DE2115694B2 - PROCESS FOR MANUFACTURING URANOXYDE CONES OR MIXED URANOXYDEPLUTONIUM OXYDE CONES WITH CONTROLLABLE POROSITY - Google Patents

Description

The invention relates to a method for producing uranium oxide spheres or mixed uranium oxide-plutonium oxide spheres, whose porosity should be controllable as precisely as possible.

In British patent specification 1067 095 is already a method of making spheres from refractory Described materials that can only be agglomerated and, in particular, from oxides of the Uranium, thorium, plutonium, berillium, aluminum, magnesium and silicon, separately or in Mixtures.

This method consists in treating a cellulose resin, if possible in the presence of an alcohol, with a to mix aqueous solution of compounds which contain the elements that make up the globules are to be produced; the resulting solution is then dropped into an alkaline solution, whereby regular beads of controllable size form.

It was also applied to the manufacture of spheres from ceramic nuclear materials with a relatively higher degree of porosity, for example as fuel in certain types of Fuel elements used in high temperature gas cooled nuclear reactors have already been proposed. Inert substances are added to the starting solution, for example during the Caking can be removed. The addition of inert substances is also in the USA patent 3,320,179 with particular reference to a sol-gel process. These fleeting strangers However, additives can complicate the reaction mechanism and affect production costs and adversely affect the purity of the final product.

In a further development of this process for the production of a porous refractory material from the Oxides of uranium or plutonium have already been proposed as the refractory material at one speed from 320 to 480 ° C per hour in one

ίο to heat a maximum temperature in the range of 1280 to 1920 ° C, hold the heated material in this range for 2 to 3 hours and then the refractory material at a rate of 320 to 480 ° C per hour to a temperature in the range of 960 to 1440 ° C and then to cool to room temperature at any rate, the atmosphere being kept inert or oxidizing for the initial part of the heating process, but a reducing atmosphere abruptly introduced at a temperature between 700 ° C and the maximum temperature and reducing for the rest of the process is held.

The material is fed to the furnace in batches.

he procedure requires a lot of working time. The average porosity of the product varied from batch to batch 5 and 6% in its density.

After this process, the atmosphere is initially oxidizing or inert up to about 650 ° C and becomes then suddenly (abruptly) reducing. However, this can lead to the outer layer of the bead is treated to a considerable extent, whereas the layers underneath are treated differently or cannot be treated at all. In addition, physical and chemical interactions take place instead, which are not necessarily controllable. The possible oxidation and the sudden reduction can result in heavier, more compact, and less porous beads.

These disadvantages are now avoided according to the invention in a method for producing uranium oxide spheres or mixed uranium oxide-plutonium oxide spheres with controllable porosity in that uranium oxide microspheres or uranium oxide-plutonium oxide microspheres in a mixture which have been obtained by dripping uranium salts into the liquids and then calcined in a suitable manner. introduced into the furnace against the current from an inert atmosphere, in particular argon or nitrogen, for 45 minutes to 2 hours at a maximum temperature of 900 to 1300 ° C, which _{varies according to the desired density for the U 3} O ₈ spheres, treated and then cooled to room temperature and that the spheres treated in this way in the inert atmosphere are then placed in the same or a different furnace against a current of a reducing atmosphere, in particular hydrogen or argon / 4% H ₂ . in-

for one to two hours at one temperature

of 1600 ° C and then cooled to room temperature, so as to be the desired reproducible porosity to come.

Preferably the microspheres were calcined in air at a temperature between 420 and 700 ° C.

As the person skilled in the art sees, the favorable results according to the invention are achieved in that

next a purely inert treatment, i.e. a purely physical treatment, is carried out which is not influenced by the surrounding atmosphere on the microspheres. Further you can see that a purely chemical treatment follows, namely reducing chemical reactions between the atmosphere and the microspheres.

In particular, the invention will work continuously. This too helps make products with more constant chemical-physical properties (with fluctuations between 1 and 2%) and with an oxygen / uranium atomic ratio not higher than 2.005.

