Generally, the atomization of UMo particles is done under vacuum or argon atmosphere, and the surface modification of these UMo particles is, usually, carried on through a further process. The techniques for surface modification of atomized UMo particles, aimed to control the Fuel/Matrix interaction, involve, in some cases, complex methodologies and often with minor effect due to the limited solubility of third elements in solid UMo alloy. The atomization and surface conditioning, applied in separate stages, may affect the efficiency of powder production process. Then, the main goal of this study is to explore the surface modification of UMo particles in liquid state or during the solidification that follows the centrifugal atomization process. Through the change of atomization atmosphere, could be possible to promote liquid/gas reactions, with a higher solubility of the modifier element in micro drops of UMo alloy, before they become solid particles. This paper presents comparative results of centrifugal atomization of UMo particles, carried out under inert argon and reactive nitrogen atmospheres. Dissolved nitrogen contents, measured by SEM-EDS analyses, reached up to 7.57 wt% at the center of under nitrogen atomized particles, very higher than 0.84 wt% of nitrogen measured at the center of UMo particle atomized under argon. The presence of uranium nitride was partially verified by conventional XRD analysis. Nevertheless, Out-of-Pile interaction test result, reveals decreasing of aluminium contents into UMo particles atomized under nitrogen atmosphere; Just 3.77 wt% of Al was the maximum content detected in the center of these particles, very lower than 29.11 wt% of Al measured inside UMo particles atomized under argon. Finally, it is possible to conclude that the atomization under reactive atmosphere may modify the surface composition and the behavior of UMo fuel particles dispersed in aluminium, for dispersion type nuclear fuel application.
The qualification of high density nuclear fuel based on UMo alloy requires, necessarily, solve the fuel/matrix interaction issues. Several articles, which cover in detail that topic, have already been written and published by nuclear fuel experts and also by researchers of the CCHEN’s Fuel Development Group [
According to the available literature, several methodologies have been developed for surface coating of UMo particles, some of them based on solid-solid reactions, as the pack cementation technique to form silicon-rich surface layers [
Specifically for Uranium-Nitrogen binary system [
Due to the solubility limitations, besides the previous observation that the effectiveness of the UN coating appeared to be small [
Being the centrifugal atomization, the most generalized method and, apparently, the most proper for UMo alloy particles production, the goal of this study is to propose the modification of the surface of UMo particles, during the atomization process, in which the alloy is, by short time, in liquid state. Depending on the feasibility of implementation, this could be a suitable surface modification process for UMo particles, considering industrialization and economic issues.
This paper reports results of reactive atomization conducted under nitrogen atmosphere. This gas is easily available, nevertheless, other reactive gas, such as methane or methane-nitrogen mixture, also could be applied as a way to promote the formation of surface layers, rich in uranium carbide or uranium carbo-nitrides, compounds initially considered as fuel for fast breeder reactor [
The atomization of cylindrical pins to produce UMo alloy particles was carried out in a laboratory scale centrifugal atomizer, described in a previous paper [
Out-of-Pile interaction test were conducted using mini compacts prepared with UMo particles dispersed in Al-4wt%Si blend. The total mass of compacts was 0.4 g. To avoid excessive oxidation of samples, the compacts were annealed, at 500˚C by 4 hours, in vacuum and inside a copper envelope. After that, the compacts were mounted in epoxy resin, sectioned and prepared to cross section examination through SEM-EDS microanalyses.
Standard Tyler meshes were used to evaluate the granulometry of UMo particles.
Using standard methodologies of hot and cold rolling, a set of 6 miniplates was manufactured, with uranium densities of 6, 7 and 8 gU/cm3. For comparative analysis, a second set of miniplates, with the same densities, was manufactured using UMo particles atomized under argon. QA&QC of miniplates includes; Industrial X-Ray, Ultrasonic Scanning Test, optical densitometry, cladding thickness measurement by Eddy current and cross section examination by SEM and optical microscopy.
