GB2354256A

GB2354256A - uranium high-density dispersion fuel

Info

Publication number: GB2354256A
Application number: GB9921855A
Authority: GB
Inventors: Chang-Kyu Kim; Ki-Hwan Kim; Se-Jung Jang; Ii-Hyun Kuk; Dong-Seong Sohn; Eng-Soo Kim
Original assignee: Korea Atomic Energy Research Institute KAERI; Korea Electric Power Corp
Current assignee: Korea Atomic Energy Research Institute KAERI; Korea Electric Power Corp
Priority date: 1999-09-15
Filing date: 1999-09-15
Publication date: 2001-03-21
Anticipated expiration: 2019-09-15
Also published as: GB9921855D0; GB2354256B

Abstract

This invention comprises of high-density fuels dispersed with atomized spherical particles and the fuel fabrication processes. Uranium and alloying metals are charged in a heat-resistant crucible having a stopper and a small orifice. The atomizer chamber is evacuated up to the vacuum degree of above 10<SP>-3</SP> torr using vacuum pumps. Uranium and alloying metals charged in the crucible 1 are melted by vacuum induction or arc heating. The alloy melt is fed through a nozzle onto a rotating disk. Then melt droplets are formed and spread by the centrifugal force of rotating disk 7. The flying fine droplets are cooled rapidly (above 10<SP>4</SP> {C /sec in cooling rate) under an inert atmosphere of argon or helium gas due to the large specific surface area. The alloy may be of composition U-(A)Q-(B)X where Q is Mo, Nb or Zr, X is Mo, Nb, Zr Ru, Pt, Si, Ir, Pd, W, Ta or Os, and wherein Q and X are different elements, A = 4 to 9 wt% and B = 0.1-4 wt%.

Description

2354256 URANIUM HIGH-DENSITY DISPERSION FUEL This invention is directed to

high-density dispersion fuel having spherical particles solidified rapidly by an atomization process. It more particularly refers to a noble method of making dispersion fuel.

Conventionally the powder of dispersion nuclear fuel is produced by alloying and comminution. Alloying metals are alloyed into ingots by induction or arc heating under a vacuum atmosphere. The as-cast ingots are heat-treated in vacuum for 100 hours at 900'C to ensure compositional homogeneity, and then quenched to form a metastable gamma phase. The ingots are machined into chips and milled under liquid argon using a hardened steel mill to obtain the appropriate particle size. The chips of uranium alloys are very pyrophoric due to its high oxidative characteristics. Thus, it is necessary to machine under a sufficient amount of cutting fluid to substantially prevent oxidation. The fuel power is contaminated by the cutting fluid. Processes of rinsing with an organic solvent such as acetone etc. and drying under vacuum atmosphere at a high temperature are required. Also, during milling the small particles containing ferrous impurities are introduced by the wear of milling machine parts. A close-up of the particle surface reveals many dark spots on the surface which energy dispersive spectroscopy has determined to be 1 iron-rich. Most of the particles containing ferrous components are removed by magnetic separation.

As it is difficult to comminute uranium alloy ingots due to its tough property, the yield of uranium alloy through a mechanical powdering process, which consists of many steps of chipping, milling, rinsing, and drying, is very low, in the range of 5 to 20%. In addition, during magnetic separation about 30% of fuel powder is lost because separated powder contains a considerable amount of fuel particles.

In the case of directly making powder from alloy ingots using a high speed lathe equipped with a rotary file, the productivity of usable powder is very low, as it yields 12 grams per hour. The yield of powder smaller than 212 g m in size, ranges 32-63% of the total powder, depending on the alloy composition. The powder is produced by grinding ingots with a tungsten/tantalum carbide tool rotating at approximately 2, 500 rpm. This process has the drawback of carbide and nitride contamination in powder due to the wear of the rotary file. Contamination levels range from 0.1-7.6% and are generally higher for uranium alloys with larger alloy contents.

2 The comminuted particles with longish and irregular shapes, arranged along the rolling or extruding direction perpendicular to heat flow, inhibit thermal conduction in fuel meat. The large specific surface area of irregular particles enhances the interaction between fuel particles and Al matrix to form uranium- aluminide (UAI,,) with low-density in the perimeter of the uranium alloy particles, and induces to thermally swell the nuclear fuel meats.

According to the present invention there is provided a method of fabricating fuel material comprising alloying uranium with one or more elements in a melt and atomising the melt so as to produce an alloy powder.

