WO1995008827A1

WO1995008827A1 - Nuclear fuel sintered body and process for producing it

Info

Publication number: WO1995008827A1
Application number: PCT/EP1994/003060
Authority: WO
Inventors: Gerhard Gradel; Alfons Roppelt; Martin Peehs; Harald Cura; Klaus Koebke; Erhard Ortlieb; Richard A. Perkins
Original assignee: Siemens Aktiengesellschaft
Priority date: 1993-09-22
Filing date: 1994-09-13
Publication date: 1995-03-30
Also published as: TW257869B; EP0720766A1; JPH09503858A; KR960705324A

Abstract

A nuclear fuel interbody of UO2 or one of the mixed oxides (U, Pu)O2, (U, Th)O2, (U, RE)O2, (U, Pu, RE)O2, (U, Th, RE)O2 and (U, Pu, Th, RE)O2 (RE = rare earth) has particles of boron and/or a chemical boron compound in the sintered matrix. To retain the boron, these particles are coated with a surface film consisting of a different substance from the sinter matrix and the particles. A nuclear reactor fuel element has a fuel rod containing such a nuclear fuel sintered body in a jacket rod.

Description

description

NUCLEAR FUEL INTERBODE AND METHOD FOR THE PRODUCTION THEREOF

The invention relates to a nuclear fuel sintered body according to claim 1, a nuclear reactor fuel element according to claim 13 and a method for producing a nuclear fuel sintered body according to claim 14.

EP-A1-0 239 843 discloses a nuclear fuel sintered body made of Uθ2 (U, Pu) θ2 and (U, Th) θ2. In the sintered matrix of this nuclear fuel sintered body, boron is a neutron poison in the chemical compound UB _X with x = 2; 4 and / or 12 and / or B4C installed. This known nuclear fuel sintered body is obtained by producing a mixture of uranium oxide powder or uranium mixed oxide powder with uranium boride or boron carbide powder and pressing it into compacts, which are then sintered in a sintering furnace in a reducing sintering atmosphere to form nuclear fuel sintered bodies. In these nuclear fuel sintered bodies, the boron is thus distributed uniformly everywhere in the sintered matrix.

Boron in uranium-containing nuclear fuel sintered bodies is a neutron absorber that can be burned off in terms of physical physics and, after a certain period of use, these nuclear fuel sintered bodies lose their property as an absorber for thermal neutrons in a nuclear reactor.

Nuclear reactor fuel elements with fuel rods which contain uranium-containing nuclear fuel sintered bodies are used in the nuclear reactor, for example, during four successive, generally equally long fuel element cycles. At the end of a fuel element cycle, some of the nuclear reactor fuel elements in the nuclear reactor are replaced by fresh, unirradiated nuclear reactor fuel elements. The fresh, unirradiated nuclear reactor fuel elements would bring about a relatively high reactivity in the nuclear reactor compared to the already irradiated nuclear reactor fuel elements. However, the boron in the nuclear fuel sintered bodies of these fresh, unirradiated nuclear reactor fuel elements initially dampens the reactivity brought about by these nuclear reactor fuel elements by initially absorbing thermal neutrons.

The nuclear fuel of fresh and unirradiated nuclear reactor fuel elements gradually burns off in the nuclear reactor by nuclear fission, but at the same time a combustible neutron absorber present in this nuclear fuel gradually burns off neutron physically, so that this neutron absorber finally has little or no thermal energy Neutrons absorb. That is why even freshly inserted unirradiated nuclear reactor fuel elements in the nuclear reactor can have about the same reactivity in the nuclear reactor during their entire service life in the nuclear reactor as the nuclear reactor fuel elements that have already undergone a fuel element cycle in the nuclear reactor.

Boron as a neutron absorber in the nuclear fuel is compared to other combustible neutron absorbers such as rare earths

Advantage if the fuel element cycles are relatively long, e.g. are longer than 12 months, since boron prevents heat build-up in the nuclear fuel.

The invention is based on the object of developing the known nuclear fuel sintered body so that there is no increase in reactivity which is too rapid and too high when starting up a nuclear reactor if this nuclear fuel sintered body is freshly introduced into this nuclear reactor in the unirradiated state .

