DE1250330B - Ver go for the production of carbon tubes - Google Patents

Description

FEDERAL REPUBLIC OF GERMANY

GERMAN

PATENT OFFICE

EDITORIAL

Int. Cl .:

C04b

German class: 80 b -8/12

Number: 1250 330

File number: G 33234 VI b / 80 b

Filing date: September 29, 1961

Opened on September 14, 1967

-5 2 /

The invention relates to a method for the production of carbon tubes, for example for Manufacture of fuel sleeves for nuclear reactors can be used.

In the German patents 1113 214 and 1158 895 a process for the production of Carbon is described by the carbonization of a body, which is produced by the recovery of cellulose or hemicellulosic material obtained from a dispersion of this material in an aqueous medium became. In this process, it has been found that during the carbonization heat treatment, considerable Shrinkage occurs.

One of the possible uses of the method of the above-mentioned patents is in Manufacture of carbon tubes, which have remarkable properties in terms of gas permeability their walls have. As stated in the latter of the aforementioned patent, can carbon tubes with such a low permeability that they have a high vacuum Withstand negligible loss for long periods of time even at elevated temperatures. These Tubes are therefore excellent for use in the construction of sleeves to receive Suitable for nuclear reactors as breeding or fissile material, as carbon can withstand higher temperatures than the more common sleeve materials, these pipe making processes are great in this regard Meaning.

It is characteristic of the pipe manufacturing processes mentioned that the materials are considerable Solidification experienced at various stages of the process, and for this reason it has been difficult proved sufficient symmetry or straightness for use as fuel sleeves thereby to achieve that the tubes during the heat treatment in the above-mentioned Patent specifications are supported in the manner described: The accuracy of the final dimensions of the pipe is important for the fuel pods. It has been shown that the mass of material during shrinkage, which accompanies the aforementioned consolidation or solidification, low resistance to plastic Has deformation and that it can therefore be shaped to some extent without its Affect properties. In particular, it has now been found that by arranging the to be shrunk Material on a mandrel can achieve straightness of the end product, which for a fuel sleeve is acceptable. Since the carbonation process heat treatment at increased Process for the production of carbon tubes

Applicant:

The General Electric Company Limited, London

Representative:

Dr.-Ing. H. Ruschke, patent attorney,
Berlin 33, Auguste-Viktoria-Str. 65

Named as inventor:

Howard Harold Walter Losty,

Hugh Soden,

Wembley, Middlesex (Great Britain)

Claimed priority:

Great Britain September 30, 1960

(33 696),

dated June 29, 1961 (23 576)

Temperature includes, it is necessary to damage the product due to different Heat movement between the mandrel and the pipe after the pipe has been shrunk onto the mandrel avoid. The solution to this problem forms one of the features of the invention.

According to the invention, a method for the production of carbon tubes, which are used, for example, for the production of fuel sleeves in nuclear reactors, with the formation of an aqueous dispersion of cellulose or hemicellulose material, regeneration of this material from the dispersion, casting of the regenerated material in the form of a tube, drying Tube and heating of the dried tube for carbonization, characterized in that, in order to regulate the mode of shrinkage of the tube, which is plastically deformable during carbonization, one moves into the dried tube

709 647/506

3 4

Tube before the carbonation stage occurs using a solid mandrel of cellulosic or hemicellulosic material

introduces, the cross-section of which is smaller than the interior of the associated possibility of deformation

Cross-section of the dried pipe, however large tion of the pipe, is the use of a core for

enough that the tube by the shrinkage of the drying stage is just as expedient,

plastic tube during carbonation 5 According to a preferred feature of the invention

stage rests against the mandrel, the shape of the is thus a first solid before the drying stage

Domes of the desired inner shape of the core with a smaller cross-section than the inner one

corresponding carbon tube, and the cross-section of the cast tube made of regenerated

The mandrel is made of a material that incorporates a cellulosic or hemicellulosic material into the

higher coefficient of thermal expansion introduced onto the cast pipe and dried this in such a way that

indicates as that of the pipe material at or near the that after removal of the liquid the rejected

Temperature at which the pipe in sufficient permanent material of the pipe on the mandrel

Dimensions is carbonized and in which the tube shrinks on the, so that it is essentially its

The mandrel shrinks on. Adjusts surface. Then the first mandrel is out

Since in order to achieve the carbonization, the tube 15 is pulled out of the dried tube and inserted

of dried regenerated material on Tempe- second mandrel made of a material with a suitable

ratures is heated above 350 ⁰ C, finds "thermal expansion coefficient and a suitable

an extensive cross-section, as described above, during the carbonation stage

Thermal expansion of the tube and the mandrel instead. Carrying out the carbonation stage in the pipe

It is therefore desirable that the mandrel be inserted from an ao.

