JAEA-Conf 2011-002 - 日本原子力研究開発機構

JAEA-Conf 2011-002
Toshimasa YOSHIIE
Research Reactor Institute, Kyoto University
Kumatori-cho, Sennan-gun, Osaka-fu, 590-0494 Japan
yoshiie@rri.kyoto-u.ac.jp
Nuclear data plays an important role for the study of materials irradiation effects. Displacement per atom
(DPA) is a commonly used parameter to estimate the displacement damage by particle irradiation on materials.
Recently the primary knock-on atom (PKA) energy spectrum analysis has been proposed to introduce effects of
cascade formation. In both cases, accurate nuclear data is required for the analysis of damage structures,
especially incident particle energy is high. In this paper, the application of nuclear data for materials irradiation
experiments is demonstrated. Then a multiscale modeling of irradiation effect in high energy proton irradiated Ni
is shown.
For the development of nuclear materials which are used under high energy particle irradiation, materials
degradation by irradiation damage is the most important factor in determining the lifetime of components in the
nuclear system. Irradiation experiments are essential to the development of such structural materials. There are,
however, several materials being developed despite the lack of appropriate irradiation test facilities, such as a
fusion reactor and an accelerator driven system (ADS). In these cases, there are two ways to investigate materials
properties. One is simulation irradiations by using other irradiation facilities. The other is computer simulations.
In these studies, the nuclear data plays an important role. In this paper, the role of nuclear data for materials
irradiation studies is demonstrated.
For simulation irradiations, we use various kinds of irradiation facilities, such as neutron irradiation, ion
irradiation and electron irradiation facilities. For these cases, one needs to translate the data to other high energy
particle irradiation environment, which requires an understanding of factors that influence generation and
accumulation of point defects. Displacement per atom (DPA) is commonly used as a damage parameter to
estimate the effect of high energy particle irradiation on materials. The accuracy of DPA strongly depends on the
nuclear data when the nuclear reaction is involved in the irradiation. DPA is, however, not sufficient to reflect
for the effect of high energy recoils such as cascade formation. The primary knock-on atom (PKA) energy
spectrum analysis [1-3] has been proposed to compensate the deficit of DPA. The analysis is based on the fact
that a large PKA forms a large cascade and the large cascade separates into several subcascades. In each
subcascade, vacancy rich area is surrounded by an interstitial rich area and point defect reactions occur in each
subcascade. In the case of fcc metals, stacking fault tetrahedra of vacancy type defects are formed directly from
subcascades at lower temperatures such as below 400 K and the PKA energy spectrum analysis is possible.
The PKA energy spectrum analysis was made by fitting the observed cascade size distribution to
calculated PKA energy spectrum. The population of each size of cascade was assigned to the cross-section of
PKA energy from its larger side by larger cascade zones. Fig. 1 is the case of Cu irradiated by 14 MeV neutrons
at room temperature. From the relation between the cascade zone size and PKA energy spectrum, the density of
energy deposition to cascade zone was estimated. The calculated cross-section (curve in the figure) was adopted
from the work by Logan and Russell [4]. From the relationship between the area and deposited energy, it was
concluded that in the case of 100keV and 800keV PKA, 0.35 eV and 0.03 eV were given, respectively, in each
atom in the cascade.
The analysis was also made by fitting the observed subcascade number distribution to calculated PKA
energy spectrum. The groups with higher number of subcascades were assumed to be produced from PKAs with
higher energy as shown in Fig. 2. The fitting of subcascade groups to PKA energy spectrum can be converted to
the relation between the PKA energy and the number of subcascades as in Fig. 3. If one supposes that each of
subcascades comes from the same energy, the subcascade formation energy is about 10 keV.