JPS6034078B2 - Fuel elements for nuclear reactors - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention provides a fuel material and/or a breeder fuel material (hereinafter abbreviated as fuel material) coated with pyrolytic carbon and/or silicon carbide to form particulate nuclei made from heavy metal oxides. The present invention relates to a fuel element (hereinafter referred to as a fuel element including a breeder fuel element) for a nuclear reactor, in particular for a high temperature nuclear reactor or a cooled fast breeder reactor.

In the case of fuel elements for nuclear reactors of this type, efforts are being made to load the fuel elements with high amounts of fuel material and to obtain a high degree of special capacity.

In order to obtain high economy in fuel elements, a high capacity with high fuel loading and maximum combustion losses of fuel material is required. However, the high operating temperatures of approximately 12500 C entail that the loading of fuel material into the fuel element must not be as high as desired in view of the capacity to be produced. As the density of fuel material in the fuel element increases, the operating temperature in the fuel and with it the temperature gradient in the fuel element increases, resulting in an increase in the interaction between particulate oxides and pyrolytic carbon. This results in disintegration of the cladding and renders the fuel element unsuitable for further use in a nuclear reactor. There are two causes for the disintegration of coated particulates. That is, this disintegration occurs because the thickness of the coating layer of the fuel particles decreases in places during the use of the fuel element.
It is based on the phenomenon that the coating layer disintegrates as a result. This has the consequence that, in particular, gaseous fission products can no longer be retained.

This phenomenon is caused by the non-uniform removal of heat that occurs during combustion losses of particulate fuel material. This phenomenon is known as the amoeba effect and results in the coating layer thickness changing depending on the temperature gradient inside the particle. At that time, this defect that occurs in the particles is
The higher the temperature of the fuel particles, which increase depending on the loading and the capacity of the fuel element, the more quickly they form.
Furthermore, the gas pressure of the carbon oxides formed during the reaction of the carbon of the coated particles with the oxides of the fuel material increases significantly during use in a nuclear reactor, to the extent that it causes significant distortion of the coating layer.

This is because the gas pressure adds to the pressure of the gaseous fission products formed in the particle nucleus under the fission process.
The gas pressure share of carbon oxide can amount to up to about 50% of the total mechanical load of the coating layer of fuel material particles, as mentioned above. As mentioned above, the height of the CO gas pressure depends on the ratio of the amount of oxygen to the fuel material of the particle nucleus. As has been observed, the gas pressure of CO varies significantly depending on the oxygen/metal oxide ratio at temperatures suitable for nuclear reactor transfer.

As the oxygen/metal ratio increases due to fuel combustion losses, the CO gas pressure also increases - and increases rapidly. It follows that this gas pressure can be very easily increased to 300 atmospheres at the operating temperatures typical in nuclear reactors. Indeed, the formation of CO can be avoided by using fuel carbides instead of fuel oxides.

However, even when fuel carbide is used as the core of the coated fuel particles, a change in the thickness of the coating layer known as the amoeba effect occurs. The reason for this effect is the temperature-dependent varying solubility of carbon in the carbide phase when carbides are used. The object of the invention is to increase the performance of a fuel element produced using fuel oxides and containing cores coated with pyrolytic carbon and/or silicon carbide at temperatures above 1250 oC; In particular, the disadvantageous effects of the amoeba effect are avoided, and furthermore the disadvantageous effects of the gas pressure of the CO formed during the reaction between the oxygen of the oxide and the carbon of the coating layer are substantially ignored. The aim is to reduce it to the extent that it can be obtained.

The present invention provides for the formation of coated fuel particle cores consisting of a mixture of fuel oxides and fuel carbides in such a way that even during combustion losses the quantity ratio of the oxides to the carbides remains within a defined range. For example, it is based on the surprising finding that there is no amoeba effect and, moreover, there is no disadvantageous significant pressure increase of the CO formed during the use of the fuel element in a nuclear reactor.

Starting from this knowledge, the object of the present invention is to provide a fuel element of the type mentioned above, in which the mixing ratio of oxides and carbides of heavy metals in the fuel particles is such that a specified amount of The solution is to have the proportions vary only within a range.