The chemical reactions gradually follow the more controllable heat treatment steps. Due to the initially purely inert treatment, the microspheres are better prepared for the subsequent purely chemical treatment than was previously conceivable; this is favorable, since one now obtains products with an oxygen / uranium atom ratio not higher than 2.005 (this is the total reduction of UO ₃ - »UO ₂ ).

The invention will now be described in more detail with reference to embodiments, numerous Modifications within the scope of the invention are of course possible.

Spheroids of uranium dioxide and mixed oxides of uranium and other nuclear materials in which the uranium oxide represented the greater part and which one according to the in the Italian patents 727, 301 and 778, 786 are calcined at a temperature which is between 420 ° C / 700 ° C varies according to the desired density in the end product.

Namely, when the calcination _{temperature is increased, the H 2} content in the calcined product is decreased, and hence the production of the low-density sintered material is easier. In this way, a differently hydrogenated product based on UO ₃ (hydrated product) is obtained.

According to patent 1 956132, with which the invention is related, the calcination temperature is fixed at 450 ° C, and this leads to considerable difficulties if one wants to obtain a low density end product.

The calcined product is successively introduced into a tunnel type continuous furnace and it is allowed to travel in proper tubes or conveyors along the major axis of the furnace, whereby it encounters zones of different temperatures, against the current of an inert, oxidizing one or reducing gaseous atmosphere.

The above-mentioned material undergoes two different heat cycles in the furnace.

The first thermal cycle consists in bringing the material in an inert atmosphere (argon, N ₂ ) to a maximum temperature, which varies between 900 ° C and 1300 ° C, for about 1 or 2 hours and then gradually bringing it to room temperature.

With this first operation, the transformation of UO ₃ into U ₃ O _{8 is obtained} at different density values; the changes in the latter are closely related to the changes in the maximum working temperature in the sense that high treatment _{temperatures lead to the formation of U 3} O ₈ at high density, whereas lower temperatures lead to the formation of products at lower density. The U ₃ O ₈ microspheres obtained after the first heat treatment undergo a second heat treatment in the same furnace or in a different furnace, which consists in placing the product in a reducing atmosphere (H ₂ , argon with 4% H ₂ ) to a maximum temperature of 1600 ° C over a period of time varying between 1 and 2 hours, the temperature then gradually increasing to

ίο is lowered to room temperature. With this second heat application, the transformation of U ₃ O ₈ into UO ₂ and the sintering of the latter are obtained. This double thermal cycle process is obviously used to obtain products with a porosity greater than 5% to 8%, whereas in order to obtain a porosity lower than these values a single thermal cycle process in a reducing atmosphere is used. With this continuous process after the

ao finding a more uniform end product is obtained with regard to the chemical-physical properties (porosity, crystallinity) as well as a considerable Stability of these properties even if the subsequent energetic heat treatment

«5 steps follow, as well as a high resistance to breaking and finally an oxygen-uranium atomic ratio of no more than 2.005 (namely the entire reduction of U ₃ O ₈ - ^ UO ₂ is obtained). These improvements come on top of the benefit brought about by the continuous heat cycle, that is, less loss of labor and a greater amount of product treated.

The following examples illustrate the invention without limiting it.

example 1

Microspheroid production from UO ₂ with a porosity equal to 27%.

The solution of a uranium salt is mixed with a thickener (e.g. methyl isopropyl cellulose) and a water-soluble alcohol compound (e.g. propylene glycol); the solution obtained is then dripped through very fine tubes into an alkaline solution (for example a 30% ammonia solution). After a sufficient period of time in the alkaline solution, the microspheroids are washed in deionized water and then dried at about 85 ° C. by means of azeotropic distillation. The dried microspheroids are then calcined in a continuous oven at a maximum temperature of 570 ° C. The calcined microspheroids consist of UO ₃ and contain hydrogen of approximately 0.08 ± 0.02 percent by weight.