The granulometry of UMo particles atomized under nitrogen and argon atmospheres, selected for miniplates manufacturing, are summarized on
The differences in particle size distribution between the two systems could be related to the presence of dissolved nitrogen in the liquid UMo alloy, which increases the melting point and decreases the density of the alloy. According to centrifugal atomization models, these variations increase the particle diameter [
The shape of the UMo particles atomized in nitrogen and argon, inspected by Scanning Electron Microscope and EDS analysis, are shown in the micrographs of
Parameter | Value |
---|---|
Gas Flow (Argon or Nitrogen) | 12 liters/minute (welding torch) |
Alloy composition | U + 7 wt% Mo (natural uranium) |
Pins diameter | 8 - 10 mm |
Mass of pins | 38 - 54 g |
Angular speed | 40,000 rpm |
Welding Machine Current Intensity | 60 amps |
Lot Identification: U7Mo-REP-NAT-L12 | |||
---|---|---|---|
Tyler Sieve | Material below* | ||
# | Mesh [µm] | [g] | [%] |
100 | 150 | 21.84 | 100.00 |
170 | 90 | 9.81 | 47.36 |
230 | 63 | 4.66 | 23.72 |
325 | 45 | 5.18 | 12.48 |
Total | ----- | 41.49 | ----- |
*Excluding the material >150 µm; Total atomized material: 36.11 g + 41.49 g = 77.6 g; Mass of particles >150 µm: 36,11g; corresponding to 46.5 wt% of total; Mass of particles <150 µm: 41,49g corresponding to 53.5 wt% of total; Mass of particles <45 µm: 5.18 g corresponding to 6.68 wt% of total.
Lot Identification: U7Mo-REP-NAT-L11 | |||
---|---|---|---|
Tyler Sieve | Material below* | ||
# | Mesh [µm] | [g] | [%] |
100 | 150 | 22.25 | 100.00 |
170 | 90 | 6.37 | 46.13 |
230 | 63 | 4.99 | 30.70 |
325 | 45 | 7.69 | 18.62 |
Total | ----- | 41.30 | ----- |
*Excluding the material >150 µm; Total atomized material: 7.52 g + 41.30 g = 48.82 g; Mass of particles >150 µm: 7.52g; corresponding to 15.4 wt% of total; Mass of particles <150 µm: 52.58g corresponding to 84.6 wt% of total; Mass of particles <45 µm: 7.69 g + 11.28 g (removed of the Lot) = 18.97 g, corresponding to 38.86 wt% of total.
oxygen content detected by punctual microanalysis on the surface of the UMo particles, verifies the formation of uranium oxide layers, with thicknesses in the order of 5 µm, as is possible to observe in the
According to the EDS results summarized in
The U-7%Mo particles atomized under nitrogen and argon atmospheres was inspected analytically with X-Ray diffraction in a diffractometer (Shimadzu XRD 6000) with CuKα radiations and divergence and reception slots of 1.0˚ and 0.3˚ mm, respectively. Diffraction patterns of particles atomized in nitrogen and argon atmospheres were done, for theta angles, between 10˚ and 120˚ degrees, with steps of 0.02˚ and 1˚ second. The X-Ray diffraction patterns of atomized U-7%Mo alloy powders are shown in
In the UMo atomized particles with sizes below 150 µm, Uγ (bcc) solid solution metastable was detected; this phase can be retained in a metastable state at room temperature by Mo addition and fast solidification [
The samples for the interaction test were prepared blending aluminium powder with UMo particles, and then pressing the mix into a die of 8 mm diameter, using a load of 260 MPa. The samples were placed into copper envelopes for interaction annealing test, carried out under vacuum for 4 hours at 500˚C. After that, the samples were mounted in epoxy resin and prepared through metallographic techniques, grinded and polished after each thermal treatment. Optical and Scanning Electron Microscopy―SEM equipped with Energy Dispersed Scanning―EDS microanalyses were used for characterization of fuel particles and interaction layer.