Since in the nuclear fuel sintered body according to the invention the particles of boron or a chemical boron compound with a Are provided surface film that holds back boron, this boron cannot escape from the nuclear fuel sintered body according to the invention. This ensures an increase in reactivity which is damped with regard to its speed and height. The surface film advantageously contains no boron at all.

Claims 2 to 12 are directed to advantageous further developments of the nuclear fuel sintered body according to claim 1. In particular, the further development according to claim 9 ensures good retention of boron, while the further development according to claim 10 ensures a largely uniform distribution of the particles of boron or chemical boron compound in the sintered matrix of the nuclear fuel sintered body enables. Claim 13 relates to a nuclear reactor fuel element with a fuel rod which contains such a nuclear fuel sintered body in a cladding tube.

The method according to claim 14 allows a relatively simple production of a nuclear fuel sintered body which contains particles of boron and / or a chemical boron compound provided with the surface film. Claims 14 and 15 are directed to advantageous developments of this method.

The invention and its advantages are explained in more detail with reference to exemplary embodiments:

UO2 powder is used as the powdered starting oxide and is obtained, for example, in accordance with the ammonium uranyl carbonate or AUC process (for example, Gemelin Handbook of Inorganic Chemistry, Uranium, Supplement Volume A3, 1981, pplOl to 104). This Uθ2 "powder has an average particle diameter of 15 to 20 .mu.m. The Uθ2 ~ powder can also by another method, such as the ammonium diuranate (ADU) process (e.g. Gmelin Handbook of Inorganic Chemistry, Uranium, Supplement A3, 1981 , pp 99 to 101), the IDR process Ren or the DC method (eg Gmelin handbook of inorganic chemistry, uranium, supplement volume A3, (1981), pp 104 to 115 "dry chemical processes"). The powdery starting oxide can also be the mixed oxides (U, Pu) 0 ₂ , (U, Th) 0 ₂ , (U, RE) 0 ₂ , (U, Pu, RE) 0 ₂ , (U , Th, RE) 0 ₂ or (U, Pu, Th, RE) θ2 with RE = rare earth (in particular at least one of the elements gadolinium, europium and samarium).

300 ppm of crystalline boron powder, the average particle diameter of which is 20 to 25 μm, is added to the Uθ2 ~ powder. The particles of this crystalline boron powder have a surface film made of metallic molybdenum, the film thickness of which is approximately 5 μm. This surface film is in a sputtering device (e.g. E. Lang, "Coatings For High Temperature

Applications ", 1983, Applied Science Publishers, London and New York, pp.79-120). In the present case, the sputtering device has an electrically conductive anode body, for example made of aluminum, on which crystalline boron powder was located. Furthermore, a cathode body is made of The crystalline starting boron powder with a mean starting particle size of about 15 μm is treated between anode and cathode body in a argon atmosphere at a direct electrical voltage of about 2000 V during a sputtering time of about 2. The crystalline boron powder rolls again and again during this sputtering on an inclined plane from the anode body, so that a uniformly thick, firmly adhering and all-round dense surface film of molybdenum is achieved on the boron powder particles.

The surface film can also consist of at least one of the metals, ruthenium, tungsten and chromium or of at least one of the alloys molybdenum-based alloy, ruthenium-based alloy, tungsten-based alloy and chromium-based alloy. It is favorable if this surface film from a

Chromium-nickel alloy or a molybdenum-niobium alloy exists. The metals of these two alloys all have one relatively small capture cross section for thermal neutrons. The metals rhenium, rhodium and hafnium and the base alloys of these metals are also suitable for the surface film, since these metals have a large initial cross section for thermal neutrons, but do not burn out neutron physically as quickly as boron Therefore, in conjunction with boron, particularly long fuel element cycles are possible.