Material is formed which is made with respect to the material, although it is not essential, it is usually

from which the pipe is made, such heat is preferred, a mandrel for the drying stage

has expansion characteristics and that the mandrel with a circular cross-section should be used. It has

a cross-section with respect to the inner transverse appears that the material of the mandrel for this

section of the pipe shows that a concern of the as stage of the process with regard to the

Rohres to the mandrel only at or near a tem-. moderately low temperature, which considerably below

temperature is made possible at which this is during 100 ° C and for drying the regenerated

Heat-shrinking of the cellulosic or hemicellulosic material required

Rohres is practically complete. The thermal expansion is not critical. The mandrel for the drying

The thinning coefficient of the mandrel material should therefore be in the 3 »stage, for example made of wood or metal, such as

Temperature range in which the pipe is sufficiently z. B. steel exist.

has shrunk to rest against the mandrel, greater Tm in view of this embodiment of the invention

than that of the pipe material. As a result, a solid mold core was known to be used

that complete contact between the molding of objects made of ceramic material

Mandrel and the tube is reached so that the mandrel 35 rial to use. For example, in the

In the final stages of the shrinking process, a US Pat. No. 2,974,388 a process for

has a restrictive effect on the pipe and thus describes the manufacture of ceramic objects,

causes the desired shape of the tube. which consists in the fact that a fusible ceramic

In this way, even when the arrangement cools down, material is mixed with the application of heat

Completion of the heating stage is sprayed on by a different 40 solid mold core, then the

Lich thermal contraction enables the mold core from the solidified ceramic material

the mandrel is free, so that tension rial either by separating the mandrel

and possible breakage of the pipe material is avoided, which is adjacent to the ceramic material.

if this becomes its plasticity when it cools, or by separating the mold core

loses and thus the mandrel after the end of the 45 of the ceramic shell due to contraction

Heating stage is easily removed from the tube by cooling and finally the ceramic

can be. mix shaped body is fired, this

A carbon tube made to the desired size during firing according to the method according to the invention does not necessarily have to shrink without any regulation of the circular cross-section . For some 50 ways of shrinking the body through application cases, it may be desirable to produce tubes on a molded body or core. According to their shape, for example with the elliptical preferred embodiment mentioned above, the schematic cross section. One such possibility is the invention from moist regenerated material is also considered, even if a dried cellulosic or hemicellulosic material is not formed on the tube with a circular cross-section as a core, but a core with a starting point for the carbonation stage is used with a smaller cross-section than the inner cross-section , cut the wet pipe into the one that was already formed

Sometimes it can be useful to clean the carbon tube before the drying stage, during which the initial

siierungsstufe i n two or_ more heating stages lent shrinkage of the tube takes place, introduced

perform so that the shrinkage is gradual 60 and remains within during the drying stage

takes place. In such cases it may be desirable to remove the pipe so that the pipe is dry

for each heating step a different mandrel due to substantially the surface of the core

use to match the type of shrinkage of the pipe .such shrinkage,

to control in a series of steps. In other cases, as in those mentioned above

In cases it may also be possible to use the same mandrel 65 patents described, the dried pipe

to be used for two or more heating stages. from regenerated material to a desired shape

Since during the drying process considerable and size are processed before the carbon

Liche shrinkage of the pipe from regenerated sierungsstufe is subjected.

In some cases it can be useful, after completion of the carbonation process, in which a or more mandrels are used, according to the invention the tube further heating at higher Temperatures, for example at temperatures of 1500 ° C or above, to be subjected to at least a partial grapple to reach it. Such further heating is carried out without supporting the tube on a mandrel. One Such further heating without a mandrel can of course only be carried out if the shrinkage of the The tube is essentially complete and there is likely to be little or no deformation of the tube will occur as a result of such further heating.