The result of interaction test, included in
EDS analyses results evidence the effect of nitrogen as diffusion barrier to avoid Al migration from matrix to inner of UMo particle during U-Al interaction annealing test. The aluminium content into UMo particles atomized under nitrogen is very lower than the content detected into UMo particles atomized in argon atmosphere,
Atomization Atmosphere | Nitrogen ( | Argon ( | ||
---|---|---|---|---|
Point of analysis | Surface (001) | Surface (002) | ||
Element | Mass (%) | Atom (%) | Mass (%) | Atom (%) |
N | 1.4 | 12.66 | 0.96 | 9.04 |
O | 4.6 | 34.49 | 4.45 | 36.76 |
Mo | 0.02 | 0.02 | 0.20 | 0.28 |
U | 93.79 | 49.97 | 94.02 | 52.16 |
Atomization Atmosphere | Nitrogen ( | Argon ( | ||||||
---|---|---|---|---|---|---|---|---|
Point of analysis | Surface (001) | Surface (002) | ||||||
Element (Mass %) | Center (2) | Center (3) | Inter zone (4) | Surface (5) | Center (3) | Center (2) | Inter layer (4) | Surface (6) |
N | 1.58 | 1.24 | 3.17 | 0.95 | 1.32 | 2.23 | 2.53 | 0.93 |
O | 1.68 | 0.93 | 3.49 | 3.64 | 3.90 | 3.17 | 5.85 | 5.27 |
Al | 2.48 | 1.87 | 1.77 | 90.47 | 3.65 | 2.20 | 1.91 | 89.86 |
Si | 0.06 | 0.15 | 0.10 | 0.41 | 0.12 | 0.04 | 0.06 | 0.88 |
U | 32.34 | 82.71 | 91.37 | 4.43 | 90.40 | 92.17 | 89.50 | 2.91 |
Mo | 61.58 | 13.10 | 0.11 | 0.10 | 0.61 | 0.19 | 0.14 | 0.15 |
minimizing the probability to promote the formation of U-Al compounds due to interaction, results completely agreed with previous studies [
The miniplates were manufactured including blending of UMo + Al-4%Si powders, compacting, assembling (compact, covers and frame), welding, hot rolling, blister test, cold rolling and QA inspections. In
According to data included in table 7, the total reduction applied to UMo miniplates was in the order of 74%, lower than U3Si2 dispersion fuel plates made of, in which the total reduction is approximately 85%. Compared with miniplates made of hydrided powder, the reduction rate and total reduction are slightly higher than those used for atomized powder miniplates [
Atomization Atmosphere | Nitrogen ( | Nitrogen ( | Argon ( | |||||
---|---|---|---|---|---|---|---|---|
Point of analysis | ||||||||
Element (Mass %) | Inter layer (3) | Center (2) | Inter layer (4) | Inter zone (3) | Center (2) | Inter layer (3) | Matrix (4) | Center (2) |
N | 2.80 | 1.75 | 7.52 | 6.14 | 7.57 | 0.33 | 0.55 | 0.84 |
O | 4.08 | 1.69 | 1.91 | 1.40 | 0.85 | 6.81 | 10.36 | 1.25 |
Al | 1.64 | 3.77 | 1.58 | 1.49 | 1.42 | 29.11 | 88.81 | 2.00 |
Si | n/d | n/d | 0.07 | 0.06 | 0.10 | 0.33 | 0.28 | 0.09 |
U | 91.29 | 90.37 | 88.29 | 88.99 | 88.00 | 63.42 | n/d | 91.54 |
Mo | 0.18 | 2.42 | 0.63 | 1.92 | 2.06 | n/d | n/d | 4.28 |
n/a: not detected.
