JAEA-Review-2010-065.pdf:15.99MB - 日本原子力研究開発機構

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Evaluation of the ZrC Layer for Coated Fuel Particles
Probed by a Positron Microbeam
M. Maekawa a) , A. Yabuuchi a) , A. Kawasuso a) and J. Aihara b)
a) Advanced Science Research Center, JAEA,
b) Division of Fuels and Materials Engineering, NSED, JAEA
Zirconium carbide (ZrC) is known as a hard and strong
material having a high melting point of 3,540 o C. The ZrC
is one of the candidates of a coating material for a fuel
particle of the Very High Temperature Gas-Cooled Reactor
1, 2)
(VHTR) . The ZrC coating layer is formed by a
chemical-vapor-deposition technique with a pyrolytic
reaction of ZrBr4, CH4 and H2 at around 1,500 o C. By
transmission electron microscope (TEM) observations,
many structural defects, such as carbon precipitates and/or
microvoids, were found in the deposited ZrC layer. It is
known that density of these defects changes drastically
depending on the gas condition. However, detailed
characteristics of defects and its behavior have not been
fully elucidated. In this study, we attempt to evaluate
open-volume type defects in the ZrC coating layer using a
positron microbeam.
Figure 1 shows the cross-sectional optical image of the
ZrC-coated particle. In this study, non-nuclear-surrogate
particles which consist of a micro-spherical kernel of
stabilized ZrO2, carbon layer and ZrC layer were used.
Table 1 shows the properties of samples. Although high
C/Zr ratio increases the growth rate, density of structural
3)
defects also increases . For the reference sample,
commercial ZrC powder (Nilaco ZR-497201) was also
measured. Doppler-broadening of annihilation quanta of
the ZrC layer were measured and characterized by S and W
Normalized W parameter
920micron
Fig. Fig. 1 1 Cross-sectional optical optical image image of
a ZrC-coated fuel particle.
of a ZrC-coated fuel particle.
1.00
0.98
0.96
0.94
0.92
0.90
Reference
Carbon
ZrO2
F4
F2
F3
1.00 1.02 1.04
F1
1.06
Normalized S parameter
JAEA-Review 2010-065
ZrC
parameters, which are defined as the peak and tail intensities,
respectively. All the S and W parameters are normalized to
the reference value. If defects exist and positrons are
trapped to them, the S parameter increases and the W
parameter decreases.
Figure 2 shows the correlation between S and W
parameters for each sample. Measured S and W parameters
of the sample with the lower defect density are closer to the
value of the reference sample. This means that the density
of structural defects observed by the TEM relates to density
of the vacancy type defects. Figure 3 shows the S
parameters as a function of C/Zr ratio. S parameter is
increased with increase of carbon and finally seems to
approach S ≈ 1.05. From a theoretical calculation, it is
confirmed that the value was corresponds to a Zr vacancy in
the ZrC crystal. By the heat treatment at 1,760 o C, S
parameter decreased to 1.03; however, it did not reach to the
reference value (1.00). This means that thermal annealing
is not effective for recovery of the defects because of the
high thermal stability of defects.
References
1) S. Ueta et. al., J. Nucl. Materials 376 (2008) 146.
2) T. Ogawa et. al., J. Am. Ceram. Soc. 75(1992) 2985.
3) J. Aihara et. al., J. Am. Ceram. Soc. 92 (2009) 197.
S parameter
1.08
1.06
1.04
1.02
1.00
Table 1 List of of samples samples.
Samp le CH4/ZrBr Growth rate Density Defect
name ratio (micron/h) (g/cm 3 ) density
F1 2.12 21 6.01 large
F2 1.50 20 6.01
F3 1.28 18 6.35
F4 1.00 14 6.5 small
1.0 1.5
C/Zr ratio
2.0
Fig. Fig 2 .2 S and W parameters for for the the each each samples samples. Fig. 3 S S parameters as as a function a function of C/Zr of C/Zr ratio. ratio.
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