DE1233507B - Mixed uranium / plutonium carbides nuclear reactor fuel - Google Patents

Description

GERMAN / # R PATENTAMT

EDITORIAL

German: 21g-21/20

Number: 1233 507

File number: U 11221 Vin c / 21 g

1 233 507 filing date: November 24, 1964

Opened on: February 2, 1967

The invention relates to mixed uranium / plutonium carbide nuclear reactor fuels, the especially for gas-cooled reactors operating at high temperatures, such as. B. for the under the Name Dragon nuclear reactor known experimental reactor, is intended.

The target coolant outlet temperature for the Dragon reactor is approximately 750 ° C, with the Fuel temperature can be 1000 ° C, and it is expected that it does not exceed 1500 ° C. It is known that metallic uranium is not used as fuel at this temperature can, so that it has been proposed to use ceramic fuels. As ceramic Fuel materials have heretofore been preferred to uranium monocarbides or uranium dicarbides.

In the Dragon nuclear reactor, the fuel consists of small integral cores with a diameter of around 250 microns, which are finely distributed in a graphite form and form what is known as a compact ao fuel. A uranium monocarbide fuel has been found to react with the graphite of the mold; it is therefore necessary to coat the uranium monocarbide with a carbide coating, e.g. B. silicon carbide to be provided to reduce this reaction. It has also been found that at a temperature of about 1600 ° C, uranium dicarbide, which is currently the preferred fuel, is in equilibrium with uranium sesquicarbide and graphite, with uranium dicarbide being stable above 1600 ° C. In a reactor of the aforementioned type, the temperature in the interior of a fuel core is in the vicinity of 1600 ° C., so that the reaction phase in the fuel core is different from that in its outer regions. Under these circumstances, a change between the dicarbide reaction phase and the mixed sesquicarbide-graphite reaction phase can occur. These two effects can create tensions that are sufficient to render the fuel core inoperable.

The invention is therefore based on the object of providing a fuel material based on uranium / plutonium carbide mixtures to create that such phase changes cannot be expected under normal operating temperatures.

It has already been proposed to use a nuclear fuel material of, for example, mixed To produce uranium / plutonium carbide through an intermediate stage including the silicide. A However, such fuel excludes the addition of silicon carbide to the uranium / Plutonium carbides.

Mixed uranium nuclear reactor fuel /
Plutonium carbides

Applicant:

United Kingdom Atomic Energy Authority,
London

Representative:

Dipl.-Ing. E. Schubert, patent attorney,
Siegen, Eiserner Str. 227

Named as inventor:

John Townsend Dalton, London

Claimed priority:

Great Britain November 25, 1963 (46 564)

The object underlying the invention is achieved in that the fuel is mixed Uranium / plutonium sesquicarbides is composed. The fuel can be 8 to 40, preferably 20 up to 22 mole percent plutonium sesquicarbide and oxygen as an impurity to a negligible extent Amount included.

If you replace part of the uranium with plutonium in a uranium-carbon system, then so shows that this stabilizes the sesquicarbide phase at high temperatures, namely occurs even in the presence of free carbon. When the uranium / plutonium atomic ratio 95: 5, the temperature rises at which sesquicarbide and carbon react to form a dicarbide, to about 1670 ° C, while with a uranium / plutonium atomic ratio of 90:10 the sesquicarbide remains stable until around 1720 ° C.

It can be assumed that, in the absence of free carbon, the sesquicarbides are at even higher levels Temperatures than those given here are stable. Since the coating of the fuel core is usually a Has thin inner layer of carbon, therefore, free carbon will be present and with the Sesquicarbide in contact, which will act on the stability of the sesquicarbide phase.