The surface film can also consist of at least one of the non-metals tantalum carbide, niobium carbide, titanium carbide, tantalum carbide, niobium carbide, titanium carbide, zirconium carbide, chro carbide, vanadium carbide, tungsten carbide, molybdenum carbide, tantalum nitride, niobium nitride, titanium nitride, zirconium nitride, zirconium nitride Vanadium silicide, tungsten silicide, molybdenum silicide, zirconium silicide, magnesium oxide, beryllium oxide, chromium oxide, calcium oxide, cerium oxide and zirconium oxide exist. In a particularly advantageous manner, the surface film consists of at least one of the substances silicon carbide and silicon nitride, preferably Si3N4. The capture cross section for thermal neutrons of these non-metals is particularly small. These non-metals are advantageously applied in the form of a surface film to the boron powder particles by precipitating the reaction product of a chemical gas phase reaction of chemical compounds which contain the components of the surface film (for example chemical vapor deposition (CVD) processes according to E. Lang , "Coating For High Temperature Applications", 1983, Applied Science Publishers, London and New York, pp.33 to 78). In this way, however, a metallic surface film can also be applied to the boron powder particles.

0.2% by weight of powdered zinc stearate is also added to the UO2 powder with the added crystalline boron powder as a pressing aid. Instead of zinc stearate, powdered aluminum distearate can also be used as a pressing aid. The UO2 powder with the added boron powder and the added pressing aid is then mixed intimately in a tumbling mixer for 15 minutes. Also, about 5% powdered U3O3 can be added to the powder in the tumble mixer as a pore former. The fact that particles of the crystalline boron powder coated with molybdenum have approximately the same density as the particles of the UO 2 powder promotes the intimate mixing of these two powders with one another and prevents segregation during the subsequent transport and storage of the mixture.

Finally, compacts are pressed from the mixture obtained by intimate mixing, which are sintered in a hydrogen atmosphere at a sintering temperature of 1750 ° C. for three hours. After cooling, the nuclear fuel sintered bodies have a specific density of approximately 10.30 g / cm ³ to 10.55 g / cm ³ while the specific density of the boron particles in the sintered matrix is 7 to 9 g / cm ³ , that is to say in the favorable range of 5 g / cm ³ to 10 g / cm ³ .

In the case of metallic, silicidic or oxidic surface film on the particles of the boron powder, in addition to the H2 atmosphere, it is advantageously also carried out in inert gas, e.g. Argon, sintered, while in the case of a carbide surface film the sintering is advantageously carried out in CO and / or CO2 and in the case of nitride surface film in nitrogen and / or a gaseous ammonia compound, so that the surface film is decomposed by the sintering atmosphere is practically avoided. Since, moreover, the sintering temperature is lower than the melting point of the surface film made of the specified metals, metal alloys and non-metals on the boron particles, this surface film remains uniformly dense.

Boron analysis of the nuclear fuel sintered bodies after sintering shows a boron concentration of 295 ppm, that is to say only a very small boron loss within the measuring accuracy. The equivalent diameter of the boron particles (diameter of a sphere whose spherical volume is equal to the boron particle volume) in the sintered matrix is in the favorable range from 5 μm to 300 μm. Furthermore, the thickness of the surface film on these boron particles, which consists of a different material than the sinter matrix and the boron particles, is in the favorable range from 0.3 μm to 30 μm. Such boron particles show practically no tendency to segregate, in particular in UO2 powder which has been obtained by the processes indicated above. No boron escapes from them during sintering.

When using powdered zirconium diboride ZrB2 as the boron-containing compound instead of the crystalline boron, similarly low boron losses were achieved during sintering.

It is advantageous if the boron concentration in the sinter matrix of the nuclear fuel sintered body is in the range from 100 ppm to 10,000 ppm; because on the one hand a strong damping of the speed and amount of the increase in reactivity can be achieved in a nuclear reactor into which such nuclear fuel sintered bodies are introduced as part of a fresh nuclear reactor fuel element, but on the other hand the formation of cracks in the sintered matrix of the nuclear fuel sintered body is avoided.

It is favorable if the isotope B] _Q in the boron in the boron or in the boron-containing chemical compounds used is enriched compared to the natural isotope composition of boron. This can be achieved in a known manner, for example by cyclotron, diffusion or separation nozzle enrichment. This isotope B _I _ _Q practically absorbs the thermal neutrons. Due to its accumulation in the boron, which is located in the sintered matrix of the nuclear fuel sintered body, the concentration of this boron can be chosen to be relatively low.

