DE2049566C3 - Process for the production of a nuclear fuel from uranium-plutonium-monocarbide or uranium-plutonium-mononitride - Google Patents

Description

The invention relates to a method for producing a nuclear fuel from uranium plutonium monocarbide or uranium-plutonium-mononitride by cold-pressing a containing uranium and plutonium compounds Starting powder with carbon to form moldings and reaction sintering of these moldings.

Today, most nuclear reactors use fuel oxides. Despite good corrosion resistance and high temperature resistance (melting point 285O ⁰ C), this fuel is not particularly suitable for certain reactor designs. This is especially true in all cases in which a high power density in the fuel rod is necessary, such as. B. with the "fast breeders" or with "fast high-flow test reactors". Here the poor thermal conductivity of uranium oxide makes itself unpleasantly noticeable. When searching for suitable fuels in particular the Brennstoffmonokarbid or the Brennstoffmononitrid with its high thermal conductivity and its relatively high melting point of 2400 ⁰ C or 280O ⁰ C seems appropriate. The high susceptibility to corrosion, e.g. B. against water, but excludes the carbide as a fuel for water or steam cooling. In addition, these properties make the production of the carbide fuel more difficult and expensive, since it is largely necessary to work in box systems with an extremely clean atmosphere. Since the sodium-cooled fast breeders do not focus on pure uranium carbide or uranium nitride, but rather a mixed carbide or mixed nitride of uranium and plutonium, the fuel costs are reduced because of its radiation hazard and toxicity these additional precautions made even more expensive.

It is for these reasons that fuel monocarbide has been found, albeit for many years known and already produced for special purposes in quantities of many tons, so far not yet widespread use.

The production of uranium carbide-plutonium carbide sintered bodies is known (magazine "atomwirtschaft", February 1969, pp. 79 and 80; US-PS 32 36 922). With these Process will either use a mixture of uranium oxide and plutonia powder or a mixture of Uranium metal powder and plutonium carbide powder mixed with graphite powder, optionally to form shaped bodies pressed and at temperatures above 1000 ° C converted to a uranium-plutonium mixed carbide. However, these methods have the disadvantage that they are very must be carried out in a pure atmosphere and due to the use of plutonium at the beginning of the Process are very cumbersome and expensive because of the special security conditions.

It was therefore the object of the present invention to provide a process for the production of uranium-plutonium monocarbide or uranium-plutonium-mononitride to be found, with the process time as late as possible work must first be carried out under plutonium safety conditions.

This object was achieved by cold pressing a starting powder containing uranium and plutonium compounds with carbon to form shaped bodies and reaction sintering of these shaped bodies, according to the invention pure uranium carbide powder or uranium nitride powder and plutonium oxide are used as the starting powder will.

According to the present invention, a pure uranium carbide or uranium nitride powder is first produced and only this is mixed with plutonium oxide and graphite powder and processed further. This allows the Manufacturing costs of uranium-plutonium-carbide or uranium-plutonium-nitride fuel are significantly reduced because only those process steps that contain the component plutonium are included in the End product which increases the cost of safety conditions in glove boxes with negative pressure have to. The uranium carbide or uranium nitride synthesis can even outside of any box systems and the production of the sinterable powder therefrom under a normal protective gas atmosphere, d. H. in one essential cheaper overpressure box system. Plutonium-specific protective measures can completely omitted for this procedural step.

Only the sinterable uranium carbide or uranium nitride powder is now introduced into the plutonium line, with Plutonia and carbon mixed, pressed and sintered. For fuel pellets with densities between 75 and 85% of the theoretical density can be plutonium in the form of a mixture of plutonium oxide and carbon are introduced for fuel pellets with densities between 85 and 95% of the theoretical density is a mixture prereacted at temperatures between 1300 and 1400 ° C

Plutonia and carbon used. During the actual sintering process, the Synthesis of plutonium carbide or plutonium nitride takes place. By mixing the components and appropriate Sintering results in a homogeneous distribution of the plutonium carbide or plutonium nitride in the uranium carbide or uranium nitride or sufficient mixed crystal formation ensures another, the manufacturing costs The lowering advantage of this invention is that the evaporation losses of plutonium are lower than in the normal production of uranium-plutonium-carbide or uranium-plutonium-nitride fuel tablets, where the synthesis and sintering in two different manufacturing steps at each 1600 to 1650 ° C can be carried out. '5

To achieve a further reduction in manufacturing costs, the uranium carbide powder can be granulated or ungranulated be coated with plasticizers. The carbide powder treated in this way can then be used without Protective gas - as described - can be processed into green compacts.