In one example of a method according to the invention, it becomes lignin-free, bleached and purified Half-cellulose pulp or pulp and crushed in a grinding device of the usual Hollander type, used in the papermaking industry is hydrated. The resulting viscous, aqueous Dispersion is then fed to a high performance centrifuge (e.g. Sharple centrifuge) where the Most of the water is separated from the mass, which comes from the centrifuge in the form of a soft, wet tube 'is pulled out. To the distance'der To facilitate tube out of the centrifuge, it will run through the tank of the centrifuge for a few hours hot air is sent in order to pre-dry the lower part of the tube sufficiently that this The end of the tube can be grasped to pull the tube out of the centrifuge.

The tube removed from the centrifuge is placed at its pre-dried end in a drying chamber suspended, which is fed with hot circulating air. When such a tube dried in this way it shrinks to about 50% of the volume of the original moist tube, and if the tube is otherwise unsupported, it will necessarily not shrink evenly, so that the dried tube may be considerably curved. As it is convenient if the material Machined at this stage, it can be seen that any curvature or even oval deformation the tube can mean that significant amounts of material are consumed during the machining process to be wasted. In addition, such distortions can also cause the dried tube is not at all suitable for machining.

Therefore, in the method chosen as an example, a steel mandrel of circular cross-section with a Diameter which is the expected final inside diameter of the tube in the dry state approximated, inserted into the tube when it is hanging in the drying chamber. The drying of the tube is ended after about 48 hours and takes place with air at a temperature of 70 ° C. In the dried In its condition, the tube is a tough and uniform solid product made from solid hemicellulosic material consists. If necessary, this tube can be machined for the purpose of to reduce the material thickness, through which decomposition products during the subsequent carbonation step have to escape. An advantage of using a dome for the drying step is that the machining is done by rotating the tube between the Mandrel tips can be made, as the dried tube is thereby essentially the desired Maintains internal shape.

The dried tube is then placed in a charring furnace with an inner mandrel made of a steel, as described in German Patent 1158,895. A suitable material for the mandrel is a steel specified in publication No. 970, 1955 of the British Standards Institution under the code name EN 58 with the weight composition: 17 to 20% chromium, 7 to 10% nickel, 2% manganese, Minimum amount 0.2% silicon, maximum amount 0.15% carbon, titanium. Corresponding to four times the amount of carbon and the remainder iron. The. Dimensions of the dome are determined in the manner described later. The tube with the mandrel is stored in a very simple way and without any compression in a pressure vessel , which is cleaned from the fl oat, by f.yft and filled with nitrogen, which has a pressure of about 100 atm . The Uefälj is then slowly heated to a temperature in the range from 350 to 450 ° C over a period of about 48 hours, the pressure being increased in the course of

As this warming rises to about 200 at.

In Fig. 2 of the drawing of German patent specification 1158 895 is the change in dimensions indicated as a result of consolidation or densification that occurs when the temperature of the heat treatment of regenerated semi-cellulosic material increased. From this curve it can be seen that after If the pressure treatment described ends, the product experiences further shrinkage its temperature rises above 360 ° C, as is necessary for the complete carbonation Treatment is necessary. It is therefore essential that the dimensions of the dome are such that the tube is in contact with it at temperatures close to the maximum temperature of the heat treatment, during which the shape should be directly influenced. It can be seen that a cathedral, which for Use during higher temperature treatments is suitable, only a very loose storage for the tube during this pressure treatment. Nevertheless, such a support of Be useful. Certainly it has no deleterious effect, and it is likely to be foreseen the mandrel is at the beginning of the carbonization treatment, d. H. at the beginning of the pressure treatment, is in place.

When the pressure treatment is finished, the pressure is released in the manner described in the cited patent specification 1158 895, ie by gradually reducing the pressure at a rate of about 21 kg / cm ² for every quarter of an hour while the pressure vessel is at an increased Temperature is maintained. When normal atmospheric pressure is reached, the vessel is cooled and then opened to remove the tube in a partially carbonized form. The tube is then transported on its dome to a vacuum drying oven for the subsequent stage of heat treatment. During this last-mentioned treatment, the consolidation or solidification process continues, the degree of shrinkage being considerable. Towards the end of the treatment, the shrinkage is such that the tube contracts to the surface of the dome.

From the curve mentioned in the patent specification 1158 895 it can be seen that when the contact of the Mandrel surface takes place at a temperature of 500 to 600 ° C, still further shrinkage during of the subsequent temperature rise occurs. The material of the tube is still plastic at this stage deformable, whereby after reaching a temperature of about 750 to 800 ° C, the inner surface the tube fits closely to the surface of the dome. The heat treatment is as a result up to continued at such temperature and then the tube and mandrel assembly is allowed to cool.