It is advantageous to insert the nuclear fuel sintered bodies according to the invention into a cladding tube, usually made of a zirconium-based alloy or stainless steel, of a fuel rod. bring and seal this cladding gas-tight. This fuel rod is advantageously part of a nuclear reactor fuel element for a nuclear reactor. Such a nuclear reactor fuel element is advantageously provided for a light water nuclear reactor, in particular for a pressurized water nuclear reactor or a boiling water nuclear reactor.

Experiments on such a cladding tube, which simulated the conditions in a nuclear reactor, showed that boron does not escape from the nuclear fuel sintered bodies even at temperatures of 800 ° C. and higher.

Claims

claims

1. Nuclear fuel sintered body made of UO2 or one of the mixed oxides (U, Pu) θ2 »(U, Th) 0 ₂ , (U, RE) 0 ₂ , (U, Pu, RE) 0 ₂ , (U, Th, RE ) 0 ₂ , and (U, Pu, Th, RE) 02 (RE = rare earth) with particles made of boron and / or a chemical boron compound in the sintered matrix and with a surface film on these particles made of a different material than the sintered matrix and the particles exist and retain boron.

2. Nuclear fuel sintered body according to claim 1, wherein the surface film consists of a metal and / or a metal alloy.

3. Nuclear fuel sintered body according to claim 2, wherein the

Surface film consists of at least one metal from the group molybdenum, rhenium, ruthenium, rhodium, tungsten and chromium.

4. Nuclear fuel sintered body according to claim 2, wherein the surface film of at least one metal alloy

Group of molybdenum-based alloy, rhenium-based alloy, ruthenium-based alloy, rhodium-based alloy, tungsten-based alloy and chromium-based alloy.

5. Nuclear fuel sintered body according to claim 4, wherein the

Surface film consists of at least one metal alloy from the group of chromium-nickel alloy and molybdenum-niobium alloy.

6. Nuclear fuel sintered body according to claim 1, wherein the

Surface film made of at least one substance from the group tantalum carbide, niobium carbide, titanium carbide, zirconium carbide, chromium carbide, vanadium carbide, tungsten carbide, molybdenum carbide, tantalum nitride, niobium nitride, titanium nitride, zirconium nitride, vanadium nitride, tantalum silicide, tungsten silicide, niobium silicide, ni-silicon silicide, ni-silicon silicide, ni-silicon silicide, ni-silicon silicide, ni-silicon silicide, ni-silicon silicide, ni-silicon silicide , Molybdenum silicide, zirconium oxide, magnesium oxide, beryl lium oxide, chromium oxide, calcium oxide, cerium oxide and zirconium oxide.

7. Nuclear fuel sintered body according to claim 1, wherein the surface film of at least one material from the group

Silicon carbide and silicon nitride, preferably Si3N4 _^ be¬.

8. Nuclear fuel sintered body according to claim 1, wherein the equivalent diameter of the particles in the range of 5 microns to

300 μm.

9. Nuclear fuel sintered body according to claim 1, wherein the thickness of the surface film is 0.3 microns to 30 microns.

10. Nuclear fuel sintered body according to claim 1, wherein the specific density of the particles is 5 g / cm ³ to 10 g / cm ³ .

11. Nuclear fuel sintered body according to one of claims 1 to 10, wherein the boron concentration in the range of 100 ppm to

10,000 ppm.

12. Nuclear fuel sintered body according to one of claims 1 to 11, in which the isotope B] _Q is enriched in boron compared to the natural isotope composition.

13. Nuclear reactor fuel element with a fuel rod which contains a nuclear fuel sintered body in a cladding tube according to one of the claims 1 to 12.

14. A process for producing a nuclear fuel ^{sintered body} according to one of claims 1 to 12 from powdered starting oxide, to which the particles of boron and / or the chemical boron compound provided with the surface film are mixed and which is pressed into pressed bodies which are pressed on ¬ are finally sintered.

15. The method according to claim 14, wherein a pressing aid is added to the powdered starting oxide before pressing.

16. The method according to any one of claims 14 and 15, in which the sintering is carried out in a sintering atmosphere which, in the case of a metallic, silicidic or oxidic surface film on the particles, hydrogen and / or an inert gas, in the case of a carbidic surface film on the particles CO and / or CO2 and, in the case of a nitridic surface film, contains nitrogen and / or an ammonia compound on the particles.