Due to the high susceptibility of the uranium-plutonium-carbide mixed crystal to corrosion it is very difficult to fill the carbide fuel into cladding tubes and thereby the To keep the content of adsorbed gases and oxygen on the surface of the fuel low. this applies especially when - as with the Sch elles breeders - there is only a low fuel density between 80 and 90% of the theoretical maximak-n density is permissible and thus a fuel of high specific surface in the form of porous sintered bodies. Attempts by Additions of z. B. nitrogen to produce a uranium carbonitride or uranium plutonium carbonitride, which is more stable against corrosion attack. This resulted in nitrogen-carbon ratios of approx. 1: 4 and higher also to an extensive success, however, this amount of nitrogen is neutron-physically for the in The question of upcoming reactor concepts may be too high.

The present invention now enables the stabilizing influence of Fremdzusä'zen such. B. Nitrogen can be obtained even with such small amounts that it is still tolerable from a neutron point of view are. This is achieved by using the foreign additives as a kind of shell around the uranium-plutonium-carbide grains and thereby largely shield the carbide from the outside.

Particularly suitable additives are nitrogen, sulfur or phosphorus. These substances form as so Fuel nitride, sulfide or phosphide with the monocarbide complete mixed crystals, whereby it it is also possible to use a single-phase nuclear fuel in the form of a modified monocarbide produce that promises particularly favorable reactor behavior. The task of this cladding layer made of z. B. Fuel nitride is not, as is known from the "coated particles", the prevention of the Cracked gas outlet.

The fuel encased according to the invention has many- * ° more the advantage that with very low nitrogen, sulfur or phosphorus contents both a stabilizing Effect on the water and oxygen content of the atmosphere as well as the single-phase nature of the fuel is reached. The content of additional elements can be less than 1% by weight, based on the plutonium carbide content, the thickness of the protective layer is between 2 and 20 µm. The RiIrI shows the exemplary structure of a plutonium carbide core treated in this way.

example 1

Uranium carbide powder with a surface area of 1.0 to 2.0 m ² / g is homogenized in a mixer with a mixture of plutonia and carbon (20% by weight). Because of the carbon content, the fuel tablets can then be pressed without lubricant at pressing pressures between 2 and 5 t / cm ² to form tablets between 50 and 80% of the theoretical density. The sintering of these compacts is carried out at temperatures of about 1600 ^c C under vacuum or argon

Example 2

Urankarbidpulver having a surface area of from 0.6 to I, 2m ² / g is a pre-reacted and at 1300-1400 ⁰ C then ground to a surface area of 0.6 to Um-Vg mixture of plutonium oxide and carbon (15 wt .-% ) homogenized in a mixer. The fuel tablets are compressed at compression pressures between 2 and 5 t / cm to form pellets with densities between 50 and 80% of the theoretical density. These compacts are sintered at temperatures between 1600 and 1700 ° C in a vacuum or argon.

Example 3

Uranium carbide powder with a surface area of 1.0 to 2.0 m ² / g is agglomerated to form secondary particles with diameters between 100 and ΙΟΟΟΟμΓη. This agglomerate is homogenized in a mixer with a mixture of plutonia and carbon. The fuel tablets are pressed without lubricant at pressures between 1 and 5 t / cm ² to form green compacts with densities between 50 and 80% of the theoretical density. Sintering takes place at temperatures between 1600 and 1700 ° C under vacuum or argon.

Example 4

Uranium carbide powder with a surface area of 1.0 to 2.0 m ² / g is homogenized with a mixture of plutonia and carbon (20% by weight) in a mixing egg. The mixture of plutonium oxide and carbon is adjusted in such a way that a substoichiometric product with a metal content of approx. 5% by weight is formed. The mixture of uranium carbide powder. Plutonium oxide and carbon are then pressed at pressures between 1 and 5 t / cm ² to form green compacts with densities between 50 and 80% of the theoretical density. Sintering takes place at temperatures between 1600 and 1700 ° C under vacuum. Towards the end of the sintering, the resulting metal portion, which is preferably enriched at the grain boundaries of the plutonium carbide, is nitrided ^{in a nitrogen stream between 700 and 1800 ° C.}