Incidentally, the nuclear combustion loss increases as the effective fuel component content of the fuel element increases, and the carbide content increases with the progress of nuclear combustion. It has been found to be particularly advantageous if the proportion of fuel carbide in the mixture of oxide and carbide amounts to about 3 to 20%. For example, in the case of fuel elements with particles whose cores are a mixture of uranium oxide and uranium carbide, with a carbide content of 7% by weight, the disadvantageous effects of the amoeba effect can also be combusted by gas pressure. There is also no CO- or CO2- gas generated during losses. If the proportion of carbide is more than 20%, the desired effect will be reduced, and the occurrence of the amoeba effect can no longer be effectively avoided. The lower limit of 3% is the minimum proportion that can be produced in the method for producing nuclear fuel particles. Instead of a mixture of uranium oxide and uranium carbide, a mixture of uranium thorium monoxide and uranium-thorium carbide or a mixture of uranium-plutonium monoxide and uranium-plutonium carbide may also be used. It was accompanied by a purpose.

A further very advantageous formation of the fuel element according to the invention consists in that in the core of the coated particles, the carbide content in the interior of the particulate core is greater than that in its periphery. Fuel elements with the structure of coated fuel particles have the advantage that they do not cause undesirable effects when using fuel carbides, and that the increased vapor pressure of the heavy metals inside the particles causes the fuel substance to precipitate on the outside of the particles. have. A convenient method of manufacturing a fuel element according to the present invention is to mix a sinterable fuel oxide powder with a caking material such as aluminum stearate or the like, strain the mixture, and then strain the mixture. The powder particles are finely divided into very small powder particles, and then the caked powder particles are subjected to jaw granulation using a drum-type jaw granulation device to form fuel particles with spherical surfaces. It is classified by means of a sieve, after which the caking material is removed and then sintered, and then it is coated and introduced as a loose mass or as a composite into a coating made of metal or graphite or from graphite. There are things that can be embedded in the matrix.

In this case, powder particles which have been caked to form a carbide are mixed with graphite powder in an amount such that the carbon content corresponds to a predetermined carbide component, and the fuel particles thus produced are sinter.

If the fuel particles are to be formed in such a way that the carbide content inside the particulate core is greater than that in its periphery, in the case of the method described above, the caking powder particles are combined with carbon to form the carbide. mixing with graphite powder in an amount such that the content corresponds to a predefined carbide proportion and then a predefined average diameter of the fuel particles formed during jaw granulation; As soon as the
The semi-occupied powder particles containing fuel oxide powder are loaded into a drum jaw granulator, and the fuel particles so formed are sintered in an argon-hydrogen atmosphere to produce carbide. It has turned out to be more advantageous to react the finely divided fuel oxide, which has been consolidated and then sintered, with the graphite contained in the particle cores. In this case, if the average kernel diameter is to be looOAm, the average diameter when adding powder particles still containing only fuel oxide powder during jaw granulation is about 500-630 m. is preferable. Another variant for producing a fuel element according to the invention in which the carbide content inside the grain core is higher than that in its periphery is to first hydrate the fuel oxide used and to The process of colloidizing an object, gelling the sol formed at the time into spherical fine particles, and further washing the gel particles formed at that time many times, and then condensing into tight particles, and then competitive The method consists of loosening the aggregated microparticles and embedding them in a coating made of metal or graphite or in a matrix made of graphite, either in the inverted state or as a bond.

In this case, the sol formed by colloidization is mixed with carbon in an amount corresponding to the carbon content of the predefined carbide component of the fine particles, and the washed gel particles are sintered in a hydrogen-containing atmosphere and then The sintered particulate fuel oxide is reacted with carbon contained in the particulate. Example 1 0.5% of U02 (N03)2 1 molar solution, 1.3% of HN03 1 molar solution and 0.2% of (NH2)2
COO. The 5 molar solutions are combined to form an aqueous solution.

