Ⅲ. Thermophysical properties of U-Mo/Al(-Si) dispersion fuel
3.1. Sample fabrication
3.1.2. Sample fabrication of fresh U-Mo/Al dispersion fuels
Atomized U-7Mo powder and high purity Al and Al-5Si powders (> 99.99% purity, < 45 μm) were prepared at Korea Atomic Energy Research Institute (KAERI). The high density of U-7Mo powder was produced from the depleted uranium using a centrifugal atomization process [72]. The U- Mo alloys are melted, and the liquid U-Mo are dropped on the rapidly rotating disk. The liquid U-Mo is spread out, which is solidified very rapidly into numerous tiny particles with isotropic γ-U phase. It is known that the U-Mo particles produced by this process have more superior characteristics such as a spherical shape, narrow size distributions, high density and fine grain structure compared to the comminuted powders, which provide better irradiation performances. The upper limit of the U-7Mo particle size was set at 150 μm and the portion of U-Mo particles smaller than 45 μm was controlled to be less than 20 wt.%. Figure 10 (a) shows some U-Mo particle size distribution data, which has a sigmoidal distribution. All powders were fabricated at KAERI using the same sieve condition, thus the particle size distribution should be nearly identical. Using the accumulated particle distribution data, an empirical fitting equation was obtained as follows:
0.063 60.1 0
100 1
x
p x
N
e
(49)where Np is the percentage of the number of U-Mo particles and x is the U-Mo particle size in μm. By integrating Eq. (49), the percentage of the number of U-Mo particles, Np was obtained as a function of particle size. The distribution has a monomodal shape as seen in Figure 10 (b).
0 30 60 90 120 150
0 20 40 60 80 100
DU-7Mo LEU-6Mo LEU-7Mo LEU-10Mo LEU-Mo-Os Fiiting equation
Accumulated percentage of particles (%)
Particle Size (m)
0 20 40 60 80 100 120 140 160
0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8
Percentages of particle numbers (%)
Particle Size (m)
(a) (b)
Figure 10. The particle size distribution as a function of particle size; (a) accumulated percentage of particles and (b) percentage of particle numbers.
The mean particle size can be calculated as follows:
0
Max p
P p
p D
p
D N
D
N
(50)where Np is the percentage of the number of U-Mo particles and Dp is the U-Mo particle size in μm.
Hence, the mean particle size of the U-Mo powders was obtained as approximately 60 μm.
Using the powders, a total of six types of fuel meat samples were fabricated by adopting three different uranium loadings (5.0, 6.5, and 8.0 gU/cm3) and two types of matrix materials (Al and Al- 5wt.%Si). The sample fabrication followed the typical plate-type fuel fabrication only except that the samples were hot-extruded instead of hot-rolled.
The sample fabrication procedures are summarized as follows [75]:
1. The atomized U-7Mo powder was mixed with the Al and Al-5Si powders to obtain 5.0, 6.5, and 8.0 gU/cm3. Table 5 shows the amounts of U-7Mo, Al, and Al-5Si powders.
2. The mixture of U-7Mo/Al and U-7Mo/Al-5Si powders was homogenized for 2 hours at 75 rpm using a shake mixer.
3. The mixtures were compacted into a small size of plate using 300 TON press (KSHM-300 Press, Dongwon M Tech as seen in Figure 11 with a pressure of about 622 MPa held for 240 s and depressed for 60 s. The size of the compact was 91.2 × 24.2 × 3.5 (mm) and each fuel type was compacted eight times as seen in Figure 12 (a).
4. The compact was pre-heated for 30 min at 400 °C in an Ar container. This temperature was selected to suppress IL formation. Then, the compact was hot-extruded with an extrusion ratio of ~2 into a rod 1000 mm in length and 10 mm in diameter. This process was repeated three to five times to improve particle homogeneity (Figure 12 (b)).
5. For laser flash analysis (LFA), disc specimens 1 - 3 mm in thickness and ~10 mm in diameter were cut from the rod. The sample thickness distribution over the surface was uniform, with variation ≤ 0.4%, satisfying the requirements of ASTM-E1461 [74] (Figure 12 (c)).
6. For differential scanning calorimetry (DSC), specimens ~1 mm in thickness and ~6.5 mm in diameter were made (Figure 12 (d)). For the DSC, the bulk samples were made as thin and flat as possible to improve heat transfer between the sample and the holder. Because homogeneity decreases as the sample thickness decreases, however, an optimum thickness was selected. The ideal specimen thickness was empirically found and corresponds to a specimen mass of 100 - 200 mg, depending on uranium loading, for the same disc diameter (6.5 mm).
Figure 11. Images of hot-extrusion machine in KAERI.
Figure 12. Material images at various sample preparation steps: (a) after compaction, (b) after extrusion, (c) LFA sample, and (d) DSC samples cut from thin discs.
Table 5. U-7Mo, Al, and Al-5Si powders used in this study
U-Mo particle homogeneity and the surface smoothness of the sample decreased as the uranium loading in the fuel meat and Si content in the Al matrix increased. U-7Mo/Al and U-7Mo/Al- 5Si samples with uranium loadings of 5.0 and 6.5 gU/cm3 were fabricated without any significant difficulties, although some pores and cracks were occasionally found. After several extrusion cycles, these defects were found to be eliminated. For samples with 8.0 gU/cm3 uranium loading, however, some unacceptable defects were observed partially. In this case, the samples were made by hot-pressing the samples at 400 °C in an Ar environment. The fabrication of 8.0 gU/cm3 U-7Mo/Al-5Si samples were more difficult due to the increased brittleness in the matrix caused by the Si addition. A high number of scratches and cracks on the surface and the front part of the extruded rod was observed. After a careful examination of the extruded rod, samples were cut from the least damage portion of the rod.
A total of 6 samples were able to be obtained for the measurements.
Figure 13. A schematic diagram for the sample extrusion.
Atomized U-7Mo powder
Al-(0,5)Si powder
Mixing (5.0, 6.5, 8.0
g-U/cm3) Compaction Extrusion
(dia. 10 mm)
Sample Fabrication Fuel type U-loadings U-7Mo powder Al powder Al-5Si powder
(gU/cm3) (g) (wt.%) (g) (wt.%) (g) (wt.%)
U- 7Mo/Al
5.0 333.265 74.15 116.204 25.85 - -
6.5 433.585 81.13 100.822 18.87 - -
8.0 533.977 86.19 85.53 13.81 - -
U- 7Mo/Al-
5Si
5.0 332.886 74.29 - - 115.221 25.71
6.5 433.275 81.25 - - 100.019 18.75
8.0 533.628 86.30 - - 84.746 13.70