JAEA-Conf 2011-002 - 日本原子力研究開発機構

JAEA-Conf 2011-002
Figure 6. Comparison of damage structures in thin foil irradiated Au between (a) fusion neutron irradiation to
0.017 dpa at 563 K and (b) fission neuron irradiation to 0.044 dpa at 573 K.
Figure 7. Comparison of PKA energy spectra between fusion neutrons and fission neutrons in Au.
A spallation neutron source is a coupling between a target and a proton accelerator. High energy proton
(GeV) irradiation produces a large number of neutrons in the target. The beam window and the target materials
are thus subjected to a very high irradiation load by source protons and generated spallation neutrons. At present,
there are no window materials that can operate for the desired period of time without deterioration of mechanical
properties.
Irradiation experiments are essential to the development of such structural materials. Recently, strong
spallation neutron sources, SNS in the United States and J-PARC in Japan have been completed. These facilities,
however, are not designed for materials irradiation experiments. Therefore computer simulations play an
important role in predicting the behavior of materials.
In this section, changes in mechanical property of Ni after irradiation with 3 GeV protons were calculated
using multi-scale modeling [6]. A proton energy of 3GeV was chosen to simulate the J-PARC spallation neutron
source. Ni is the simplest model material of austenitic stainless steels, which are currently used as the proton
beam window. The multi-scale modeling code consisted of four parts.
The first part covered nuclear reactions based on the PHITS code [7]. This part calculated the interactions
between high energy protons and nuclei in the target from 10 -22 s and estimated secondary particles and PKAs.
The second part simulated atomic collisions between particles without nuclear reactions. Because the energy of
particles was high, the PKA energy spectrum analysis was employed according to the procedure by Satoh et al.
[8]. In each subcascade, the direct formation of clusters and the number of mobile defects were estimated using
molecular dynamics and kinetic Monte-Carlo methods. The third part considered damage structural evolution,
estimated using reaction kinetic analysis. The development of damage structures affects the mechanical
properties of target materials. The fourth part estimated mechanical property changes using three-dimensional
discrete dislocation dynamics (DDD). Stress-strain curves of high energy proton irradiated Ni were thus obtained.