The expansion coefficients of the mandrel material and the tube material must be selected in such a way that that different contraction occurs during the cooling process, so that when the arrangement is removed from the furnace the mandrel is only loosely held in the tube and easily withdrawn can. Attention must also be paid to the degree of heat contraction to be expected in the tube and for the material mentioned in the present example the curve is of change of the coefficient of thermal expansion with the treatment temperature, as experimentally for with the appropriate Temperature treated material was determined, shown in the drawing, including the Values of the expansion of steel EN 58 and graphite at the corresponding temperatures are given are. This curve shows that when the temperature rises, the expansion coefficient of the at a suitable temperature treated material decreases and approaches the value of graphite. Above about 550 ° C the coefficient is smaller than that of the steel EN 58 of the dome, so that even if significant Interference between the mandrel and the tube occurs at temperatures higher than 550 ° C, the mandrel contracts away from the tube wall as it cools.

After cooling to room temperature, the tube is removed from the furnace and the mandrel is pulled out. This shows that the tube has a surface design that is that of the mandrel surface corresponds exactly, and that the tube is only out of round to 0.0254 mm, if substantially completely round mandrel is used. There is little or no further consolidation and shrinkage occurs at higher temperatures, there is no need for further. Storage of the tube during the subsequent graphitization stage. However, it may be useful to a certain extent to provide support in order to avoid any possible risk of an occurrence during graphitization Reduce warpage to a minimum.

In a special procedure, in accordance with that described in the example Procedure was carried out, tubes were made of regenerated hemicellulosic material and dried in the manner described above, the mean inner diameter of the dried, so-called green tubes was 18.67 mm. The tubes were placed on mandrels made of steel EN 58 with a diameter of 12.7 mm at room temperature treated. The tubes were first subjected to the above-described pressure treatment up to 450 ° C and were then cooled and put on their thorns in a vacuum furnace for the final stage of the Carbonation treatment transported. The abutment of the tubes on the thorns was at 770 ° C perfect, and the heat treatment was finished at that temperature. The tubes were cooled down and removed from the furnace, and the mandrels withdrawn without difficulty. The tubes were finally heated to temperatures in the range from 1500 to 2000 ° C without internal support, to cause partial graphitization. Although slight further shrinkage during this When heating occurred, there was no noticeable distortion of the tubes. The tubes also turned out to be as free from cracks or other defects.

The dimensions of a typical generated with the method defined above tube at three stages, namely in the so-called green state, after being heated to the contact temperature of 770 ° C and after a final heat treatment at 1500 ⁰ C are given in the following table. The term "curvature" means the maximum gap between the outer surface of the tube and a flat reference surface, measured at the center of the tube, and the "inner ovality" is the difference between the maximum inner diameter and the minimum inner diameter of the tube.

After heating dimensions tube heating up

1500 ⁰ C 30

length

Middle

Inside diameter
Medium wall thickness.

Curvature ...
Inner ovality

Green
tube After this
Heat mm mm 150.88 106.17 18.57
0.889 12.98
0.635
0.127
0.127

104.14

1.2.72 0.610 0.152 0.152

The finished tube obtained with the method described in the example above can be used in a manner as will be understood by those skilled in the art to represent part of an envelope or Sleeve for nuclear fuel, such as uranium or uranium oxide, to form what is machining to match of the tube would require, with suitable end caps in a gas-tight manner for the purpose of sealing attached to the fuel. As a result of its ceramic nature, uranium oxide is difficult to process likely if the fit is to provide intimate thermal contact between the fuel and the sleeve. In this However, the invention provides a clever solution to this problem.

It is usually desirable from a heat transfer point of view, not just this to establish intimate contact between fuel and sleeve material, but also to ensure that the thickness of the sleeve wall is kept small, so that the neutron absorption by the Sleeve material is kept to a minimum. This limitation arises with a graphite sleeve the thickness due to the small neutron capture cross-section of the graphite. It is therefore possible the advantage of the characteristic consolidation or compression feature of the pulp mentioned or Semicellulosic material to be characterized herein

utilize 'that the dry regenerated material is machined in such a way that it shrinks directly onto a ceramic fuel coating in the later heat treatment stages, the fuel filling in this way the ver. used the mandrel for the final heat application stage forms according to the invention. The dimensions of the sleeve and the fuel fill would be such be that at the irradiation temperature of the fuel in a nuclear reactor, the sleeve contains the fuel * ° would enclose tightly enough to give the desired heat transfer characteristic.

