JAEA-Review-2010-065.pdf:15.99MB - 日本原子力研究開発機構
JAEA-Review-2010-065.pdf:15.99MB - 日本原子力研究開発機構
JAEA-Review-2010-065.pdf:15.99MB - 日本原子力研究開発機構
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1-29<br />
Irradiation Hardening in Ion-irradiated Hafnium<br />
Y. Chimi and J. Ogiyanagi<br />
LWR Long-term Reliability Research Unit, NSRC, <strong>JAEA</strong><br />
Pure hafnium metal is used as one of the neutron<br />
absorbers for control rods in commercial light water reactors,<br />
due to long nuclear lifetime and good corrosion resistance.<br />
In recent years, many cracks on the stainless steel sheath of<br />
control rods using hafnium plates have been found in<br />
Japanese boiling water reactors. The cracks might be<br />
caused by irradiation-assisted stress corrosion cracking<br />
(IASCC) due to the tensile stress originated from the<br />
irradiation growth of hafnium. However, the irradiation<br />
behavior of hafnium has been hardly reported in literature so<br />
far. We have investigated the defect production behavior<br />
of hafnium irradiated with energetic electrons, and the<br />
displacement threshold energy for hafnium was estimated as<br />
E d = 28 eV 1) . In the present work, in order to study the<br />
irradiation behavior of mechanical properties in hafnium, the<br />
irradiation hardening in hafnium irradiated with energetic<br />
heavy-ions has been investigated.<br />
Polycrystalline hafnium plates and rods were used as<br />
specimens. Hafnium plates with ~1 mm in thickness were<br />
cut into small pieces (~5 mm 5 mm), and hafnium rods<br />
with ~5 mm and ~3 mm in diameter (denote as “rod-1” and<br />
“rod-2”, respectively) were cut into disk shape with ~1 mm<br />
in thickness. A plane surface of each specimen was<br />
mechanically and chemically polished.<br />
Ion irradiation of the specimen was performed with<br />
12.0 MeV 56 Fe 4+ ions at 290 °C up to the ion fluence of<br />
9.4 10 15 cm -2 by using a 3-MV tandem accelerator in<br />
TIARA, <strong>JAEA</strong>-Takasaki. The projected range of the ions<br />
is ~2.5 m and the maximum damage cross-section is<br />
2.5 10 -15 cm 2 at a depth of 2.4 m from the specimen<br />
surface, based on the calculation using SRIM code (using<br />
1)<br />
Ed=28 eV ). Therefore, the maximum irradiation dose and<br />
dose rate are estimated to be ~23 dpa and ~7 10 -4 dpa/s,<br />
respectively. In order to eliminate the effect of temperature<br />
history of the specimen, a part of each specimen was<br />
masked by thick metal plate not to be irradiated.<br />
For observation of irradiation hardening in the<br />
ion-irradiated specimen, indentation tests were performed at<br />
room temperature using the UMIS-2000 (CSIRO, Australia)<br />
ultra micro-indentation testing system with Berkovich-type<br />
indenter tip. Indentation depth of 500 nm (corresponding<br />
to indentation load of around 25 mN) was adopted to<br />
observe only the damaged region in the specimen.<br />
Indentation measurements were carried out ~10 times at<br />
intervals of 20 m in both the irradiated and un-irradiated<br />
regions for each specimen.<br />
The results of micro-indentation hardness for<br />
ion-irradiated hafnium specimens are shown in Fig. 1 as a<br />
function of ion fluence. For hafnium plate specimen, the<br />
values of micro-hardness are higher than those for rod<br />
<strong>JAEA</strong>-<strong>Review</strong> <strong>2010</strong>-065<br />
- 33 -<br />
specimens because of the difference of texture caused by the<br />
different manufacturing processes between plate and rod<br />
materials. A large increase in micro-hardness was<br />
observed at the ion fluence of ~ 4 10 14 cm -2 , corresponding<br />
to ~1 dpa at a peak of damage profile. Above ~ 4 <br />
10 14 cm -2 , micro-hardness shows gradual increase with ion<br />
fluence, and has not been saturated even at 9.4 10 15 cm -2<br />
(~ 23 dpa).<br />
For comparison, results for a hafnium rod specimen<br />
irradiated at 190 °C are also shown in Fig. 1. The<br />
micro-hardness for 190 °C irradiation is a little smaller than<br />
those for 290 °C irradiation, whereas the values of<br />
micro-hardness in the un-irradiated region are in the same<br />
range for both 190 °C and 290 °C irradiations.<br />
Irradiation hardening behavior we have observed for pure<br />
hafnium metals is strongly concerned with the evolution of<br />
point defects, defect clusters and dislocation loops, which is<br />
affected by the irradiation temperature, dose and dose rate.<br />
In order to evaluate and clarify the irradiation behavior<br />
including irradiation hardening and also irradiation growth,<br />
it is necessary to understand the relationship between<br />
mechanical property change and microstructural evolution.<br />
Reference<br />
1) Y. Chimi et al., <strong>JAEA</strong> Takasaki Ann. Rep. 2008 (2009)<br />
37.<br />
Micro-hardness (GPa)<br />
5<br />
4<br />
3<br />
Hf plate<br />
Hf rod-1<br />
Hf rod-2<br />
Hf rod-1(190°C)<br />
0 5<br />
Ion fluence (10<br />
10<br />
15 cm -2 )<br />
Fig. 1 Ion fluence dependence of micro-hardness<br />
for ion-irradiated hafnium (Hf) plate and rods.<br />
Dotted lines are guides for the eye.