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

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Effects of Radiation Damage and Helium on Swelling and
Microstructure of EHP Ni-base Superalloy
G. H. Kim a) , K. Shiba a) , I. Ioka a) , T. Sawai b) and S. Yamashita c)
a) Nuclear Engineering Research Collaboration Center, JAEA,
b) Research Coordination and Promotion Office, NSED, JAEA,
c) Division of Fuels and Materials Engineering, NSED, JAEA
1. Introduction
Cladding materials of a nuclear reactor are required to
have the long excellent performance under the irradiation
environment. In this experiment, irradiation performance
of extra high purity (EHP) Ni-base superalloys designed as
the MA doped MOX fuel claddings for the sodium cooled
fast reactor was examined. In this alloy, impurities, such
as C, O, N, P, S were reduced to less than 100 ppm in total
to improve workability, irradiation embrittlement, and intergranular
corrosion.
Neutrons in the FBR core produce about 1 or
2 appmHe/dpa in high nickel alloys. It is less than that in
fusion reactors or than that even in light water reactors
considering the two-step reaction of Ni-58 in stainless
1, 2)
steels . Small amount of helium may, however, affect
the microstructural evolution especially in EHP alloys where
C, O, N impurities are extremely low. Because cavity
nucleation in such high purity alloys is very difficult due to
3)
the high surface energy of cavities and helium gas pressure
will drastically change the stability of cavity nuclei.
Dual-ion-irradiation is, therefore, essential for the adequate
evaluation of EHP alloys under radiation environment.
This work is focused on investigating the effects of
helium injection on the cavity development and the
dislocation evolution in EHP Ni-base superalloys by the
irradiation experiments using a dual beam of iron and
helium ions.
2. Experimental procedures
The EHP alloy (Fe-40Ni-25Cr-1.5Ti-1.5Al) was used for
the irradiation experiments. The irradiations of samples
were carried out by using Takasaki Ion Accelerators for
Advanced Radiation Application (TIARA) facility.
Ions of 10.5 MeV Fe 3+ were injected to produce radiation
damage and 1.05 MeV He 2+ ions were implanted to simulate
the He production. Single (Fe 3+ ) and dual (Fe 3+ + He 2+ )
ion beam irradiation were conducted up to ~50 dpa and
~150 appmHe at 825 K. The irradiation of helium was
controlled using an Al foil energy degrader to implant over
the depth range from about 0.9 to 1.5 m. The nominal
displacement damage and ion-implantation depth were
calculated using TRIM code. Thin foils for transmission
electron microscopy (TEM) were fabricated with a focused
ion beam (FIB) micro-sampling system. TEM observation
was carried out for the evaluation of cavity and dislocation
structures.
3. Results and discussions
Figure 1 shows the cross-section views of single and dual
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ion irradiated EHP alloys up to 50 dpa at 825 K. Under
single beam irradiation, high density of dislocation structure
was observed in the depth region 0-2.2 m of the projected
range of iron ion. Cavities were not founded. On the
other hand, for dual beam irradiation to EHP alloy, the
density of dislocation structure relatively decreased in the
projected range of iron ion. But, the presence of helium
produced many small polyhedral cavities of 25 nm in
average diameter in the depth region of 0.9-1.5 m. In this
region, the displacement damage level and the He ion
concentration increased from 15 dpa and 100 appm to
33 dpa and 150 appm, respectively. This result means that
helium exceeding 100 appm can produce cavities.
In future, void swelling and microstructural evolution of
dual-ion beam irradiated EHP Ni-base superalloy will be
analyzed at the lower dose.
References
1) A. A. Bauer et al., J. Nucl. Mater. 91 (1972) 42.
2) L. R. Greenwood, J. Nucl. Mater. 137 (1983) 115.
3) E. H. Lee et al., J. Nucl. Mater. 79 (1979) 83.
Fig. 1 Bright field images of (a) single and (b) dual ion
irradiated EHP alloy up to 50 dpa at 825 K.