GB1564079A - Process for the production of uranium plutoniumixed carbides - Google Patents

Description

(54) PROCESS FOR THE PRODUCTION OF URANIUM-PLUTONIUM-MIXED CARBIDES (71) We, KERNFORSCHUNGSZENTRUM KARLSRUHE GESELLSCHAFT MIT BESCHRAENKTER HAFTUNG, formerly, GESELLSCHAFT FUER KERNFORS CHUNG mbH of 75 Karlsruhe 1, Postfach 3640, Germany (Fed. Rep); a Germany body corporate do hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement: The present invention relates to a process for the production of uranium-plutoniummixed carbides, in which a mixture of uranium dioxide, plutonium dioxide and graphite is subjected to a carbothermal reduction.

Whilst the production of uranium carbide is characterised by technological difficulties with regard to extraction of possibly phase-pure products, additional problems arise during the production of uranium-plutonium-mixed carbides by the peritectic decomposition of plutonium carbide. Besides weight losses, there also occurs a carbon enrichment of the samples which leads to sesquicarbide formation. In pure plutonium carbide, from about 157 K considerable weight losses are already observed, whilst in samples containing uranium carbide having above 30 mol % uranium carbide in solid solution, the peritectic decomposition of the plutonium carbide is clearly reduced. Quantiative investigations resulted for the plutonium steam pressure: of pure PuC . log Ppu = 4.686 - 18091/T (atm) (1761-1907 K) of U085Pu0el5C : log Ppu = 5.47 - 23000/T (atm) (1700-2000 K) The effects of carbide decomposition on the production of specific carbide fuels are manifold, e.g. the specific Pu/U+Pu ratio, and also the stoichiometric composition, changes during the manufacturing process. Therefore, for carbide manufacture it is necessary to have an exact knowledge of the rates of plutonium loss which are, in any case, plant-related. For industrial production of mixed carbide fuel, moreover, economic and safety technological questions arise. Fissionable material occurring as plutonium loss during production remains in the processing plant and on the one hand collects in the plant regions which are particularly inaccessible; on the other hand the plutonium quantities not returnable to the manufacturing process increase the manufacturing costs, which in processes applied heretofore amount to substantially between 1.5 and 2% by weight.

The full synthesis process proceeds as the most simple of the known processes from a mixture of a metal oxide with carbon. Due to the common carbothermal reduction a mixed carbide is extracted which is then available as starting product for the production of pellet fuel by sintering. The whole production process is thus schematically described by the following equations: x U02 + (1-x) PuO2 + 3 C o (UxPul-xc) + 2 CO (reduction) (uxpul-xc) < UxPul c (sintering) The object of the invention is now to provide a process which at least minimizes the disadvantages of known processes and reduces plutonium losses.

According to the present invention there is provided a process for the production of uranium-plutonium-mixed carbides, in which a mixture of uranium dioxide, plutonium dioxide and graphite is subjected to a carbothermal reduction, characterised by the process steps in the established sequence: a) grinding a mixture of uranium dioxide, plutonium dioxide and graphite for a grinding period of from 1 to 3 hours; b) heating the ground mixture of a) to a temperature of 1600 K or above until the onset of carbothermal reduction for the mixed carbide core formation indicated by the commencement of carbon monoxide production; c) cooling the reaction mixture to a temperature in the region of from 1300 K to 1550 K; d) maintaining a constant temperature after cooling, until completion of a first part stage of the carbothermal reduction recognized by the abatement of carbon monoxide production, and e) continued heating of the reaction mixture to form a single phase, substantially oxygen-free uranium-plutonium-mixed carbide at a temperature of from 1800 K to 1850 K, completion of a second part stage of the carbothermal reduction occurring therewith and subsequent cooling of the end product.