A homogenization treatment and a mechanical comminution of alloy ingots are not required. An investigation has been carried out for applying this atomization process to the development of high-density dispersion fuels. Many kinds of advantages have been obtained as follows; 1) direct formation of meta-stable y-U phase, 2) process simplification 3) minimization of fabrication space, 4) improvements to uranium yield, fuel productivity, powder purity, and fuel formability, 5) higher thermal conductivity in the real heat flow direction, 6) the decrease of asfabricated porosity, and a smaller thermal swelling.

3 Embodiments of the present invention will now be described by way of example with reference to the following drawings, in which:

Figure I is a schematic diagram of a centrifugal atomizer, and Figure 2 is a block diagram of this invention.

This invention comprises of high-density fuels dispersed with atomized spherical particles and the fuel fabrication processes. Comparing with the conventional method, this has many advantages for the direct formation of meta-stable 7, -U phase, the process simplification, the minimization of fabrication space, the improvements of production yield, fuel productivity, powder purity and fuel formability, a higher thermal conductivity in real heat flow direction, the decrease of as-fabricated porosity, and a smaller thermal swelling.

Uranium and alloying metals are charged in a heat-resistant crucible having a stopper and a small orifice. The atomizer chamber is evacuated up to the vacuum degree of above 10'3 torr using 4 vacuum pumps. Uranium and alloying metals charged in the crucible are melted by vacuum induction or arc heating. The alloy melt is fed through a nozzle onto a rotating disk. Then melt droplets are formed and spread by the centrifugal force of rotating disk. The flying fine droplets are cooled rapidly (above 104 'C/sec in cooling rate) under an inert atmosphere of argon or helium gas due to the large specific surface area.

Fig 1. shows a schematic diagram of the centrifugal atomizer, and Fig. 2 shows a block diagram of this invention. The equipment of this invention is composed of the heat-resistant crucible I having a stopper and an orifice 11., the high-frequency generator 'Z for heating the crucible, the vacuum pump system 4 evacuating the chamber 3 to a proper vacuum degree, the gas supply valve 5 providing with the cooling gas 10 in the chamber 3, the check valve 6 discharging the over-pressurized gas outside the chamber, the rotating disk 7, the container 8, collecting the produced powder, and the cyclone 9 - collecting the very fine powder.

The crucible I is located in the chamber and surrounded by induction coil. Alloying metals are charged in the crucible. The chamber is evacuated up to about 10-3 torr by a vacuum pump system. The crucible is heated -by induction method. High frequency electric power is supplied to the coil from a generator. The melt is discharged by lifting the stopper and fed through the orifice onto the rotating disk. At the same time the cooling gas of Ar or He is supplied in a downward direction from the nozzles at the middle chamber. The flow rate of the cooling gas is controlled by adjusting valves. The melt is spread with forming droplets by the centrifugal force of the rotating disk. The flying melt droplets are rapidly solidified by cooling gas due to a large specific surface area. The solidified particles slide along the inclined wall of the chamber, 3 into the powder container 8 at the bottom of the chamber 3. The check valve 6 located between the cyclone and the chamber discharges the used cooling gas 10 by the over-pressure of the chamber.

This invention is explained as the following example (1). In the case of preparing U-8wt.%Mo alloy powder, uranium and Mo metal are weighed in proportion to the alloy composition and charged into a crucible. The crucible I and insulation are assembled properly. The atomizer chamber is evacuated up to about 10 -3 torr using a vacuum system. Then the crucible is heated by switching on the generator. When the crucible temperature reaches 2001C higher than the melting point. The disk 7 is rotated to about 30,000 rpm using an electric motor 71. By lifting the stopper the melt is poured on rotating disk through an orifice. The melt is spread by the centrifugal force of the rotating disk, and forms fine droplets, which fly through the downward injecting cooling gas toward the wall of the chamber. The fine droplets are rapidly solidified into the meta- stable 7-U phase at a cooling rate of about 104 C /sec. The atomized powder is collected in the container 8 at the bottom of the chamber 3 The median particle size is about 65/im, and the portion of powder below 125pm in size is about 95%. Then the 6 atomized powder is blended with aluminum powder and compacted into pellets. The pellets are preheated at 4201C and extruded into fuel meats under an inert atmosphere.