Example 5

Uranium carbide powder with a surface area of 1.0 to 2.0 m ² / g is produced by grinding in a vibrating ball mill with decalin as the grinding liquid. Stearic acid is already added in the mill in amounts of 0.2 to 1.0% by weight of the dry grist. The grinding liquid is removed by vacuum distillation drying. The powder produced in this way can be handled in air. This powder is combined with a mixture of plutonium oxide and carbon in one

Mixer homogenized. The fuel tablets are pressed at pressures between 1 and 5 t / cm ² to form green compacts with densities between 50 and 80% of the theoretical density. The stearic acid is driven off at temperatures between 400 and 800 ^° C., the sintering at temperatures between 1600 and 1700 ^° C. under vacuum or argon.

Example 6

Granulated uranium carbide powder with a surface area of 1.0 to 2.0 m ² / g, produced by dry grinding in an attritor mill, is inactivated in a rotary evaporator with the aid of stearic acid. Here, decalin with an addition of 5 to 10% by weight of stearic acid is injected into the evacuated and rotating vessel containing the uranium carbide powder. While the decalin evaporates and is immediately removed from the vessel, the stearic acid covers the uranium carbide granules with a protective layer. The powder prepared in this way is homogenized in air with a mixture of plutonium oxide and carbon in a mixer. The fuel tablets are pressed at pressures between 1 and 5 t / cm ² to form green compacts with densities between 50 and 80% of the theoretical density. The stearic acid is driven out at temperatures between 300 and 800 ^° C., the sintering at temperatures between 1600 and 1700 ^° C. under vacuum or argon.

Example 7

Uranium nitride powder with a surface area of 1.0 to 2.0 m ² / g is homogenized in a mixer with a mixture of plutonia and carbon (20% by weight). The ratio of plutonia and

Carbon is adjusted in such a way that the reaction under nitrogen would produce plutonium nitride. The fuel tablets are pressed at pressures between 2 and 5 t / cm ² to form compacts with densities between 50 and 80% of the theoretical density. The sintering of these compacts carried out at temperatures from 1600 to 1700 ⁰ C under nitrogen. The cooling takes place below 1450 C under vacuum or argon.

Example 8

Uranium nitride powder with a surface area of 1.0 to 2.0 m ² / g is produced by grinding in an attritor mill with decalin as a grinding liquid. Stearic acid is added in the mill in amounts of 0.2 to 1.0% by weight of the dry grist. The grinding liquid is removed by vacuum distillation. The powder produced in this way can be handled in air. This powder is pre-reacted with an at 1300-1400 ⁰ C and then homogenized on a surface area of 0.6 to I, 2m ² / g of ground mixture of plutonium oxide and carbon (20 wt .-%) in a mixer. The mixture of plutonium oxide and carbon was adjusted so that plutonium nitride is formed during the reaction under nitrogen. The fuel tablets are compressed at compression pressures between 2 and 5 t / cm ² to form pellets with densities between 50 and 80% of the theoretical density. The sintering of these compacts carried out at temperatures from 1600 to 1700 ⁰ C under nitrogen. The cooling takes place at temperatures below 1450 ^° C. under vacuum or argon.

1 sheet of drawings

Claims (6)

Patent claims: 1. Process for the production of a nuclear fuel from uranium-plutonium-monocarbide or uranium-plutonium-mononitride by cold pressing a starting powder containing uranium and plutonium compounds with carbon to form shaped bodies and reaction sintering of these shaped bodies, thereby characterized in that the starting powder is pure uranium carbide powder or uranium nitride powder and plutonia is used. 2. The method according to claim 1, characterized in that the pure uranium carbide powder or Uranium nitride powder is inactivated with the aid of chemical substances' 5. 3. The method according to claim 1, characterized in that that first the pure uranium carbide powder or uranium nitride powder is produced and then with a mixture of pre-reacted plutonia and carbon is mixed. 4. The method according to claim 1, characterized in that first the pure uranium carbide powder or granulated uranium nitride powder is produced. 5. A method for producing a nuclear fuel ² S fes from uranium plutonium monocarbide according to claim I, characterized in that the initially produced pure uranium carbide powder mixed with a reacted, ground, sub-stoichiometric mixture of plutonium oxide and carbon, binder-free to fuel tablets pressed and the fuel metal formed during the reaction sintering is nitrided towards the end of the sintering. 6. The method according to claim 5, characterized in that the substoichiometric set Mixture of plutonia and carbon is prereacted.