It has been found that the carbothermal reduction of a mixture of UO2, PU2 and graphite takes place in two stages. In the first stage, there is formed a mixture of oxide, oxicarbide (U,Pu)(Ox,Cl-x) and sesquicarbide, which still contains carbon in varying proportions7 and in the second stage this mixture then comprises the required mixed carbide. Thus, the effectiveness of the first reaction stage is dependent upon the powder preparation, whilst the effectiveness of the second stage on the other hand is independent thereof.

Experiments, in which the starting materials are first mixed and ground each for a period of 2, 4, 8, 16 or 32 hours, have shown that in the samples with the longer grinding period the carbothermal reduction sets in at a lower temperature than in the other samples. Moreover, it was possible to observe that the separation of the two reaction stages became clearer with an increasing grinding period and the reproductability of the process is improved.

To have a deeper insight into the operations during the carbothermal formation of the uranium and plutonium carbides, a number of samples were subjected to radiographic phase analysis according to different reactions. These investigations have shown that the reaction behaviour of uranium dioxide-plutonium dioxide-graphite mixtures, have to be slit up into three fundamental cases: a) in the case of mixed, and not ground, powder the two above-mentioned reaction stages for the uranium- and the plutonium-containing phases occur substantially independently of one another in parallel.

b) in powders which were mutually ground for 36 hours, the mixed crystals (U0.8Pu0.2)O2 forms before the inset of carbide formation. Then in the first reaction stage there occurs the formation of (U,Pu)(Ox,Cl.x) and (U,Pu)2C3, which in the next stage react mutualy with the residual free carbon to form (U,xPuo.)C.

c) if the powders are not ground for such a long period, though no pure mixed oxide is any longer produced. the carbide formed, dependent upon the grinding period, however, contains mixed carbide to a greater proportion.

On account of the intermediate products, it has to be assumed that in the case of the synthesis of the mixed carbide, the various stages are determined by the plutonium, since in the samples in which the two starting dioxides were present only in a mixed state, the carbothermal reduction of both oxides occurring substantially independently of one another, it is also comprehensible that the kinetics of this operation is determined by the compounds of the excess uranium present.

Whilst long grinding periods have been found effective in reducing the plutonium losses, such procedure accounts for an increase in expenditure in time, costs and apparatus for industrially applicable processes. For this reason for optimising the process further investigations were carried out which are described below.

The investigation results of the high activation energy at the start of the reaction in samples which were made of powders only ground for a short period, permit the conclusion to the expiration of a core forming operation. This core forming in a uranium dioxide-graphite mixture ground for 36 hours obviously occurs already during the grinding process.

The hypothesis of the core formation may be proved in that a mixture of uranium dioxide, plutonium dioxide and graphite ground for two hours is heated until the carbothermal reduction process sets in and then suddenly cooling it. If on reheating the reaction occurs at a substantially low temperature, core formation is present. During the first reheating the reaction sets in at about 1655 K. After cooling and reheating the reaction began at 1390 K, thus 265 K lower. This is a clear proof of the existence of core formation.

Independently of the powder preparation, a single phase monocarbide, Iow in oxygen, may be produced only at temperatures above 1800 K.

The invention is described by way of an example and a diagram for the reaction course of this example shown in the accompanying drawing. The invention, however, is not limited to this example and this diagram.

Example: 274 mg of a mixture of U02, PuO2 and graphite, in which the ratio of UO2 to PuO2 amounted to 80 to 20 (percent by weight), and the quantity of graphite corresponded to a quantity stoichiometrically required for conversion, were at first ground for 2 hours and then for 80 minutes with a constant heating up rate (approx. 18 degrees/minute) were brought to a temperature of 1753 K. The noticeable CO reduction showed the onset of the carbothermal reduction (first part stage). The reaction mixture thereupon was suddenly cooled off to 1543 K and kept at this temperature for 4.5 hours. With a rapid further heating up to 1823 K the carbothermal reduction was terminated (end of second part stage). The temperature was held for a period of 60 minutes for forming a single phase, substantially oxygen-free, U0 8PUo 2C and subsequently slowly cooled off by shutting down the furnace.

furnace.