The calcined microspheroids are placed in aluminum oxide containers and then introduced into a horizontal type continuous furnace and undergo two heat treatments in a row.

In the first of these heats, the calcined microspheroids are treated with argon in the following way: temperature increase of 300 ° C / h up to a temperature of 1070 ° C; Dwell at this temperature for 45 minutes and then cool to room temperature at a rate of 175 ° C / h. The product obtained in this way consists of U ₃ O ₈ microspheroids with a density of 5.5 ± 0.15 g / cm ^3. In the second _{heating step, the U 3} 0 ₈ microspheroids are treated with argon /

4% H ₂ treated in the following way: temperature increase of about 220 ° C / h up to 1570 ° C; Dwell at this temperature for about 1 hour 30 minutes and then cool to room temperature at a rate of about 500 ° C / h. The product thus obtained consisted of UC ^ microspheres with a density of 8.05 g / cm ³ (equal to a porosity of about 27%) with an oxygen / uranium atom ratio not higher than 2.005. The density of the product obtained is quite stable and shows increases of between 2 and 3 % when the product is treated again for 1 hour at 1550 ° C in a reducing atmosphere.

Example 2

Production of UO ₂ microspheres with a porosity of about 20 %.

The procedure was as in Example 1, except that the calcination was carried out at a maximum temperature of ao 550 ° C. (instead of 570 ° C.), the calcined product being UO ₃ microspheres with a hydrogen content of about 0.11 ± 0.02 Weight percent received.

The first heat cycle was carried out with argon under the same conditions as in Example 1, but with a maximum temperature of 1130 ° C to 1150 ° C instead of 1070 ° C. The product of the first heat cycle consisted of U ₃ O _g microspheres with a density of 5.95 ± 0.15 g / cm ³ .

The second heating step is the same as that described in Example 1 and leads to an end product of UO ₂ microspheres with a density of 8.80 g / cm ³ (which corresponds to a porosity of about 20 % ) and an oxygen / uranium atom ratio which is not higher than 2.005.

Example 3

Production of UO ₂ /10% PU-O ₂ microspheres with a porosity of about 20 %.

The procedure was carried out as in Example 2, with the exception that the transition from argon to argon / 4 % hydrogen took place at 900.degree.

The measured porosity is found to be 20% ± 1%.

Comparative example:

Total amount of microspheres produced about 2 kg.

The starting point was UO ₂ with a nominal density of 85%. Work was carried out once according to the older proposal (German patent specification 1956132) and once according to the invention.

The older proposal worked with 8 batches of around 250 grams each. The ones in the The density achievable in individual batches fluctuated between 78 and 90% of the theoretical value. the Density deviations in the 2 kilograms of the product obtained were even greater.

According to the invention, small tubs of about 250 grams each were used continuously. These small containers or vats were inserted into continuous tunnel ovens and the two exposed heat steps or guides referred to above. The density of in each tub contained microspheres varied between 82.5 and 88% of the theoretical value, which is a Means improvement of 100%. There were practically no differences from tub to tub.

Claims (2)

Patent claims: 1. A process for the production of uranium oxide or mixed uranium oxide-plutonium oxide balls with controllable porosity, characterized in that uranium oxide microspheres or uranium oxide / plutonium oxide microspheres in a mixture which have been obtained by dripping uranium salts into liquids and then suitably calcined in the furnace Stream of an inert atmosphere, especially argon or nitrogen, introduced, treated for 45 minutes to 2 hours at a maximum temperature of 900 ° C to 1300 ° C, which _{varies according to the desired density for the U 3} O ₈ beads, and then treated be cooled to room temperature, and that the spheres so treated in the inert atmosphere are then introduced into the same or a different furnace against a current of a reducing atmosphere, in particular hydrogen or argon / 4% H ₂ , for one to two hours at one Heat-treated at a temperature of 1600 ° C and then at room r temperature must be cooled in order to achieve the desired reproducible porosity. 2. The method according to claim 1, characterized in that the microspheres in air at a temperature between 420 and 700 ° C were calcined.