Granulated mixture of pile 203 and 5 wt% Pt (50-1
After adding the catalyst consisting of 0.001 m particle size), the solution is reduced with the introduced gas. This reduction reaction ends after about 2 hours. Thereafter, the catalyst is removed from the furnace. 13.8 g of pure formic acid and an equal amount of a mixture of an aqueous solution of 9 molar NH3 and an aqueous solution with a N2 ratio of 1.5 molar are sequentially added to the reduced solution. The addition of the solution mixture consisting of NH3 and N24 is terminated as soon as the pH value of the reduced solution increases to approximately 7-7.5. The precipitate of U02 hydrate formed in this case is separated from the solution by suction filtration and washed with distilled water for approximately 10 minutes until the ions can no longer be detected. The U02-hydrate is dissolved in 0.5% distilled water and colloidized by adding 0.1% of a 1 molar HN03 aqueous solution under a nitrogen gas atmosphere with constant stirring. The resulting black U0
2-sol in a container maintaining an argon gas atmosphere pressure of 20-30 Tom at the start.
Concentrate under Nawa Azusa until the concentration is 2 molar. 4. In the U02-sol prepared in this way. female carbon,
Mix the mixture until it becomes uniform. This carbon-containing sol is
0.5 to 1 secretion at log charge under 10q0
Mix with 90 molar hexamethylene-tetramine solution
Add dropwise into a gelling column containing paraffin-oil with a temperature of ~95 qo. Particles are formed from the sol by the gelation that takes place.

The particles are then freed of adhering paraffinic oil in a well known washing step with petroleum ether. The gel particles are then added to a concentrated aqueous NH3 solution and left for about 2 hours. Then, by suction furnace filtration, N
Separate the H3-aqueous solution. The still moist particles were heated stepwise from 1200 to 50 qC per hour in a tube furnace under an atmosphere consisting of argon containing 4% hydrogen by volume.
Sintered at elevated temperatures up to 140,000 to form a tight U0
2- Nucleate.

By including hydrogen in the furnace atmosphere, the uranium oxide is stoichiometrically reduced to UC2, thereby obtaining a carbon-free outer spherical region of this core.

This treatment was followed by 10 at a temperature of 1800-1950q0
- Heat in a tantalum crucible for 1 to 4 hours under reduced pressure of 2 Ton. The grain cores produced according to this example have an average uranium carbide content of 15% by weight. Example 2 A mixture consisting of 0.1 molar U02(N03)2 solution and 0.4 molar Th(N03)4 solution is added to the 1 molar NH3 solution while stirring.

The (U,TH)02 hydrate produced by this common precipitation process is passed through a suction furnace and washed with distilled water for about 10 minutes to prevent it from containing ions. This is followed by colloidalization by mixing 0.7 in distilled water and adding 0.1 to 0.2 in a 1 molar aqueous solution of HN03. The dark red (U,Th)02-sol formed in this way is concentrated to a concentration of 2 molar in a vessel at 9000 °C. In the (U,Th)02-sol prepared in this way, 0
．． Mix 7 g of carbon until uniform.

The further treatment of this sol corresponds to the implementation steps listed in Example 1. This particle nucleus is U02-Tho2
- Consists of a mixture of mixed crystals and mixed carbides of these heavy metals. The carbide component is on average 3.5% by weight.
The present invention relates to a nuclear reactor fuel and/or breeder fuel element according to the claims, but also includes the following embodiments.

'1) In the fuel element according to claim 1,
The fuel carbide component in the oxide-carbide mixture is about 3 to
The above fuel element being 20%.

(1) The fuel element according to claim 1 and the above-mentioned item '1', wherein the mixture comprises uranium dioxide and uranium carbide.

(3) The fuel element according to claim 1 and item {1-- above, wherein the mixture comprises uranium thorium oxide and uranium thorium carbide.

{4) The fuel element according to claim 1 and item m above, wherein the mixture comprises uranium-plutonium oxide and uranium-plutonium carbide.

Claims (1)

[Claims] 1. For nuclear reactors, especially for high-temperature nuclear reactors, in which there is a fuel material in the form of particulate nuclei made of heavy metal oxides and coated with pyrolytic carbon, silicon carbide or a mixture thereof. In a fuel element for a cooled or cooled fast breeder reactor, a heavy metal oxide of one of the group consisting of uranium oxide, uranium/thorium oxide and/or plutonium oxide, together with the respective heavy metal carbide, is added to said fuel. Fuel element for a nuclear reactor as described above, characterized in that, depending on the nuclear combustion losses of the element, the proportion of heavy metal carbides is present as a mixture varying within a range of about 3 to 20%. 2. A nuclear reactor fuel element according to claim 1, characterized in that the carbide content inside the grain core is greater than the carbide content in the surrounding area.