricfetetea

^Per

Prun ^ cher Etarchmesser) Prerequisites calculated - ^{what is 8% more} than achieved ? ^{etm mt} ^ ° ^{em Te5Icn} en which together _{j0 is} started

Claims (6)

Patent claims: 1. A process for the production of uranium carbide spheroids, which are filled into molded bodies made of refractory material for fuel elements, with a layer of pyrolytic material that retains fission products, a first thin layer of pyrolytic carbon being applied to the spheroids gradually at low temperature, followed by the Application of a second outer layer of refractory material retaining fission products on the uranium carbide ^{spheroids coated with the pyrolytic carbon at a higher temperature of above 1500 °} C., uranium monocarbide being used as the substance for the spheroids, the first layer of pyrolytic carbon at a temperature between 1200 and 1400 ⁰ C is deposited on the spheroids, so that there is still no reaction between the applied layer and the uranium monocarbide spheroids, and when the temperature is increased to above 1500 ⁰ C, the first applied layer also takes place the uranium monocarbide reacts to form uranium dicarbide, a cavity being created in the form of a gap between the spheroids and the applied second layer of refractory material retaining fission products, according to patent 1471511, characterized in that the first layer of pyrolytic carbon on particles of a composition in the range from UCij to UCi.b is applied and that when the second layer of fission products-retaining, refractory material is applied at a higher temperature from the pyrolytic carbon of the first layer and the uranium carbide particles, uranium dicarbide of the composition UC135 is formed. 2. The method according to claim 1, characterized in that the uranium carbide particles used have a composition of UC1.4 to UC1.6. 3. The method according to claim 1 or 2, characterized in that the first carbon ^{coating is deposited at a temperature in the range from 1000 0} C to 1400 ⁰ C and that the outer layer on the refractory material retaining the fission products at a temperature above 1450 ⁰ C is deposited. 4. The method according to claim 1, 2 or 3, characterized in that an initial layer of Carbon thicker than that used to convert uranium carbide particles to uranium dicarbide is required to be deposited on the uranium carbide particles, creating a residual internal A layer of carbon remains on the particles as part of the coating. 5. The method according to claim 1, 2, 3 or 4, characterized characterized in that the initial carbon coating is applied to the uranium carbide particles by pyrolysis deposited by methane. 6. The method according to claim 1, 2, 3 or 4, characterized characterized in that the initial carbon coating is applied to the uranium carbide particles by pyrolysis is deposited by acetylene. The invention relates to a process for the production of uranium carbide spheroids, which are filled into molded bodies made of refractory material for fuel elements, with a layer of pyrolytic material that retains fission products, a first thin layer of pyrolytic carbon being applied to the spheroids in stages at a low temperature, followed by applying a second outer layer of cleavage products zuruckhaltendem refractory material on the article coated with pyrolytic carbon Urancarbidsphäroiden at a higher temperature of above 150Q ⁰ C, being used as substance for the spheroids Uranmonocarbid, wherein the first layer of pyrolytic carbon at a Temperature between 1200 and 1400 ⁰ C is deposited on the spheroids, so that there is still no reaction between the applied layer and the uranium monocarbide spheroids, and when the temperature is increased to above zo 1500 ⁰ C the first applied layer reacts with the uranium monocarbide to form uranium dicarbide, creating a cavity in the form of a gap between the spheroids and the applied second layer of refractory material that retains fission products, according to patent 14 71 511. In the case of coated particles which are bonded directly to the coating layer, the latter can in particular on cooling the coated particles from the coating temperature to room temperature chen. When it cools down, the layer is subjected to considerable stress, due to of the difference in the coefficient of thermal expansion between the particles and the layer additional stress occurs if the coated particles exceed the coating temperature reheated and then cooled from that temperature in final manufacture, the one Dispersion and embedding of the coated particles in a dense matrix of a refractory material in Brings rod shape with it. Such damage can be avoided by removing low density particles (i.e., particles, which have a high degree of porosity) and that a thick layer on the Particle is provided. If the particles are sufficiently strong (i.e. thick enough) they will crack limited to the particles due to thermal cycling. Layers of silicon carbide, for example, and of sufficient thickness act as a barrier to both gaseous as well as metallic fission products, which during the irradiation of a fuel assembly, containing such coated particles. Hold layers of pyrolytic carbon gaseous fission products return, and metallic fission products are made through the dense matrix retained non-fissile refractory material in which the coated particles are finely divided. fio must be the outer layer of the particles of sufficient Thickness to avoid mechanical damage during the final manufacture of a rod from a dense matrix material, in soft in the coated Particles are embedded as a dispersion to be able to withstand. The optimal thickness of the outer layer, which is used for the purpose of fission product restraint and for Achievement of resistance to mechanical