The analysis of the cooled end product showed a plutonium loss of only approximately 0.5% by weight.

The process in accordance with the invention also provides the advantage that it permits a considerable economy of electric energy by the reduction of the reaction temperature.

WHAT WE CLAIM IS: 1. A process for the production of uranium-plutonium-mixed carbides in which a mixture of uranium dioxide, plutonium dioxide and graphite is subjected to a carbothermal reduction, characterised by the process steps in the established sequence: a) grinding a mixture of uranium dioxide, plutonium dioxide and graphite for a grinding period of from 1 to 3 hours; b) heating the ground mixture of a) to a temperature of 1600 K or above until the onset of carbothermal reduction for the mixed carbide core formation indicated by the commence- ment of carbon monoxide production; c) cooling the reaction mixture to a temperature in the region of from 1300 K to 1550 K; d) maintaining a constant temperature after cooling until the completion of a first part stage of the carbothermal reduction noticeable by the abatement of carbon monoxide production; and e) continued heating of the reaction mixture to form a single phase, substantially oxygen-free uranium-plutonium-mixed carbide at a temperature of from 1800 K to 1850 K, completion of a second part stage of the carbothermal reduction occuring therewith and subsequent cooling of the end product.

2. A process for the production of uranium-plutonium-mixed carbides, substantially as hereinbefore described, exemplified and illustrated in the accompanying drawing.

**WARNING** end of DESC field may overlap start of CLMS **.

Claims (2)

**WARNING** start of CLMS field may overlap end of DESC **. The invention is described by way of an example and a diagram for the reaction course of this example shown in the accompanying drawing. The invention, however, is not limited to this example and this diagram. Example: 274 mg of a mixture of U02, PuO2 and graphite, in which the ratio of UO2 to PuO2 amounted to 80 to 20 (percent by weight), and the quantity of graphite corresponded to a quantity stoichiometrically required for conversion, were at first ground for 2 hours and then for 80 minutes with a constant heating up rate (approx. 18 degrees/minute) were brought to a temperature of 1753 K. The noticeable CO reduction showed the onset of the carbothermal reduction (first part stage). The reaction mixture thereupon was suddenly cooled off to 1543 K and kept at this temperature for 4.5 hours. With a rapid further heating up to 1823 K the carbothermal reduction was terminated (end of second part stage). The temperature was held for a period of 60 minutes for forming a single phase, substantially oxygen-free, U0 8PUo 2C and subsequently slowly cooled off by shutting down the furnace. furnace. The analysis of the cooled end product showed a plutonium loss of only approximately 0.5% by weight. The process in accordance with the invention also provides the advantage that it permits a considerable economy of electric energy by the reduction of the reaction temperature. WHAT WE CLAIM IS:

1. A process for the production of uranium-plutonium-mixed carbides in which a mixture of uranium dioxide, plutonium dioxide and graphite is subjected to a carbothermal reduction, characterised by the process steps in the established sequence: a) grinding a mixture of uranium dioxide, plutonium dioxide and graphite for a grinding period of from 1 to 3 hours; b) heating the ground mixture of a) to a temperature of 1600 K or above until the onset of carbothermal reduction for the mixed carbide core formation indicated by the commence- ment of carbon monoxide production; c) cooling the reaction mixture to a temperature in the region of from 1300 K to 1550 K; d) maintaining a constant temperature after cooling until the completion of a first part stage of the carbothermal reduction noticeable by the abatement of carbon monoxide production; and e) continued heating of the reaction mixture to form a single phase, substantially oxygen-free uranium-plutonium-mixed carbide at a temperature of from 1800 K to 1850 K, completion of a second part stage of the carbothermal reduction occuring therewith and subsequent cooling of the end product.

2. A process for the production of uranium-plutonium-mixed carbides, substantially as hereinbefore described, exemplified and illustrated in the accompanying drawing.