The choice of the thickness of the finished sleeve for the purpose of creating strength, the degree of influence is reasonable with the fuel is obvious to the professionals. After the wrapping process and at room temperature, the fuel could not be tightly enclosed, which is likely to be the case. However, the required Contact on the fuel assembly can be achieved if »o this receives an elevated temperature in the reactor. '

The advantage of the last-described method lies in the fact that the ceramic fuel not to be machined accurately in order to achieve thermal contact with the shell needs because the inner surface of the shell or sleeve is affected by the fuel itself during the carbonization heat treatment is shaped so that the desired contact is made. It might be useful to provide wedge devices to facilitate alignment to ensure the fuel filling in the sleeve, in the event that it is loose when it is cold adjusts. .

To ensure the properties of the fuel substance or for any other reason it can be necessary to the upper treatment temperature of the sleeve in the process described above using the fuel itself as a mandrel. However, it has been shown that the Sleeve material, which is produced from solid half-cellulose in the manner described above, also then has a very high degree of impermeability when in the ungraphitized state. For reasons of stability, it is likely necessary to make the arrangement so that the final heat treatment is carried out at a higher temperature than expected is reached by the fuel in the irradiated state in the reactor.

The invention does not apply to the treatment of wood pulp, half-cellulose or similar materials yet limited to the creation of impermeable materials. Although emphasis on making straighter Tubing is not intended to apply the invention to these particulars To restrict product type.

Claims (9)

Claims: 60 1. Process for the production of carbon tubes, which are used, for example, for the production of fuel sleeves for nuclear reactors, by forming an aqueous dispersion of cellulosic or hemicellulosic material, regenerating this material from the dispersion, pouring the regenerated material into the shape of a tube, drying the tube and heating the dried tube relative to the {carbonization, characterized in that füan introduced to control the ¹ Schrumpfüngsweise the deformable during carbonization pläStisdi tube in the dried Rohr VOF the Kärbönisierungsstufe a fixed mandrel whose cross-section is smaller than the inner cross-section of the dried tube, however, large enough that the tube rests against the mandrel due to the shrinkage of the plastic tube during the carbonization step, the shape of the dome corresponding to the desired internal shape of the carbon tube to be produced, and the mandrel being made of one material t, which has a higher coefficient of thermal expansion than that of the tube material at or near the temperature at which the tube is sufficiently carbonized to abut the mandrel with shrinkage. 2. The method according to claim 1, characterized in that the carbonization of the pipe is carried out in several heating stages. 3. The method according to claim 2, characterized in that the dried tube in the first carbonization stage in an inert atmosphere to a temperature of about 350 to 450 ° C and under a pressure of up to 200 atmospheres and in a second ' Stage is heated in a vacuum furnace to a temperature of about 750 to 800 ° C. 4. The method according to claim 2 or 3, characterized in that one for each heating stage Another mandrel is used to increase the type of shrinkage of the pipe in several stages steer. 5. The method according to claim 2 or 3, characterized in that for two or more heating stages the same mandrel is used. 6. The method according to claim 1 to 5, characterized in that a before the drying stage first solid mandrel with a smaller cross-section than the inner cross-section of the cast one Tube made of regenerated cellulose or Heml · cellulose material is introduced into the tube and this is dried so that when the liquid is removed, the remaining material of the Tube is shrunk onto the mandrel and essentially adapts its surface and that then the first core is pulled out of the dried tube and a second mandrel with it a cross-section and made of a material with a coefficient of thermal expansion as in FIG Claim 1 stated before carrying out the carbonation step in the dried tube is introduced. 7. The method according to claim 1 or 6, characterized in that the dried tube from regenerated material is processed to the required shape and size before the Is subjected to carbonation stage. 8. The method according to claim 1 to 7, characterized in that the tube after completion the carbonation process in which one or more mandrels are used, one further heating to a temperature of at least 1500 ° C is subjected to at least to achieve partial graphitization of the pipe without being supported on a mandrel. 709 647/506 9. Modification of the method according to one of the preceding claims 1, 2, 3, 5, 6 and 7, characterized in that a mandrel is used during the carbonization of the tube made of regenerated cellulose or hemicellulose material from a ceramic fuel for nuclear reactors, such as. B. uranium oxide is used. References considered: U.S. Patent No. 2974,388. 1 sheet of drawings 709 647/506 9.67 © Bundesdruckerei Berlin