Another example (H) is as follows; this invention can be applied to uranium alloy dispersion fuels of U-(A)-(B)X (here, Q: Mo, Nb, Zr; X: Mo, Nb, Zr, Ru, Pt, Si, Ir, Pd, W, Ta, Os etc.; QX; (A)=4-9 wt.%, (B)=0.l-4wt. %) alloys including U-(A)Q alloy. In the case of preparing U-5wt.%Mo-2wt. %Ir alloy powder, U, Mo and Ir element are weighed in proportion to the alloy composition and charged in a ceramic crucible. Thereafter the atomizer chamber is evacuated up to above 10-3 torr using a vacuum pump system in the same way as the atomization process of U-8wt.%Mo. After the alloy melt is superheated to about 2001C higher than the melting point, the rotation of disk is started and increased to about 30,000 rpm. The fuel particles are produced with rapid solidification effect (a cooling rate of above 104 C/sec) under an inert atmosphere (10). Spherical U-(A)Q(B)X (here, Q: Mo,, Nb, Zr; X: Mo, Nb, Zr, Ru, Pt, Si, Ir, Pd, W, Ta, Os, etc.; QX; (A)=4-9wt.%, (B)=0.1-4wt.%) alloy powder of 30-55% in volume percentage is blended with aluminum powder, and then compacted into billets. The billets are preheated at 3701C, and extruded under inert atmosphere into a ftiel meat.

Thus, uranium alloy powders are obtained directly from the melt.

The following are the merits obtained by this technology.

First, the powder fabrication by atomization method is excellent in 7 the yield and the productivity. The fabrication processes such as mechanical comminution of ingots, rinsing and drying chips for the removal of cutting fluid components, and magnetic separation can be eliminated.

Second, the gamma phase of uranium alloy is formed directly from the melt by the rapid cooling effect.

Third, atomized particles have a spherical shape, which gives many kinds of beneficial effects on fuel performance such as: a smaller interaction swelling between fuel particles and matrix, a better thermal conductivity in the real heat flow direction, and improving the formability of fuel meat.- Fourth, the powder quality is pure because there are no chances for contamination from the cutting fluid and grinding tools.

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Claims

Claims

I. A method of fabricating fuel material comprising alloying uranium with one or more elements in a melt and atomising the melt so as to produce an alloy powder.
2. A method according to claim 2 wherein the atomising of the melt comprises applying the melt to a rotating surface.
3. A method according to either claim I or 2 wherein the atomising of the melt comprises forming droplets of the melt and solidifying said v droplets.
4. A method according to claim 3 wherein the solidifying of said droplets comprises forming meta-stable -phase U.
5. A method according to any one of claims 1 to 4 wherein the alloying comprises preparing an alloy of composition U-(A)Q, wherein Q is Mo, Nb or Zr and (A)= 4-9 wt.%.
6. A method according to any one of claims I to 4 wherein the alloying comprises preparing an alloy of composition U-(A)Q-(B)X, J wherein Q is Mo, Nb or Zr and (A)= 4-9 wt.%, wherein X is Mo, Nb, Zr, Ru, Pt, Si, Ir, Pd, W, Ta or Os, wherein Q and X are different elements and wherein (B)= 0. 1-4 wt.%.

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7. A method according to either claim 5 or 6 further comprising blending the alloy powder with aluminium power, wherein the alloy power comprises 30-55% by volume.
8. Fuel material comprising powdered alloy of composition U-(A)Q obtained by atomising the alloy when molten, wherein U has body-centred cubic configuration and Q is Mo, Nb or Zr and (A) = 4-9 wt.%.
9. Fuel material comprising powdered alloy of composition U-(A)Q(B)X obtained by atomising the alloy when molten, wherein U has bodycentred cubic configuration, Q is Mo, Nb or Zr and (A) = 4-9 wt.%, X is Mo, Nb, Zr, Ru, Pt, Si, Ir, Pd, W, Ta or Os, X and Q are different elements and (B) is 0.1-4 wt.%.
10. A fuel material according to either claim 8 or 9 further comprising aluminium power.
11. High-density fuels dispersed spherical U-(A)Q, and U-(A)Q-(B)X alloy powders (Q: Mo, Nb, Zr element; X: Mo, Nb, Zr, Ru, Pt, Si, Ir, Pd, W, Ta, Os, etc. minor addition element; Q#X; (A)= 4-9wt.%, (B)=O. 1-4wt.%) having meta-stable y -U (body-centered cubic) phase in volume percentage of 30-55% which are produced from the melt by the atomization method.
12. The fabrication processes of high-density dispersion fuels including the atomized spherical powders claimed in claim 1.
13. A method of fabricating fuel material substantially as herein described with reference to Figures I and 2.
14. Apparatus configured to perform the method according to any one of claims 1 to 7.
15. Apparatus for fabricating fuel material substantially as herein described with reference to Figures 1 and 2.

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