Miniplate Identification | UMo-97 (argon) | UMo-102 (argon) | UMo-103 (nitrogen) | UMo-104 (nitrogen) | UMo-105 (nitrogen) | UMo-106 (nitrogen) | UMo-107 (nitrogen) | UMo-108 (nitrogen) | ||
---|---|---|---|---|---|---|---|---|---|---|
Uranium Density gU/cm3 | 6.0 | 8.0 | 6.0 | 6.0 | 7.0 | 7.0 | 8.0 | 8.0 | ||
UMo Fuel Mass [g] | 5.93 | 6.40 | 5.93 | 5.93 | 6.18 | 6.18 | 6.40 | 6.40 | ||
Mass (Al + 4% Si) [g] | 1.52 | 1.05 | 1.52 | 1.52 | 1.27 | 1.27 | 1.05 | 1.05 | ||
Compact Metrology | 22.41 | 22.41 | 22.41 | 22.40 | 22.40 | 22.40 | 22.40 | 22.40 | 22.40 | |
18.0 | 18.0 | 18.0 | 18.0 | 18.0 | 18.0 | 18.0 | 18.0 | 18.0 | ||
2.47 | 2.47 | 2.47 | 2.80 | 2.47 | 2.50 | 2.64 | 2.46 | 2.47 | ||
Meat Metrology | 86.2 | 86.2 | 86.2 | 86.7 | 86.5 | 83.3 | 83.1 | 81.9 | 82.0 | |
18.8 | 18.8 | 18.8 | 18.0 | 18.0 | 18.0 | 18.0 | 18.0 | 18.8 | ||
Thickn. (mm) | 0.65 | 0.65 | 0.69 | 0.68 | 0.67 | 0.66 | 0.64 | 0.64 | ||
Meat Volume (cm3) | 1.04 | 1.05 | 1.08 | 1.06 | 1.00 | 0.99 | 0.94 | 0.99 | ||
Volume fraction matrix | 0.50 | 0.39 | 0.50 | 0.50 | 0.45 | 0.45 | 0.39 | 0.39 | ||
Miniplate metrology | Length (mm) | 130.17 | 130.21 | 130.42 | 130.51 | 130.40 | 130.24 | 130.36 | 130.21 | |
Wide (mm) | 51.32 | 50.64 | 50.92 | 50.53 | 50.69 | 50.63 | 50.55 | 50.82 | ||
Thickn. (mm) | 1.44 | 1.44 | 1.43 | 1.43 | 1.43 | 1.42 | 1.43 | 1.42 | ||
Starting Thickness (mm) | 5.80 | 5.48 | 5.80 | 5.76 | 5.63 | 5.58 | 5.42 | 5.40 | ||
Total Reduction (%) | 75.1 | 73.8 | 75.3 | 75.2 | 74.7 | 74.5 | 73.7 | 73.7 | ||
The results of ultrasonic scanning test, carried out in eco-pulse mode, reveal the absence of non-bonding areas. Besides, is possible to observe an UT waveform, characteristic of UMo dispersion fuel. Images of UT inspection, corresponding to UMo-103 miniplate is included in
Results of industrial X-Ray inspection are shown in
The improvement in the homogeneity can be related with the modification in size distribution of UMo particles. As has been mentioned before, the addition of nitrogen to UMo alloy may produce larger particles and more narrow size distribution. On the other hand, the particles with spherical shape obtained by Rotating Electrode Process are easily segregated, or relocated, during the mixing with aluminium powder and even after, in the matrix, during the rolling process [
The centrifugal atomization of U-7wt% Mo alloy, conducted under nitrogen atmosphere, was achieved without significant differences in the process, compared with the atomization in argon.
The particle size distribution was the main difference between particles atomized under argon and nitrogen. The presence of nitrogen could increase the melting point and decrease the density of UMo alloy [
The surface of the UMo particles atomized in argon, analyzed by XRD, reveals the presence of UO2 and γ-U. In case of particles atomized under nitrogen, the same compounds were detected, and additionally, peaks of uranium nitride UN.
UMo/Al interaction test confirms that the nitrogen addition can decrease but not eliminate this phenomena, nevertheless, the content of Al into the UMo particles was lower in particles atomized under nitrogen atmosphere.
The authors wish to express their acknowledgments to CCHEN’s Nuclear Fuel Section Staff.
LuisOlivares,JaimeLisboa,JorgeMarin,MarioBarrera,AlbertoNavarrete, (2016) Atomization of UMo Particles under Nitrogen Atmosphere. World Journal of Nuclear Science and Technology,06,43-52. doi: 10.4236/wjnst.2016.61004