GB2126773A

GB2126773A - Process for the production of nuclear reactor fuels

Info

Publication number: GB2126773A
Application number: GB08323643A
Authority: GB
Inventors: Karl Richter; Gerard N Kramer
Original assignee: European Atomic Energy Community Euratom
Current assignee: European Atomic Energy Community Euratom
Priority date: 1982-09-06
Filing date: 1983-09-02
Publication date: 1984-03-28
Also published as: FR2532778B1; DE3327921A1; IT1167643B; GB2126773B; LU84367A1; IT8348879A0; FR2532778A1; GB8323643D0

Abstract

A process for the production of nuclear reactor fuels of the MX type, wherein M represents an actinide metal and X represents carbon and/or nitrogen, which process comprises mixing at least one actinide oxide with carbon, pressing and carbo-thermally reducing the mixture, optionally under nitrogen, characterised in that the reduction product is directly subsequently compressed and then thermally treated at high temperature. The carbon may be omitted and the reduction effected by nitrogen alone.

Description

SPECIFICATION Process for the production of nuclear reactor fuels The invention relates to a process for the production of nuclear reactor fuels of the MX type, wherein M represents an actinide metal and X represents nitrogen and/or carbon.

For fast breeder reactors, great importance is generally attached to fuel of the MX type (e.g.

Uranium-plutonium carbide) because of their higher breeding rate and shorter doubling time in comparison with oxide fuel. Mixed carbon nitrides also come into consideration.

Technological production is principally carried out by "carbo-thermal" reduction -i.e. thermal reduction with carbon-of uranium oxideplutonium oxide-carbon mixtures. These oxidecarbon powder mixtures are pressed to form pellets, tablets, discs, etc. and under vacuum, inert gas or nitrogen, are heated for several hours at temperatures of about 1 ,4000C to 1 8000 C.

There are thus formed sintered bodies of the desired chemical composition with a density of 4060% of their theoretical density (TD).

However, for use in the reactor, pellet densities > 80% TD are required. Therefore, the sintered bofies with 4060% TD are comminuted and ground (i.e. by hammer mills, ball mills, vibrating mills, etc.). As a result of grinding, the surface area of the powder is enlarged and the reactivity of the powder is increased. Subsequently, these powders are pressed into the desired form and sintered. The pellet densities are then between 80 and 97% TD.

The' process stage of comminution and grinding results not only in a certain amount of equipment and time expenditure, but also in substantial dust formation inside the working space in contact with the product (i.e. glove-box apparatus). The high dust level inside the working space firstly leads to substantial radiation exposure of personnel, and secondly is associated with a safety risk, since fine powders of MX fuels are pyrophoric.

Furthermore, because the powder products has a high affinity for oxygen, this process stage has to be carried out under an inert-gas atmosphere with very low residaul 02 and H20 contents (generally < 50 ppm).

Most technologically-relevant processes have as a basis the principle of carbo-thermal reduction of oxides, and are based on the pressing-sintering technology (i.e. mixing of MO2+C, pressing; carbo-thermal reduction; crushing-grinding of the reaction product MX; pressing; and sintering).

We have found that the reaction product can be subsequently compressed directly, i.e. without previous comminution and grinding, and then treated thermally at high temperature, e.g.

1,400--1,8000C.

Like most MX-nuclearfuel production processes, this "direct-pressing process" is based on the carbo-thermal reduction of oxides or mixtures thereof. The oxide-carbon mixtures are pressed in a specific form adapted to the geometry of the end product and then treated thermally (at 1 ,300-1 ,8000C under vacuum, inert gas, nitrogen, or a nitrogen-inert gas mixture). After the thermal treatment, the reaction product is the desired chemical composition is in the form of pellets of predetermined size and with a material density of 4060% TD. The individual pellets are then charged directly into a normal cold press and subsequently compressed therein, i.e.

in contrast to the conventional processes without previous comminution and grinding.

Subsequently, the pressings are brought to the desired density and size by subsequent sintering at 1 ,400-1 ,8000C under vacuum, nitrogen, or an inert gas-nitrogen mixture.

As a result of this process, the "comminutiongrinding" process stage which was used in existing processes, is dispensed with. The accompanying drawing shows schematically the present "New Process" as compared with the "Hitherto Conventional Process".

Accordingly, the following advantages are achieved in relation to the existing production processes: -lower expenditure in respect of equipment and time; -the handling of MX-type powder with its great affinity for oxygen is eliminated; -dust levels are reduced practically to zero.

The two proceding points result in a lower expenditure on the protective gas atmosphere necessary for handling the material, since the affinity for oxygen of MX fuels is dependent on its specific surface area. The danger of spontaneous ignition by powder or dust is obviated, radiation exposure of personnel is considerably reduced, and fissile material-flow control is facilitated.

Furthermore, the dies of the presses can be charged with pressings instead of powders. This leads to: -simple and rapid charging of the dies, -simple and readily-controllable material flow; -short (i.e. shallow) and cheaper dies as a result of the low compaction ratio; -no metallic impurities which could arise in the hitherto conventional comminution and grinding processes; -considerable variation possibilities with respect to fuel structure (e.g. pore size, subsequent-sintering behaviour) by varying the corresponding parameters of the starting materials.

The new process has already been tested practically. For example, uranium-plutonium carbide fuels with different fuel densities and residual oxygen contents have been produced and characterised.

Pellet density:78-86% TD. There are even samples with 90% TD; Oxygen content: > 100 ppm and about 2000 ppm; There are ceramograpic slides of the fuels.

Although carbo-thermal reduction usually requires the use of carbon, in certain situations the carbon can be omitted, and nitrogen can be used alone as the reducing species, to give MX wherein X is nitrogen alone.

Claims

1. A process for the production of nuclear reactor fuels of the MX type, wherein M represents an actinide metal and X represents carbon and/or nitrogen, which process comprises mixing at least one actinide oxide with carbon, pressing and carbo-thermally reducing the mixture, optionally under nitrogen, characterised in that the reduction product is directly subsequently compressed and then thermally treated at high temperature.

2. A process as claimed in Claim 1 wherein the thermal treatment takes place at a temperature of from 1,400 to 1 ,80O0C.

3. A process as claimed in claim 1, substantially as hereinbefore described with reference to the accompanying drawing.

4. Nuclear reactor fuel in the form of pellets, tablets and discs when produced by a process as claimed in any one of the preceeding claims.