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

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Irradiation Hardening in Ion-irradiated Hafnium
Y. Chimi and J. Ogiyanagi
LWR Long-term Reliability Research Unit, NSRC, JAEA
Pure hafnium metal is used as one of the neutron
absorbers for control rods in commercial light water reactors,
due to long nuclear lifetime and good corrosion resistance.
In recent years, many cracks on the stainless steel sheath of
control rods using hafnium plates have been found in
Japanese boiling water reactors. The cracks might be
caused by irradiation-assisted stress corrosion cracking
(IASCC) due to the tensile stress originated from the
irradiation growth of hafnium. However, the irradiation
behavior of hafnium has been hardly reported in literature so
far. We have investigated the defect production behavior
of hafnium irradiated with energetic electrons, and the
displacement threshold energy for hafnium was estimated as
E d = 28 eV 1) . In the present work, in order to study the
irradiation behavior of mechanical properties in hafnium, the
irradiation hardening in hafnium irradiated with energetic
heavy-ions has been investigated.
Polycrystalline hafnium plates and rods were used as
specimens. Hafnium plates with ~1 mm in thickness were
cut into small pieces (~5 mm 5 mm), and hafnium rods
with ~5 mm and ~3 mm in diameter (denote as “rod-1” and
“rod-2”, respectively) were cut into disk shape with ~1 mm
in thickness. A plane surface of each specimen was
mechanically and chemically polished.
Ion irradiation of the specimen was performed with
12.0 MeV 56 Fe 4+ ions at 290 °C up to the ion fluence of
9.4 10 15 cm -2 by using a 3-MV tandem accelerator in
TIARA, JAEA-Takasaki. The projected range of the ions
is ~2.5 m and the maximum damage cross-section is
2.5 10 -15 cm 2 at a depth of 2.4 m from the specimen
surface, based on the calculation using SRIM code (using
1)
Ed=28 eV ). Therefore, the maximum irradiation dose and
dose rate are estimated to be ~23 dpa and ~7 10 -4 dpa/s,
respectively. In order to eliminate the effect of temperature
history of the specimen, a part of each specimen was
masked by thick metal plate not to be irradiated.
For observation of irradiation hardening in the
ion-irradiated specimen, indentation tests were performed at
room temperature using the UMIS-2000 (CSIRO, Australia)
ultra micro-indentation testing system with Berkovich-type
indenter tip. Indentation depth of 500 nm (corresponding
to indentation load of around 25 mN) was adopted to
observe only the damaged region in the specimen.
Indentation measurements were carried out ~10 times at
intervals of 20 m in both the irradiated and un-irradiated
regions for each specimen.
The results of micro-indentation hardness for
ion-irradiated hafnium specimens are shown in Fig. 1 as a
function of ion fluence. For hafnium plate specimen, the
values of micro-hardness are higher than those for rod
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specimens because of the difference of texture caused by the
different manufacturing processes between plate and rod
materials. A large increase in micro-hardness was
observed at the ion fluence of ~ 4 10 14 cm -2 , corresponding
to ~1 dpa at a peak of damage profile. Above ~ 4
10 14 cm -2 , micro-hardness shows gradual increase with ion
fluence, and has not been saturated even at 9.4 10 15 cm -2
(~ 23 dpa).
For comparison, results for a hafnium rod specimen
irradiated at 190 °C are also shown in Fig. 1. The
micro-hardness for 190 °C irradiation is a little smaller than
those for 290 °C irradiation, whereas the values of
micro-hardness in the un-irradiated region are in the same
range for both 190 °C and 290 °C irradiations.
Irradiation hardening behavior we have observed for pure
hafnium metals is strongly concerned with the evolution of
point defects, defect clusters and dislocation loops, which is
affected by the irradiation temperature, dose and dose rate.
In order to evaluate and clarify the irradiation behavior
including irradiation hardening and also irradiation growth,
it is necessary to understand the relationship between
mechanical property change and microstructural evolution.
Reference
1) Y. Chimi et al., JAEA Takasaki Ann. Rep. 2008 (2009)
37.
Micro-hardness (GPa)
5
4
3
Hf plate
Hf rod-1
Hf rod-2
Hf rod-1(190°C)
0 5
Ion fluence (10
10
15 cm -2 )
Fig. 1 Ion fluence dependence of micro-hardness
for ion-irradiated hafnium (Hf) plate and rods.
Dotted lines are guides for the eye.