The sesquicarbide is the stable phase at low temperatures, while even at a little higher Temperatures the stable material made from a mixture of sesquicarbides in equilibrium, Composed of dicarbides and carbon; at still

709 507/309

Claims (3)

higher temperatures, around 2000 ° C, however, the dicarbides with an excess of unreacted carbon represent the stable material. A uranium / plutonium sesquicarbide, which contains about 8 to 40 mol percent plutonium in the presence of free carbon, is up to one Stable temperature of around 1700 ° C, which increases the safety margin considerably compared to a uranium carbide fuel. Such a material is therefore particularly suitable as a reactor fuel. In order to better understand the subject matter of the present invention, reference is made to the drawing which illustrates the stability of mixed uranium / plutonium sesquicarbides with various proportions of plutonium. The drawing shows the mole percent of plutonium (as plutonium dicarbide) in a uranium / plutonium dicarbide alloy as a function of temperature. The curve represents the boundary between a stable sesquicarbide phase in the presence of carbon and a mixture of sesquicarbides, dicarbides and carbon. The curve rises sharply at its origin, then becomes flatter and slowly falls again at the end. The curve runs at a temperature of about 1700 ° C with a plutonium content of 8 to 40 mol percent. The area below the curve represents the equilibrium state of the sesquicarbide phase with carbon, while the area above the curve represents a state in which there is a mixture of carbide phases. The maximum of the curve is at a temperature of approximately 1750 ° C, where the plutonium content is 20 to 22 mole percent. The mixed sesquicarbides can be prepared by any of the known methods; so z. B. by arc melting the metals in the amounts corresponding to the carbon fractions or by so-called immersion sintering, in the manner as described in British patent specification 914 153, or by reacting the oxides and the carbon in the respective fractions with one another. When producing the sesquicarbides by means of one of the above-mentioned processes, however, care must be taken to ensure that the oxygen content in this product is kept low, since it is known that the oxygen in a uranium dicarbide works in this dicarbide, since the oxygen is at room temperature, i. H. at about 20 ° C, is stable. Similar phenomena could occur with the mixed uranium / plutonium carbides. In the manufacture of the sesquicarbides by means of the arc melting process, a mixture of dicarbides and monocarbides can separate out when the melt cools, although carbon is added in the correct proportions in order to obtain the sesquicarbides. In such a case, the sesquicarbides can be obtained if the mixture of dicarbides and monocarbides is kept in equilibrium at a temperature point which is immediately below the curve and which corresponds to the amount of plutonium used in each case. In the example given below, the sesquicarbide was obtained from arc-melted dicarbides and therefore contains free carbon, although the method described can be modified and used to make sesquicarbides from a mixture of dicarbides and monocarbides. Example 12.75 g of uranium and 2.25 g of plutonium in the form of a cube with a side length of 3 mm were placed in the water-cooled copper melting tank of an arc melting furnace. 1.50 g of graphite, also in the form of a cube with a side length of 3 mm, were also placed in the melting tank. This mixture was then melted and caused to react by igniting an electric arc between the mixture and a graphite electrode in an argon atmosphere. After one minute, the arc was switched off and the mixture was cooled. The button-like material made by arc melting was then turned over and melted again. This turning over and remelting was repeated six times in order to have the guarantee that a homogeneous uranium-plutonium-carbon alloy is produced. A subsequent test showed that the product obtained by arc melting and reaction was a mixed uranium / plutonium dicarbide. This mixed dicarbide was then held in a vacuum oven at a temperature of 1700 ° C for 6 hours at a pressure of 10 ~ s mmHg. After cooling the material, its metallurgical examination showed that it consisted of dispersed graphite in a solid solution of uranium and plutonium sesquicarbides. This manufacturing process also shows that the sesquicarbides are stable at around 1700 ° C, even in the presence of graphite. This property of the mixed sesquicarbides is likely to be in contrast to the properties of a uranium-carbon system, in which the dicarbides achieve a stable form above 1600 ° C in the presence of sufficient carbon. Patent claims: 1. Nuclear reactor fuel made from mixed uranium / plutonium carbides, characterized in that that it is composed of mixed uranium / plutonium sesquicarbides. 2. Fuel according to claim 1, characterized in that it contains 8 to 40 mole percent plutonium sesquicarbide contains. 3. Fuel according to claim 1 and 2, characterized in that it contains oxygen as an impurity Contains negligible amounts. Considered publications:
German interpretative document No. 1100195. 1 sheet of drawings 709 507/309 1.67 © Bundesdruckerei Berlin